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COLLECTED 
SCIENTIFIC PAPERS 



CAMBRIDGE UNIVERSITY PRESS 

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COLLECTED 
SCIENTIFIC PAPERS 



BY 



JOHN HENRY POYNTING, Sc.D., F.R.S. 

Mason Professor of Physics in the University of Birmingham 
Formerly Fellow of Trinity College, Cambridge 



CAMBRIDGE 

AT THE UNIVERSITY PRESS 

1920 



PREFACE 

IN the summer of 1914 Sir Oliver Lodge summoned a meeting of the 
colleagues and friends of the late Professor John Henry Poynting to 
consider how best a suitable memorial could be established to perpetuate his 
memory. The following committee was appointed to carry out the purpose 
of the meeting : Sir Oliver Lodge (chairman), Guy Barlow, Neville Chamber- 
lain, P. F. Frankland, Sir R. T. Glazebrook, R. S. Heath, George Hookham, 
Sir Joseph Larmor, Sir Napier Shaw, Sir J. J. Thomson, Sir Richard Threlfall, 
and T. Sydney Walker, with G. H. Morley as Secretary and G. A. Shakespear 
as Treasurer. 

A fund was opened and subscriptions were invited, and the Committee 
decided that there could be no better memorial than the publication in a 
collected form of the Scientific Papers of John Henry Poynting, and the 
distribution of copies to certain scientific institutions throughout the world. 

The task of editing and proof correction was gladly undertaken by his 
junior colleagues G. A. Shakespear and Guy Barlow who have throughout 
had the benefit of the advice of Sir Oliver Lodge. 

Biographical and critical notices by Sir Oliver Lodge, Sir Joseph Larmor, 
Sir J. J. Thomson and G. A. Shakespear have been inserted as an introduction 
to the volume. 

The papers have been arranged in groups, with the object of bringing 
together those dealing with kindred subjects. In each group the papers 
are in chronological order. 

In such collections there is inevitably a certain amount of repetition or 
overlapping, since a subject is often dealt with from more than one point of 
view — the strictly scientific and the popular — but since this variety of treat- 
ment is helpful, in cases of doubt the decision has generally been given in 
favour of inclusion. 

The popular discourses and general articles have for the most part been 
relegated to the last section of the volume. 

His books, and other publications which are easily accessible, are not 
included; but a complete list of his works in chronological order is given 
at the end of the volume. 

The papers have been reproduced as originally published except for small 
verbal corrections here and there. In certain cases mistakes have been 



y| PREFACE 

corrected after reference to the original manuscripts which were kindly put 
at our disposal by Mrs. Poynting ; but, in all cases, corrections of any import- 
ance have been indicated in footnotes, and editorial comments have been 
included in square brackets and marked Ed. 

We wish to acknowledge most warmly the unstinted generosity with which 
permission to republish the several articles has been given both by joint 
authors and by the original publishers, including the representatives of the 
following : 

The Birmingham Natural History and Philosophical Society. 

The British Association. 

Le Bureau International des Poids et Mesures. 

The Electrician. 

The Encyclopaedia of Religion and Ethics (Messrs T. and T. Clark). 

The Hibbert Journal. 

The Indiarubber Journal. 

The Inquirer. 

The Mason College Magazine. 

The Manchester Literary and Philosophical Society. 

Nature. 

The Philosophical Magazine. 

The Royal Astronomical Society. 

The Royal Society. 

The Royal Statistical Society. 

The Royal Institution. 

La Societe de Physique. 
: , The Committee was fortunate in securing the services of the Cambridge 
University Press as publishers, and the editors take this opportunity of 
thanking them for their unfailing courtesy and for invaluable help in proof- 
correcting. 

G. A. SHAKESPEAR. 
GUY BARLOW. 



The University. Birmingh.\m. 
1920. 



A PERSONAL NOTE 

John Henry Poynting was a man admired of all who knew him, beloved 
of all who knew him well. 

Of somewhat less than middle height and sturdy thickset build, his general 
appearance was suggestive rather of rural than academic interests, »but even 
a casual observer would probably have been struck with the sense of power 
indicated by his fine head. His face, which was of a meditative cast when 
in repose, lighted with a genial and friendly warmth when he was conversing 
with friends, and his grey-blue eyes were expressive of the kindly gentleness 
of his nature. 

In habit he was methodical, and indeed the condition of his health was for 
many years such that he could not have accomplished so much had he not 
economised effort with method. (He told me once that he had never been 
able to do more than six hours' useful work a day.) He had, moreover, 
great power of concentration. 

He was a remarkably clear thinker and had that characteristic insight 
into fundamental ideas which intuitively distinguishes between hypothesis 
and fact ; and it was probably for this reason that he viewed with suspicion 
some of the more recent developments of mathematical physics. He withheld 
his judgment when the experimental foundations were either wanting or else 
inadequate to bear the superstructure erected upon them. 

Himself a man of wide interests and sympathies, and with a finely balanced 
sense of proportion, he was keenly alive to the danger of that too-exclusive 
specialisation which so frequently makes a man incapable of conversation 
except in his awn particular line of work. He felt the need of guarding against 
atrophy of the spiritual side of his character in his outlook on life, and sought 
in the reading, of fiction, and even more of poetry, the complement to the 
intellectual stimulus of scientific work. Indeed of poetry he read much, 
though he spoke of it but rarely even with his intimate friends ; but those 
to whom he opened his mind on such matters knew his deep admiration 
for many of the English poets. Shelley, Keats and still more Wordsworth, 
appealed to him strongly, for he himself was imbued with that love of Nature 
which inspired them. 

He was probably never happier than when living in the beautiful 
Alvechurch district of Worcestershire, where he found an unfailing source 
of pleasure in the rolling landscape which stretched away into the distance 



Viii A PERSONAL NOTE 

around his upland home. He would walk in the fields with some friend, 
here pointing out the haunt of a rare wildflower, there showing, with all the 
interest of a schoolboy, the nest of an uncommon bird, or discussing the 
effect of atmosphere on the landscape. The conversation would wander 
from the botanical afiinities of the tway-blade to the mechanism whereby 
the grasshopper performs his prodigious leaps, or from the theory of the 
grinding action of a cider-press to the causes and prevention of crime in large 
cities. To such occasions his equable temper and ready sense of humour 
lent a rare charm. 

Among his outdoor recreations cycling through the country lanes held 
a high place. Indoors, in his later years, he derived much pleasure from 
a piano-player ; and in the evenings, when too tired to read, he would often 
amuse himself with a game of "patience." 

In politics he was a liberal, and perhaps the thing for which he stood 
more strongly than anything else was freedom and liberty of thought; the 
thing of which he was most intolerant was bigotry, and indeed in an intimate 
aquaintance of 20 years' duration the only time T ever heard him speak of 
any man with bitterness was in reference to a case of religious intolerance. 

The soundness of his judgment being well known, his advice was often 
sought ; and, whether the matter were small or great, he always gave of his 
best, earning thereby the gratitude of many. 

He had a great sympathy with humanity in general and especially with 
the poorer classes. As a magistrate he tempered judgment with mercy and 
his experience in this capacity confirmed him in the opinion that delinquents 
in general were as often sinned against as sinning. 

The comradeship of his home-life was ideal, while to his students he was 
an object of admiration and of affectionate regard in a degree which perhaps 
they alone can appreciate. 

His ability may to some extent be judged from his pubhshed work, but 
his personal worth and charm are truly measured by the affection with which 
he inspired those who knew him best. * 

G. A. S. 



OBITUARY NOTICES 

[From Nature, vol. xciii, p. 138, with additions.] 

On the evening of Monday, March 30, 1914, surrounded by his family 
John Henry Poynting passed quietly away. A memorial service was held in 
Birmingham on the Thursday following, and was attended by representatives 
of many universities and learned societies, including Sir J. J. Thomson, 
Sir Joseph Larmor, Dr Glazebrook, Sir William Tilden, Prof. W. M. Hicks, 
Dr W. N. Shaw, and of course by many colleagues and councillors of the 
University in which he occupied a chair, as well as by a large number of 
private citizens and friends. For he was a man universally beloved. 

At the memorial service, the following true words concerning him were 
spoken by the Rev. Henry Gow, who knew him well : 

"We remember that he did work to make him famous throughout the 
world of science which gave him a high place amongst the discoverers of truth ; 
but we remember much more than that. We remember how he loved life, 
how interested he was in little things, how he delighted in children, in flowers, 
and in birds; what confidence and affection he inspired, how free he was 
from claims of self and from uneasy egotism; how much happiness he felt 
and gave. We remember his wise judgments, strong character, cheerful 
courage, his delightful humour, and a certain peaceful beauty and childlike 
joyousness of spirit behind all his multifarious gifts. He rejoiced to be the 
friend as well as the teacher of the young. He kept his heart free from all 
bitterness and disillusion which come so often to us in our later years. He 
knew and felt always how beautiful and great a thing it was to be alive." 

He was born on September 9, 1852, at Monton, near Manchester, son 
of the unitarian minister of that place. His first education was at home, 
but the years 1867 to 1872 he passed at Owens College, Manchester, graduating 
B.Sc. at the London University, and proceeding, in 1872, to Trinity College, 
Cambridge, where he was bracketed third wrangler in 1876. 

He was then appointed demonstrator at Owens College by Balfour Stewart, 
and began a life-long friendship with Sir J. J. Thomson, who was at that time 
a student. In due time Poynting became a fellow of Trinity, and in 1880 
was appointed to the professorship of physics at Birmingham, which he 
held to the day of his death. 

The four first professors of the Mason College, which was opened by 
Huxley in 1880 (who delivered, on this occasion, a notable address, reprinted 



X OBITUARY NOTICES 

as the first of his collected essays), were Sir Wm. Tilden, Prof. M. J. M. Hill, 
Dr T. W. Bridge, who died a few years ago, and Poynting. In this same year 
Poynting married Miss M. A. Cropper, daughter of the Rev. J. Cropper, of 
Stand, near Manchester. In 1887 he received the Sc.D. of Cambridge, and 
in 1888 the fellowship of the Royal Society. In 1893 the Adams prize 
was awarded to him, and in 1899 he presided over Section A of the British 
Association at Dover. This meeting was memorable for the clear discovery 
of the separate existence of electrons, which was announced to Section A 
bv Sir J. J. Thomson on an occasion when many members of the French 
Association, meeting simultaneously at Boulogne, had come over for friendly 
fraternisation. 

In 1905 Poynting became president of the Physical Society, and was 
awarded a Royal medal by the Royal Society " for his researches in physical 
science, especially in connection with the constant of gravitation and the 
theories of electro-dynamics and radiation." In this brief summary an 
immense amount of work is referred to. The work for which he is locally 
best known was his determination of the Newtonian constant of gravitation 
by the very accurate use of an ordinary balance with an adjustable mass 
under one or other of the arms — a determination which is popularly called 
" weighing the earth." His account of it appears in the Phil. Trans, for 1891. 
It is a classical memoir of its kind, and very instructive to the physical 
student, but the papers on electro-dynamics eclipse it in value. These were 
''communicated" to the Royal Society in 1884 and 1885 respectively, their 
titles being "On the Transfer of Energy in the Electromagnetic Field," 
and "On the Connection between Electric Current and the Electric and 
Magnetic Inductions in the Surrounding Field." 

The memoir on the transfer of energy aroused universal attention. The 
paths by which energy travels from an electromotive source to various parts 
of a circuit were displayed, and their intricacies unravelled, for the first time ; 
idottihj of energy might legitimately be urged as a supplement to conservation 
(see a paper by the present writer in Phil. Mag., June, 1885) ; and it is to these 
papers that we owe that fundamental generalisation, connecting mechanical 
motion with electric and magnetic forces, which is known all over the world 
as "Poynting's Theorem." 

The following letter from Sir Joseph Larmor to the writer expresses a 
mathematician's view of the importance of this subject: 

St John's College, Cambridge, 10th May, 1915. 

•'Nobody before Poynting seems to have thought of tracing the flux of 
energy in a medium elasticaUy transmitting it, and where the whole process 
is therefore exposed to view. The line of flow is a ray in optics: thus it 
includes a dynamical aspect of that conception added on to and of course 
consistent with the Huygenian or rather Young-Fresnelian one. The electric 



OBITUARY NOTICES XX 

and optical ray is implicitly in Maxwell's equations, and is only a corollary 
to them. But in any other kind of elastic transmission, e.g. waves in an 
elastic-solid medium, a corresponding theory can be worked out. 

I take it this idea is Poynting's main contribution, and it clarified many 
things, especially electrical." 

A great expansion of this note is contained in a remarkable paper On the 
Dynamics of Radiation which Sir Joseph Larmor communicated to the Inter- 
national Congress of Mathematicians meeting at Cambridge in August, 1912. 
This paper is so intimately associated with Poynting's work, and so pleased 
him when he saw it, that I have asked and obtained permission to include 
extracts from it in this volume; they will be found at the end of the 
Section dealing with the Pressure of Light. 

" The essential characteristic of an electrodynamic system is the existence 
of the correlated fields, electric and magnetic, which occupy the space sur- 
rounding the central body, and which are an essential part of the system; 
to the presence of this pervading aethereal field, intrinsic to the system, all 
other systems situated in that space have to adapt themselves. When a 
material electric system is disturbed, its electrodynamic field becomes 
modified, by a process which consists in propagation of change outward, 
after the manner of radiation, from the disturbance of electrons that is 
occurring in the core. When however we are dealing with electric changes 
which are, in duration, slow compared with the time that radiation would 
require to travel across a distance of the order of the greatest diameter of 
the system — in fact in all electric manifestations except those bearing directly 
on optical or radiant phenomena — complexities arising from the finite rate 
of propagation of the fields of force across space are not sensibly involved : 
the adjustment of the field surrounding the interacting systems can be taken 
as virtually instantaneous, so that the operative fields of force, though in 
essence propagated, are sensibly statical fields. The practical problems of 
electrodynamics are of this nature — how does the modified field of force, 
transmitted through the aether from a disturbed electric system, and thus 
established in the space around and alongside the neighbouring conductors 
which alone are amenable to our observation, penetrate into these conductors 
and thereby set up electric disturbance in them also? and how does the 
field emitted in turn by these new disturbances interact with the original 
exciting field and with its core ? For example, if we are dealing with a circuit 
of good conducting quality and finite cross section, situated in an alternating 
field of fairly rapid frequency, we know that the penetration of the arriving 
field into the conductor is counteracted by the mobility of its electrons, whose 
motion, by obeying the force, in so far annuls it by Newtonian kinetic reaction ; 
so that instead of being propagated, the field soaks in by diffusion, and it does 
not get very deep even when adjustment is delayed by the friction of the vast 
numbers of ions which it starts into motion, and which have to push their way 
through the crowd of material molecules; and the phenomena of surface 
currents thus arise. If (by a figure of speech) we abolish the aether in which 
both the generating circuit and the secondary circuit which it excites are 
immersed, in which they in fact subsist, the changing phases of the generator 
could not thus establish, from instant to instant, by almost instantaneous 



Xii OBITUARY NOTICES 

radiant transmission, their changing fields of force in the ambient region 
extending across to the secondary circuit, and the ions in and along that 
circuit would remain undisturbed, having no stimulus to respond to. The 
aethereal phenomenon, viz., the radiant propagation of the fields of force, 
and the material phenomenon, viz., the response of the ions of material bodies 
to those fields, involving the establishment of currents with new fields of their 
own, are the two interacting factors. The excitation of an alternating current 
in a wire, and the mode of distribution of the current across its section, depend 
on the continued establishment in the region around the wire, by processes 
of the nature of radiation, of the changing electromagnetic field that seizes 
hold on the ions and so excites the current ; and the question how deep this 
influence can soak into the wire is the object of investigation. The aspect 
of the subject which is thus illustrated, finds in the surrounding region, in the 
aether, the seat of all electrodynamic action, and in the motions of electrons 
its exciting cause. The energies required to propel the ions, and so establish 
an induced current, are radiant energies which penetrate into the conductor 
from its sides, being transmitted there elastically through the aether; and 
these energies are thereby ultimately in part degraded into the heat arising 
from fortuitous ionic motions, and in part transformed to available energy 
of mechanical forces between the conductors. The idea — introduced by 
Faraday, developed into precision by Maxwell, expounded and illustrated 
in various ways by Heaviside, Poynting, Hertz — of radiant fields of force, 
in which all the material electric circuits are immersed, and by which all 
currents and electric distributions are dominated, is the root of the modern 
exact analysis of all electric activity." 

Poynting's work on radiation appeared partly in the Phil. Trans, for 1904 
and partly in the Phil. Mag. for 1905. In these memoirs the tangential 
pressure of radiation is analysed and demonstrated; and it is shown, both 
theoretically and experimentally, that a beam of light behaves essentially as 
a stream of momentum, and gives all the mechanical results which may thus 
be expected, though of a magnitude exceedingly minute. Nevertheless, he 
goes on to show that these radiation-pressures, however small, are of much 
consequence in astronomy, and have many interesting and some conspicuous 
results. A noteworthy part of the radiation memoirs, however, is independent 
of considerations of pressure or momentum, and gives a means of determining 
the absolute temperatures of sun and planets, and of other masses in space, 
in a singularly clear and conclusive manner. 

A complete list of his publications is given below, but special mention 
must be made here of the important series of text-books on physics, 
written in conjunction with his friend. Sir J. J. Thomson. 

He took great interest also in the philosophical aspects of physical science, 
and his help is acknowledged by Prof. James Ward in connection with the 
publication of that notable series of Gifford Lectures entitled Naturalism, 
and Agnosticism. Poynting was strongly inclined, almost unduly, to limit 
the province of science to description, and to regard a law of nature as nothing 
but a formulation of observed correspondences. He wished to abolish the 



OBITUARY NOTICES XUl 

idea of cause in physics. In some of this he may have gone too far ; but his 
rebellion against an excessive anthropomorphism which had begun to cling 
around the notion of natural laws, as if they were really legal enactments to 
be obeyed or disobeyed by inert matter almost as if it possessed will-power 
and could exercise choice, some substances being praised as good radiators 
while others are stigmatised as bad — most gases being admittedly unable to 
reach a standard of perfection held out to them as Boyle's law, though a 
few of excessive merit might surpass it, — Poynting's revolt against this kind 
of attitude to laws of nature, though doubtless more than half humorous, 
was in itself wholesome. Some of his philosophic views may be read, as a 
Presidential Address to Section A of the British Association for 1899 (infra 
p. 599) ; but I think it useful and legitimate to extract a few sentences 
from that address and quote them here, as an illustration of his mode of 
approaching the misty region where physics and metaphysics intertwine: 

" To take an old but never- worn-out metaphor, the physicist is examining 
the garment of Nature, learning of how many, or rather of how few different 
kinds of thread it is woven, finding how each separate thread enters into 
the pattern, and seeking from the pattern woven in the past to know the 
pattern yet to come.... So, as we watch the weaving of the garment of Nature, 
we resolve it in imagination into threads of ether spangled over with beads 
of matter. We look still closer, and the beads of matter vanish; they are 
mere knots and loops in the threads of ether." 

And then, a few pages further on, when dealing with the interaction of 
Matter and Mind : 

"Do we, or do we not, as a matter of fact, make any attempt to apply 
the physical method to describe and explain those motions of matter which 
on the psychical view we term voluntary? 

Any commonplace example, and the more commonplace the more is it 
to the point, will at once tell us our practice, whatever may be our theory. 
For instance, a steamer is going across the Channel. We can give a fairly 
good physical account of the motion of the steamer. We can describe how 
the energy stored in the coal passes out through the boiler into the machinery, 
and how it is ultimately absorbed by the sea. And the machinery once 
started, we can give an account of the actions and reactions between its 
various parts and the water, and if only outsiders will not interfere, we can 
predict with some approach to correctness how the vessel will run. All 
these processes can be likened to processes already studied — perhaps on 
another scale — in our laboratories, and from the similarities prediction is 
possible. But now think of a passenger on board who has received an 
invitation to take the journey. It is simply a matter of fact that we make 
no attempt at a complete physical account and explanation of those actions 
which he takes to accomplish his purpose. We trace no lines of induction 
in the ether connecting him with his friends across the Channel, we seek no 
law of force under which he moves. In practice the strictest physicist 
abandons the physical view, and replaces it by the psychical. He admits 
the study of purpose as well as the study of motion." 



Xiv OBITUARY NOTICES 

In other words he recognises Mind and Purpose as dominant over and 
in a different category from Matter and Mechanism. 

In psychical phenomena Poynting was, I judge, an agnostic, but on the 
question of a materialistic or naturalistic explanation of mental phenomena 
he expresses himself thus, in the Dover Section A address above referred to : 

"It appears to me that the assumption that our methods do apply, and 
that purely physical explanation will suffice to predict all motions and changes, 
voluntary and involuntary, is at present simply a gigantic extra-polation, 
which we should unhesitatingly reject if it were merely a case of ordinary 
physical investigation. The physicist when thus extending his range is 
ceasing to be a physicist, ceasing to be content with his descriptive methods 
in his intense desire to show that he is a physicist throughout." 

But I must not delay further on his scientific work ; the man himself 
was even more than his work. When the Mason College became the University 
of Birmingham Poynting was elected Dean of the Faculty of Science; in 
that capacity his quiet wisdom and efficiency were very manifest, and keen 
was the regret of all his colleagues when, some twelve years later, failing 
health necessitated his yielding this office to another. His judgment was 
as sound as his knowledge, and his conspicuous fairness endeared him to 
colleagues and the members of his staff. By the latter it is not too much 
to say that he was regarded with affectionate veneration ; one of them writes 
to me as follows : 

" As to his character it is impossible to give the right impression to those 
who did not know him well. I consider him a man of very extraordinary 
ability, which might have carried him much farther if it had been associated 
with more self-assertion. But it was largely this modesty and self-suppression 
which created a very unusual degree of affection in those who had the privilege 
of knowing him intimately. I always associate him in my mind with Faraday 
and Stokes." 

As a lecturer and teacher he was admirable, and the respect in which he 
was held by his peers was noteworthy. I am glad to remember that so 
recently as the 1913 meeting of the British Association, some of the greatest 
physicists in the world, who were staying with me — Prof. H. A. Lorentz, 
Lord Rayleigh, and Sir Joseph Larmor — went to his house one evening, 
and met there in his study Sir J. J. Thomson and Dr Glazebrook, who were 
staying with him; thus constituting an appropriate and representative 
gathering, and giving him a pleasure which he remembered to the end of 
his life. 

There is much more that might be said ; but let his position in the world 
of science be what it may, we in the University of his mature life knew him 
well, and know him best as an admirable colleague, a staunch friend, and a 
good man. 

0. J. L. 



OBITUARY NOTICES XV 



[From the Proceedings of the Royal Society, A, vol. xcii, 1914.] 

John Henry Poynting, the youngest son of the Rev. T. E. Poynting, 
Unitarian Minister at Monton, near Manchester, was born there on September 
9, 1852. He received his earlier education at the school kept by his father 
and then went, in 1867, to the Owens College, which his elder brother, C. T. 
Poynting, who was for many years Unitarian Minister at Fallowfield, near 
Manchester, had just left. Poynting must have received a good grounding 
in Mathematics at his father's school, as he gained a Dalton Entrance Exhibi- 
tion in Mathematics before entering the College. Owens College in those 
days was in a modest building, once the residence of Richard Cobden, in 
Quay Street, Deansgate. Neither the amenities of the locality nor the 
accommodation in the building were anything to boast about, but few educa- 
tional institutions before or since, whatever their equipment or surroundings, 
have had a more efficient staff than Owens College in the old Quay Street 
days. As an old Quay Streeter, the writer can speak from personal experience. 
The cramped space was not an unmixed disadvantage. We were so closely 
packed that it was very easy for us to get to know each other. Arts students 
and Science students jostled against each other continually; a crowd of 
Mathematicians would be waiting outside the doors of a lecture room for it 
to discharge a Latin or Greek class, and thus one of the chief difficulties of 
non-residential colleges, the lack of social intercourse between the students, 
was almost absent. 

The professors at Owens in Poynting's time were : Barker for Mathematics, 
of whom Poynting always spoke in terms of the highest appreciation, a feeling 
shared by all his pupils, for no abler or more conscientious teacher of Mathe- 
matics than Thomas Barker ever lived; Jack, another great teacher, was 
Professor of Natural Philosophy; Roscoe of Chemistry, and Williamson of 
Natural History; on the literary side. Greenwood, the Principal, was Pro- 
fessor of Classics, Ward of English History and Literature, Jevons of Logic, 
and that very lovable man, Theodores, lecturer on Modern Languages. At 
that time Owens had not the power of granting degrees, and most of the 
students prepared for the examinations of the University of London. In 
those days these covered a very wide range of subjects, and Poynting, who 
took the London degree, must have attended the lectures of all these professors. 
He was second at the London Matriculation in 1869, obtained Second Class 
Honours in both Physics and Mathematics in the First B.Sc. examination 
in 1871, and took the B.Sc. degree in 1872. In the spring of 1872 he obtained 
an entrance scholarship at Trinity College, Cambridge, and came into residence 
at Cambridge in October. At Cambridge he pursued the normal course of 
one destined for high honours in the Mathematical Tripos. He read with 



Xvi OBITUARY NOTICES 

Routh, he obtained his Major Scholarship in due course, like many of the 
reading men of his time at Trinity he joined the Second Trinity Boat Club 
and rowed in the first boat in 1875 ; the fortunes of that once famous club 
were, however, then declining and it came to an end in 1876. He took his 
degree in the Mathematical Tripos of 1876 as Third Wrangler, bracketed with 
Mr Trimmer, of Trinity College, a very brilliant man who suffered from 
persistent ill-health and died within a few months of taking his degree. As 
Dr Glazebrook and Dr Shaw both graduated in the same Tripos and Lord 
Rayleigh was the additional Examiner, Physics was well represented on 
this occasion. 

After taking his degree Poynting came back for a short time to the Owens 
College, which was now in the buildings it at present occupies, and demon- 
strated in the Physical Laboratory under Prof. Balfour Stewart, who had 
succeeded Jack as Professor of Natural Philosophy shortly before Poynting's 
departure for Cambridge. 

On his election to a Fellowship at Trinity College in 1878, Poynting 
returned to Cambridge and began, in the Cavendish Laboratory under Clerk 
Maxwell, those experiments on the mean density of the earth which were 
destined to occupy so much of his time for the next 10 years. 

He remained at Cambridge until 1880, when he was elected to the Chair 
of Physics in Mason College, Birmingham (now the University of Birmingham), 
which had just been founded; this post he held until his death. The year 
that he went to Birmingham, he married the daughter of the late 
Rev. J. Cropper, of Stand, near Manchester. 

He threw himself whole-heartedly into the arduous duties connected with 
the starting of a new University College, the preparation of his lectures and 
the equipment of the physical laboratory, and, as was his wont, without any 
bustle or hurry he soon had things working efficiently. And so in the efficient 
discharge of his duties as a Professor, in successful original research, in the 
fulfilment of municipal duties, the time passed placidly on, the only cloud 
on an almost idyllic domestic life being his somewhat indifierent health, 
the first threatenings of the disease from which he ultimately died. To see 
if a country life would suit his health better than a town one, the Poyn tings 
moved from Edgbaston to Fox Hill, Alvechurch, a house about 12 miles 
out of Birmingham. There was a small farm attached to the house and 
Poynting entered into farming most heartily, though I am afraid he did 
not derive much pecuniary profit from it. But even farming when the 
agricultural depression was most acute could not impair his good temper or 
ruffle his equanimity. If the farm did not yield money, it gave new interests 
and experiences, and if something was always going wrong, at any rate it 
drove away monotony. The quietness and simplicity of the life were 
thoroughly to the taste of Mrs. Poynting and himself. Life in the country 



OBITUARY NOTICES XVll 

too gave free scope to his taste for Natural History, in which he always took 
great interest ; he was a keen and excellent observer, and a favourite conten- 
tion of his was that physicists were somewhat too much inclined to confine 
their observations to experiments made in the laboratory and did not 
sufficiently avail themselves of the opportunities of studying the physical 
phenomena going on in the sky, the sea, and the earth. The taste for Natural 
History was a family one; his brother, the late Mr. F. Poynting, was an 
excellent ornithologist, devoting himself especially to the study of the eggs 
of British birds, of which he made most careful and accurate water-colour 
drawings — some of these have been reproduced in his book The Eggs of 
British Birds. 

The Poyntings stayed at Foxhill until 1901, when, his health much 
improved, they returned to Edgbaston. His life at this time was a busy 
one, for in addition to the work demanded from him as the head of a large 
and successful School of Physics, he acted as the Dean of the Faculty of 
Science, was a Justice of the Peace, and for some time Chairman of the 
Birmingham Horticultural Society. He had also to plan and superintend the 
erection of a new physical laboratory when his department was transferred 
from its old quarters to the new buildings of the University of Birmingham. 
He went with the British Association to Canada in 1909, when it met at 
Winnipeg, and gave one of the evening lectures ; his subject was the Pressure 
of Light, on which he had been experimenting for several years. He went 
the trip to Vancouver and back and seemed thoroughly to enjoy the visit. 
The pressure of light was also the subject of a lecture which he gave in French 
at Paris before the French Physical Society at Easter, 1911. 

In the spring of 1912 a severe attack of influenza was followed by a 
recrudescence of diabetes, a disease from which he had suffered for some time, 
and he was ordered to take a long rest; he was, in consequence, away from 
Birmingham for two terms. On his return to Birmingham he seemed much 
better, he took an active part in the meeting of the British Association held 
there in September, 1913, and he and Mrs. Poynting entertained a large party 
of physicists at their house in Ampton Road, and it then seemed as if he 
might hope to enjoy many years of useful work. Another attack of influenza 
in the spring of 1914 brought on a very severe attack of diabetes, and he 
died on March 30, 1914. 

It is difficult to attempt to say what Poynting was to his friends without 
using terms which must appear exaggerated to those who did not know him. 
He had a genius for friendship, and a sympathy so delicate and acute that 
whether you were well or ill, in high spirits or low, his presence was a comfort 
and a delight. During a friendship which lasted for more than thirty years, 
I never saw him angry or impatient and never heard him say a bitter or unkind 
thing about man, woman or child. 

p. c. w. h 



Xviii OBITUARY NOTICES 

He took pleasure in many things, in music, in literature, for he was a 
lover of books and a collector in a modest way, in novels of all kinds, good 
and bad. He was fond of the country, and especially of North Wales, where 
he spent most of his vacations, but happiest of all when at home with his 
family. Throughout his life he took considerable interest in Philosophy, 
and a discussion of the philosophical basis of Physics formed part of his 
Presidential Address to Section A at the Dover Meeting of the British 
Association. Views similar to those he there expressed are now held by 
many ; he had formed his years before, when but few in this country agreed 
with them. The excellence of his work received many recognitions, though 
not in my opinion so many as it deserved. He was elected a Fellow of the 
Royal Society in 1888, received a Royal Medal in 1905, served on the Council 
from 1909 to 1911 and was Vice-President in 1910-11. He received the 
Adams Prize from the University of Cambridge in 1893, the Hopkins Prize 
from the Cambridge Philosophical Society in 1903. He was President of 
Section A when the British Association met at Dover in 1899 and was President 
of the Physical Society in 1909-11. 

He was in great request as an Examiner in Physics and no one excelled 
him at this work, his long experience of students, his judgment and common 
sense, the charitable view he took of the limitations of a student's knowledge, 
and the fact that he was never afraid of setting easy papers, made him an 
eminently fair and discriminating examiner. He was very successful as a 
teacher of students of all kinds, those who only took Physics as a subsidiary 
subject as well as those who made it their life's work, the latter he inspired 
with an enthusiasm for research, with some of his own skill in accuracy of 
measurement and with the desire for thoroughness in their work. 

Poynting's Scientific Work. 

This may be divided into four groups : {a) studies on gravitational attrac- 
tion, {b) on the change of state, (c) on the transfer of energy in the electro- 
magnetic field, and (d) on the pressure of light. 

Gravitational Attraction. 

His experiments on the mean density of the earth were commenced in 
Cambridge in 1878 but it took twelve years' steady work before he obtained 
a result with which he was satisfied. The method used was to measure the 
attraction between two known masses A and B by suspending A from one 
of the arms of a balance of the ordinary type and finding the increase in 
weight produced when B was brought underneath it. The balance used in 
the later experiments was one built specially for the experiment by Oertling 
and had a beam 123 cm. long. With a balance of this size the difficulties 
arising from air currents proved very formidable. Poynting fully recognised 



OBITUARY NOTICES XIX 

the advantage of Boys' short torsion balance method in this respect and said 
that if he were designing the apparatus again, instead of using an exceptionally 
large balance for the sake of being able to suspend large masses, he should 
go to the other extreme and make the apparatus as small as possible. At 
the same time, as he points out, the magnitude of the effects produced by the 
air currents made their detection easy, whereas they might have been over- 
looked and not allowed for had they been smaller. The final results (Phil. 
Trans., A, vol. clxxxii, p. 565, 1891) he obtained for A, the mean density 
of the earth, and G, the gravitational constant, were 

A = 54934, 

G = 6-6984 X 10-8. 

Poynting's long investigation incidentally added considerably to our know- 
ledge of the technique of accurate weighings. 

With the co-operation of Gray he made a series of most interesting experi- 
ments (Phil. Trans., A, vol. cxcii, p. 245, 1899) to see if the attraction between 
two quartz crystals was the same when the axes of the crystals were parallel 
as when they were crossed. The method he used was a very ingenious 
application of the principle of forced oscillations, which was so effective that, 
though one sphere was only about 1 cm. in diameter and the other about 
6 cm., the experiments showed that the attractions in the two positions 
could not differ by as much as one part in 10,000. Later he made with 
Phillips a series of experiments to see if weight depended on temperature, 
using as in his first experiments a balance of the ordinary type; the result 
of these was (Proc. Roy. Soc, A, vol. Lxxvi, p. 445, 1905) that between 15° C. 
and 100° C. the change is not greater than 1 in 10^ and between 16*6° C. 
and - 186° C. it is not so great as 1 in 10i« per 1° C. 

Change of State. 
The problem of the change of state was one in which he took especial 
interest, and it was the subject of one of his earliest papers (Phil. Mag. (5), 
vol. XI, p. 32, 1887). His way of picturing this change was to suppose that 
from the surface of a liquid or solid particles were continually breaking free, 
so that through each unit of area of the surface there was a constant escape 
of molecules. This loss was balanced by the passage from the vapour above 
the solid of some of the gaseous particles which struck against its surface, 
so that when there was equilibrium the flow out from the liquid or solid was 
balanced by the flow inward from the gas. The proportion of gaseous mole- 
cules which after striking the surface passed across to the solid or liquid state 
he assumed to be the same for a solid as for a liquid and to be independent 
of the temperature, so that it could be measured by the vapour pressure. 
Thus at the same temperature the flow across water would be proportional 
to the vapour pressure of water, that across ice to the vapour pressure of 

62 



XX OBITUARY NOTICES 

ice, thus ice could only be in equilibrium with water when the vapour pressure 
over ice is equal to that over water. 

Poynting supposed that the mobility of the molecules in liquids and solids 
is increased by pressure — the pressure as it were squeezing the molecules 
out : the amount of the increase depending on the density of the substance, 
diminishing as the density increases. Thus, if pressure increases the escape 
of the molecules from a liquid, a liquid under pressure will evaporate more 
freely, and so for it to be in equilibrium with its vapour the vapour pressure 
must be higher than that over the normal liquid; from the equilibrium 
between water and its vapour in a capillary tube, he found that if Bp is the in- 
crease in the vapour pressure produced by applying a pressure P to the liquid, 
Sp =- Po/p, where o- is the density of the vapour and p that of the liquid. 

Poynting applied this conception of mobility to the case of solutions, 
taking the view that the molecules of the salt formed aggregates with some 
of the water molecules and thus diminished their mobility thereby diminishing 
the number of water molecules which passed from the liquid state through 
each unit of area of surface per second. The mobility of pure water is thus 
greater than that of the solution, so that if the two are separated by a semi- 
permeable membrane more molecules will pass from the water to the solution 
than from the solution to the water, and the water will flow into the solution. 
To prevent this flow the mobility of the molecules of water in the solution 
must be increased by the application of a pressure that will make the mobility 
of the solution equal to that of pure water; this pressure is the osmotic 
pressure. Since under this pressure the mobility of the solution is equal to 
that of pure water the vapour pressure in equilibrium with the pressed 
solution will be the vapour pressure over pure water, so that another definition 
of osmotic pressure would be the pressure required to raise the vapour 
pressure over the solution to that over pure water. On the assumption 
that the presence of one molecule of salt to n of water would diminish the 
mobihty of the water in the proportion of (n — l)/w, which would be the case 
if a molecule of salt imprisoned one and only one molecule of water, Poynting 
showed that the osmotic pressure on his theory would be the pressure exerted 
by the salt molecules if they were in the gaseous state and occupying the 
volume of the solution. Though this theory does not connect the electrical 
properties of solutions with the properties associated with osmotic pressure 
so readily as the dissociation theory, it is so simple and fundamental that 
it helps to give vividness and definiteness to our picture of the processes 
operative in solutions. 

Transfer of Energy. 
The researches by which Poynting is most widely known are those 
published in the papers " On the Transfer of Energy in the Electromagnetic 
Field" (Phil Trans., A, 1884), and "On Electric Currents and the Electric 



OBITUARY NOTICES * XXI 

and Magnetic Induction in the Surrounding Field" (Phil. Trans., A, 1888). 
He says in the first paper, " The aim of this paper is to prove that there is a 
general law for the transfer of energy, according to which it moves at any 
point perpendicularly to the plane containing the lines of electric and 
magnetic force, and that the amount crossing unit of area per second of this 
plane is equal to the product of the two forces multiplied by the sine of the 
angle between them divided by 47r, while the direction of the flow of energy 
is that in which a right-handed screw would move if turned round from the 
positive direction of the electromotive to the positive direction of the magnetic 
intensity." He shows from the equation of the electromagnetic field that 
the rate of increase in the energy inside a closed surface is equal to 

1 1|[? (R^ -Qy) + m (Py - Ra) + n (Qa - PjS)] dS, 

where dS is an element of the closed surface, I, m, n the direction cosines of 
the normal to the surface, P, Q, R the components of the electromotive 
intensity, and a, jS, y those of the magnetic force. This expression may be 
regarded as showing that the energy flows across the surface, the components 
of flux being 

^(R^-Qy), ^{Py-Ra). ^(Oa-P^); 

the vector which has those components is now universally known as Poynting's 
vector; it is at right angles to both the electric and magnetic forces and is 
proportional to the product of these forces and the sine of the angle between 
them. Thus when we can draw equipotential surfaces for both the electric 
and magnetic forces the energy flows along the lines of intersection of the 
two sets of surfaces. Poynting illustrates this theorem by applying it to 
the following cases : a constant current flowing along a straight wire, a con- 
denser discharged by short-circuiting the plates by a wire of great resistance, 
a voltaic battery, a thermo-electric circuit. 

The magnitude of the change in the point of view consequent on the 
principles brought forward in this paper is perhaps shown most clearly in the 
case of the discharge of the condenser and the transference of the energy 
which before the discharge was distributed between its plates into heat in 
the discharging circuit. Before the publication of this paper the general 
opinion was that the energy was transferred along the wire much in the same 
way as hydraulic power is carried through a pipe. On Poynting's yiew the 
energy flows out from the space between the plates and then converges 
sideways into the wire, where it is converted into heat, the paths of the energy 
being those represented in the figure. 

As shown in this figure the paths of energy near the wire are at right 
angles to it. This is not so unless the wire is such a bad conductor that the 



xxu 



OBITUARY NOTICES 



lines of electric force in its neighbourhood run parallel to it; if for example 
the current through the wire were an alternating one with very high 
frequency the electric force near the wire would be at right angles to it. In 
this case the energy would flow parallel to the wire but outside it. 

In the second paper Poynting, taking the view that the electromagnetic 
field consists of distributions of lines of electric and magnetic force, discusses 
the question of the transfer of energy from the point of view of the movement 
of these lines. He applies the same considerations to the question of the 
residual charge in Leyden jars in his fascinating and instructive paper on 
"Discharge of Electricity in an Imperfect Insulator"' {Phil. Mag., vol. v, 1886, 
p. 419). Poynting's vector occurs as a quantity of fundamental importance 
in many theories of electromagnetic action in which the subject is approached 




from a point of view somewhat different from the one he adopted. It appears, 
for example, as a measure of the momentum per unit volume when the electro- 
magnetic field is regarded as a mechanical system and the properties of the 
field as the result of the laws of motion of such a system. It appears, too, 
when we regard magnetic force as the result of the motion of tubes of electric 
force, the direction of motion of these tubes being parallel to Poynting's vector. 

Pressure of Light. 

For some years before his death Poynting devoted much attention to the 
question of radiation and the pressure of light. On the theory of this subject 
he published {Phil. Trans., A, vol. ecu) a very valuable paper, in the first 
part of which he discusses the application of the fourth-power law of radiation 
to determine the temperature of planets (in this he found afterwards he had 
been anticipated by Christiansen). Among other interesting results he 
arrived at the conclusion that the temperature of Mars must be so low that 
life, as we know it, would be impossible on its surface, this result was criticised 



OBITUARY NOTICES XXUl 

by Lowell, but Poynting maintained his ground in a paper published in the 
Philosophical Magazine, December, 1907. The second part of the paper in 
the Philosophical Transactions contains investigations of the repulsive force 
between two hot spheres which arises from the radiation from the one tending 
to repel the other. He showed that if the bodies are in radiation equilibrium 
with the Sun at the distance of the Earth from it, the repulsive effect will be 
greater than the gravitational attraction between them if their radii are 
less than 19-6 cm., if their density were that of water; if they were made 
of lead the corresponding radius would be 1-78 cm. Thus if Saturn's rings 
consisted of very small particles it is possible that the effect of radiation 
might make them repel instead of attract each other. He considers at the 
end of the paper the effect produced by radiation on the orbits of small bodies 
round the Sun and shows that this would ultimately cause them to fall into 
that body. To quote his own words : " The Sun cannot tolerate dust. With 
the pressure of his light he drives the finest particles altogether away from 
his system. With his heat he warms the larger particles. They give out this 
heat again and with it some of that energy which enables them to withstand 
his attraction. Slowly he draws them to himself and at last they unite with 
him and end their separate existence." {Pressure of Light, "Romance of 
Science" Series.) 

He made important contributions to the experimental side of the subject, 
thus with Dr. Barlow he established the existence of the tangential force 
produced when light is reflected from a surface at which there is some absorp- 
tion, and also the existence of a torque when light passes through a prism. 
They also succeeded in demonstrating the existence of the recoil from light 
of a surface giving out radiation : an account of these experiments was given 
in the Bakerian Lecture for 1910 {Proc. Roy. Soc, A, vol. lxxxiii, p. 534, 1910). 
These investigations involved the detection of exceeding minute forces and 
gave ample scope for Poynting's skill in devising methods and apparatus. 
He had exceptionally good mechanical instincts and an excellent knowledge 
of the capabilities of instruments; the result was that the apparatus he 
designed was always simple and effective. 

In addition to papers published in scientific journals and the Transactions 
of Societies he wrote The Mean Density of the Earth : the Adams Prize Essay 
for 1893, The Pressure of Light ("Romance of Science" Series) and The Earth 
(Cambridge University Press). Of the Text Book of Physics written in con- 
junction with J. J. Thomson he wrote the whole of the volumes on Sound and 
Heat and of the first volume of Electricity and Magnetism and the chapters 
on Gravitation in the Properties of Matter. His writings exhibit to the 
full the clearness, simplicity and thoroughness which was characteristic of 
all his work. 

J. J. T. 



Xxiv OBITUARY NOTICES 

[From the Philosophical Magazine for May, 1914, with additions.] 

Although Prof. Poynting, whose loss will be universally deplored, graduated 
with high distinction in mathematics at Cambridge, coming out as third 
Wrangler in the Tripos of 1876, his interest seems always to have lain in 
the direct elucidation of physical laws and principles rather than in the 
evolution and exposition of their consequences by analysis. When he came 
to Cambridge, in 1872, he was already largely trained in the niceties of 
refined experimentation ; and after graduation he embraced an early chance 
to resume experimental work at Manchester. The founding of the Mason 
University College gave him the opportunity of organising a laboratory 
of his own. Much work about this time was concerned with instrumental 
improvements, such as the design of polarimeters and other optical apparatus ; 
and to the same period belong studies in chemical physics, such as the eluci- 
dation of osmotic pressure, theories to which in later years he returned with 
conviction, and which, though perhaps not yet fully appreciated, should 
not be lost sight of, in view of his proved insight into fundamental problems 
in other domains. 

An exami^le of the latter class is the memoir on the transfer of energy 
in the electromagnetic field, Phil Trans., 1884, culminating in the famous 
result that will go down to posterity as Poynting's Theorem, which not only 
specifies the path of transfer of electric energy from one material system to 
another through the aether, but also as a very special case gives for the first 
time (strange to say) the dynamical specification of a ray of light. 

At about the same time 0. Heaviside, and a little later Hertz, were engaged 

upon this aspect of electric transmission as an elastic effect propagated from 

body to body across the aether, in place of the older aspect of electric charges 

in movement, each carrying its field of disturbance along with it, — which latter, 

rejuvenated ten years later by exact conceptions of the agency of electrons, and 

duly modified for chano^e of acceleration, now includes the whole field of view. 

But times not being yet ripe for that, he pursued his subject in 1885 in another 

Phil. Trans. Memoir "On the connexion between electric current and the 

electric and magnetic inductions in the surrounding field," which tracks out 

the relations of a current circuit by the graphical device of the motion of 

what are now known as Faraday tubes of force, a type of visualisation of 

the phenomena which is at the present time once more widely in favour. 

Afterwards he broke new ground in the experimental determination of 
the constant of gravitation — the problem of weighing the earth — which had 
been solved by Cavendish with his accustomed genius by aid of Michell's 
principle of balancing by torsion. To Poynting's mind an ordinary balance 
with lever and scale-pans gave at least equal promise of practical accuracy, 
and his long continued experimental investigations, which were summed 



OBITUARY NOTICES XXV 

up in a Phil, Trans. Memoir of 1892 and a Cambridge Adams Prize Essay 
of 1893, were the starting-point of a new interest in this subject which opened 
up into many methods more or less cognate to his own. By this time, however, 
the torsion method had renewed its power through the discovery of the pro- 
duction and properties of quartz fibres by C. V. Boys, whose remarkable 
subsequent investigation with small-scale apparatus was generously acknow- 
ledged by Poynting as the last word on the subject. 

The resource thus acquired in refined dynamical experimenting was to 
reap further successes in a more untilled field. The ancient idea of a pressure 
exerted by light, so obvious on the corpuscular view of optics, had been revived 
by Maxwell on a foundation of an accompanying electric stress in the trans- 
mitting medium. Its mere existence, as distinct from an analysis of its 
propagation to the place where it is in evidence, was already indubitably 
involved in the Amperean forces on the electric currents induced in the surface 
of a reflector, once the principles of the electric theory of light are admitted. 
It had assumed some importance in its application, notably by FitzGerald, 
to the elucidation of the mysterious phenomena of comets' tails. To Poynting, 
this pressure exerted by a ray coming say from a distant star, far out of reach 
of direct dynamical effect, involved that the ray carried momentum along 
with it, and that the pressure effect was of the nature of a thrust exerted 
along the ray arising from the transfer of this momentum. After long efforts, 
the disturbances arising from gaseous convection as a whole, and from the 
radiometric molecular effect, were eliminated by Lebedew, and were com- 
pensated by balance against each other by Nicholls and Hull, about the 
same time, and the Maxwell value of the normal component of the pressure 
was fully verified. But Poynting's line of thought led him straight to a 
tangential component of the thrust as well as a direct component, and he 
noted that the former could be investigated without much trouble from the 
gas-effect. This idea led to many beautiful determinations in conjunction 
with his assistants and students. His idea of convected momentum also 
led him in another direction to the conclusion that the pressure on a receding 
surface must be less than on one at rest; it also suggested that by reaction 
a moving radiating body would be accelerated by its own radiation— an 
impossible result which is corrected by recognising that its radiation is greater 
towards the direction of its motion than towards other directions, which 
leads to retardation on the whole. As this effect depends on extent of surface, 
it is greater in proportion for small bodies. Thus he was led to consider 
clouds of cosmic dust revolving orbitally round the sun, each particle heated 
by his rays to an equilibrium temperature of the space where it is, retarded in 
its motion by the reaction of its own exchanges of radiation, and thus gradually 
sucked into the sun. This clearance of solar spaces from dust must be a 
prominent feature in views of stellar cosmogony ; the calculation of the time 



XXvi OBITUARY NOTICES 

that would be required aptly illustrates his latent mathematical power, 
which was never unduly obtruded ; and the whole Memoir is an example of 
that simplification of reasoning and reduction to its lowest terms which is- 
suggestive of the depth of vision that belongs to genius. When the theory 
of electrons came to be developed into Maxwell's channels by Lorentz and 
others, it appeared at once that the stress argument, on which he had based 
radiation pressure, was in default, and the natural first conclusion was 
against the objective existence of the stress as thus specified in favour of 
some type too complex for simple expression. But later Poincare and 
Abraham introduced the idea of grouping the refractory outstanding terms 
as a distribution of electric momentum, specified very simply as the vector 
product of the aethereal and magnetic inductions. The stress in the medium 
is thus taken to be the sole operating cause : it is unbalanced, and so reveals 
itself partly in a distribution of mechanical forces exerted on the material 
bodies that are present, and partly in storage and expenditure of mechanical 
momentum of some latent type throughout the aether. This latter agrees 
precisely with the momentum of radiation elucidated on very simple inde- 
pendent grounds by Poynting. Interest in the subject is thus stimulated, 
and the problems now under discussion as to whether the effect is in all cases 
simply momentum, and whence arises the subsidiary travelling inertia which 
is implied in it, become of pressing interest. The application of the stress 
method to calculation over a boundary surrounding a material system' leads 
in fact to an additional result — that when the system gains energy SE of 
electric type, its effective mass increases by SE/c^, where c is the velocity of 
radiation : but this is less important practically, and would not for example 
affect sensibly the clearance of cosmical dust above mentioned,— though the 
idea that energy possesses inertia naturally assumes prominence in general 
relativity theory. An experimental and theoretical incursion into the 
different field of the elongation of a wire due to its torsion was probably 
prompted primarily by these problems ; tiiough not perhaps strictly pertinent 
to them, it opened up new views in the theory of elastic solids under stresses 
so great that mere superposition of strains no longer holds good. 

The formulation of an exact notion of the temperature of space, above 
indicated, is but one phase of his interest in the theory of natural radiation ; 
and It seems but yesterday that he was discussing, in private correspondence, 
with his usual acuteness and judgment and no sign of failure of powers, the 
theory of Stefan's law and its other fundamental relations. 

J. L. 



CONTENTS 

PAGE 

PREFACE V 

A PERSONAL NOTE vii 

OBITUARY NOTICES ix 

PART I 

THE BALANCE AND GRAVITATION 

ART. 

1. On the Estimation of Small Excesses of Weight by the Balance 

from the Time of Vibration and the Angular Deflection of the 
Beam . 1 

[Manchester Lit. Phil. Soc. Proc. 18, 1879, pp. 33-38. Read December 10, 1878.] 

2. On a Method of using the Balance with great delicacy, and on its 

employment to determine the Mean Density of the Earth . 7 

[Roy. Soc. Proc. 28, 1879, pp. 2-35. Received June 21, 1878.] 

3. On a Determination of the Mean Density of the Earth and the 

Gravitation Constant by means of the Common Balance . 43 
[Phil. Trans. A, 182, 1892, pp. 565-656. Received May 13. Read June 4, 1891.] 

4. An Experiment in Search of a Directive Action of one Quartz 

Crystal on another. By J. H. Poynting and P. L. Gray, B.Sc. . 137 
[Phil. Trans. A, 192, 1899, pp. 245-256. Received September 27. 
Read November 17, 1898.] 

5. An Experiment with the Balance to Find if Change of Temperature 

has any Effect upon Weight. By J. H. Poynting and Percy 

Phillips, M.Sc 149 

[Roy. Soc. Proc. A, 76, 1905, pp. 445-457. Received July 12, 1905.] 

6. On a Method of Determining the Sensibility of a Balance. By 

J. H. Poynting and G. W. Todd, M.Sc 162 

[Phil. Mag. 18, 1909, pp. 132-135. Read June 25, 1909.] 

PART II 

ELECTRICITY 

7. On the Law of Force when a Thin, Homogeneous, Spherical Shell 

exerts no Attraction on a Particle within it . . . .165 
[Manchester Lit. Phil. Soc. Proc. 16, 1877, pp. 168-171. Read March 6, 1877.] 

8. Arrangement of a Tangent Galvanometer for lecture room purposes 

to illustrate the Laws of the Action of Currents on Magnets, and 

of the Resistance of Wires 168 

[Manchester Lit. Phil. Soc. Proc. 18, 1879, pp. 85-88. Read April 1, 1879.] 



XXViii CONTENTS 

ART. PAGE 

9. On the Graduation of the Sonometer ... . . 170 

[Phil. Mag. 9, 1880, pp. 59-64. Read before the Physical Society, 
December 13, 1879.] 

10. On the Transfer of Energy in the Electromagnetic Field . . 175 
[Phil. Trans, lib, 1884, pp. 343-361. Received December 17, 1883. Read 

January 10, 1884.] 

11. On the Connection between Electric Current and the Electric and 

Magnetic Inductions in the Surrounding Field . . .194 

[Phil. Trans. 176, 1885, pp. 277-306. Received January 31. Read 
February 12, 1885.] 

12. Discharge of Electricity in an Imperfect Insulator . . . 224 
[Birmingham Phil. Soc. Proc. 5, (1885), pp. 68-82. Read December 10, 1885.] 

13. On the Proof by Cavendish's Method that Electrical Action varies 

Inversely as the Square of the Distance 235 

[British Association Report, 1886, pp. 523-524.] 

14. On a Form of Solenoid-Galvanometer 237 

[Birmingham Phil. Soc. Proc. 6, (1888), pp. 162-167. Read May 10, 1888.] 

15. On a Mechanical Model, illustrating the Residual Charge in a 

Dielectric 242 

[Birmingha?n Phil. Soc. Proc. 6, (1888), pp. 314-317. Read November 8, 1888.] 

16. Electrical Theory. Letters to Dr Lodge 245 

[Electrician, 21, 1888, pp. 829-831.] 

17. An Examination of Prof. Lodge's Electromagnetic Hypothesis . 250 

[Electrician, 31, 1893, pp. 575-577, 606-608, 635-636.] 

18. Molecular Electricity ....... 269 

[Electrician, 35, 1895, pp. 644-647, 668-671, 708-712, 741-743.] 



PART III 

WAVE PROPAGATION— RADIATION— PRESSURE OF 
LIGHT— AND RELATED SUBJECTS 

19. Note on an Elementary Method of Calculating the Velocity of Pro- 
pagation of Waves of Longitudinal and Transverse Disturbances 

by the Rate of Transfer of Energy 299 

[Birmingham Phil. Soc. Proc. 4, (1885), pp. 55-60. Read November 8, 1883.] 

Radiation in the Solar System : its Effect on Temperature and its 

Pressure on Small Bodies 3Q4 

[Phil. Trans. A, 202, 1903, pp. 525-552. Received June 16. Read 
June 18, 1903.] 



20 



CONTENTS XXIX 

ART. PAGE 

21. Note on the Tangential Stress due to Light incident obliquely on 

an Absorbing Surface 332 

[Phil. Mag. 9, 1905, pp. 169-171. Read at Section A, British Association, 
Cambridge, August, 1904.] 

22. Radiation-Pressure 335 

[Phil. Mag. 9, 1905, pp. 393-406. Presidential Address, delivered at the 
Annual General Meeting of the Physical Society, February 10, 1905.] 

23. On Prof. Lowell's Method for Evaluating the Surface-Temperatures 

of the Planets ; with an Attempt to Represent the Effect of Day 
and Night on the Temperature of the Earth .... 347 
[Phil. Mag. 14, 1907, pp. 749-760.] 

24. The IVEomentum of a Beam of Light 357 

[Atti del IV Congresso internazionale dei Matematici (Rome), 3, 1909, 
pp. 169-174.] 

25. On Pressure Perpendicular to the Shear-Planes in Finite Pure 

, Shears, and on the Lengthening of Loaded Wires when Twisted 358 

[Roy. Soc. Proc. A, 82, 1909, pp. 546-559. Read June 24, ]909.] 

26. The Wave-Motion of a Revolving Shaft, and a Suggestion as to the 

Angular Momentum in a Beam of Circularly Polarised Light . 372 
[Boy. Soc. Proc. A, 82, 1909, pp. 560-567. Read June 24, 1909.] 

27. Preliminary Note on the Pressure of Radiation against the Source : 

The Recoil from Light. By J. H. Poynting and Guy Barlow, 

D.Sc 380 

[British Association Beport, 1909, p. 385.] 

28. Bakerian Lecture. The Pressure of Light against the Source : The 

Recoil from Light. By J. H. Poynting and Guy Barlow, D.Sc. 381 
[Boy. Soc. Proc. A, 83, 1910, pp. 534-546. Read March 17, 1910.] 

29. On Small Longitudinal Material Waves accompanying Light- 

Waves 394 

[Boy. Soc. Proc. A, 85, 1911, pp. 474-476.] 

30. On the Changes in the Dimensions of a Steel Wire when Twisted, 

-^ and on the Pressure of Distortional Waves in Steel . . . 397 

[Boy. Soc. Proc. A, 86, 1912, pp. 534-561. Read March 21, 1912.] 

31. The Changes in the Length and Volume of an India-Rubber Cord 

when Twisted 424 

[The India-Bubber Journal, October 4, 1913.] 

Appendix by Sir J. Larmor on the Momentum of Radiation . 426 



XXX CONTENTS 

PART IV 

LIGHT 

ART. PAGE 

32. On a Simple Form of Saccharimeter 435 

[Phil. Mag. 10, 1880, pp. 18-21.] 

33. On the Law of the Propagation of Light. By J. H. Poynting 

and E. F. J. Love, B.A. 438 

[Birmingham Phil. Soc. Proc. 5 (1887), pp. 354-363. Read March 31, 1887,] 

34. Haze 446 

[Nature, 39, 1889, pp. 323-324.] 

35. A Graphical Method of Explaining the Diffraction Bands at the 

Edge of a Shadow ......... 449 

[Birmingham Phil. Soc. Proc. 7 (1890), pp. 210-219. Read November 5, 1890.] 

36. On a Parallel -Plate Double-Image Micrometer .... 455 

[Roy. Astr. Soc. Monthly Notices, 52, 1892, pp. 556-560.] 

37. Historical Note on the Parallel-Plate Double-Image Micrometer 460 

[Eoyal Astr. Soc. Monthly Notices, 53, 1893, p. 330.] 

38. A Method of Making a Half -Shadow Field in a Polarimeter by two 

inclined Glass Plates ......... 462 

[British Association Report, 1899, pp. 662-663.] 

PART V 

MISCELLANEOUS 

39. Change of State: Solid-Liquid 454 

[Phil. Mag. 12, 1881, pp. 32-48, 232.] 

40. Note on a Method of Determining Specific Heat by Mixture . 481 
[Birmingham Phil. Soc. Proc. 4 (1883), pp. 47-54. Read November 8, 1883.] 

41. Osmotic Pressure 4gg 

[Phil. Mag. 42, 1896, pp. 289-300.] 

42. Musical Sands 4gg 

[Nature, 77, 1908, p. 248.] 

PART VI 

STATISTICS 

43. The Drunkenness Statistics of the Large Towns in England and 

Wales ^g^ 

[Manchester Lit. and Phil. Soc. Proc. 16, 1877, pp. 211-218. Read 
April 3, 1877.] 



CONTENTS XXXI 

ART. PAGE 

44. The Geographical Distribution of Drunkenness in England and 

Wales. By J. H. Poynting and John Dendy, Jun. . . 504 

{^Fourth Report from the Select Committee of the House of Lords on 
Irdemperance 1878. Appendix R, pp. 580-591.] 

45. A Comparison of the Fluctuations in the Price of Wheat and in the 

Cotton and Silk Imports into Great Britain .... 506 
[Statistical Society Journal, 1884. Read before the Statistical Society, 
15 January, 1884.] 



PART VII 

ADDRESSES AND GENERAL ARTICLES 

46. Change of State: Fusion and Solidification .... 538 

[Birmingham, Phil. Soc. Proc. 2 (1881), pp. 354-372. Read May 12, 1881.] 

47. Overtaking the Rays of Light 552 

[Mason College Magazine, 1, 1883, pp. 107-111.] 

48. University Training in our Provincial Colleges. An Address 

delivered at the Mason Science College, Birmingham, Oct. 2, 

1883 557 

49. The Growth of the Modern Doctrine of Energy. Address to the 

Mason College Physical Society, March 26, 1884 ... 565 

50. The Electric Current and its Connection with the Surrounding Field 576 

[Birmingham Phil. Soc. Proc. 5 (1887), pp. 337-353. Read March 10, 1887.] 

51. The Foundations of our Belief in the Indestructibility of Matter 

and the Conservation of Energy. A Criticism of Spencer's 
'First Principles,' Part II, Chaps. IV, V, and VI . . . 588 

[Midland Naturalist, 12, 1889. Read before the Sociological Section of the 
Birmingham Natural History and Microscopical Society, November 22, 1888.] 

52. Presidential Address to the Mathematical and Physical Section 

of the British Association (Dover), 1899 .... 599 

[British Association Report, 1899, pp. 615-624.] 

53. A History of the Methods of Weighing the Earth. Presidential 

Address delivered to the Birmingham Philosophical Society, 
October 19, 1893 613 

[Birmingham Phil. Soc. Proc. 9 (1894), pp. 1-23.] 

54. The Mean Density of the Earth [Letter] 628 

[Nature, 48, 1893, p. 370.] 

55. Recent Studies in Gravitation. Address: Royal Institution of 

Great Britain, February 23, 1900 629 

[Roy. Inst. Proc. 16, 1900-02, pp. 278-294.] 



XXXll 



CONTENTS 



ART. 

56. 



57. 



58. 



59. 



60. 



61. 



62. 



63. 



64. 



65. 



66. 



67. 



68. 



69. 



70. 



284-300. 



Le mode de propagation de I'Energie et de la tension Electrique 

dans le champ Electromagnetique .... 
[Rapport presente au Congres International de Physique de 1900, 3, pp. 

Paris, Gauthier-Villars.] 
The Transformation and Dissipation of Energy . 

[The Inquirer, 1902, pp. 627-628.] 
Molecules, Atoms and Corpuscles .... 

[The Inquirer, 1902, pp. 740-741, 772-773.] 

The Pressure of Light 

[The Inquirer, 1903, pp. 195-196.] 
Mysteries of Matter. Radium at the British Association 
[The Inquirer, 1903, pp. 635-636.] 

A City University [Letter] 

[The Inquirer, 1903, p. 660.] 

The Universities and the State 

[The Inquirer, 1903, p. 779.] 

Physical Law and Life 

[Hibhert Journal, 1, 1903, pp. 728-746.] 
Radiation in the Solar System. Afternoon address delivered at 
the Cambridge meeting of the British Association, August 23, 

1904 

[Nature, 70, 1904, pp. 512-515.] 

Radiation-Pressure [Letter in correction to above address] 

[Nature, 71, 1904, pp. 200-201.] 
Radiation-Pressure. Presidential Address to the Physical Society 
of London, February 1905. See Part III, Art. 22 . . . 
Some Astronomical Consequences of the Pressure of Light. Dis- 
course delivered at the Royal Institution on May 11, 1906 
[Nature, 75, 1906, pp. 90-93.] 

George Gore, 1826—1908 

[Roy. Soc. Proc. 84, 1911, pp. xxi-xxii.] 

Atomic Theory (Mediaeval and Modern) 

[Encyclopaedia of Religion and Ethics, 2, 1909, pp. 203-210.] 
Quelques experiences sur la Pression de la lumiere. Address to 
the French Physical Society, March 31, 1910 .... 
[Bulletin des seances de la Societe frangaise de Physique, 1, 1910.] 

POSTSCRIPT (1918). Retardation by Radiation Pressure: A 
correction. By Sir Joseph Larmor, F.R.S 

BIBLIOGRAPHY 

INDEX 



PAGE 

645 

658 
664 
673 

677 
682 
683 
686 

699 

708 

711 
712 

722 
724 

742 



754 

758 
764 



PART I. 

THE BALANCE AND GRAVITATION. 

1. 

ON THE ESTIMATION OF SMALL EXCESSES OF WEIGHT BY 
THE BALANCE FROM THE TIME OF VIBRATION AND THE 
ANGULAR DEFLECTION OF THE BEAM. 

[Manchester Lit. Phil Soc. Proc. 18, 1879, pp. 33-38.] 

[Read Dec. 10, 1878.] 

While working last year on an experiment to determine the mean density 
of the earth by the balance, I had to measure such an exceedingly small 
difference of weight that I could not at that time estimate it by means of 
a rider, but was obliged to adopt the method described in this paper. Stated 
generally, it consists in treating the balance as a pendulum. Knowing the 
nature of the pendulum (that is, its moment of inertia) and its time of vibration, 
we can calculate what force acting at the end of one arm of the beam will 
produce a given angular deflection. It is, in fact, an application to the common 
balance of the method which has always been used with the torsion-balance 
when it has been necessary to calculate the forces measured in absolute 
measure. I cannot find any record of a previous application of the method ; 
and as it might be of use in very delicate weighings or in verifying the small 
weights in a laboratory, I have thought it worth while to give a full account 
of it. 

When small quantities of the second order are neglected and the 
oscillations are of the first order, it will easily be found that the equation 
of motion of the beam of the balance is 

(mP + ^^) 6 + (2Ph + Mgk) d = ap, (1) 

p. o. w. 1 



2 ON THE ESTIMATION OF SMALL EXCESSES OF WEIGHT 

where MP = moment of inertia of beam about central knife-edge, 
M = mass of beam, 
a = half length of beam, 
p = weight of either pan and the mass in it, 
h = distance of Hne joining terminal knife-edges below the central 

knife-edge, 
k = distance of centre of gravity of beam below central knife-edge, 
p = small excess in one pan, 

6 = angular deflection in circular measure produced by p, 
9 = gravity. 
If 6* = 0, we have the position of equihbrium given by 

ft- ^P (2) 

2Ph + Mgk ^ ^ 

The semiperiodic time is 



MI 



^^^\/ mrrwk ^^^ 

From equations (2) and (3) we can eliminate 2Ph -f- Mgk, obtaining 

^MgP + 2Pa^d ... 

P^TT^ -^ -^ (4) 

■^ ag t^ ^ 

From this expression it appears that, if we know the moment of inertia 

of the beam, its length, and the weight at each end, we can find the excess 

p from the time of vibration and deflection. 

The results given in this paper were obtained with a 16-inch chemical 

balance by OertUng. The exact length of the half beam {a) measured by 

a dividing-engine is 20-2484 centimetres. 

To find the Moment of Inertia MP of the Beam. The simplest way 

theoretically would appear to be this. Find the times of vibration t^, t^, 

and the deflections 6^, d^, due to the same excess p with two different loads 

Pj, P2 in each pan. Equating the values of p given for the two by equation 

(4) we have 

MgP + 2P^a^ d^ti" 



MgP + 2P2a2 d^t^^ ' 
an equation which will give MgP in terms of known quantities ; but on trial 
it was found that a very small proportional error in the observed time made 
a large error in the value of MgP ; and the following method, that usually 
adopted in magnetic observations, was employed in preference. A stirrup 
was suspended by a platinum wire, and its time of vibration (^1) against the 
force of torsion (/x) of the wire was observed. The moment of inertia of the 
stirrup being S, we have 

tj 

fX 



ON THE ESTIMATION OF SMALL EXCESSES OF WEIGHT 3 

The time of vibration (^2) was then observed when a cylindrical brass bar of 
known moment of inertia (B) was inserted in the stirrup. We now have 

The bar was then removed and the balance-beam inserted in its place ; and 
the time of vibration (/^) gives 

t^^=-(S+MI^). 

From these three equations, eliminating S and /j,, we obtain 

3JP Bjh'-t^) 

Now Eg was calculated from the weight and dimensions of the bar to be 
6332-83 (in centimetres and grammes). The observed times were 

t^ = 3-6792 sees., t^ = 4-495 sees., ^3 - 7-1483 sees. 

From these values we find 

M^/2 = 35651-6*. 

To measure 9. The angle of deflection was measured by the number of 
divisions of the scale which the pointer moved over. As the length of the 
pointer is 32-1006 centimetres, while 20 divisions of the scale measure 2-5658 
centimetres, a tenth of a division, in terms of which the deflection was 
measured, corresponds to an angle of 0-0003996. The oscillations were 
observed from a distance of six or eight feet by a telescope. The resting- 
point (i.e. the point where the balance would be in equilibrium) was found in 
the usual way by observing three successive extremities of two swings and 
taking the mean of the second and the mean of the first and third. Five 
determinations of the resting-point were usually made with the excess to be 
measured alternately added and removed. From these five, three values of 
the deflection {n) due to the excess were calculated in a manner which will be 
seen from the example below. 

The Time of Vibration. This was found from several determinations of 
the time of ten oscillations. The method will be seen from the example. 
No correction was needed for the resistance of the air as long as the vibrations 
did not exceed two divisions of the scale. When, however, they were much 
more than that, the time of vibration was found to increase with the arc. 
As the time of vibration frequently changes slightly, probably through 
variations of temperature, it was usually observed before- and after the 
determination of the deflection (n) and the mean of the two taken as the true 
time. 

* To this a small correction should be added if the adjusting-bob is not in its lowest position. 
This amounts to 7-6 for each turn of the screw, and may therefore in general be neglected. 

1—2 



4 ON THE ESTIMATION OF SMALL EXCESSES OF WEIGHT 

The following example of the determination of the value of a centigramme- 
rider by placing it halfway along the beam will sufficiently explain the details 
of the method. 



Time of Vibration at Commencement. 



No. of 
vibration 


Observed time of 

passage of pointer 

through resting- 

point 


No. of 
vibration 


Observed time of 

passage of pointer 

through resting- 

point 


1 

Time of 10 
vibrations 




Pointer apparently moving 


from left to right 






2 

4 
6 


h. m. s. 
11 15 36 
11 16 1 
11 16 26-5 
11 16 52 


10 
12 
14 
16 


h. m. s. 
11 17 43 
11 18 8 
11 18 33-5 
11 18 59 


s. 

127 
127 
127 
127 




Mean value of 10 vibrations 





127 




Pointer ajiparently moving 


from right to left 




1 
3 
5 

7 


11 15 49 
11 16 14 
11 16 39-5 
11 17 5 


11 
13 
15 
17 


11 17 56 
11 18 21 
11 18 46 
11 19 11-5 


127 
127 
126-5 
126-5 




Mean value of 10 vibrations 




126-75 




Mean of means = 126-875 ; 


t^ = 12-6875 sees. 





Determination of Deflection (n). 



Excess 
weight 


Extremities 
of oscillation 


Resting- 
point 


Mean of preceding 
and succeeding 
resting-points 


Deflection 
due to 

excess 


Added 


109 

96 
109 


102-5 






Removed 


93 

40 
92 


6625 


102-25 


35-25 


Added 


152 

53 
150 


102 


66-75 


36 


Removed 


80 


67-25 


102-5 


35-25 


Added 


147 

60 
145 


103 







Mean value of n = 35-83. 



ON THE ESTIMATION OP SMALL EXCESSES OF WEIGHT 



Time of Vibration at End. 



I 



No. of 
vibration 


Observed time of 

passage of pointer 

through resting- 

point 


No. of 
vibration 


Observed time of 

passage of pointer 

through resting- 

point 


Time of 10 
i vibrations 




Pointer apparently moving 


from left to right 





2 
4 
6 


h. m. s. 
11 26 19 
11 26 44-5 
11 27 10 
11 27 35-5 


10 
12 
14 
16 


h. m. s. 
11 28 27 
11 28 53 
11 29 18 
11 29 44 


s. 

128 
128-5 
128 
128-5 




Mean value of 10 vibrations 





128-25 




Pointer apparently moving 


from- right to left 




1 
3 
5 

7 


11 26 32-5 ; 
11 26 58 i 
11 27 23-5 
11 27 49 


11 
13 
15 
17 


11 28 39 
11 29 5 
11 29 30-5 
11 29 56-5 


126-5 
127 
127 
127-5 




Mean value of 10 vibrations . 


.. 


127 




Mean of means = 127-625 


; ^2 = 12-7625 sees. 





Remembering that one-tenth of a division of the scale is an angle of 
•0003996 in circular measure, formula (4), expressed in milligrammes, becomes 



f = '% 0-3996 — {Mgl^ + 2Pa2). 
^ t^ ag ^ 



In our present example' 



n = 35-83, 

^ = ^^t^2= 12-725 sees., 



Mgl^ = 35651, 

2Pa^ = 94704, 

p = 5-724 milligrammes. 

The length of time occupied in this determination was not quite a quarter 
of an hour. 

* For this, as for several other cases, I removed the pans and hung the weights directly by 
fine wires from the suspending-pieces. By this means the resistance of the air was very much 
diminished. 



6 ON THE ESTIMATION OF SMALL EXCESSES OF WEIGHT 

The following table contains a series of results which I have obtained of 
the weight of two centigramme-riders, the first of which was accidentally 
destroyed after the conclusion of the fourth determination. As the rider 
was always placed at division 5 on the beam, the values given in the table 
are double those actually obtained. 



No. of 
experi- 
ment 


Mgl^ + 2Pa2 


t in 
seconds 


n 


Weight of 
rider in milli- 
grammes 


Mean value 


1 


145364 


8-921 


13-458 


9-78^ 




2 
3 


309356 
519769 


17-65 
20-435 


25-49 
19-12 


10-05 1 
9-55 1 


9-96 milligrammes 


4 


130355 


1310 


34-71 


10-47 j 




5 


130355 


12-87 


36-6 


11-44) 




6 


130355 


12-72 


35-5 


11-35 






7 
8 


130355 
130355 


12-725 
1281 


35-83 
35-5 


11-45 
11-20 




11-35 milligrammes 


9 


130355 


12-903 


36-37 


11-31 






10 


454405 


19-406 


22-08 


10-58J 







2. 

ON A METHOD OF USING THE BALANCE WITH GREAT DELICACY, 
AND ON ITS EMPLOYMENT TO DETERMINE THE MEAN 
DENSITY OF THE EARTH. 

[Roy. Soc. Proc. 28, 1879, pp. 2-35.] 
[Received June 21, 1878.] 

In the ease and certainty with which we can determine by the balance 
a relatively small difference between two large quantities, it probably excels 
all other scientific instruments. 

By the use of agate knife-edges and planes, even ordinary chemical balances 
have been brought to such perfection that they will indicate one-milHonth 
part of the weight in either pan, while the best bullion-balances are still more 
accurate. The greatest degree of accuracy which has yet been attained was 
probably in Professor Miller's weighings for the construction of the standard 
pound, and its comparison with the kilogramme, in which he found that the 
probable error of a single comparison of two kilogrammes, by Gauss's method, 
was TiWWoo*^ P^^^ ^^ ^ kilogramme*. (Phil. Trans. 1856.) 

But, though the balance is peculiarly well fitted to detect the relatively 
small differences between large quantities, it has not hitherto been considered 
so well able to measure absolutely small quantities as the torsion-balance. 
The latter, for instance, was used in the Cavendish experiment, when the 
force measured by Cavendish was the attraction of a large lead sphere upon 
a smaller sphere, weighing about 1 Jibs., the force only amounting to 5 000^0000" ^^ 
part of this weight, or about 50^0^^ P^^^ ^^ ^ grain. 

The two great sources of error, which render the balance inferior to the 
torsion-balance in the measurement of small forces, are : 

1. Greater disturbing effects produced by change of temperature, such as 
convection-currents and an unequal expansion of the two arms of the balance. 

2. The errors arising from the raising of the beam on the supporting 
frame between each weighing, consisting of varying flexure of the beam and 
inconstancy of the points of contact of the knife-edges and planes. 

* Even so far back as 1787, Count Rumford used a balance which would indicate one in a 
million and measure one in seven hundred thousand. {Phil. Trans. 1799.) 



8 ON A METHOD OF USING THE BAIjANCE 

The disturbances due to convection-currents interfere with the torsion- 
balance as well as with the ordinary balance, though they are more easily 
guarded against with the former, by reason of the nature of the experiments 
usually performed with it. They might, perhaps, as has been suggested by 
Mr. Crookes, be removed from both by using the instruments in a partial 
vacuum, in which the pressure is lowered to the 'neutral point,' where the 
convection- currents cease, but the radiometer- effects have not yet begun. 
But a vacuum-balance requires such comphcated apparatus to work it, that 
it is perhaps better to follow the course which Baily adopted in the Cavendish 
experiment. He sought to remove the disturbing forces as much as possible, 
and to render those remaining as nearly uniform as possible in their action 
during a series of experiments, so that they might be detected and eliminated. 
For this purpose the instrument was placed in a darkened draughtless room, 
and was protected by a thick wooden casing gilded on its outer surface. 
Most of the heat radiated from the surrounding bodies was reflected from the 
surface of the case by the gilding. The heat absorbed only slowly penetrated 
to the interior, and was so gradual in its action, that, for a considerable time, 
the effect might be supposed nearly uniform. Under this supposition it was 
then eliminated by the following method of taking the observations. The 
resting-point (that is, the central position of equilibrium, about which the 
oscillations were taking place) of the torsion-rod, at the ends of which were the 
small attracted weights, was first observed when the two large masses pulled 
it in one direction. The masses were then moved round to the opposite side, 
when they pulled the rod in the opposite direction and the resting-point was 
again observed. The masses were then replaced in their original position 
and the resting-point was observed a third time. These three observations 
were made at equal intervals of time; if, then, the disturbing effect was 
uniform during the time, the mean of the first and third observations gave 
what the resting-point would have been, had the rod been pulled in that one 
direction at the same time that it was actually observed when pulled in the 
opposite direction. The difference between the second resting-point and the 
means of the first and third might, therefore, be considered as due to the 
attractions of the masses alone. 

In the experiments of which this paper contains an account, I have en- 
deavoured to apply this method of introducing time as an element to the 
ordinary balance. But, before it could be properly applied, it was necessary 
to remove the errors due to the raising of the beam between successive 
weighings, as they could not be considered to vary in any uniform way 
with the time. I think I have effected this satisfactorily, by doing away 
altogether with the raising of the beam by the supporting frame, between 
the weighings. For this purpose I have introduced a clamp underneath one 
of the pans, which the observer can bring into action at any time, to fix that 
pan in whatever position it may be. The weight can then be removed from 



WITH GREAT DELICACY, ETC. 9 

the pan, and another, which is to be compared with it, can be inserted in its 
place without altering the relative positions of the planes and knife-edges. 
The counterpoise in the other pan, meanwhile, keeps the beam in the same 
state of flexure. The pan is then undamped and the new position about 
which it oscillates is observed. The only changes are due to the change in 
the weight and the effect of the external disturbing forces ; the latter we may- 
consider as proportional to the time, if sufficient precautions have been 
taken, and by again changing the weights and again observing the position 
of the balance, we may eliminate their effects. 

Though the method when applied to the balance does not yet give such 
good results as Baily obtained from the torsion-balance — partly, I believe, 
because I have not yet been able to apply all his precautions to remove 
external disturbing forces — ^it still gives better results than would have been 
obtained without it. This may be seen by the numbers recorded in the tables, 
where a progressive motion of the resting-point may be noticed, in most cases 
in the same direction, during a series of experiments. Even when this is 
not the case, the method at once shows when the disturbing forces are 
irregular, and when we are justified in rejecting an observation on that account. 

I give in this paper two applications of the method, one to the comparison 
of two weights, the other to the determination of the mean density of the 
earth. The latter is given only as an example of the method, but I hope 
shortly to continue the experiments with a large bullion-balance, for the 
construction of which I have had the honour to obtain a grant from the 
Society. The balance is now in course of construction, by Mr. Oertling, of 
London. 

Description of the Apparatus. 

The balance which I have employed is one of Oertling's chemical balances, 
with a beam of nearly 16 inches, and fitted with agate planes and knife-edges. 
It will weigh up to a little more than 1 lb. To protect it from sudden changes 
of temperature, the glass panes of the case are covered with flannel, on both 
sides of which is pasted gilt paper, with the metallic surface outwards. This 
case is enclosed in another outer case, a large box of inch deal, lined inside 
and out with gilt paper. The experiments have been conducted in a darkened 
cellar under the chemical laboratory at Owens College, which was kindly 
placed at my disposal by Professor Roscoe. As the ceilings and floors of the 
building are of concrete, any movement near the room causes a considerable 
vibration of the floor and walls. It was necessary, therefore, to support the 
balance independently of the floor. For this purpose, six wooden posts 
(A, B, C, D, Ey F, Fig. 1) were erected resting on the ground underneath and 
passing freely through the floor to a height of 6 feet 6 inches above it. They 
are connected at the top by a frame like that of the table, and stayed against 
each other to give firmness. The wider part of the frame, near the posts 



10 



ON A METHOD OF USING THE BALANCE 



E and F, is boarded over to form a table for the telescope {t, Fig. 1) and scale 
(s), by which the oscillations of the balance are observed. The box con- 
taining the balance rests on two cross-pieces, on the narrower part, ABCD, 
of the frame, with the beam parallel to AD, and its right end towards the 
telescope. 

In order to observe the position of the beam, a mirror, 1 J inches by f inch, 
is fixed in the centre of the beam, and the reflection of a vertical scale (s, 
Fig. 1) in this is viewed with a telescope (t) placed close to the scale. The 
light from the scale passes through two small windows cut in each of the 
cases of the balance and glazed with plate glass. The position of the beam 




Fig. 1. 

is given by the division of the scale upon the cross-line on the eyepiece of the 
telescope. The scale, which was photographed on glass, and reduced from 
a large scale, drawn very carefully, has 50 divisions to the inch. These are 
ruled diagonally with ten vertical cross-lines. It is possible to read, with 
almost certainty, to a tenth of a division, or ^i^th of an inch. Since the 
mirror is about 6 feet from the scale, a tenth of a division means an angular 
deflection of the beam of about 3"*. 

The scale is illuminated from behind by a mirror (m), several inches in 
diameter, which reflects through it a parallel beam from a paraffin lamp (Q. 

* The numbers on the scale run from below upwards, so that an increase in the weight in the 
right-hand pan is indicated by a lower number on the scale. 



WITH GREAT DELICACY, ETC. 



11 



A plate of ground glass between the scale and mirror diffuses the light evenly 
over the scale and, by altering the position of the mirror, any desired degree 
of brilliancy may be given to the illumination of the scale. A screen (not 
shown in Fig. 1) prevents stray light from striking the balance-case. 

This method of reading — which, of course, doubles the deflection — has 
been so far sufficiently accurate for my purpose; that is to say, the errors 
arising from other sources are far greater than those arising from imperfections 
of reading. But in a long series of preliminary experiments I used the 
following plan to multiply the deflection still further. A rather smaller 
fixed mirror, ah, is placed opposite to and facing the beam-mirror, AB, fixed 
on the beam, and a few inches from it. Suppose the beam-mirror to be 
deflected from the position BL, parallel to ah, through an angle, 6, to the 
position AB. If a ray, PQ, perpendicular to ah strikes AB at Q, it will make 
an angle 6 with QM, the normal at Q, and will be reflected along QR, making 
an angle 29 with its original direction, and therefore with the normal RO, 
at R, when it strikes it. If it be reflected again to AB at S, it will make an 
angle 3^ with the normal SN, and the reflected ray, ST, will make an angle 
4^ with the original direction, PQ, of the ray. It may be still further 




reflected between the two mirrors, if desirable, each reflection at the mirror, 
AB, adding 2^ to the deflection of the ray. I have, for instance, employed 
three reflections from the beam-mirror, so multiplying the deflection six times. 
In this case, one division of my scale, at the distance at which it was placed 
from the beam, corresponded to a deflection of 7" in the beam, and this could 
be subdivided to tenths by the eye. The only limit to the multiplication 
arises from the imperfection of the mirrors and the decrease in the illumination 
of the successive reflections*. 

The chair of the observer is placed on a raised platform, and a small table 
rising from the platform and free from the frame on which the instruments 
rest, is between the observer and the telescope. On this he can rest his note- 
book during an experiment. As the differences of weight observed are some- 
times exceedingly minute, the balance is made very sensitive— usually 

* This method was used in the seventh and eighth series here recorded. Two reflections 
from the beam-mirror were employed, giving four times the actual deflection. 



12 



ON A METHOD OF USING THE BALANCE 



vibrating in periods between 30 sees, and 50 sees. The value of a division of 
the scale cannot be determined by adding known small weights to one pan, 
as the deflection would usually be too great. Any approach of the observer 
to the case causes great disturbances, so that the ordinary method of moving 
a rider an observed distance along the beam is inapplicable. In some experi- 
ments made last year I calculated the force equivalent to the small differences 
in weight, in absolute measure, by observing the actual angular deflection 
and the time of vibration. With a knowledge of the moment of inertia of 
the beam and treating it as a case of small oscillations, it was possible to 
calculate the value of the scale. But the observations and subsequent 
calculations were so complicated that the following method of employing 
riders was ultimately adopted. 

{a) 




(h) 



(c) 



id) 





A small bridge about an inch long (Fig. 2 a) is fitted on to the beam. The 
sides of the bridge are prolonged about half an inch above the arch which 
fits on to the beam, as shown in the end view (Fig. 2b). In each of these 
sides are cut two V-shaped notches directly opposite to each other, one of 
the opposite pairs being 6-654 millims. (about J inch) distant from the other 
pair. Two equal riders of the shape shown in Fig. 2 c are placed across the 
bridge, and are of such a size that they will just fit into the bottom of the 
notches. When one of these rests across the bridge the other is raised up 



WITH GREAT DELICACY, ETC. 



13 



from it. The lowering of one rider and the raising of the other corresponds 
therefore to a transference of a single rider from one pair of notches to the 
other. The length of the half beam being 202-716 millims. and the distance 
between the notches 6-654 millims., this transference will be equivalent to 
the addition to one pair of 0-03282 of the weight of the rider used. As I have 
generally used a centigramme-rider this means 0*3282 mgm. 

Two levers I, I' (Fig. 2d), with hooks h, h' are used to raise one rider while 
the other is lowered. These levers are worked by two cams c, c' on a rod R, 
which is prolonged out of the balance-case to the observer. By turning this 
rod round, the one lever is raised while the other is depressed. The hook 
at the end of the raised lever picks up its rider while the other hook deposits 
its rider on the bridge, and then sinks down between the raised sides (as 
shown in Fig. 2d), leaving the rider resting freely on the bridge. 

The levers are so adjusted that the beam even in its greatest oscillations 
never comes in contact with the hooks. 

This arrangement might probably be still further perfected by introducing 
two small frames for the riders to rest upon, the frames resting on the beam 
by knife-edges. It would then be certain that the movement of the riders 
was equivalent to a transference from one knife-edge to the other, whereas 
the rider at present may not rest exactly over the centre of the notch. But 
I find that I get fairly consistent results by lowering the rider somewhat 
suddenly so as to give it sufficient impetus to go to the bottom of the notch, 
and have not therefore thought it necessary as yet to introduce more com- 
plicated apparatus. 

In place of the right-hand pan of the usual shape, another of the shape 
shown in Fig. 3 a is employed. To the centre of the pan underneath is 



I 



^ n /// 




(a) 



(&) 
3. 



(c) 



attached a vertical brass rod which passes downwards through the bottom 
of the inner case of the balance. To the under side of this case is attached 



14 



ON A METHOD OF USING THE BAI^ANCE 



the clamping arrangement before referred to. This consists of two sliding 
pieces (Fig. ia, s, s) working horizontally in a slot cut in a thick brass plate 
which is fastened to the case. Through a circular aperture in this plate (the 
slot is not cut through the whole thickness of the plate, but only as shown in 
Fig. 46) and about the middle of the slot hangs the rod r attached to the 
scale-pan. 




(a^ 




By means of right and left handed screws on a rod R, which is prolonged 
out of the case to the observer, these two sliding- pieces can be made to 
approach, and clamp the rod, or to recede and free it. By having the opposite 
surfaces of the sliding- pieces and the rod polished and clean, it is possible 
to clamp and unclamp without producing any disturbance. The clamp is 
of great use also to lessen the vibrations when they are too large, as it may be 
brought into action at any moment, and on releasing carefully the beam will 
start again from rest without any impetus. It may be used too to increase 
the vibrations by releasing suddenly, when the beam will have a slight impetus 
in one direction or the other. 

The weights which I have compared are two brass pounds avoirdupois, 
made for me by Mr. Oertling, and marked A and B respectively. They are 



WITH GREAT DELICACY, ETC. 



15 



of the usual cylindrical shape with a knob at the top (Fig. 36). Two small 
brass pans (Fig. 3 c) with a wire arch by which they can be suspended, are 
used to carry them; these are called respectively X and Y. I found on 
beginning to use them that there was too great a difference between A and 
B. I therefore adjusted them by putting a very small piece of wax upon A, 
the lighter. But the difference between them increased by 0-0782 mgm. in 
two days, which I thought was probably due to the wax. After the fourth 
series I therefore removed it and scraped B till it was more nearly equal 
to A. The weighings I — IV have, however, been retained, for though the 
differences on different days vary they are fairly constant on the same day. 

The weights are changed by the following apparatus which has been 
designed to effect the change as simply and quickly as possible. 

A horizontal ' side-rod ' or Hnk {ss, Fig. 5) is worked by two cranks (c, c, 
Fig. 56), which are attached to the axles of two equal toothed wheels {t, t) 




16 ON A METHOD OF USING THE BALANCE 

with a pinion (p) connecting them. A second pinion (q), on a rod prolonged 
out of the case to the observer, gears with one of the toothed wheels. By 
turning this rod the toothed wheels are set in motion, both in the same 
direction, moving the horizontal 'side-rod' from the right say upwards and 
over to the left. A pin (pn) stops its motion downwards further than is shown 
in Fig. 6 a. Near each end of the rod is cut a notch, and across these are 
hung the pans carrying the weights. The apparatus is fastened to the floor 
of the case between the central upright, supporting the beam, and the scale- 
pan, *the side-rod being perpendicular to the direction of the beam, and 
exactly over the centre of the pan. In Fig. 5 a, one of the weights B is sup- 
posed to be resting on the scale- pan (the wires suspending the pan from the 
beam not being shown), the side-rod having moved down so far below the 
wire of the smaller pan carrying the weight that it leaves it quite free. If, 
now, it is desired to change the weights the rod R is turned, setting the wheels 
in motion, the side-rod moves up, picks up B — the notch catching the wire — 
then travels over round to the extreme right, when A will be just over and 
nearly touching the scale-pan. By continuing the motion slightly A will 
be gently deposited on the pan, and the side-rod will move slightly down 
leaving the weight quite free. On the scale-pan are four pins, turned slightly 
outwards, acting as guides for the small pan, and ensuring that it shall always 
come into the same position. The wheels and pinions are of such a size that 
two revolutions of the rod just suffice to change one weight for the other. 

It will be seen that all the manipulation required from the observer during 
a series of weighings is the simple turning of three rods, which are prolonged 
out of the balance case to where he is stationed at the telescope. By turning 
one of these he can change the position of the rider on the beam by a known 
amount, and so find the value of his scale. By turning a second he clamps 
the scale-pan, and so steadies the balance while the weights are changed by 
turning a third rod. I have made this arrangement not only because it 
seems as simple as possible to secure the end required, but also because it 
seemed more applicable to a vacuum-balance (with which I hope ultimately 
to test it). 

I take this opportunity of expressing my thanks to Mr. Thomas Foster, 
mechanician of Owens College, for his aid in the construction of the apparatus, 
and in the planning of many of its details. 

Method of conducting a Series of Weighings. 
After the counterpoise has been adjusted so that the beam swings nearly 
about its horizontal position, the frame is lowered so that the balance is 
ready for use. The pan is then clamped and the balance is left to come to 
a nearly permanent state of flexure if possible, sometimes for the night or 
even longer. The lamp is lighted usually half-an-hour or more before begin- 
ning to observe, so that its effect on the balance may attain a more or less 



WITH GREAT DELICACY, ETC. 17 

steady state. It is necessary also to wait some time after coming into the 
room, for the opening of the door will always cause a considerable and 
immediate deflection of the beam. When a sufficient time has elapsed, the 
observations are commenced with a determination of the value of one scale- 
division by means of the riders. The three extremities of two successive 
oscillations are observed with one of the riders resting on the beam. These 
are then combined as follows : The mean of the first and third is taken, and 
the mean again of this and the second, this constituting the 'resting- point,' 
that is, the position of equilibrium of the beam at the middle of the time. 
For instance, in weighing No. I (see tables at the end) the three extremities 
of successive oscillations were 280-5, 312-0, and 286-0 (column 2). The resting- 
point was taken as 

280- 5 + 286-0 + 2x31 2-0 _ ^97.62 

the rider on the beam being the right-hand one denoted by R (column 1). 
The balance is then clamped, and the other rider is brought on to the beam 
while the first is taken up. The resting- point is again observed. In No. I 
it was 270-05. The balance is again clamped, and the first rider again brought 
on to the beam, and, on unclamping, the resting-point again observed. In the 
same weighing it was 296-75. These three are sufficient to give one deter- 
mination of the deflection due to the transference of a rider. This will be 
the difference between the second resting-point and the mean of the first 

'^97-62 + 296-75 
and third. For instance, '^ ^ 270-05 = 27-13 divisions. This 

number is found in the fifth column. 

This process is continued, the resting- points being combined in threes till 
several values of the deflection due to the rider have been obtained, and the 
mean of these is taken as the true value. This plan of combining the resting- 
points requires that the observations should be taken at nearly equal intervals. 
After a little practice it will always take the observer about the same time 
to go through the same operations of clamping, changing the riders, unclamping, 
clamping again to lessen the vibrations about the new resting-point, and then 
beginning to observe, and I have considered that this was a sufficiently correct 
method of timing the observations. 

When a series has been taken it will at once be seen whether they were 
begun too soon after entering the room, or whether any irregular disturbing 
force has acted. For instance, in weighing No. II, determination of one 
scale-division, the first resting-point is so much lower than the succeeding 
with the same rider that evidently the balance was still affected by my 
entrance into the room. It was, therefore, rejected. Again, in weighing 
No. Ill, determination of the difference between the weights, the fourth 
resting-point was much lower than the others with the same weight in the pan. 

p. o. w. 2 



18 ON A METHOD OF USING THE BALANCE 

The resting-points, when the other weight was in the pan, showed no similar 
sudden drop of such magnitude. This observation was, therefore, rejected 
as being affected by some irregular disturbance. 

When the value of the deflection is determined, the value of one scale- 
division is at once found by dividing -3282 mgm. by the number of divisions 
of the deflection, since the change of the sides is equivalent to the addition 
of -3282 mgm. to one pan. 

The determination of the difference between the weights is then begun. 
This is carried on in a precisely similar manner, the only difference being 
that the rod changing the weights is now turned round in place of the rod 
changing the riders. I have usually taken a greater number of observations 
of the difference between the weights than of the deflection due to the riders, 
as the former is somewhat more irregular than the latter. This irregularity 
I believe to arise from slight differences of temperature of the two weights, 
and perhaps from air currents caused by their motion inside the case. They 
do not seem to be due to any fault in the clamping arrangement, since that 
is employed equally in both, and the changing of the weights, if effected gently, 
does not move the beam at all. 

When the deflection has been determined, it is multiplied by the number 
of milligrammes corresponding to one scale-division, and this, of course, gives 
the difference between the weights. I have interchanged the weights in the 
two pans X and Y, between the series of weighings, in order to make the 
experiments like those conducted in the weighings for the standard pound. 
But my object has not been to show at all that the method gives consistent 
results day after day, and, in fact, the difference between the weights has 
varied. For instance, according to weighings I and II, A — B = -0446, 
while, according to weighings III and YV, A — B = -0116. There is a greater 
difference between these than can be accounted for by errors of experiment, 
and it probably arose from the small piece of wax with which I made A nearly 
equal to B. The difference between the weights when measured to such 
a degree of accuracy as that which I have attempted, will, no doubt, vary 
from time to time, partly with deposits of dust, partly with changes in the 
moisture in the atmosphere, and so on. 

But I think the numbers which are given in the tables are sufficient to 
show that the difference between two weights in any one series of weighings 
can be measured with a greater degree of accuracy than has hitherto been 
supposed possible. I give in the tables a full account of the weighings, each 
series containing a determination of the value of one scale-division and 
a determination of the difference between the weights. The greatest deviation 
of any one of a series from the mean of that series of differences is always 
given. This I consider a better test of accuracy of weighing than the probable 
error. What is wanted in weighing is rather a method which will give at 



I 



WITH GREAT DELICACY, ETC. 19 

once a good determination of the difference between two weights. But 
I may state, that if the error of any one of a series be taken as its difference 
from the mean of that series, the probable error of a single determination of 
the difference between the weights in the first four series is -4344 of a division, 
or -0054 mgm., that is, s4-ituuuwu^^ ^^ ^^^ *otal weight, while the greatest 
error is 1-8 divisions, or -0224 mgm., that is, ^ ooo^oooo ^^ ^^ *^® ^otal weight. 
It may be remarked that these weighings were all made during peculiarly 
unfavourable weather when there were frequent heavy showers, causing 
sudden changes of temperature, and thus seriously affecting the working of 
the balance. In the series V — VIII the greatest error is only -^ ooo^oooo ^^ 
the total weight, the weather having improved considerably. 

On the Employment of the Balance to determine the Mean Density 

of the Earth. 

In the Cavendish experiment, the attraction of a large sphere of lead of 
known mass and dimensions upon another smaller sphere, also of known mass 
and dimensions, is measured when the two are an observed distance apart. 
Comparing this attraction with the weight of the small sphere — that is the 
attraction of the earth upon it — and knowing the dimensions of the earth, 
we can deduce the mass of the earth in terms of the mass of the large lead 
sphere, and so obtain its mean density. The torsion-balance, which was 
invented for the purpose by Mitchell, the original contriver of the experiment, 
has hitherto been used to determine the force exerted by the mass upon the 
small sphere. In the arrangement here described, I have replaced the torsion- 
balance by the ordinary balance, and have so been able to compare the 
attraction of a lead sphere with that of the earth upon the same mass 
somewhat more directly. The results which I have obtained have no value in 
themselves, but they serve as an example of the employment of the balance for 
more delicate work than any which it has as yet been supposed able to perform. 

The method is shortly this: A lead weight (called ''the weight') weighing 
452-92 grms. (nearly 1 lb.) hangs down by a fine wire from one arm of a balance, 
from which the pan has been removed, at a distance of about six feet below it, 
and is accurately counterpoised in the other pan, suspended from the other 
arm. A large lead mass (called 'the mass') weighing 154,220-6 grms. 
(340 lbs.) is then introduced directly under the hanging ' weight.' The 
attraction of this mass increases the weight slightly and the beam is deflected 
through an angle which is observed. The value of this deflection in milH- 
grammes is measured by the employment of riders in the manner described 
above, and so the attraction of the ' mass ' is known. The increase of the 
weight caused- by the 'mass' has been in my experiments about -01 of a 
milligramme, or 4500^0000 ^^ ^^ *^® whole weight. 

The balance which I have used is that which I have described above. 
It was placed in the same room and in the same position as in the weighing 

2—9. 



20 ON A METHOD OF USING THE BALANCE 

experiments. The same method was used to observe the oscillations with a 
single mirror on the beam. The scale was a simple one etched on glass and 
not diagonally ruled. It had about 50 divisions to the inch, and the numbers 
increased from above downwards, so that an increase in the weight hanging 
from the left arm was indicated by a lower number on the scale. 

The ' weight' which is suspended by a very fine brass wire from the left arm, 
passing through a hole in the bottom of the balance-case, hangs in a double 
tin tube, 4 inches in diameter, to protect it from air-currents. At the bottom 
of the tube is a window, through which can be seen the bottom of the 'weight' 
as it hangs. The 'weight' is 4-248 centims. in diameter and is gilded. The 
' mass ' is a sphere of an alloy of lead and antimony. It was cast with a ' head ' 
on and then accurately turned. Its vertical diameter is 30-477 centims. 
(about 1 foot). The specific gravity of a specimen of the metal was found to 
be 10-422. Its weight given by a weighing-machine is 340 lbs. about, and this 
agrees very nearly with the weight calculated from the specific gravity. I am 
obliged to accept this as the true weight provisionally, until it is found more 
correctly by the large balance referred to above and now being constructed. 

This mass (Fig. 1, M) is placed in a shallow wood cup at one end of a 
2-inch plank, 8 inches wide and 6 feet 1 1 inches long, mounted on four flanged 
brass wheels, and serving as a carriage for it (Fig. 1). A plank about 12 feet 
long nailed to the floor in a direction perpendicular to the beam of the balance, 
as shown in Fig. 1, pp, acts as a railway for the carriage, and a firm stop at 
each end prevents the carriage from running off the rail. The distance be- 
tween the stops is rather less than twice the length of the carriage, and the 
' weight ' hangs down from the balance exactly midway between the stops. 
The 'mass' is placed on the carriage so that it is exactly under the 'weight' 
when the carriage is at one end of its excursion against one of the stops. An 
empty cup (c. Fig. 1) of the same dimensions as that in which the ' mass ' 
rests is placed at the other end of the carriage, and is just under the 'weight' 
when the carriage is against the other stop. By this arrangement no correction 
is needed for the attraction of the carriage upon the ' weight ' or counterpoise, 
and the eflect caused by the removal of the carriage from one end of its 
excursion to the other is entirely due to the difference of attractions of the 
'mass' upon the 'weight' and counterpoise in its two positions. The position 
of the 'mass' when directly under the 'weight' is called its 'in' position, and 
that when it is at the other end of its excursion is called the ' out ' position. 
The length of the excursion is 5 feet 7-3 inches. 

To draw the carriage along the rail a vertical iron shaft with a wood 
cyhnder at the lower end pivots on the floor, and is prolonged up to the 
level of the observer as he sits at the telescope with a handle by which he 
can turn it. The two ends of a rope which winds round the cylinder pass 
through pulleys on the stops, and are attached to the ends of the carriage. 



WITH GREAT DELICACY, ETC. 



21 



The observer can then move the 'mass' with great ease by turning the handle, 
even while looking through the telescope. 

When a series of observations is made, the general method is this. The 
deflection (r) due to the transference of a rider from one notch to the other 
on the beam is first observed exactly in the manner before described, the 
mean of four or five values being taken as the true value. Then the deflection 
(n) due to the difference of attraction of the 'mass' in its two positions is found 
in exactly the manner in which the difference between two weights is found, 
except that now when three successive extremities of oscillations have been 
observed for a resting-point the ' mass ' is moved from one position to the other 
where the weights were changed in the former experiments, the clamp not 
being brought into action. The second extremity of the oscillation which 
is proceeding while the 'mass' is moved, is observed as the first of the next 
three. When nine or more resting-points have been observed they are com- 
bined in threes, and the mean of the resulting values of the deflection n is 
used in the subsequent calculation. 

This deflection is, of course, less than that which would be observed were 
there no attraction on the counterpoise, and were the 'out' position of the 

A 
^ B 




'mass' at an infinite distance. To find the factor/ by which the deflection n 
due to the change of position of the ' mass ' must be multiplied in order to 
reduce it to the deflection which would be observed under these conditions, 
let AB be equal and parallel to the beam of the balance at the level of the 
counterpoise of which B is the centre. Let C be the centre of the ' weight,' 
D that of the 'mass' in its 'in' position, E that of the 'mass' in its 'out' posi- 
tion. Draw BF, DF, parallel to AD, AB. Let ju, = the mass of the 'mass.' 
The vertical attraction of the 'mass' in its 'in' position will be 

jM fiBF 

CD^ BD^ ' 
The vertical attraction in its ' out ' position will be 

^CD ixBF 
CE^ BE^' 



22 ON A METHOD OF VSINQ THE BALANCE 

The difference between these is actually observed, viz. : 

^ ( CD^.BF CD^ CD^ . BF 

C&\ BD^ CE^ BE^ 

The factor by which we must multiply the observed difference to reduce it 

to the attraction of the 'mass' on the 'weight' in its 'in' position is therefore 

CB^ . BF CD^qJDKBF 
*^~ "^ BD^ '^CE^ BE^ 
= 1-0185 
since CD = 22-13 centimetres. 
BD = 192-03 
BF = 187-70 
CE = 172-36 
BE = 257-09 

The values of r and n being observed, the distance d between the centres 
of the 'mass' and 'weight' is then measured by adding to 17-362 centims. (the 
sum of their radii) the distance from the top of the 'mass' to the bottom of 
the 'weight' as measured by a cathetometer. 

It now remains to explain the calculation of the mean density A from the 
observed values of r, n, and d. We have 

/ X increase in weight observed _ Attraction of 'mass' on 'weight' when 'in' 
Weight of 'weight' Attraction of earth on 'weight' 

But the increase in weight is ^„^.^.,^ mgms., since the distance between 

^ r X 202716 ^ 

the notches is 6-654 milHms., and the half beam 202-716 millims. The weight 
of the 'weight' is 453-29 grammes. 

The attraction of the 'mass' per gramme of the 'weight' 

_ Volume X density 

(distance d between centres of 'mass' and 'weight')^' 
_ Mass in grammes 
~ d^ ' 

_ 154220-6 
~ I^ • 

The attraction of the earth is similarly 

A X inR {1 + M- (f M - e) cos2 A}, 
where A = mean density of the earth, 

R = earth's polar radius in centimetres, 
j^ _ centrifugal force at the equator 
Equatorial gravity ' 

€ = ellipticity, 
A = latitude. 



WITH GREAT DELICACY, ETC. 



23 



The logarithm of the coefficient of A when R is in inches is 9-0209985 
(Astron. Soc. Mem., vol. 14, p. 118), or if R is in centimetres it is 94258322. 
Inserting these values in the equation we obtain 

154220-6 

453290 



A = 



£ 

^ttR {1 + M - (f M - e) cos2 A} 



fx 



n 6654 



r 202716 



^""^^^ 



where 
and 



C = 



154220-6 X 453290 x 202716 



iTvR {1 + M - (f M - e) cos2 A} X / X 6654 
log C = 1-8951337, 
.-. log A = 1-8951337 
+ log r 

— log n 

- 2 log d. 

The following table is an account of an experiment made on May 30th, 
1878, and will serve as a specimen of the method of making the observations. 
It is the best which I have yet made in the closeness with which all the values 
of n agree with each other. 





VII. May 


30, 1878. Determination of 


r. 


Eider on 
beam 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting -points 


Differences 


R 


260-9 

250-9 
260-6 


255-82 


— 


— 


L 


"214-6 

205-0 
212-3 


209-22 


256-74 


47-52 


R 


271-9 

244-7 
269-4 


257-67 


210-31 


47-36 


L 


214-3 

209-2 
212-9 


211-40 


257-62 


46-22 


R 


249-7 

263-8 
253-0 


257-57 


212-23 


45-34 


L 


204-9 

220-7 
206-0 


213-07 




— 



Mean r = 46-61. 



24 



ON A METHOD OF USING THE BALANCE 



Determination of n. 



Position 
of mass 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting-points 


Differences 
= n 


In 


216-8 

208-1 
215-9 

210-9 

216-1 
211-0 


212-22 


— 


— 


Out 


213-52 


212-27 


1-25 


In 


213-9 

210-8 
213-8 


212-32 


213-61 


1-29 


Out 


212-7 

214-6 
212-9 


21370 


212-41 


1-29 


In 


211-5 

213-6 
211-3 


212-50 


213-78 


1-28 


Out 


215-9 

211-8 
216-0 


213-87 


212-60 


1-27 


In 


210-2 

2150 
210-6 


212-70 


213-93 


1-23 


Out 


216-7 

2116 
216-1 


214-00 


212-73 


1-27 


In 


209-9 

215-4 
210-4 


212-77 


214-08 


1-31 


Out 


217-0 

211-4 
216-9 


21417 
213-20 


212-98 


M9 


In 


209-8 

216-2 
210-6 


— 


— 



Mean n = 1 26. 



WITH GREAT DELICACY, ETC. 25 

At the close of the experiment d was found to be 22-226 centimetres. 
We have therefore 

log A = log 

+ log 46-61 
- log 1-26 
-2 log 22-226 

= 1-8951337 I 

' +1-6684791 I 

i- 0-1003705 
-2-6937226 
= 0-7695197. 
.-. A =5-882. 

I have made in all eleven experiments with this method. The resulting 
values of A are 

1 May 20 5-393. 



2 ... 


. . . ,, 


23 ... 


... 5-570. 


3 ... 


5) 


24 ... 


... 4-415. 


4 ... 


5, 


28 ... 


... 7-172. 


5 ... 


. . 5) 


29 ... 


... 5-109. 


6 ... 


,5 


29 ... 


... 6-075. 


7 ... 


,, 


30 ... 


... 5-882. 


8 ... 


,, 


30 ... 


... 6-336. 


9 ... 


June 5 ... 


... 5-977. 


10 ... 


. . . 5j 


5 ... 


... 5-580. 


11 ... 




6 ... 


... 5-100. 



The resulting mean value of the mean density of the earth is 5-69. 

If the eleven determinations be supposed to have equal weight, the 
probable error of their value is 0-15. 

The various determinations differ very much among themselves, but they 
seem to me sufficiently close to justify the hope that with a large balance 
and a large weight, which will not be so easily affected by air-currents, and 
with greater precautions to prevent those air-currents, a good determination 
of the mean density of the earth may ultimately be obtained by this method. 



26 



ON A METHOD OP USING THE BALANCE 



I. June 12. Determination of 1 Scale- Division. 



Rider on 
beam 


Extremities of 

three successive 

oscillations 


Resting- 
point 


Mean of pre- 
ceHing and 
succeeding 

resting -points 


DifiEerence 
due to R - L 


L 


280-5 

312-0 
286-0 


297-62 


— 


— 


R 


253-1 

284-9 
257-3 


270-05 


297-18 


27-13 


L 


306-2 

288-5 
303-8 


296-75 


268-68 


28-07 


R 


258-8 

275-0 
260-5 


267-32 


294-81 


27-49 


L 


285-8 

298-7 
288-3 


292-87 


266-79 


26-08 


R 


272-9 

260-6 
271-0 


266-27 


292-88 


26-61 


L 


296-1 

290-0 
295-5 


292-90 


— 


— 



Mean R - L = 27-08 divisions. 

0-3282 
1 division = ' - = 0-01212 milligramme. 



WITH GREAT DELICACY, ETC. 



27 



Determination of {B -{- X) — (A -\- Y). 



Weight in 
pan 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting -points 


Difference 


A+ Y , 


254-5 

272-9 
257-3 


264-40 


— 


— 


B + X 


267-5 

249-8 
264-8 


257-97 


263-27 


5-30 


^ + F 


258-5 

265-4 
259-3 


26215 


257-04 


5-11 


B + X 


259-3 
, 253-6 

258-0 


256-12 


261-87 


5-75 


A+ Y 


253-5 

268-5 
255-9 


261-60 


255-63 


5-97 


B + X 


244-6 

264-5 
247-0 


255-15 


261-87 


6-72 


A+Y 


273-6 

252-0 
271-0 


262-15 


255-85 


6-30 


B + X 


252-7 

260-0 
253-5 


256-55 


262-92 


6-37 


A+Y 


275-0 

253-7 
272-4 


263-70 


— 


Mean 

difiference 

593 



.-. {B + X) - {A+ Y) = 0-01212 X 5-93 = 0-0718 milligramme. 
Greatest deviation from the mean = 0-82 division = 0-0099 milligramme. 



The weather during this series of weighings was very unfavourable, with 
frequent heavy showers. 



ON A METHOD OF USING THE BALANCE 



II. June 13. Determination of 1 Scale-Division. 



Rider on 
beam 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting -points 


Difference 
due to R - L 


L 


302-8 

299-9 
302-9 


301-37 


This is rejected as it is so 
much lower than the 
succeeding 


R 


321-8 

335-7 
326-8 


330-00 


— 


— 


L 


299-6 

308-7 
301-0 


304-50 


330-68 


26-18 


R 


322-9 

337-5 
327-6 


331-37 


305-10 


26-27 


L 


299-5 

310-9 
301-5 


305-70 


331-77 


26-07 


R 


325-2 

337-1 
329-3 


332-17 


305-90 


26-27 


L 


290-4 

319-4 
295-2 


306-10 


— 



Mean R - L = 26-20 divisions. 
.*. 1 division = 0-01252 milligramme. 



The weather was as unfavourable as on the previous day. 

The weights were changed shortly before the commencement of this series 
and the balance then worked so irregularly that for some time I was unable 
to begin the rider- determination. Even then the first resting-point had to 
be rejected. 



WITH GREAT DELICACY, ETC. 



29 



Determination of (A+ X)— {B+ Y). 



Weight in 
pan 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting -points 


Diflference 


B+ Y 


307-8 

326-0 
311-6 


317-85 


These are all rejected, as the 
motion was so irregular. 
The weights had been 
\ changed a short time be- 
fore, and had probably not 
reached an uniform tem- 
perature 


A + X 


293-7 

307-8 
295-8 


30127 


B+ Y 


309-6 

322-3 
312-6 


316-70 


A + X 


295-5 

309-6 
297-6 


303-07 


B+ Y 


304-1 

322-7 
308-1 


314-40 


A + X 


289-3 

304-4 
291-5 


297-40 


— 


— 


B+ Y 


305-5 

3150 
307-5 


310-75 


297-35 


13-40 


A + X 


294-2 

300-1 
294-8 


297-30 


309-95 


12-65 


B+ Y 


304-6 

312-7 
306-6 


309-15 


296-51 


12-64 


A + X 


290-0 

300-7 
291-5 


295-72 


— 


— 



Mean {A + X) - {B + Y) = 12-89 divisions. 
.-. {A + X) - (B + Y) = 0-1614 milligramme. 
Greatest deviation from the mean = 0-51 division = 0-0062 milligramme. 



30 



ON A METHOD OF USING THE BALANCE 



III. June 13. Determination of 1 Scale- Division. 



Rider 
on beam 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting-points 


Difference 
due to R - L 


R 


301-3 

290-6 
299-7 


295-55 


— 


— 


L 


313-7 

328-7 
318-1 


322-30 


296-38 


25-92 


R 


293-6 

300-4 
294-5 


297-22 


322-47 


25-25 


L 


311-0 

331-6 
316-4 


322-65 


297-88 


24-77 


R 


281-6 
313-2 

286-2 


298-55 


323-57 


2502 


I. 


310-1 

335-8 
316-3 


324-50 


1 298-62 


1 
25-88 


R 


287-8 
308-1 
' 290-8 


298-70 


— 


- 



Mean R - L = 25-37 divisions. 



0-3282 
1 division = o^-qy = 0-01293 milligramme. 



WITH GREAT DELICACY, ETC. 



31 



Determination of {A-\- X) — {B-^Y). 



Weight in 
pan 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting.points 


DiflEerence 


A-\- X 


303-3 

294-5 
301-8 


298-52 


— 




B+ Y 


303-6 

315-8 
306-2 


310-35 


297-07 


13-28 


A + X 


288-9 

301-5 
290-6 


295-62 


— 


— 


B+ Y 


300-2 

311-6 
302-5 


306-47 


This is evidently due to some 
irregular and short disturb- 
ing cause, and is rejected 


A^ X 


28S-8 

304-2 
291-1 


297-07 


— 


— 


B+ Y 


301-5 

319-3 
303-9 


311-00 


297-16 


13-84 


A + X 


291-7 

3020 
293-2 


297-22 


310-33 


13-11 


B+ Y 


3010 

316-8 
304-1 


309-67 


— 


— 



Mean {A + X) - {B + Y) = 13-40 divisions. 

.-. (A + X) - (B + Y) = 0-1732 milligramme. 

Greatest deviation from the mean = 0-44 division = 0-0057 milligramme. 



32 



ON A METHOD OF USING THE BALANCE 



IV. June 14. Determination of 1 Scale-Division. 



Rider on 
beam 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting-points 


DifEerence 
due to R - L 


R 


252-5 

253-4 
251-7 


252-25 


This was taken soon after enter- 
ing the room. It is so much 
lower than the succeeding 
that it is rejected 


L 


290-8 
273-7 

288-8 


281-75 


— 


— 


R 


242-4 

266-7 
245-1 


255-22 


281-96 


26-74 


L 


272-6 

289-7 
276-7 


282-17 


255-89 


26-28 


R 


259-8 
253-9 

258-7 


256-57 


282-67 


26-10 


L 


286-2 
280-0 

286-5 


28317 


— 


— 



Mean R - L = 26-37 divisions. 

0-3282 
,-. 1 division = = 0-01244 milligramme. 



Being interrupted, I could not continue the series of rider- determinations 
further. 



WITH GREAT DELICACY, ETC. 



33 



Determination of (B + X) — (A+ Y). 



Weight in 
pan 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- i 

resting -points 

1 


A+ Y 


291-7 

275-0 
289-5 


282-80 


— 


— 


B+ X 


261-0 

280-3 
263-4 


271-25 


282-85 


11-60 


A+ Y 
B + X 


285-7 

280-4 
285-1 


282-90 


271-66 


11-24 


280-2 
265-3 

277-5 


272-07 


283-90 


11-83 


A+ Y 


294-9 

276-1 
292-5 


284-90 


273-43 


11-47 


B + X 


276-0 

273-9 
275-4 


274-80 


286-40 


11-60 


A+ Y 


267-3 

305-4 
273-5 


287-90 


275- n 12-79 


B+ X 


278-5 
272-9 

277-4 


275-42 


289-28 i 13-S6 


A+ Y 


296-6 

285-5 
295-1 


290-67 


— — 



Mean {B + X) - [A + Y) ^ 12-06 divisions. 
.-. (5 + Z) - (^ + 7) = 0-1500 milligramme. 
Greatest deviation from the mean =1-8 divisions = 0-0224 mgm. . 



The previous determination oi{B+ X)— (A+ Y) was -0718 mgm. The 
difference is too great, -0782 mgm., to be accounted for by errors of experiment. 
There must have been some deposit on one of the weights, either of dust or 
moisture. I therefore took them out, cleaned and adjusted them by scraping 
B till nearly equal to A, and removing the wax from A. 



p. c. w. 



34 



ON A METHOD OF USING THE BALANCE 



V. June 14. Determination of 1 Scale-Division. 



Rider on 
beam 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting -points 


Difference 
due to R - L 


R 


212-6 

230-3 
214-7 


221-97 


— 


1 


L 


252-3 

242-5 
251-2 


247-12 


222-16 


24-96 


R 
L 


225-0 

220-5 
223-6 


222-40 


246-67 


24-27 


238-6 

252-8 
240-7 


246-22 


221-87 


24-35 


R 


224-9 

218-7 
223-3 


221-40 


245-98 


1 
24-58 


L 


249-7 
242-2 

248-9 


245-75 


221-26 


24-49 


R 


226-8 

216-7 
224-3 


221-12 




! 



Mean R - L = 24-53 divisions. 

Q.Q902 

1 division = - ^ = 0-01339 milligramme. 



WITH GREAT DELICACY, ETC. 



35 



Determination of {B+ Y)— (A+ X). 



Weight in 
pan 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting-points 


DifiEerenoe 


A + X 


206-3 

212-7 
206-8 


209-62 


This was rejected as being so 
much higher than the rest 


B+ Y 


209-0 

206-4 
208-6 


207-60 




— 


A + X 


212-4 

204-0 
211-3 


207-92 


20771 


21 


B+ Y 


206-8 

208-8 
206-9 


207-82 


207-88 


-06 


A + X 


211-8 

204-4 
210-8 


207-85 


207-64 


•21 


B+ Y 


209-1 

206-1 
208-6 


1 

207-47 207-86 

i 
1 


-39 


A + X 


210-7 

205-5 
209-8 


207-87 


207- 11 


-76 


B+ Y 


208-5 

205-3 
207-9 


206-75 207-53 


•78 


A + X 


209-7 

205-1 
208-9 


1 
207-20 206-45 

i 


-75 


B+ Y 


203-3 

208-7 206-15 
203-9 


— 


— 



Mean {B + Y) - {A + X) = 0-45 division = 0-00602 milligramme. 
Greatest deviation from the mean = 0-39 division = 0-00522 mgm. 

A and B had here been cleaned and B readjusted by scraping. A small 
vessel containing calcium chloride was put inside the balance to dry the air. 
This improved the action of the clamp, diminishing the cohesion. 

3—2 



36 



ON A METHOD OF USING THE BALANCE 



VI. June 17. Determination of 1 Scale-Division. 




Mean R - L = 23-02 divisions. 
1 division = -k^^;^ = 0-01425 milligramme. 



WITH GREAT DELICACY, ETC. 



37 



Determination of (5+ X)— (A-{- Y). 



Weight in 
pan 


Extremities of 
oscillations 


Resting- 
point 


Mean of pre- 
ceding and 
succeeding 

resting -points 


Difference 


B+X 


216-9 

214-6 
216-6 


215-67 


— 


— 


A+Y 


256-7 

233-8 
253-9 


244-55 


216-08 


28-47 


B + X 


220-1 

213-7 
218-5 


216-50 


244-90 


28-40 


A+Y 


254-8 

236-7 
252-8 


245-25 
217-65 


217-07 


28-18 


B + X 


221-5 

214-5 
220-1 


245-48 


27-83 


A+Y 


257-5 
235-3 

254-8 


245-72 


217-37 


28-35 


B+X 


220-9 

214-0 
219-5 


217-10 


245-68 


28-58 


A+Y 


246-8 

244-5 
246-8 


245-65 


216-83 


28-82 


B + X 


223-7 

210-5 
221 -6 


216-57 


— 


— 



Mean {B + X) - {A + Y) = 28-38 divisions = 0-4043 milligramme. 
Greatest deviation from the mean = 0-55 division = 0-00784 milligramme. 



38 



ON A METHOD OF USING THE BALANCE 



VII. June 18. Determination of 1 Scale-Division. 



Rider on 
beam 

R 


Extremities of 
oscillations 


Resting- 
point 


Mean of pre- 
ceding and 
succeeding 

resting -points 


Difference 
due to R - L 


171-6 

190-7 
172-7 


181-42 


— 


— 


L 


2103 

225-3 
212-4 


218-32 


180-47 


37-85 


1 R 


189-2 

171-7 
185-5 


179-52 


217-54 


38-02 


L 


223-2 

210-8 
222-3 


216-77 


179-04 


37-73 


1 

R 


1850 

173-2 
182-9 


178-57 


216-17 


37-60 


1 

1 L 


220-2 

211-3 
219-5 


215-57 


178-22 


37-35 


1 

1 - 

1 


189-1 

168-8 
184-8 


177-87 

1 


— 


— 



Mean R - L = 37-71 divisions. 



Q.Q2Q9 

■. 1 division ^ 07^ = 0-00870 milligramme. 



WITH GREAT DELICACY, ETC. 



Determination of (B-{- Y)— {A-\- X). 



Weight in 
pan 


Extremities of 
oscillations 


Resting - 
point 


Mean of pre- 
ceding and 
succeeding 

resting -points 


Difference 


B+ Y 


208-5 

221-7 
209-5 


215-35 


— 


— 


A + X 


246-9 

214-4 
244-1 


229-95 


216-22 


13-73 


B+ Y 


219-2 

215-6 
2180 


217-10 


230-91 


13-81 


A + X 


240-8 

224-0 
238-7 


231-87 


217-12 


14-75 


B+ Y 


226-1 

209-4 
223-7 


217-15 


232-24 


1.509 


A + X 


221-8 

242-3 
224-1 


232-62 


216-96 


15-66 


B+ Y 


211-5 

221-8 
212-0 


216-77 


232-21 


15-44 


A + X 


219-0 

243-0 
222-2 


231-80 


— 


— 


B+ Y 


208-3 

2290 
209-9 


21905 


This sudden change of resting - 
point must be due to some 
irregular disturbance. It is 
therefore rejected. It was 
slowly returning to nearly its 
former values 



Mean {B + Y) - {A + X) = 14-75 divisions = 0-12831 milligramme. 
Greatest deviation from the mean = 1-02 division = 0-0089 mgm. 



The great difference between the result here and that in series V is probably- 
due to deposit of dust. The new mirrors had to be fixed up just before the 
experiment began, and the doors were open for some time. At the conclusion 
of the weighing I found a good deal of dust on the weights. 



40 



ON A METHOD OF USING THE BALANCE 



VIII. June 19. Determination of 1 Scale- Division. 



Rider on 
beam 


Extremities of 
oscillations 


Resting- 
point 


Mean of pre- 
ceding and 
succeeding 

resting -points 


Difference 
due to R - L 


L 


235-8 

222-3 
233-3 


228-42 


This is so much higher than 
the rest, probably through 
being observed soon after I 
entered the room, that it is 
rejected 


R 


197-0 

181-2 
193-9 


188-32 


— 


— 


L 


228-2 

222-7 
228-0 


225-40 


188-24 


37-16 


R 


197-4 

180-7 
193-9 


188-17 


i 
225-46 1 37-29 


L 


233-6 1 

218-0 225-52 
232-5 j 


i 
187-63 ' 37-89 

i 


R 


191-7 

183-3 187-10 
190-1 


225-28 


38-18 


L 


230-2 

220-3 
229-4 


225-05 


187-35 37-70 


R 


193-8 

182-5 
191-8 


187-60 


— 





Mean R - L = 37-64 divisions. 

0*3282 
1 division = -^ - = 0-00872 milligramme. 



WITH GREAT DELICACY, ETC. 



41 



Determination of {B -\- X) — (A -\- Y). 



Weight in 
pa,n 


Extremities of 
oscillations 


Resting, 
point 


Mean of pre- 
ceding and 
succeeding 

resting -points 


Difference 


A-\- Y 


245-3 

238-7 
244-2 


241-72 


In one observation not recorded 
just before this the clamp 
had been loose, and the 
scale-pan had slipped, and 
the resting-point was thereby 
changed. The disturbance 
had apparently not subsided 
when this was taken, it is 
therefore rejected 


B + X 


193-0 

185-7 
191-4 


188-95 


— 


— 


A+ Y 


226-6 

243-3 
229-2 


235-60 


188-25 


47-35 


B+ X 


182-2 

192-8 
182-4 


187-55 


235-22 


47-67 


A+ Y 


229-4 

239-2 
231-6 


234-85 


187-72 


47-13 


B+ X 


194-4 

182-5 
192-2 


187-90 


234-71 


46-81 


A+ Y 


239-8 

229-7 
2391 


234-57 


187-75 


46-82 


B + X 


194-7 

181-6 
192-5 


187-60 


234-51 


46-91 


A+ Y 


230-6 

237-4 
232-4 


234-45 


187-50 


46-95 


B + X 


186-0 

1890 
185-6 


187-40 


— 





Mean (B + X) - {A + Y) = 47-09 divisions = 0-4100 miUigramme. 
Greatest deviation from the mean = 58 division = 000506 mgm. 



42 ON A METHOD OF USING THE BALANCE WITH GREAT DELICACY, ETC. 



Summary. 



Greatest deviation 
Series Mgms. from mean in 

milligrammes 



II 
III 

IV 



I (B+Z)-(^+y) = -0n8 -00991 

I (A+X)-{B+Y) = -\U4: -0062 (^ ^ ^ 

- (^ + Z)-(B+r) = -1732 •0057) ^^^^^g 

(B+X)~(A+Y) = -\m) -0224 ( ^ °^ ^ 



V (5+y)-M + Z) = -0060 -0052) 

VI (B+X)-(A+Y) = -4043 -0078 / ^ " ^ + ^051 mgm. 

VII (B+y)-M + Z) = -1283 -00891 

VIII (,B+X)-(A+Y) = -4100 -0051 ( ^ " ^ + ^^^^ ""8™- 

The greatest error — that is the greatest deviation of any one value from 
the mean of its series — in the first four series is yoooVttoo^^ ^^ ^ pound. The 
greatest error in the four series V — VIII is n oWoooo^^ ^^ ^ pound. 



3. 



ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH 
AND THE GRAVITATION CONSTANT BY MEANS OF THE 
COMMON BALANCE. 

[Phil. Trans. A, 182, 1892, pp. 565-656.] 

[Received May 13. Read June 4, 1891.] 

I. Account of Apparatus and Method. 

In a paper printed in the Proceedings of the Royal Society, No. 190, 1878 
(vol. 28, pp. 2-35)*, I gave an account of some experiments undertaken in 
order to test the possibihty of using the Common Balance in place of the 
Torsion-Balance in the Cavendish Experiment. The success obtained seemed 
to justify the intention expressed in that paper to continue the work, using 
a large bullion-balance, instead of the chemical balance with which the pre- 
liminary experiments were made. 

As I have had the honour to obtain grants from the Royal Society for 
the construction of the necessary apparatus, I have been able to carry out 
the experiment on the larger scale which appeared likely to render the 
method more satisfactory, and this paper contains an account of the results 
obtained. 

At the time I was making the preliminary experiments the late Professor 
von Jolly was already employing the balance for gravitation investigations 
(W iedemann'' s Annalen, vol. 5, p. 112), though I was not aware of the fact. 
Later he published an account {Wied. Ann., vol. 14, p. 331) of a determination 
of the Mean Density of the Earth by the use of the Balance. Still more 
recently Drs. Koenig and Richarz have devised a method of using the balance 
for the same purpose {Nature, vol. 31, pp. 260 and 475), and I believe that 
their work is still in progress. It might appear useless to add another to 
the list of determinations, especially when, as Mr. Boys has recently shown, 
the torsion-balance may be used for the experiment with an accuracy quite 
unattainable by the common balance. But I think that in the case of such 
a constant as that of gravitation, where the results have hardly as yet begun 

* [Collected Papers, Art. 2.] 



44 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

to close in on any definite value, and where, indeed, we are hardly assured 
of the constancy itself, it is important to have as many determinations as 
possible made by different methods and different instruments, until all the 
sources of discrepancy are traced and the results agree. 

The apparatus for the experiments described in this paper was first set 
up in the Cavendish Laboratory at Cambridge through the kindness of 
Professor Clerk Maxwell. After spending some months in working at the 
experiment, but without much success beyond the detection of some sources 
of error, I left Cambridge, and ultimately the apparatus was again set up at 
the Mason College, Birmingham. The difficulties in carrying out the work 
with any approach to exactness have been far greater than were anticipated, 
and many times work has been begun and results have been obtained, but 
examination has shown them to be affected by large errors which could be 
traced and eliminated by further improvements in the apparatus. 

At the beginning of 1890, however, the apparatus was brought into fair 
working order, and during the course of the year I made a number of experi- 
ments with the results recorded in this paper. 

The Princifle of the Experiment. 

The object of the experiment, in common with all of its class, may be 
regarded, primarily, as the determination of the attraction of one known 
mass M on another known mass M' a known distance d away from it. The 
law of universal gravitation states that when the masses are spheres with 
centres d apart this attraction is GMM'/d^, G being a constant — the gravita- 
tion constant — the same for all masses. Astronomical observations fully 
justify the law as far as M'jd^ is concerned. They do not, however, give the 
value of G, but only that of the product GM for various members of the solar 
system. 

To determine G we must measure GMM'/d^ in some case in which both 
M and M' are known, whether they be a mountain and a plumb-bob, as in 
Maskelyne's experiment, the surface strata and a pendulum-bob, as in Airy's 
experiment, or two spheres of known mass and dimensions, as in all the 
various forms of Cavendish's experiment. 

Knowing the gravitation constant G, we may at once find the mean 
density of the earth A. For if 7 be the volume of the earth— regarded as 
a sphere of radius i?— the weight of any mass M', being the attraction of the 
earth on it, is 

GVAM'/R^ 

But if g is the acceleration of gravity the weight is also expressible as M'g. 
Equating these we get 

A = gR^jGV. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 45 



Method of Using the Common Balance. 

In using the common balance to find the attraction between two masses, 
perhaps the most direct mode of proceeding would consist in suspending 
a mass from one arm of a balance by a long wire, and counterpoising it in 
the other pan. Then bringing under it a known mass, its weight would be 
slightly increased by the attraction of this mass. The increase would be the 
quantity sought if the attracting mass had no appreciable effect before its 
introduction beneath the hanging mass, and if, when beneath it, the effect 
on the balance could be neglected. This is very nearly the principle of the 
method used by von Jolly, and it is that of the method used in the prehminary 
experiments referred to above, in which a mass of 453 grms. of lead was hung 
from one arm of a chemical balance (about 40 centims. beam) by a wire 
1-8 metres long, and was attracted by a mass of 154 kilogrms. of lead. But 
the attraction to be measured was exceedingly small, rather less than 0-01 
milligrm., and it therefore appeared advisable to use a much larger balance 
with a larger hanging mass so that the attraction might be made comparable 
with the weight of exactly determined riders. Other anticipations as to 
proportionate increase of sensibility and diminution of effect of air-currents, 
have hardly been justified in the way I expected, though, by the ultimate 
form of the apparatus, they have, I think, been more than realised. 

With increase in the length of beam, a differential method became 
appHcable, by means of which the attraction of the mass on the beam w^as 
eliminated, and the necessity for prolonging the case to allow of a long 
suspending wire was removed. This will be seen from a consideration of 
Fig. 1. Let A, B represent equal masses suspended from the two arms of the 
balance, and let M be the attracting mass put first under A, the position of 
the beam being noted. If M is then placed under B its attraction is not only 
taken away from A but added to B, so that the tilting of the beam is that due 
to nearly double the attraction to be measured. Of course there are what 
we may term cross-attractions, in the first position, of M on B, and in the 
second position, of ilf on ^, but these may be allowed for in the calculations. 
We cannot give any mathematical expression for the attraction of M on the 
beam and suspending wires, owing to their irregularity of shape. But this 
attraction is eliminated if a second experiment is made in which A and B 
are raised equal known distances to A' and B' . For the difference between 
the two increments of weight on the right, is due solely to the alteration of 
the positions of A and B relative to M, the attraction on the beam remaining 
the same in each. From the observed effect of a known alteration of distance 
the attraction at any distance can be found. 

This is, shortly, the method adopted. The arrangement was ultimately 
complicated by the addition of a second mass m. Originally the mass M 



46 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

was alone on a turn-table which revolved about a vertical axis immediately 
under the central knife-edge of the balance. And some experiments which 
I made led me to suppose that mere change of position of the mass did not 
affect the level of the balance. However, after a complete determination 
in 1888 of the mean density, when I supposed that the work was finished, 
an examination of the results showed some curious anomahes, which I could 
only ascribe to a tilting of the whole floor on the displacement of the mass. 
Making new tests as to the effect of removal of the mass, I found that the 




Fig. 1. Elevation of balanoe-room and observing-room. The front of the case is removed, 
and the front pillar is not shown. The pointer and mirrors are at the back. 



previous tests had been quite wrong in principle, and that there was a very 
appreciable effect quite visible in the telescope when the masses A and B 
were removed, and M was removed from one side to the other, the slope of 
the floor changing by an angle comparable with a third of a second. If this 
had been absolutely constant in amount, the differential method would have 
eUminated it ; but, probably, it varied sKghtly in successive motions of the 
turn-table, and the results showed that there was also a secular change, the 
amount of tilt gradually increasing. This secular change was probably due 
to increasing rigidity of the floor, so that it tilted over bodily, moving the 
supports of the balance with it, an increase partly due, perhaps, to the 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 47 

pressure of the building, which had only been erected ten or twelve years, 
but chiefly, I think, to a gas-engine recently erected next door. When this 
was doing heavy work, the vibrations were very plainly felt, and no doubt 
they greatly aided the floor in 'settling down.' A second balancing mass 
m was therefore added, half as great as M, and on the opposite side of the 
turn-table, but twice as far from the axis. The resultant pressure was now 
always through the axis, and I could detect no tilting of the floor when the 
turn-table was moved. Of course the balancing mass acted somewhat to 
reduce the effect of the larger attracting mass, but in a calculable ratio. 

Finally, in order to eliminate or reduce the effect of any want of symmetry 
in the moving parts or in the masses, a second set of experiments was made 
with all the masses turned over and moved from left to right, and the mean 
of the first and second sets was taken. 

I now proceed to a detailed description of the various parts of the apparatus 
and the mode of experiment. 

The Balance-Room. The balance-room is in the basement of the Mason 
College, immediately under my room, and about 20 metres from the street. 
On one side were three windows looking on to a small courtyard, entirely 
surrounded by high buildings, but the windows have been bricked up. On 
the two adjacent sides are two other rooms, and on the opposite side a closely 
fitting door opening on a short corridor with doors at each end. There is no 
chimney in the room, and only an opening in the ceihng through which the 
balance was observed from the room above. The floor is of brick, resting on 
earth, and is very firmly laid. 

The temperature of the room was taken by means of a thermometer with 
a protected bulb at the end of a long wooden rod hanging down from the 
room above. The thermometer was about 6 feet from the floor, near one 
end of the case, and it could be rapidly pulled up into the room above and 
read by the observer before its temperature sensibly varied. The tempera- 
ture never appeared to vary so much as 0-1° C. in the course of two or three 
hours. 

The Balance-Case and its Supports. The case (Fig. 1) is a large cabinet of 
IJ inch wood, 1-94: metres high, 1-63 metres wide, -61 metre deep, with three 
large doors in front giving access to the hanging masses and riders, and a small 
door at the back near the mirror hereafter described. It is lined inside and 
out with tinfoil, and under each of the suspended masses is a double bottom 
with a layer of wool between, making a total thickness of about 1 J inches or 
4 centims. At the top is a small window about 10 centims. square, through 
which the oscillations of the beam were observed. On each side within the 
case are placed three horizontal partitions, like shelves, to hinder circulation 
of the air. 

The larger attracting mass and the attracted masses are gilded, and it is 



48 03Sr A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

possible that some advantage may arise from having the surface of the case 
of different metal. For if it, too, were gilded, it would readily absorb 
radiation from the large mass, and when the inside temperature changed, 
the suspended masses would readily absorb radiation from the inner surface 
of the case. But gold probably absorbs considerably less of tin radiation 
than it absorbs of gold radiation, and so temperature changes are probably 
lengthened out more than if the case were gilded. 




Plan of turn-table, girders, pillars, and balance-case. w. Window 
in case. c. Usual position of cathetometer. 



It was necessary to support the case so that the attracting masses could 
be moved about underneath it, and also to make it independent of the floor. 
Two brick pillars, 58 centims. x 36 centims. and 56 centims. high, were 
therefore built on thick beds of concrete under the floor, and about 3J metres 
apart. They rise up free from the bricked floor. Stretching between them 
are two parallel iron girders (g, g), about 30 centims. apart, and with their 
under side 56 centims. above the floor. The balance-case is placed across 
the middle of these girders (see plan, Fig. 2), with its under surface level with 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 49 

that of the girders. The square base-plate of the balance is placed on the 
girders on three levelling screws. Two horizontal screws attached to the 
girders bear against each edge of the base-plate, so that it can be adjusted 
and fixed in any position. 

To lessen vibration one tier of bricks is removed from each pillar, and in 
its place are inserted eight cyhndrical blocks of indiarubber (^, i, Fig. 1), 
originally 7-5 centims. diameter and 7-5 centims. high. These crushed down 
almost 1 centim. at once, but have not shown any further measurable con- 
traction in the coursfe of several years. Their effect in deadening vibration 
has been surprisingly great. 

The Turn-table. On a bed of concrete, and quite free from the brickwork 
of the floor, is a circular rail of cast iron, 1-3 metres in diameter. On this, on 
conical brass wheels and pivoted at the centre, runs the turn-table, about 
1-5 metres in diameter. This is made of wood and covered with tinfoil. It 
is like a wheel with a flat circular rim, and with four flat spokes arranged as 
a cross. It is as nearly symmetrical as possible, and at opposite ends of a 
diameter are placed two shallow cups, in either of which the large attracting 
mass may rest. The centres of these cups are a distance apart, equal to the 
length of the balance-beam. There are cut slots through the bottom of each 
cup, so that the bottom of the mass can be seen for the purpose of measuring 
the vertical diameter. 

Two beams, 2-74 metres long, run across the turn-table 26 centims. apart, 
with the cups between them, and across the ends are two boards, each with 
a circular hole 12 centims. in diameter, and in either of these the smaller, 
or balancing mass, may rest. These beams are braced by brass rods to brass 
uprights at their middle points to diminish bending. 

The turn-table is moved by an endless gut rope passing round it, and 
fixed at one point of the rim. The two sides of the rope pass over pulleys 
on to a drum in the room above. There are stops on the circular rail, against 
which come brass pieces on the turn-table when the masses are in position 
at either end of the motion. The drum can be turned easily by the observer 
at the telescope. Since the knife-edges and planes of the balance are of steel, 
all other moving parts of the apparatus were made free from iron. As an illus- 
tration of the necessity of this, I may mention that for some time I used what 
I supposed to be a brass wire rope to move the turn-table, but on looking out 
for the explanation of some irregularities, I found that the brass was wrapped 
round a core of steel wire, which acquired poles at the highest and lowest 
points in the position in which it always rested between different sets of 
weighings. These poles had quite an appreciable action on the balance-beam. 

The Balance. This is of the large bullion-balance type, with gun-metal 
beam and steel knife-edges and planes. It was made specially for the 
experiment by Mr. Oerthng, with extra rigidity of beam. Its performance 

p c. w. 4 



50 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

has shown the great excellence of the design. The central knife-edge is 
supported on a steel plane by a framework rising 107 centims. above the 
base-plate, and the usual moveable frame can be raised or lowered from out- 
side the case, fixing the beam or setting it free to oscillate. The beam has 
often been left free to oscillate for months at a time, with the full load of 
20 kilogrms. on each side, but I have no reason to suppose that the knife- 
edges have suffered at all. 

The length of the beam was measured by taking the length of each half 
separately by a beam-compass, and the mean of several measurements gave 
123-329 centims. as the total length. The standard scale used throughout 
was that of a cathetometer made by the Cambridge Scientific Instrument 
Company. This scale has been verified at the Standards Office, and taking 
its coefficient of expansion as ewoo' ^^ ^^J ^® regarded for our purpose as 
perfectly correct at 18°, any errors being at that temperature much less than 
the errors of experiment. Comparing the beam-compass with this scale, it was 
found that -06 centim. must be subtracted, reducing the length to 123-269 
centims. Now both beam and scale are of gun-metal and may, therefore, 
without serious error, be assumed to have the same coefficient of expansion, 
so that this is the length of the beam at 18°. At 0° it is 123-232 centims. 

Mirrors, Telescope, and Scale. At first a mirror was attached to the 
centre of the beam and the reflection of a scale in it was observed, either 
in the ordinary method or in the method described in the former paper 
(Roy. Soc. Proc, vol. 28, 1879)*, where a second fixed mirror is used to throw 
the ray of light a second, or even a third time back on to the moving mirror, 
each return increasing the deflection of the ray. But it was then necessary 
to make the time of vibration very long, and even when the time was three 
minutes, the tilt due to the attraction, i.e. the change of resting-point, did 
not amount to more than two or three scale-divisions. Now certain 
irregularities observed when the apparatus was first set up at Cambridge, 
led to experiments on the time taken by heat to get through the case in 
sufficient quantity to affect the balance, and I found that a coil of copper 
wire placed close under the case on one side (the bottom of the case being 
then solid, 1 inch thickness), heated by a current yielding 100 calories per 
minute, began to produce an appreciable disturbance on the balance in about 
10 minutes, doubtless by the creation of air-currents from the heated floor 
of the case. It appeared advisable, therefore, to reduce the time of a complete 
experiment to less than this if possible, and, consequently, the time of a single 
swing very much below 3 minutes. This could only be done if at the same 
time the optical sensibility were very greatly increased. 

The employment of what may be termed the double-suspension mirror 
method due I beUeve to Sir WilHam Thomson, and used by Messrs. G. H. and 

* [Collected Papers, Art. 2.] 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 51 

Horace Darwin in their experiments on the Lunar Disturbance of Gravity 
{Brit. Assoc. Rep., 1881), has very satisfactorily solved the problem, giving 
a greatly increased deflection on the scale, even when the time of oscillation 
is as short as twenty seconds. 

This method, which deserves to be more generally known and applied for 
the detection of small motions, consists in suspending a mirror by two threads, 



Microscope sijage 



Bnrad^xt 





1 



Tir^ 



«q 



Varus ivorkUn^ 
im dashpot 

Fig. 3. Double-Suspension Mirror (half size). 

one from a fixed point, the other from the point which moves. The angle 
through which the mirror turns for a given motion of the latter point is 
inversely as the distance between it and the fixed point, so that by diminishing 
this distance the sensibility of the arrangement may be almost indefinitely 
increased. 

To apply it to the balance, a small bracket (Fig. 3) is fixed to the ordinary 
pointer of the balance, about 60 centims. below the central knife-edge. This 
projects horizontally at right angles to the axis of the beam, and it is bevelled 
at the edge. Close to it is another bevelled edge attached to a microscope- 

4—2 



52 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

stage movement which is fixed on to the central pillar of the balance. 
A thread of silk (as supphed for the Kew magnetometer) is fastened to the 
stage, passes over the bevelled edge, through two eyes (e, e) on a Hght frame 
holding the mirror, up over the bevelled edge of the bracket, and is fastened 
to the bracket. The microscope-stage movement allows the distance between 
the threads to be adjusted, and also enables the azimuth of the mirror to 
be altered. 

Of course, if the mirror were weightless, it would not affect the sensibihty 
of the balance, and the threads might be brought very close together. But 
the weight of the mirror — it is silver on glass, 56 milhms. x 38 miUims. x 10 
miUims. — has a considerable effect on the sensibility, diminishing it with de- 
crease of distance between the points of suspension. In practice it has been 
found convenient to work with the threads parallel, and from 3 to 4 miUims. 
apart, the time of swing one way being adjusted to about 20 seconds. 
A less time hardly suffices for a correct determination and record of the scale 
reading. Taking 4 milHms. as the distance, and supposing the bracket to 
be 600 millims. below the knife-edge of the balance, the mirror evidently turns 
through an angle 150 times as great as that through which the beam turns. 

The drawback to this method of magnification is that the mirror has its 
own time of swing and is easily disturbed. The swings of the mirror and the 
disturbances are, however, effectually damped by having four light copper 
vanes attached to the end of a thin wire, projecting down from the mirror 
and working in a dashpot with four radial partitions not quite meeting in 
the centre, one vane being in each compartment. I found that mineral lubri- 
cating oil is very suitable for the dashpot, as the surface keeps quite clean and 
there is little evaporation. The swings of the balance are also very greatly 
damped by this arrangement, but the effect of this will be discussed later. 

The telescope and scale are in the room over the balance-room (see Fig. 1), 
a hole being cut through the floor, and a small glass window being fixed in 
the top of the case. As the suspended mirror is in a vertical plane it is 
necessary to have an inclined mirror fixed in front of it to direct the light 
from the scale horizontally on to it and back again to the telescope. With 
the magnification used it was necessary, for good definition, to have an ex- 
ceedingly good inclined mirror, and several were rejected before a suitable 
one was obtained. That finally used is a silver on glass oval mirror, 
60 milHms. x 40 milhms., by Browning. The glass window in the case is 
optically worked and carefully adjusted to be normal to the path of the light. 

The telescope has a 3-inch object-glass of about 4 feet focal length. It is 
fixed on a brick pillar, on one of the brick arches which form the ceiHng 
of the balance-room, and it rises free from the floor of the observing-room. 
To destroy vibration one course of bricks is replaced by blocks of india- 
rubber. The scale has 50 divisions to the inch (say J milhm.), ruled diagonally, 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 53 

and divided to tenths by cross-lines. It is photographed on glass from a scale 
drawn on paper with very great care, 50 inches long (say 127 centims.), and 
with 500 divisions. The photograph is ^^^th of this length, and only the 
central part of the scale, about 60 divisions in length, has been used. The 
diagonal ruling enables a tenth of a division to be read with certainty, and 
the readings recorded in the Tables, pp. 98-128, are in tenths. Though the 
lines appear somewhat coarse, I have not been able to find another scale 
equal to it in distinctness and in ease of reading. As all the results depend 
on the ratio of measurements, taken almost simultaneously, of deflection 
due to attraction and rider respectively, in the same part of the scale, I have 
not thought it necessary to calibrate it. 

The scale is fixed horizontally on the end of the telescope close to the 
object-glass with a piece of ground glass over it. It was illuminated in 
general by an incandescent lamp placed above it, once by an Argand burner. 

The distance from the scale to the mirror and back is about 5 metres. 
It follows that 1 division of the scale corresponds to an angular motion of 
the mirror through -0001 radian. But this is at least 150 times the angle 
through which the beam turns for the same defiection. So that 1 scale 
division implies an angular motion of -0000006 radian, or -^" , in the beam. 
As the total length of swing in Table III is never more than 12 divisions, the 
angular vibrations of the beam are at the most about l"-6, and the hnear 
vibrations of the masses, since the half beam is about 60 centims., are at 
the most about -005 millim. This shows that it is quite unnecessary to 
consider any change of distance due to vibration. The greatest deviation 
from the mean in any of the series of weighings recorded is about 1 per cent, 
of the rider-value, corresponding to about -^-^th. of a division, or an angle 
of -^" in the beam, and a distance of -00004 miUim., say goo'ooo inch, in the 
motion of the masses. This seems to show that the method is accurate as 
well as sensitive. 

Determination of the Value of the Scale-Divisions by means of Riders. This 
was done by means of centigramme-riders (Fig. 4), these being the least 
weights which appeared capable of sufficiently accurate determination. 
Instead of transferring the same rider from point to point, it was much 
easier to use two equal riders, and to take one up while the other was 
being let down a given distance from it. The distance selected was about 
2-5 centims., since the deflection due to the transfer of one centigramme 
so far along the beam was nearly equal to that due to the greatest attraction 
to be measured. 

At first the riders when on the beam rested in V-notches in a pair of parallel 
brass strips fixed on and parallel to the beam. But this plan was soon 
abandoned, as there was no certainty about the position of the rider in the 
notches. The riders were then supported in little wire frames, each hung 



54 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

by two cocoon-fibres from the edges of a plate fixed to the beam, the edges 
being parallel to the central knife-edge. The only objection to this method 
was the very considerable time spent in replacing the fibres after the breakages 
which occurred on dusting or any readjustment of the balance. 




Fig. 4. Rider, actual size, and end of lifting-rod, 




r^ 



^JX 



Fig. 5 a. Subsidiary rider-beam, 66, attached to centre Fig. 5 6. Wire frames depending like 
of balance-beam, BB, by plate jp just above central scale-pans from ends of 66, Fig. 5 a, 

knife-edge, k (half size). side and end views (half size). 

Ultimately a small subsidiary beam, about 2-5 centims. long, was attached 
to the centre of the balance-beam just above the knife-edge (Fig. 5a), the 
scale-pans being represented by small wire frames in which the riders could 
rest (Fig. 56). These frames depend from agate pieces resting on steel points 
at the extremities of the subsidiary beam in the way now usually adopted 
in delicate assay-balances. This mode of supporting the riders appears to 
be perfectly satisfactory. 

To raise or lower the riders two short horizontal lifting-rods parallel to the 
beam move up and down within the supporting wire frames with a nearly 
parallel motion, and on them are two metal pieces with their upper surfaces 
shaped so that the riders rest on them without swinging (Fig. 4, r). They 
are the extremities of L-shaped projections from a jointed parallelogram- 
framework (Fig. 6), supported on an upright in front of the subsidiary beam. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 56 

The framework is moved by a tongue engaging with it, and projecting from 
a horizontal rod, which rotates about its axis in bearings, one within the case 
and the other outside. The rod is turned through an angle of about 30° 
between stops by an endless string passing upwards and round a wheel in 
the observing-room. 

The parallelogram-framework and the bearing of the rotating rod within 
the case are both supported, independently of the case, from the ceihng. At 
first they were supported respectively on the central pillar of the balance 
and on the case ; but when the increase of optical sensitiveness enabled me 
to detect small irregularities, I realised how essential it was for accurate 
weighing that all parts of the apparatus moved from the outside should be 
supported quite independently of the balance. Even the string moving the 




Fig. 6. Lifting-rods to raise or lower riders (half size). 



rod transmitted great and continual vibration. The rod and the framework 
with the lifting-levers were, therefore, supported by iron rods coming down 
from the ceiling through holes in the top of the case, large pieces of cardboard 
stretching from these rods over the holes to hinder the passage of dust into 
the case. Once or twice in the course of prehminary experiments irregularities 
were traced to accidental contact of outside bodies with the case. 

It appeared just possible that there might be electrification of the riders 
by friction with the lifting-rods, especially when they were supported by 
cocoon-silk. It was, therefore, advisable that the surface of the lifting-rods 
should be of the same kind as that of the riders. As the latter are silver wire 
gilded, the lifting-rods are also gilded. It may not be uninteresting to note 
here a curious phenomenon which occurred during some early preliminary 
experiments. The shaped pieces on the Hfting-rods were then of wood 
covered with gold leaf, put on with ordinary paste. After they had been on 
for some months, I obtained some very various results for the deflection due 
to the riders, and on examining the lifting-rods I found that a number of long 



56 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



needle-growths projected from the wood pieces and interfered with the 
supporting wire frames. At first I thought these were organic, but my 
colleague, Professor Hillhouse, examined them and found that they were 
crystalhne. Doubtless, the hygroscopic paste set up electric action between 
the gold leaf and the brass to which the wood pieces were attached, and the 
crystals were probably zinc sulphate. The wood was then replaced by brass 
gilded, and no further difficulty of the kind was experienced. 

The length of the subsidiary beam was kindly determined for me by 
Mr. Glazebrook at the Cavendish Laboratory. The steel points are hardly 
sharp enough to determine the distance to 1 in 10,000, but the mean of the 
results is sufficiently exact. The following are Mr. Glazebrook's determina- 
tions ; the four points being denoted by a, h,c,d\ 



Date 

1889 July 4 
Julv 11 
July 12 


Temperature 


Number of 
readings 


, , Number of , , , 
«t°^ readings ^ ^ ^^ ^ 


22^5 
21-5 
23 


6 
3 
3 


inches \ inches 
•9985 i 6 ^9979 
•9990 i 3 j ^9982 
•9988 3 ^9979 



These are in terms of a gun-metal standard of which the error is only 3 in 
100,000 at 0°, and, therefore, for my purpose negUgible. The beam is of 
brass, and we may assume with sufficient exactness that it has the same 
expansion as the standard. The temperature may, therefore, be left out of 
account. The mean value of \{ah + cd) is therefore -9983375 inch, or taking 
2-539977 centims. to the inch we obtain 

Length of beam at 0°, 2-53575 centims. 

There is an advantage in fixing this beam at the centre, which should be 
noted here. Suppose the riders are not quite equal, but have values w and 
w + S. Let the two ends of the subsidiary beam be distant a and a -f I from 
the central knife-edge. Then the effect of picking up the rider w from the 
nearer, and letting down the rider -ii; + S on the further end, is equivalent to 
putting at unit distance 

{w + h)(a-\-l)-wa==wl-\-h{a-\-l) = wl (l + ^^ ^-\ , 

or the error hjw is multiplied by {a + 1)11, and, if the beam is not central, 
(a -f l)jl may be greater than 1, so that the error is magnified. 

If, however, the small beam is central, I is equal to — 2a, and the error is 
multipHed by + J. 

If the riders are interchanged and the weighings are then repeated, the 
mean result is the same as if riders with the mean value were used, for 



w[a + l)- {w-\-Z)a = wl~ Sa 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 57 



and the mean of this and the above is 



(-1) 



The Attracting and Attracted Masses. These are all made of an alloy of 
lead and antimony, for the sake of hardness, the specific gravity in each case 
being about 10-4. They were made at various times and places, the large 
attracting mass M being made more than 12 years ago by Messrs. Storey, of 
Manchester. The smaller balancing mass m was made in 1889 by Messrs. 
Heenan, of Manchester and Birmingham. These were both cast with a 
'head' on, and then turned. The attracted masses A and B were made by 
Messrs. Whitworth, and subjected to hydraulic pressure before turning. The 
dimensions have been measured from time to time, and there is no evidence 
of any sensible change of shape. 

The larger mass M and the attracted masses A and B were weighed at 
the Mint through the kindness of the Deputy Master and Professor Koberts- 
Austen. For the weight of the balancing mass m, I am indebted to Messrs. 
Avery, of Birmingham. The large mass M has suffered two accidents since 
it was weighed, once being slightly cut into by a saw during some alteration 
of the case, and once being scratched by coming into contact with a piece 
of metal fixed to the turn-table in taking it out of its place. The saw-cut was 
carefully filled in with lead, and the scratch removed only a fraction of a 
gramme, as was determined by taking a mould of the hollow. I should be 
glad to think that the determination of the attraction was sufficiently exact 
to make reweighing necessary, but I am afraid that the alteration in weight 
is very far beyond the important figures, and I therefore take the original 
weight as sufficiently near the truth. The masses A and B have been gilded 
since the original weighing, but I carefully determined their increase of weight 
by the balance used in the gravitation experiment. 

The values given below in the second column are the true masses. In 
the third column are the masses of M and m, less the air displaced by them, 
this being taken as 18-4:1 and 9-2 grms. respectively. It will be shown later 
that the true masses of A and B and the reduced masses of M and m may be 
used in the calculation of the result. 





True mass in 
grammes 


Mass less that of air 
displaced 


M 
m 
A 
B 


153407-26 
76497-4 
21582-33 
21566-21 


153388-85 

76488-2 



58 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Suspension of the Attracted Masses. Each of the attracted masses is 
drilled through along a diameter, the hole being -6215 centim. in diameter, 
and a brass rod (Fig. 7) terminating in an eye e below, is passed through the 
hole. The mass is secured in position by a nut n working in a screw-thread 
cut for a short distance in the rod. An exactly 
similar rod terminating in a similar eye e', and 
with a similar nut n', is fastened end to end to 
this by a union u. The nuts and the inner sides 
of the enlargements for the eyes are hollowed 
out so as to fit exactly on to the spheres. 

From the ends of the balance beam hang 
down stout brass wires terminating in hooks. 
If these hooks are passed through the eyes e' 
the attracted masses are close to the floor of 
the balance-case, and their centres are adjusted 
to be about 32 centims. from the centre of the 
large attracting mass when under either of them. 
If the masses are turned over so that the hooks 
pass through the eyes e, they are about 
30 centims. higher or at nearly double the 
distance, the length ee' being about 48 centims. 
The rods being perfectly symmetrical about the 
union u, the attraction on them is the same in 
either position. The weight of each is about 
212 grms., or about j-J^ of the attracted mass, 
so that any small variation in their position 
would produce a negligible variation in the 
total attraction. By the differential method, 
the attraction on them entirely disappears from 
the results. 

The Mode of Support of the Attracting Masses 
M and m. This has already been described 
when describing the turn-table. 

The Riders. Four centigramme-riders, A, B, 
C, D, of silver wire gilt were made by Mr. Oerthng 
of the form shown in Fig. 4. These were weighed 
in 1886 at the Bureau International des Poids 
et Mesures, by M. Thiesen. The following is an extract from the certificate : 

' Densite et volume. Comme densite on a accepte celle de 1' argent, et par 
consequent comme volume de chacun des cavahers, 0-0010 miUihtre. 

' Deterynination des poids des cavaliers. L'etude des poids de ces quatre 
cavaliers a ete faite par M. le Dr. Thiesen, adjoint du Bureau International, 




Fig. 



7. Suspender for Attracted 
Mass (one-fourth size). 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 59 

charge de la section des pesees. M. Thiesen au moyen de la balance Stuchrath, 
destinee a des poids au dessous du gramme, a d'abord determine les differences 
entre les quatre cavaliers pris deux a deux dans les six combinaisons possibles, 
et ensuite la difference entre 1' ensemble des quatre cavaliers et le poids de 
40 milligrms. de la serie du Bureau, serie en platine iridie recemment 
etalonnee par M. Thiesen. Les comparaisons ont ete faites du 19 au 29 Mars, 
1886. 

' Resultats. De I'ensemble de ces comparaisons resultent les poids : 

A = 10-1247 milligrms. 
B = 10-0615 
0=10-1196 
Z)= 10-1262 

' L'incertitude de ces determinations ne depasse pas 0-001 milhgrm.' 
A and D were selected for use as being the nearest to each other in value. 
B and C were kept untouched in boxes till 1890. In the various experiments 
made between 1886 and the final weighings, A and D had necessarily been 
handled to some extent, especially through the frequent breaking of the silk 
fibre suspension used before the subsidiary beam described above, and it 
appeared possible that their weights might be altered. It was also necessary 
to determine whether an appreciable amount of dust was deposited on them 
in the course of several weeks as it was inconvenient to dust them frequently. 
The riders B and C might be assumed to have the same weight as in 1886, 
and could be taken as standards. 

To make the weighings a 16-inch chemical balance was arranged with a 
double-suspension mirror on exactly the principle already described for the 
large balance. The apparatus was put together quickly with materials at 
hand, and might easily be greatly improved. It is only described here to 
show how accurate the method is, even with such rough apparatus, and that 
it is applicable to a small as well as a large balance. 

A cork sliding on the pointer with a horizontal needle stuck in it, served 
to support one thread of the mirror; a stand with a projecting arm — one 
made to hold platinum wires in a Bunsen flame — served to support the other 
thread. A wire with a small copper vane depended from the mirror and was 
immersed in an oil dashpot. The telescope and a miUimetre-scale were on 
a level with the mirror about 2 metres distant on one side of the balance. 
Two brass strips, parallel to each other and the beam, were fixed on the top 
of one arm of the beam, and in each of these were two V-notches in which 
centigramme-riders could rest. Two levers, worked by cams on a rod rotated 
by the observer, picked one rider up and let down the other, so that the effect 
was equivalent to the transfer of 1 centigrm. from one notch to the other. 
Their distance apart was such that this was equivalent to the addition of 



60 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

•3284 mimgrm. to one pan of the balance. This was the arrangement 
described in my former paper. Attached to one pan was a pair of brass 
strips parallel to each other, and such that the riders A, B, C, or D, would 
just rest across them. Two lifting-rods worked up and down between these 
strips, so that of the two riders to be compared, one could be picked up at the 
instant the other was let down. The lifting-rods were worked by a rod 
rotated by the observer and supported quite independently of the balance, 
and of the slab on which it rested. By this plan the value of the scale-divisions 
and the shifting of the centre of swing on changing the weights to be compared, 
could all be determined without raising the beam of the balance between the 
successive weighings, an essential condition, I beheve, for exact work. 

The weighings were made in the large room of the Physical Laboratory, 
and no precaution was taken to protect the balance-case beyond placing 
a board in front of it. The room is draughty and subject to great variations 
of temperature, so that the weighings were made under very disadvantageous 
circumstances. One result of this was a rapid and sometimes very great 
change of resting-point in the course of a few hours, so that the scale passed 
out of the field of view. In order to bring it back without opening the case, 
two glass tubes passed through the top of the case, almost down to the scale- 
pans, and small bits of wire could be dropped through these on to either pan 
as needed. Caps fitted on to the tubes to prevent draughts. This plan 
appears worthy of mention, as it suggests a mode of determining the value 
of a scale-division when a balance is either too sensitive for riders or has no 
special arrangement for their accurate use. If a piece of wire weighing, say, 
1 milligrm. is cut into say ten nearly equal parts, and if these are dropped 
on to the two pans alternately the shiftings of the centre of swing will be to 
and fro, about equal distances, due to about -1 milligrm., but the sum of the 
shiftings will be that due to 1 miUigrm., and the balance at the end will be 
nearly in the same position as at the beginning. 

The following is an abstract of the comparisons of the riders. They were 
made soon after the first determinations of attraction on February 4, when 
A and D had not been dusted for three months. 

In each case three extremities of swing were observed, and the centre of 
swing was determined from these by the graphic construction described later 
(p. 72). 

The centres of swing were combined in consecutive threes in the usual 
way to give the differences in scale-divisions. 

Thus, in the first series, the successive centres of swing with D and A 
alternately in the scale-pan were 



D 


A 


D 


A 


D 


!31 


223 


217 


211-9 


208 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 61 

whence (D - A)^ = ^ - 223 = + 1-0 division, 

fn A^ 017 223 + 211-9 ^ .^ .. . . 

(D — ^)2 = 217— = — 0-45 division, 

(D - A)^ = ^1+^ _ 211-9 = + 0-6 division. 

Mean D — A = -SS division. 
Successive values of the differences alone are given below. 
The time of swing one way was about 16 seconds. 

February 16, 1890. 
(1) Comparison of A and D, undusted. 

Deflection due to -328 milligrm., 83-45, 82-45, 84-45 divisions. Mean 83-45 
divisions. 

D- A = 1-0, - -45, + -06 division. Mean -38 division; 
therefore D = .4 + -0015 milligrm. 

(2) Comparison of A undusted, D dusted. 
Value of scale-division taken as in the last. 

D- A = -'6, + -3, - -1, - -4, + -25. Mean - -09 division ; 
therefore D = A — -0004 milligrm. 

Februanj 17, 1890. 

(3) Comparison of A and D, both dusted. 
Value of scale-division taken as below (4). 

B- A = -'I, - -2, + -3, - -3, - -8. Mean - -22 division; 
therefore B -= A — -0008 milhgrm. 

(4) Comparison of C and B. 
Beflection due to -328 milligrm., 85-35, 85-4, 84-65. Mean 85-13 divisions. 
B-C = + -15, -00, + -05, - -15, + -05, + -3, + -05, - -05, - -35, 
-05, -35, -45, -50, -2. Mean -114 division; 
therefore D = C + -00044 milligrm. 

February 18, 1890. 

(5) Comparison of C and B repeated. 

Beflection due to -328 milligrm., 92-75, 92-3, 91-65. Mean 92-23 divisions. 

B-C= -35, - -05, - -8, - -95, - -1, + -05, 0, - -15, - -1, + -05. 

Mean — -17 division; 

therefore B = C — -0006 milligrm. 



62 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

Combining this with the last, and weighting them in the ratio of the 
numbers of determinations in each, 

D = C + (-00044 X 14 -- -0006 x 10) h- 24 = - -0000 milHgrm. 

(6) Comparison of A and D. 

Value of scale-division taken as above, -328 milHgrm. = 92-23 divisions. 

D-A = -45, -25, -1, - -2, - -1, -35, -25, -45, -60, -5, -5, -55, -5, -75, -55, 

•1, -05, -45, -10, -30, -8, -9, -35, -2, -30, -55, -50, -35, -45, -45. 

Mean -378 division; 

therefore D = A + -00134 milligrm. 

Examining the values obtained in (1), (2), and (3), it will be seen that 
no trustworthy evidence is given of a difference due to dusting. Any existing 
difference was probably under -002 milHgrm., and since the weighings on 
February 4, before dusting, were made with the attracted masses in the upper 
position, when the attraction was only one-fourth of that on which the final 
results depend, we may safely neglect the effect. After this the riders were 
dusted more frequently, so that we may probably assume their values more 
constant. 

The comparisons of C and D, and of A and D, in (4), (5), and (6), were 
made more carefully. That of A and D in (6) is much the best of the series, 
the air in the laboratory happening to be steadier while it was made. The 
range between the greatest and least values of the difference is one scale- 
division, or -0036 miUigrm., and the different results are grouped fairly 
closely about the mean. 

The centres of swing and the differences are plotted in Diagram VIII 
(p. 136). I do not claim that these results show any remarkable accuracy 
when compared with those obtained at the Bureau International des Poids 
et Mesures, but remembering how rough the apparatus was, and how little 
precaution was taken to ward off air-currents, I have not the slightest doubt 
that, with special design of apparatus and more suitable locality, the results 
could be very greatly improved, and the accuracy carried far beyond anything 
hitherto reached. As they stand, they seem to show the value of the com- 
bination of a short time of swing with optical magnification. 

The results of comparisons (4), (5), and (6), is, that if C has its Paris value, 
viz., C = 10-1196 miUigrms., then, A = 10-1183 milligrms., and D = 10-1196 
milUgrms. ; whence 1{A ^ D) ^ 10-119 milhgrms. This value may be used 
in calculating the result, since the riders were interchanged before Set II was 
taken. 

The losses experienced since 1886 by A and D are respectively, by A 
-0064 miUigrm., and by D -0066 milligrm., i.e., they have diminished by 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 63 

practically equal amounts. This was to be expected as they have probably 
received equal amounts of rough usage. 

The substitution of the subsidiary beam for the cocoon-fibre suspension 
of the riders having greatly diminished the handling to which they were 
subjected, I have not thought it necessary to weigh them again during the 
work. 



Linear Measurements. 

In the mathematical theory it will be shown that the lengths required 
are those marked in Fig. 14, viz., the horizontal distances, L and I, and the 
vertical distances, D^ D^ , d^d^, H^ H^ , h-^h^. 

The Horizontal Distances. Except when estimating the moment of the 
rider, the distance L is really that between the 
verticals through the centre of M and the centre 
of the more distant attracted mass. But the 
verticals through the centre of M in each position 
so nearly passed through the centre of the mass 
above it, and, therefore, through the knife-edge 
from which it hung, that L was taken as equal 
to the length of the beam (p. 50). 

The accuracy of this adjustment was secured 
as follows. A horizontal cross-piece was fixed on 
the top of each attracted mass, with two horizon- 
tal cards at its two ends, each with a portion of 
a circular arc on it, with radius equal to that of 
the large mass M, and with centre over that of 
the attracted mass (Fig. 8). A plumb-line was 
then hung just in front of the case, and the 
balance was moved by the horizontal screws 
bearing against the base-plate until the plumb- 
line always appeared to touch the circular arc 
above, when it appeared to touch the large mass 
below. The adjustment was not quite perfect, 
but the error in the worst case was probably 
not more than 1 millim., and certainly less than 
2 millims. Such an error in the horizontal distance is negligible. 

The distance I had different values for the two positions occupied by m 
on the turn-table. Calling these values l-^ and l^ respectively, l-^ + l^ was found 
by measuring a, the inside distance between M and m, arranged as in Set II, 
and 6, the inside distance between them, when m was put on the same side 
of the turn-table as M, and adding to a + h the sum of the diameters of M 
and m in the radial direction of the turn-table as taken by square calipers. 




Fig. 8. Plumb-line 
Adjustment of Masses. 



64 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

The following are the values in terms of the cathetometer-scale already 
referred to, the temperature being 15° C. : 

a = 157-01 

h= 33-95 

Diameter of M = 30-52 

m= 24-23 



therefore h + k- 245-71 

The value of l^ - l^. was found by measuring the shortest distance of m 
from the wall when respectively in the first and the second positions on the 
turn-table. It was found that 

whence h = 122-91o 

l^ = 122-795. 

We may obtain from these measures an independent value of the radius 
of the circle in which the centre of M moves. With perfect adjustment this 
should be IL = 61-66 at 18°. 

It is equal to a + radius of M + radius of m - l^ , or, by the above 
measures, 

= 157-01 + 15-26 + 12-115 - 122-795 

= 61-59, 
which is only -07 centim. less than \L. 

Inasmuch as the wood probably expanded less than the cathetometer- 
scale, while the metal expanded more, I have assumed as a rough approxi- 
mation that the total expansion equalled that of the scale, so that the values 
of /j and I2 are correct at 18° (see p. 50). No importance is, however, to be 
attached to this temperature-correction. 

The Vertical Distances. At the conclusion of each set of weighings with 
the attracted masses in a given position, the vertical distances between the 
top of the attracting masses and the bottom or top of the attracted masses 
(accordingly as they were in the upper or lower position) were measured by 
the cathetometer already referred to. 

This instrument is of the well-known design of the Cambridge Scientific 
Instrument Company, and is especially adapted for measuring differences of 
level at different distances in different vertical planes. It reads to -002 centim. 
The account of these measurements will be found in Table II (p. 89, et seq.). 

To find the distances D, f/, H, h (Fig. 14), it was necessary to add to the 
actual distances measured the sum or difference of the vertical radii of the 
attracting and attracted masses, and, therefore, the vertical diameters of all 
the masses were measured. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 65 



For this purpose I used a cathetometer which has lately been constructed 
for me by Messrs. Bailey, of Bennett's Hill, Birmingham. I have to thank 
Mr. Potts, of that firm, for his care in 
its construction, and also for the trouble 
which he has taken in the construction 
and alteration of much of the apparatus 
used throughout the work recorded in 
this paper. As the cathetometer is, I 
beheve, new in design and satisfactory 
in its performance, it appears worthy of 
description. 

The Cathetometer used to measure 
Vertical Diameters (Fig. 9). There are 
two telescopes, one to sight the upper 
the other to sight the lower of the 
points between which the vertical height 
is required. There is no scale on the 
instrument, but after the telescopes are 
fixed to sight the two points the instru- 
ment is turned round a vertical axis, so 
that the telescopes sight a vertical scale 
at the same distance from them as the 
points. In general, of course, the cross- 
wire will appear to lie between two 
divisions, but by means of the fine 
adjustment, to be described below, the 
two nearest scale-divisions are brought 
in succession on to the cross-wire, and 
by interpolation the reading correspond- 
ing to the point first sighted by the 
telescope is determined. 

The telescopes are fixed on collars 
running up and down the main pillar, 
which has a section of the form shown in Fig. 10 (shaded). 

The guides consist of three knobs, k, k, on the inside of the collar, two 
sliding in a vertical V-groove and one on a plane, both groove and plane 
being at the back of the pillar. A screw, s, clamps the collar in any position. 
Gut strings running up over pulleys and supporting counterpoises, sliding 
on the thinner pillars (see Fig. 9), are attached to the collars so that these 
move easily. At first springs were used to keep the knobs always in contact, 
but I found it much better to remove these and trust merely to hand-pressure 
to keep the collars in the proper position before clamping with the screw s. 

p. c. w. 6 




Fig. 9. 



Cathetometer used to measure 
Vertical Diameters. 



66 



ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



The fine adj ustment is secured by the use of a piece of plate-glass {g, Fig. 10), 
placed in the front of each object-glass and capable of rotation about a 




Fig. 10. Section of pillar and collar of new Cathetometer. 
5, clamping screw, k, k, guiding knobs, g, glass plate for fine 
adjustment, turning on axis hh, with pointer at %> perpendicular 
to plane of figure. 

horizontal axis, hh. A pointer is fixed on the end of this axis at f, and at 
its end is a small glass plate with a scratch on it moving close against a straight 
scale. If the plate is initially normal to the optic axis of the telescope, on 
turning it through an angle </>, the ray which now comes along the optic 
axis has been shifted, by transmission through the plate, parallel to itself, a 
distance t sin ((/> - j/f)/cos xjj, where t is the thickness of the plate and ifj is 
the angle of refraction within it (see Fig. 11). 




Fig. 11. Section of fine adjustment plate. 

For small angles this shifting happens to be nearly proportional to tan</), 
and, therefore, to the reading on the straight scale. To show how nearly 
this is the case the following table gives the shifting for angles of 5°, 10°, 
and 20°, with a thickness oit = I centim. and a refractive index a -= I: 



ngle 


Shifting* 


5= 


itan 5°(1 + -00042) 


10° 


itanlO°(l + -00160) 


20° 


itan20°(l + -00526) 



The error in taking the shifting as proportional to tanc/) is, up to 20°, 
quite negUgible in ordinary telescope-cathetometer work. If it is desirable 
to have greater accuracy, it is probably best to use a table of corrections 
to the tangent ; but it is possible to get an exact scale thus : 

* [These expressions are given with the wrong sign in the original paper. A sHght correction 
has also been made in the values of the numerical coefiicients. Ed.] 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 67 



Let OP, Fig. 12, represent the pointer of length /*, making an angle </> 
with a line MN. Let a pointer PM jointed to this at P be of length /ur, 
and let its extremity M move on the Hne MN. Drawing OD at right 
angles to MN, if s is the shifting, we have 

sin (<^ — ijj) 



or 



OD^OP 
_ rs 

s = ~OD. 
r 



cos l/j 



Probably the practical difficulties in the use of such an arrangement would 
render it troublesome and uncertain. 

The plate is used as follows: Adjust it normal to 
the optic axis of the telescope, and move the telescope 
till the required point is brought as near to the cross- 
wire as is possible by the hand. Clamp the telescope, 
and then turn the plate till the point is exactly on the 
cross-wire. Read on its scale the position of the pointer 
attached to the plate. Repeat these operations with 
the other telescope on the other point, then turn the 
instrument about its vertical axis till the telescopes 
sight the vertical scale placed at the same distance 
away as the two points. Looking through one of the 
telescopes the cross- wire is in general not exactly on a 
division. Turn the plate so that first the nearest 
division above, and next the nearest division below, is 
on the cross-wire. Reading the position of the pointer 
in each case, interpolation gives us the reading on the 
vertical scale corresponding to the position of the 
pointer when the cross- wire was between the two scale 
divisions. Doing this for each telescope the difference 
between the two points is found in terms of the vertical scale. 

The plates I have used are about 9 millims. thick, and the pointers about 
9 centims. long. They move over scales such that 25 to 27 divisions corre- 
spond to a shifting of 1 milhm. The lower scale is graduated from to 50, 
the upper from 50 to 100, to prevent confusion. The 50 divisions occupy 
a distance of 66 millims. 

It will be observed that in this form of instrument the 'level error' is 
practically entirely obviated. It can only come in if the scale is not at the 
same distance as the height to be measured, and may then be made neghgible 
in practice by levelling the telescopes. Indeed, the uncertainty of measure- 
ment appears only to depend on the uncertainty with which the cross-wire 

5—2 




Fig. 12. 



68 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

can be brought to the proper point, that is, it depends only on the magnifying 
power and definition of the telescopes used. 

To illustrate the use of the instrument, a full account of the determinations 
of the vertical diameters is given in Table II. Below are the results, and for 
the sake of showing that there has certainly been no great change in shape, 
I give results obtained with a cathetometer more than 10 years earlier at the 
Cavendish Laboratory at Cambridge. 

1890 1880 

centims. centims. 

Large attmcting mass J/ ... 30-526 30-5192 

Small „ „ m ... 24-176 

Attracted mass .4 15-8203 15-8166 

B 15-7829 15-7842 

The diameters of M and m in a horizontal direction parallel to a radius 
of the turn-table measured by square calipers were 

M = 30-52 centims. 
m = 24-23 „ 

Temferature-Correction. Though the expansion of the masses was to be 
expected of an unimportant amount, I thought it advisable to attempt to 
measure it, in case there might be anything anomalous. One of the attracted 
spheres, B, was for this purpose placed between two vertical levers, in a tank 
through which could be run a continuous stream of cold or warm water. 
These levers depended from horizontal rods which could rock or slightly 
rotate on fine point-suspensions. This was, in fact, a kind of double Lavoisier 
and Laplace apparatus. The motion of each lever was shown by another 
lever of about the same length, rising vertically up from each horizontal axis, 
and serving as the moving support for a double-suspension mirror in which 
was viewed the reflection of a millimetre-scale. Two telescopes and one scale 
were used for the two mirrors, though it would not have been difficult to 
arrange one telescope and two scales. The value of one scale-division was 
determined by inserting a piece of thin glass between the sphere and each 
lever in turn. The method is exceedingly sensitive, but I have not been 
able to make it exact, owing to the warping produced in the rods due to 
unequal temperatures. 

The measures of the expansion varied between -0000214 and -0000277, 
both vertical and horizontal diameters (in the position in the balance) being 
tested. The true value is probably nearly -000025 or 1/40000. It will, 
therefore, lead to no appreciable error if we take the expansion as equal to 
that of the scale of the cathetometer, say 1/60000 (see p. 92, Table II). 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 69 



Determination of the Attraction by the Balance. 

When the balance is used to measure such small forces and weights as 
those with which we are here concerned, it must be left swinging on its knife- 
edge throughout any set of weighings in which the deflections are to be com- 
pared one with another. For there is not the slightest reason to suppose 
that if the beam is Hfted up and let down again, its new position of equilibrium 
will coincide with the old. And again, the beam, especially with such loads 
as the attracted masses, is put into a state of considerable strain, and continues 
to alter its shape sensibly for hours, and probably even days, after the masses 
are put on to it. I have, therefore, always left the beam free for at least 
two or three days before commencing work with the balance, and it has of 
course remained free during the course of each day's work. The balance- 
room was never entered just before any weighing, as it took many hours 
for the disturbance due to entrance and interference with the case to die 
away. 

When the turn-table supporting the attracting masses is moved half round, 
from one stop to the other, the bulk of the attraction is taken away from one 
attracted mass and put on to the other. The balance, being free, is slightly 
tilted over to the side on which is the larger attracting mass. But the 
deflection in the apparatus as arranged is so very small — at the most only 
10 scale-divisions — that errors of reading can only be neutralised by making 
a great number of successive measures. 

Probably other errors are also largely eliminated, such as those due to 
the deposition of dust particles, shaking, change of ground level, and varying 
air-currents. Of such errors I have found those due to varying air-currents 
by far the worst. Sometimes — especially in autumn and winter — the balance 
will move quite irregularly through more than a scale-division, and continue 
to move to and fro in this way for days or weeks. When in such an unsteady 
condition it is useless for accurate work. In spring and summer, however, 
it is much more steady as a rule, and frequently the scale can hardly be seen 
to move. I have never worked when on looking into the telescope for some 
time the irregular movements appeared to be more than a fraction of a tenth, 
i.e., a fraction of one of the diagonal divisions, though, doubtless, irregularities 
comparable with a tenth of a whole division have often made their appearance 
in the work. It is perhaps not safe to ascribe these always to air-currents. 

I have always found the air steadiest in warm quiet weather, with a slowly 
rising temperature in the balance-room, and most unsteady after a sudden 
fall of temperature. As the alteration of temperature spreads downwards, 
this is fully in accord with Lord Rayleigh's observation that when the air is 
steady the ceiling is warmer than the floor, and that when it is unsteady the 



70 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

floor is the warmer of the two. In the observing-room I had a gas stove 
often kept burning day and night, in the hope that the higher temperature 
it produced in the ceihng of the balance-room below might steady the air. 
But the vertical walls of the balance-room interfered with the action of the 
ceihng, and often produced unsteadiness. 

A door opening or shutting anywhere in the building had a visible though 
transient effect, doubtless through an air- wave. In a high wind the balance 
was always unsteady, partly, I suspect, through rushes of air into and out 
of the case with sudden pressure-changes, and partly through changes of 
grouDd-level, with variations of wind-pressure against the building. 

At all times there was a march, in one direction or the other, of the centre 
of swing. This was especially marked soon after the frame was lowered and 
the beam left free. As already remarked, readings were not taken till changes 
due to change in strain of the beam had subsided. But the march was very 
appreciable at other times, as will be seen from the diagrams. Perhaps the 
change was sometimes due to tilting of the ground, with barometric variation, 
since the balance was a very dehcate level, and sometimes due to the change 
in buoyancy of the air affecting the two sides unequally, though I have not 
been able to make out any direct connection between barometric height and 
position of centre of swing. I believe that the explanation is to be sought 
for the most part in unsymmetrical effect on the beam of sHght changes of 
temperature, for I have frequently noticed that a rising temperature produced 
an upward march, and a falling one a downward march. This explanation 
is supported by the following table (p. 71) of observations of the centre of 
swing, extending from May 9 to May 22, 1890, the balance being free, and the 
balance-room undisturbed meanwhile. 

The relation between temperature and centre of swing is represented in 
Diagram IX (p. 136). 

Of course, after a change in the position of the attracting masses or of the 
riders, the balance does not at once settle in a new position of equilibrium, 
but oscillates about it. Inasmuch as the balance never rests in this position, 
it is better to term it the centre of swing rather than the equihbrium position 
or resting-point. The dashpot used to damp the vibrations of the mirror 
reflecting the scale serves also to damp those of the balance-beam, and they 
die down rapidly. Instead of waiting, however, to observe directly the point 
on which they are closing in, it is much more exact, and also saves much time, 
to find the centre of swing, as with an undamped balance, from the extremities 
of the swmgs. I have always observed and recorded four extremities of three 
successive swings, occupying in all a httle more than a minute. 

Notwithstanding the very considerable damping, the successive lengths 
of swing are still in geometrical progression, but the rate of reduction is too 
great to allow the ordinary approximation, in which the geometrical is assumed 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 71 



I 



I 









Temperature 




Date, 1890 


Time 


Centre of 




Barometer 










swing 


Balance- 
room 


Observing- 
room 




May 9 


11.5 a.ni. 


1360 


12°0 




13-4 


739-8 




12.55 p.m. 


1330 


120 


15-0 


739-2 


„ 12 


11.15 a.m. 


133-8 


12-05 


14-5 


738-6 




1.15 p.m. 


131-9 


12-05 


15-8 


738-3 




2.40 p.m. 


133-7 


12-05 


16-6 


738-1 




Stove left on all night of 12th-13th 




„ 13 


11.0 a.m. 


181-7 


12-6 


17-5 


740-2 




12.35 p.m. 


181-5 


12-6 


18-4 


740-3 




3.15 p.m. 


185-0 


12-7 


18-6 


740-3 




Stove turned off 






5.25 p.m. 


189-4 


12-7 


16-5 


740-5 


„ 14 


11.20 a.m. 


167-4 


12-6 


14-3 


745-8 




1.10 p.m. 


165-5 


12-6 


14-4 


746-0 


„ 15 


11.5 a.m. 


156-8 


12-4 


13-8 


749-5 




2.45 p.m. 


160-0 


12-4 


140 


749-3 


„ 16 


1.25 p.m. 


158-3 


12-4 


14-0 


744-3 


„ 17 


8.50 p.m. 


171-0 


12-55 


13-7 


741-9 


„ 19 


10.30 a.m. 


174-3 


12-55 


14-0 


743-5 




6.5 p.m. 


181-8 


12-6 


14-0 


741-0 


„ 20 


11.30 a.m. 


175-2 


12-75 


14-0 


739-8 




1.5 p.m. 


173-7 


12-75 


141 


740-0 




5.20 p.m. 


172-3 


12-7 


13-8 


741-7 


,, 21 


11.10 a.m. 


172-7 


12-6 


13-7 


751-9 


„ 22 


11.30 a.m. 


192 about 


12-85 


14-1 


757-6 



to be an arithmetical progression. The exact method of determining the 
centre of swing is as follows : 

Let a, h, c, d be four successive readings of extremities of swing, and let 
X be the reading of the required centre. 

Let the constant ratio of each swing length to the next be A. 

Then a-x^X(x- b), (1) 

X — 6 = A (c — x), (2) 

c-x = X{x-d) (3) 

Eliminating A from (1) and (2), we may readily obtain x in the form 

and from (2) and (3) 

x=b+-, — i'~y ., (5) 

(c — 6) + (c — d) 



72 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

With no disturbances and no errors of reading, the values of x in (4) and 
(5) will coincide ; but usually there is some small difference, the result of error 
or disturbance, and it is better to find both and take the mean. A third 
value might be obtained from (1) and (3); but it appears unadvisable to 
combine directly observations so far separated in time. 

These formulae lend themselves to easy arithmetical treatment, especially 
with the aid of a shde-rule; but the following graphic method of finding the 
centre of swing is much less tiring and quite sufficiently exact. 

Let the line OA, Fig. 13, represent the scale; its zero, and A, B, C, D 
the points distant respectively a, b, c, d from 0. 

Let O'C be a parallel line, B\ C, D' being points 
opposite to B, C, D respectively. Let AB' and BC 
intersect in iiTi . Draw X^K^X^^' perpendicular to OA. 
Then Zj is the centre of swing given by equation (4). 

For 

AX^ AX, K,X, ^ X^^X.B 
X.B'X.'B'" K,X,' Z/C ZiC 

i.e., Zj is the point dividing AB and BC in the same 
ratio. Similarly if BC and CD' intersect in K2, and 
X2K2X2 be drawn perpendicular to OA, X2 is the 
point given by equation (5). 

The third point given by equations (1) and (3) is 
obtained fronj the intersection of AB' and CD', but 
evidently a small error in C or D' may considerably 
alter the position of this point, and it is better not to 
use it. 

The construction was carried out thus: a large 
opal glass plate, 10 in. x 11 in., was etched with cross- Fig. 13. 

lines 10 to the inch, so as to present the appearance 

of ordinary section-paper. The glaze was taken of! so that pencil-marks could 
be made. A diagonal line ran at 45° across the plate through the corners of 
the inch squares, and this was always taken as the line BC in the figure. 
Taking any convenient horizontal line, usually, of course, far below the plate, 
as zero, each inch represented a scale-division, each tenth a diagonal division. 
The values of b and c fixed the hues to be taken as OA, O'C, and on these were 
marked the points A, C, B', U. A long glass shp, with a straight scratch 
on it, was then laid across from A to B' so that the scratch passed through 
A and B' , and its intersection A\ with the diagonal BC w^as x-^ from the zero 
hue. The slip was then laid with the scratch passing through C and U , and 
its intersection K^ with BC gave x^. It will be observed that all the actual 
construction for a set of readings of the balance-swings consisted in marking 
four points on the plate. 




THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 73 



The following cases, the first of very regular, the second of very disturbed 
swing, will serve to compare the results by this exact method with those 
obtained from the ordinary arithmetic-mean method. At the same time they 
will show how nearly constant is the ratio of swing decrease. 



Date and 
Number, 1890 


Scale-read- 
ings in 
diagonal 
divisions 


Length of 
swing 


Ratio of 

each to 

preceding 


Centre of swing, 
exact 


Centre of swing, 

approximate 

a 4- 26 -H c 

4 


May 4, No. 3 


865 
939 
896 
921 


74 
43 
25 


•581 
•581 


911-8 
91b8 


909-75 
913-0 


Mean 911^8 


Mean 911-375 


Sept. 17, No. 45 


1118 
1053 
1093 
1068 


65 
40 
25 


-615 
-625 


1077-8 
1077-6* 


1079-25 
1076-75 


Mean 1077-7 Mean 1078-0 



In finding the attraction the observations were always made in the same 
order, the determination of the scale-value of rider and attraction being 
sandwiched so that each might be equally affected by any comparatively 
slow changes. Starting with the initial position, the attracting masses and 
riders were so arranged that, on moving either, the balance was deflected in 
the same direction and over the same part of the scale. 

The following was the order of proceeding always observed, the column 
headed ' Centre of swing ' being supposed to contain the values of the position 
in each case determined from four swing extremities as just explained : 

Centre of swing. 

(1) Initial position *i 

(2) Riders moved r^ 

( 3 ) Riders moved back to initial position i^ 

(4) Masses moved round m^ 

(5) Masses moved back to initial position i^ 

(6) Riders moved ^2 

(7) Riders moved back to initial position i^ 

(8) Masses moved round Mg 

and so on. 

To minimise the effect of progressive changes these observations were 
always combined in threes in the following way. Denoting the scale-value 
of rider by R, and of attraction by M : 

* [The original has 1078-6 which is evidently a slip ; hence this is not really a good example 
of a 'very disturbed' swing. Ed.] 



74 ON A DETEEMINATION OF THE MEAN DENSITY OF THE EAETH AND 

From (1), (2), (3) Ri = ri-'^^, 

„ (3), (4), (5) M, = m,-'^\ 

„ (5), (6), (7) R2 = r^~'^\ 

and so on. 

These again were combined in threes, so that (the notation being continued) 
the successive values of attraction /rider are 

i?2 + jB2 ' 2i?2 -^2 "^ -^3 

The successive centres of swing i-^, r-^, i^, m-^, etc., correspond to instants 
of time following each other at intervals of about 2 minutes, rather more than 
1 minute being taken up in making and recording the four readings for each, 
and the rest in making the change of position in rider or mass and waiting 
for the next readings. It will be seen that each value of M or i^ is based 
on three successive centres of swing, the w^eighings extending over about 
6 minutes, while each value of M/R is based on seven successive centres of 
swing determined in about 14 minutes. 

A series of readings was usually continued for about 2 or 3 hours. The 
temperature in both observing and balance rooms was read at the beginning 
and end of the series, and the barometric height was also observed. As soon 
as possible after the desired number of determinations was completed with 
the attracted masses in one of the two positions, the vertical distances between 
attracting and attracted masses were measured by the cathetometer in the 
manner explained in Table II, and the position of the attracted masses was 
then altered. 

A full account of all the weighings is given in Table III, and the results 
are represented in Diagrams I-VI (pp. 130-135). The three upper rows of 
points in each diagram represent the centres of swing, those in the initial 
position being marked • . After movement of the rider they are marked x , 
and after movement of the masses they are marked o. The base-lines for 
the different rows are altered to save space, as described on the diagrams, for 
on the scale adopted the rider series would always be about 10 inches 
above or below the initial series. In Diagram I the rider and mass series 
are also brought down and superposed on the initial series, so that each of 
the three has the same average height. It will be seen that all three are 
affected by the same disturbances. The advantage of the short time of 
swing and the mode of combining the results in threes will be realised more 
easily from this superposition. 

The base-hne may be regarded as a time-scale, as the instants corresponding 
to successive centres of swing were almost exactly equidistant. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 75 

In each case, under the representation of the centres of swing, are plotted 
the resulting values of MjR, and at the side will be found a representation of 
the distribution of results about the mean. 

Assuming that each day's mean value is correct, and that the differences 
for different days are to be set down to variation of distance, etc., we can 
find the distribution of all the values about the mean by simply superposing 
the marginal curves at the side of the figures. The result fairly shows the 
accuracy as far as the weighing alone is concerned. It is represented in 
Diagram VII, where A is the mean value of the attraction in the lower, and 
a that in the upper position. A and a are brought near together to save 
space, but really they should be 40 inches apart. It will be seen that the 
range is about 2 per cent, of ^ — a on each side of the mean, or taking the 
value of ^ — a in milHgrammes weight as about \ milHgrm., and the load 
on each side as 20 kilogrms., the range is about 1/3 x 10^ of this load on each 
side of the mean. 

A comparison of the values of MjR in Diagrams I and II, shows a very 
curious similarity in the fluctuations, and at first I was inchned to think 
there was some common external disturbance producing these fluctuations. 
But an analysis of the two sets of values appeared to show that the resemblance 
is merely accidental. When the values of M and R are set out separately, 
it is seen that the fluctuations depend chiefly on M, of which the fluctuations 
are shghtly hke each other for the two series, while those of R are quite 
different, but such that they make the fluctuations in MjR resemble each 
other more closely than those in M alone. Further, it is not easy to see 
how fluctuations due to some external source would aflect the values of M 
equally in the upper and lower positions and not have any effect on R. Some 
periodic change of level might be suspected, but this ought certainly to be 
traced in R. I have examined all the other diagrams and plotted out the 
component values of M and R, but have found no trace of resemblance, so 
that I think the curious likeness in I and II must be set down to accident. 

There is a curious step by step descent of the centre of swing in the initial 
position on September 23, Diagram VI, which I cannot explain. It may be 
due to some error in the method of finding the centre of swing which comes 
in with a rapid march of that centre. The effect on the result is probably 
only small, for the value of MjR obtained with a march in the reverse direction 
on September 25 is very nearly the same, the two values being 

September 23 -2112753. 

25 -2112533. 

The following is a hst of the weighings recorded, with the distances 
measured and the mean values of the attraction: 



76 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Date 
1890 



Position of 
attracted 



Feb. 4 Upper 

April 30 and May 4 Lower 
May 25 Upper 



Date 
1890 



Position of 

attracted 

masses 



July 28 Lower 

Sept. 17 Lower 

Sept. 23 and 25 ... Upper 



Set I. 

No. of values 
oiM/R 

50 

100 

50 

Set IL 

No. of values 
oiM/E 

25 
25 
52 



Mean value 
of M/R 

•2142212 

1-0109685 

•2157379 



Mean value 
otM/R 

•9973168 
•9984148 
•2112647 



Dord 
in centims. 

62-318 
31-783 
62-308 



D or d 
in centims. 

32-106 
32-116 

62-708 



Horh 
in centims. 

61-416 
30-824 
61-373 



H ork 
in centims. 

30-965 
30-954 
61-566 



On the completion of Set I the four masses were inverted, and changed 
over from right to left or left to right, and the initial position was after this 
always arranged so that movement of rider or mass decreased the reading. 
This was done in order to lessen errors due to want of symmetry. If reversal 
had no effect, Set II should, with the increased distance recorded above, give 
a value of M/R in the lower position of about -990, instead of -998. The 
larger value actually found is no doubt chiefly due to a want of symmetry 
in the large attracting mass M, The effect of this want of symmetry will 
be discussed after the investigation of the mathematical formula, and an 
account will be given of an independent method of detecting it. I think 
there is still outstanding a small difference, due, perhaps, to want of symmetry 
in the turn-table or in the attracted masses. The result of the reversal shows 
how necessary it was to make it. I should have Hked to have in Set II as 
many determinations as in Set I, so that the mean should be based on values 
of equal weight. During June and July, 1890, a complete set of 100 in each 
position, upper and lower, was made; but, owing to the pressure of other 
work, I was unable to calculate the results till the completion of the set. 
I then found that the value of M/R was still more than in Set II, and, on 
plotting out the results, it appeared that occasionally the rider-value fell very 
considerably, and in an irregular way. On examination, there was Httle 
doubt that the rider came in contact occasionally with the suspending frame, 
when it was raised and should have been clear from it. Very likely tempera- 
ture-changes had brought about a displacement of the lever-apparatus. 
Comparison with Set I seemed to show that during that set no such contact 
had taken place, for there was no comparable irregularity. As it appeared 
dangerous to attempt to disentangle the good from the bad, the set of June 
and July was rejected, and Set II was taken as recorded. When I had made 
the weighings giving 50 and 52 values in the two positions respectively, the 
balance became so irregular, through the cooler weather, that it was useless 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 77 

to continue work. Rather than carry over the experiment into another 
season, when it might be necessary to repeat the whole of the work, I have 
preferred to take Set II as it stands, and give it the same weight as Set I. 
The final results are calculated from the means of Sets I and II, as explained 
hereafter. I may here state the results obtained : 

^ r ■ ^ 6-6984 

Constant of attraction "^ = — tt^— . 

Mean density of the Earth ... A = 5-4934. 

General Remarks on the Method. 

Comparing the common balance with the torsion-balance, there is no 
doubt that the former labours under the great disadvantage that the dis- 
turbances due to air-currents are greatest in the vertical direction, that of 
the displacement to be measured. But even with this disadvantage the 
common balance may, I beheve, be made to do much more than has hitherto 
been supposed possible. As an instrument in itself, apart from the external 
disturbances of air-currents, dust, etc., I believe its accuracy would be far 
beyond anything approached when these external disturbances are, as they 
always are, present to interfere with its action. I have always found that 
every precaution to ward off air-currents and external disturbance has been 
accompanied by a corresponding increase in steadiness ; and I have seen no 
sign of a limit of accuracy depending on the instrument itself. 

Besides the protection from air-currents, there are two conditions essential 
above all others for accurate work : 

1st. That during any set of weighings in which the deflections are to be 
compared with each other, the beam should be supported on its knife-edge, 
and should be under constant strain. 

2nd. That all moving parts, such as apparatus for changing riders or 
weights, should be supported quite independently of the balance or its case. 

With regard to the first condition, it seems impossible to make the 
supporting frame move so truly and with so little disturbance that the knife- 
edge shall return exactly to the same line. Even were it possible, the beam 
after raising and lowering would be practically a different beam, for, as my 
observations show, the condition of strain changes considerably after the 
load is first put on, and it would be merely a chance coincidence if the mean 
state of strain were the same during successive weighings. I have, in my 
former paper {Proceedings of the Royal Society, vol. 28, 1879)*, described one 
method of comparing weights of nearly equal value with the beam throughout 
on its knife-edge and equally strained f, and I should now only modify that 

* [Collected Papers, Art. 2. J 

t I am glad that Dr. Thiesen urges the importance of this condition {Travaux et M^moires du 
Bureau International des Poida et Mesures, vol. 5, 'ilfitudes sur la Balance'). 



78 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

method in having regard to the second condition, of which I have since 
reahsed the importance when working with the large balance and with 
increased optical sensitiveness. It is surprising to find how much disturbance 
is produced by having the moving parts of the apparatus connected with the 
balance or its case. 

As to air-currents there is no doubt that, as Professor Boys has shown, 
the greater the apparatus the greater the errors produced by them. At the 
time my apparatus was designed I did not know this, and there seemed 
to be a great advantage in making it large, as riders could be used of weight 
large enough to be measured accurately. Were I about to start with a new 
design I should certainly go towards the other extreme and make the apparatus 
small, attempting to get over the rider-difficulty by some such method as that 
explained on p. 60. For not only is a smaller apparatus kept more easily 
at a uniform temperature, and, therefore, freer from the source of air-currents, 
but it is much more handy to adjust, and even if the adjustments are not 
more accurate they will at least take much less time to make. 

At the same time it is only fair to say, on behalf of the large apparatus, 
that some errors have been magnified on a like scale till they have become 
observable, and so could be investigated and eliminated. Starting with 
a small apparatus they would probably never have been detected, and would, 
therefore, have appeared in the final result. 



II. Mathematical Investigation. 

The Value of the Attraction Expressed in Terms of the Masses and Distances, 
and the Investigation of the Effect of Want of Syynmetry in the Masses. 

Let us suppose that initially the attracting masses are in the positions 
Ml, »?i, Fig. U, the larger on the left, the smaller on the right, and that the 
attracted masses are in the lower positions A, B. When the turn-table is 
moved round so that the positions of the masses are M^, m^, the greater 
attraction is taken from the left and put on to the right. Let the centre of 
swing of the balance alter by an amount corresponding to a total change of 
vertical pull of n dynes. Assuming that a spherical mass M attracts another 
spherical mass M' when their centres are D centimetres apart with a force of 
GMM'/D" dynes, we can express the change of vertical pull due to the change 
of position of the masses as 6^ x a function F of the masses and distances. 
There is also a change of pull on the suspending-rods and the balance-beam 
which we may denote by E. 

Then n = GF + E. 

In order to eliminate E let the attracted masses be moved into their 
upper positions A', B\ and let the change on moving round the attracting 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 79 

masses be n' dynes. If / is the function of the masses and new distances 
corresponding to F, 



Subtracting 
whence 



n — n' 



G = 



F-f 



and knowing G, the mean density of the earth may be at once found in the 
manner shown later. 






Fig. 14. 

We have then to find the form of the functions F, f, and as a prehminary 
step it is necessary to find the effect of the holes bored through the attracted 
masses A, B. This may be made to take the form of a correcting factor to 
the attraction which would be exercised on them if they were spheres. 

The piece bored out in each case has radius -31 centim. This we denote 
by c. It may be taken as practically a cylinder with plane ends and length 
equal to 15-8 centims., the diameter 2r of the spheres. The intensity due to 
such a cylinder of mass /x at Z) from its centre is (Todhunter's An. Stat., 
Ed. 5, p. 292), 

2r - V{{D + r)2 + c2} + ^{(D - r)2 + c^} 



GfJL 



c^r 



which equals, to a sufficient approximation, 

GfJL 



80 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

If the mass remaining after fx is removed is A, and if the centre of the mass 
Mis D below that of A, the attraction of M on A is 
GMJJ^ji) GMji 

GMA , rM (^ ^ \ 

= ^{l -^(J + ^ig^er powers of JJl. 



1 
and the greatest value of 



N- f = -^J^^r = I % ^'^^'^ = - ^^-'^ = ■'^''''^ 



' _ / 79 \2 _ .Afil 



Then the higher powers may be neglected, and the attraction may be 

written 

GMA /, 3 c2 \ GMA 



IMAf. 3 C^\ brMA 



When A and B are in the lower position, D = 32, and 1 - 6 ^ -99986. 
When they are in the upper position, D = 62 and 1 - ^ = -99996, a value 
so near 1 that we shall in this position omit the correction, since it is only 
applied to one-fourth of the final result. 

In the cross-attractions we shall also omit the correction. 
Referring to Fig. 14 let the vertical differences of level between the centres 
of the various spheres be denoted as follows, the suffixes to M and m denoting 
their first and second positions respectively : 

A-M^ = D^, B-M^ = D^, 

B 

B 



M^=D^, 


A-M2 = D^, 


nil - H^, 


A - mi = i?/. 


m^ = H2, 


B - m^^ H^. 



When the masses A, B are placed in their upper positions, let the corre- 
sponding distances be denoted by small letters. 

Let the horizontal distance between the centres of A and B be L, being 
within sensible limits equal to that between the centres of M in its two 
positions, and to the length of the beam, and let the radius of the circle in 
which m moves be I. 

Then we have the following horizontal distances : 
A-M^^B-M^=^L, 
A — m-^ = B — m^ = I + \L, 
A — 7)12 = B — m^ ^ I — \L. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 81 



We may now write the change in vertical pull on the left by the motion 
of M from left to right, and of m from right to left, as follows — the first four 
terms representing the vertical attractions on A and Bhj M and m in their 
first position, the next four their attractions when moved round, and the last 
term E representing the change in attraction on the beam and suspending 
rods : 

( MA (1 - 6) _ MBD,' mBH^ , mAH^' 

\ Di' {D^^ + L^f 

MB(\-d) MAD^ mAH^ mBH: 



H{' 



'-r 



«."+('+ 1) 



LV)^ 



+ 



D, 



{D,'^ + L^)^ 



H,+ il-^f 



+ 



2)] 



H''^ 



'-!) 



LV)^ 



+ E. 



We may arrange all but the last term in nearly equal pairs. 

Thus the first and fifth go together, and if we put B^^- 3^ = 2D and 
Di + S = D2 — S = D, their sum is 

A B 



(?M(l-^)(^^+^^,) 



GM(l-d)\'^,{l 



= GM(l- d) 



382 
2 



Z)2 



D^V^ D^ D^^ '"J^ D^V D^ D 
(^1 + ^ + higher powers of -^^j 

^ A + B d[ ^ D^ 



Now (S/Z))2 is negligible, as will be seen by reference to the table of 
distances, p. 92, and {A — B)I(A + B) is less than jjji-Qjj, or less than S/D.* 

To a sufficiently close approximation then the sum of the two terms is 

GM (A -\-B)(l- d) 

2)2 

The second and sixth terms may also be taken together, and putting 
D^ + D^ = 2B' and Z)/ + 8' = D/ - 8' = Z)', 
we may show that to a sufficient approximation 

BD^ , .4Z)./ I _ GM {A + B)U^ 

(Z)'2+ 2,2)1 



GM 



The two pairs with m give similar results with 



H = i{H^ + H2) and 



H' = i {H,' 



H,'). 



* [A slight correction of the original, obviously required, has been made here. Ed.] 
P. c w. 6 



82 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

Now 2D=D^ + D, = A-M^-hB-M^ 

= B-M, + A-M,-=D,'-\-D,' = 2D', 

and similarly 2H = 2H' , so that we may put the expression in the form 
(M(A-^B\i\-e) _M{A + B)D m(A + B)H m(A + B)H \ 

-{- E=GF + E say. 

It is evident that we may combine experiments at different distances on 
different occasions in the same way by taking D and H to represent the mean 
values of these distances, so long as there is only a small variation from the 
mean. 

If the attracted masses are now moved into their upper positions the 
expression for the change in attraction may be at once deduced from that 
in the lower position by replacing D and H hj d and h, and omitting the 
factor 1 - e. Let it be denoted by Gf + E. 

Subtracting one expression from the other E is ehminated, and we have 

0{F-f) 

[M^ + B){l-e) M{A + B )D m{A + B)H ^ m(A + B)H 

M(A + B) , M(A + B)d , m{A + B)h m{A + B)h ^ 



d^ "^ {d^ + L^)'^ 



M-Wf M-W 



This is to be equated to the difference in the values of the change in attrac- 
tion in the two positions, as determined by the rider. 

Let 

b = the length of the small rider-beam, 
ir = the mass of each rider, 

A = mass deflection -^ rider deflection in lower position*, 
« = » ^, „ „ upper „ *, 

gjj = acceleration of gravity, or dynes weight per unit mass at 
Birmingham. 

Then . G(F j^ _ i^ - f bwgj,^ 

Whence we may find the gravitation-constant 

^ 2hwgj^ {A -a) 

where all the quantities on the right-hand are given in the tables at the end. 
* [It will be noted that this symbol is used with two distinct significations. Ed.] 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 83 

The value of gjg may be found sujSiciently nearly from the formula 
(Everett's Units, p. 21): g = 980-6056 - 2-5028 cos 2A - -000003/i, where A 
is the latitude = 52° 28' at Birmingham, and h is the height above sea-level, 
which may be taken as 450 feet, or 13,725 centims. Whence g^ = 981-21. 

Since all the operations are conducted in air, the effective masses should 
throughout be less by the mass of air each displaces. But since they all have 
nearly the same densities, and w and A -^ B appear respectively in numerator 
and denominator, it is sufficient to take their true masses, and to correct for 
air displaced in the case of M and m only. 

To obtain the mean density of the earth A, we must express the acceleration 
of gravity in terms of G and the mass and dimensions of the earth. 

The ordinary formula (Pratt, Figure of the Earth, 4th ed., p. 119) is based 
on the assumption that the earth is a spheroid. It is sufficiently correct for 
our purpose, the departure of the assumed spheroid from the actual shape 
being very small. Adding a term — 3 x 10~^h, or approximately, — 41 x 10~®, 
since the balance-room is taken as 13,725 centims. above sea-level (see above), 
the value of gravity at Birmingham may be written 

g^ = ^^|l +^-^^+(1^-^) sin^ 52° 28' - 41 X 10-« 
where 

F = volume of the earth = 1-0832 x 10^7 (Everett's Units, p. 57), 

a = mean radius of the earth = 6-3709 x 10^ {loc. oil.), 

A = mean density of the earth, 

m = equatorial ' centrifugal force ' -^ gravity = ^^ , 

€ = ellipticity of the earth = ^b 2 • 

The value of the ellipticity is taken to make the formula agree with that 
quoted above from Everett's Units. The uncertainty in the value is quite 
unimportant, for were e as low as 2^5, the error in A, introduced by taking 
it as 2^2 ' would be less than 1 in 50,000. 

Substituting for G, the value of the mean density of the earth is 

a^L{F-f) 



2bwV 



|l + I - I m + f| m - e") sin^ 52° 28' - 41 x 10-4 (A - a) 



Here, as in the value of G, w and A + B may have their true values, 
M and m their values less the mass of air displaced. 

In the foregoing investigation we have supposed that all the masses are 
homogeneous and spherical, with the exception of the borings through A and 
B. We have supposed, also, that the turn-table is exactly symmetrical about 
a vertical plane through its axis, so that its motion through two right angles 
is without effect. Doubtless, these suppositions and the formula based on 

6—2 



84 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

them are not quite true. But, if we invert all the masses and change their 
sides, or pervert the whole arrangement of them, on taking the mean of the 
results obtained in the original and inverted and perverted positions we ought 
to greatly reduce the errors. Indeed, those due to want of symmetry in the 
turn-table should evidently be quite ehminated, and those due to want of 
homogeneity in the masses should certainly be lessened. 

To show this, we shall calculate the effect of a spherical ' blow-hole,' or 
gas-cavity in M, in the first and most important term of F. This we shall 

take as being 

GM (A + B) 

on the supposition that M is homogeneous and spherical. 

If the mass of metal which would fill the blow-hole is A, supposing it to be 
placed there, the sphere is completed and its attraction is 

G(M + X){A + B) ^ 

2)2 

but the vertical attraction is less than this in reahty by the vertical component 
of the attraction of A. 

Let B be the centre of the cavity, 

P the centre of the attracted mass, 
the centre of the attracting mass, 
8 the distance of B from the centre of M, 
e the angle BOP. 

The vertical component of the attraction of A is 

GX (A +B) cos BPO 
PB^ 

but 5P2 = Z)2 + §2 _ 22)3 cos d, 

1 7? on D-Scosd 

and . cos BFU = „„ — , 

Br 

whence the attraction of A may be put 

GX(A + B){D-Scosd) GX{A + B) "^ ^ 



( D2 + §2 _ 22)8 cos d)^ ' ' dD VZ)2 + 8^ _ 2Z)8 cos 6 

GX(A + B)f^ , ,„ 8 , ,„ 8^ 



A + B)( 6 6^ 

2)2 ^^ + -^12) + ^^2 2)2 



where P^, P^, ... are zonal harmonics. The attraction of the sphere with the 
cavity is therefore 

GM(A + B)L A / „ 8 , ,„ 8^ 



2)2 |i MV^^D^^^' D^^ 'D^ 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 85 

If the mass is inverted, the vertical component is obtained by changing the 
sign of 8, and the mean of the two values is 



6MiA + B)i^_X_j^pJ^^^^p^8^ 



2)2 M\ ^D^ " D* 



■)i- 



the first power of hjD being ehminated. 

If ^ = 0, Pg ^^^ ^11 ^h^ other harmonics = 1. 

If ^ = 90°, Pg = - 1, P4 =- I, etc. 

Now, with the actual dimensions of the apparatus, (S/Z))^ cannot be so 
great as (J)^ or J, and may, of course, be much smaller. The first term of 
those involving A, therefore, is the most important, and it lies between 
+ f (A/M) (8VI>2) and - 3 (XjM) {h^jB^), changing sign for the value of 6 
given by Pg == 0. 

The second set of experiments recorded in this paper was taken after 
inversion and change of side of all the masses, and the final result obtained 
from this set differs by a little more than 1 per cent, from that obtained from 
the first set, the observed attraction being slightly greater at the same distance. 
The difierence may be due to irregularities in any or all of the masses and in 
the turn-table, and to other undetected effects, such as change of level on 
rotating the turn-table. It would be a very long task to disentangle these, 
and I have contented myself with trying to find how much must be set down 
to irregularity in the large mass M, by taking a set of weighings with it alone 
inverted. 

After the weighings on July 28, and the subsequent measures of distances, 
M was inverted only, and the other masses remained as in Set II. Some 
weeks later, on September 14, 25 values of MjR -^ A were obtained, the 
mean being -9926. The distances were B = 32-118, H = 30-978. The mass 
M was then put in its original position, as in Set II, and on September 17, 
as will be seen on referring to the tables, the value of MjR obtained was 
•9984, the distances being B = 32-117 and H = 30-955, practically the same 
as on September 14. 

Assuming that the difference in attraction is due to cavities in various 
places, and that, for each, the term ^tP^^h'^jB^ is negligible, we have, approxi- 
mately, 

2AP,8 
" MP _ 9926 
^ .SAPi8~9984' 

Whence, approximately, since B = 32, 

,>r— = -0464 centim. 
M 



86 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

This result may be tested by independent experiment. For, let the 
centre of gravity be x below the horizontal plane through the point bisecting 
the vertical diameter (i.e., the centre of figure), in the position of Septem- 
ber 14. The distance of any missing particle A from the horizontal plane is 
8 cos ^ = Pi§. Completing the sphere by the addition of all such particles, 
the centre of gravity is brought to the centre of figure, so that we have 

Mx = SAPiS, 

SAP^S 
and X = — ^-^ . 

We have, therefore, to determine the vertical distance of the centre of 
gravity from the centre of figure. 

In order to do this, a large flat-bottomed scale-pan (one belonging to the 
balance used in the gravitation-experiment) was suspended by two parallel 
wires about 8 centims. apart and 3 metres long. In the middle of the pan 
was a shallow cup about 7-5 centims. internal diameter, arranged so that it 
could turn freely but truly about a vertical axis. The mass, M, was placed 
on this cup with the diameter, which had been vertical, arranged horizontal, 
and perpendicular to the plane of the suspending wires. A vertical flat plate, 
worked by a horizontal micrometer-screw, could be brought just in contact 
with the end of the diameter, and the reading of the micrometer gave the 
position of the point of contact. The position of the scale-pan was deter- 
mined by a plumb-line hanging over one edge in front of a horizontal scale. 
On turning the cup and mass through 180°, and repeating the readings, 
knowing the weight of the scale-pan, and the position of its centre of gravity, 
X could at once be found. 

Two separate experiments gave 

X = -0536 centim., 
and X - -0516 centim., 

not very different from the value -0464 obtained from the attraction-experi- 
ments. The agreement is, I think, very close when it is noted that a difference 
of 1 in 1000 in the attraction in one of the sets of weighings would make x 
either -038 or -054. 

This result appears to justify the rejection of all terms in the expansion 
above the first, and so supports the belief that the reversal largely ehminates 
errors due to irregularity of shape. For it is in the case of M that there is 
the greatest danger of a large value for S/D, and the above experiments seem 
to indicate that even in this case it is small. 

It is, perhaps, noteworthy that the largest term rejected in the attraction 
of M, viz., ^XP^S^/MD^ is, if we give P^ its maximum value 1, 

3AS S^^3x S 
MB' D~ D'D' 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 87 

which is not greater than 

since the radius of the mass is 15. 

This is in a term about 5/4 of the final result, so that the greatest error 
which can be introduced by neglecting this term is -0025, or 1 in 400. 

In calculating the results of the experiments the means of Sets I and II 
have been taken. Equal weights have been given to each set. It would 
have been more satisfactory if the number of experiments had been the 
same in each set ; but I should have had to wait for another season to obtain 
more, and then it would, probably, have been necessary to repeat the whole 
series in both arrangements, as it is not safe to assume that the various 
disturbing causes remain the same over a wide interval of time. The second 
set, though fewer in number, are, in some respects, I beheve, better ; partly 
owing to the additional experience gained when they were taken. 

In order that the various terms in F —f may be compared, I give below 
their numerical values, as determined from the values of the masses and 
distances given in the tables. The meaning of each term in the first column 
will be seen on referring to Fig. 14. The second column contains the actual 
values ; the third column the values in terms of the lowest term, the fourth. 

Value oi F -f. 
M{A-{-B){l-e) 



2)2 

M(A + B)D 

(Z)2 + L^)^ 
miA + B)H 



m(A + B)H 

M(A + B) 
M{A + B)d 

m(A + B)h 



+ 6483938-8 416 

- 102416-3 6-6 

- 316243-3 20 

+ 15579-9 1 



1693687-2 


109 


156728-0 


10 


310695-0 


20 



^iA + B)h _ 27597.7 ].7 

{'-('-ST 

Whence F -f= 4826997-2. 



88 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



The mean value oi A - a (see Table III) is 

^_a = -791295; 

substituting these values of J -/and ^ - a in the formula for G (p. 82), we 

obtain 

6-6984^ 

substituting them in the formula for A we obtain 

A = 5-4934. 
The values given by Sets I and II, treated separately, are to two figures 
of decimals : 

Set I. A = 5-52 
Set 11. A = 5-46. 



i 



III. Tables. 
Table I. Constants of the Apparatus and Dimensions of the Earth. 



Masses 

Attracting mass M, in vacuo 
Less air displaced, say . . . 
Attracting mass m, in vacuo 
Less air displaced, say ... 
Attracted mass A, in vacuo 

55 55 -^5 ?5 

,, ,, A + B, in vacuo 
Riders each, in vacuo 



grms. 

153407-26 

153388-85 
76497-4 
76488-2 
21582-33 
21566-21 
43148-54 

0-010119 



Vertical Diameters of Masses in terms of Catheto meter -Scale correct at 18°. 

The masses are taken as having the same coefficient of expansion as the 
scale. 

centims. 

M = 30-526 
m = 24-176 
A = 15-8203 
B - 15-7829 

The diameters of the masses A and B are taken between the nuts securing 
them on the suspending wires. 

centims. 

Balance beam at 0°, L _ =123-232 

Rider beam at 0°, 6 ^ 2-53575 

L/b (as occurring exphcitly in G and A, independent of tempera- 
ture, assuming them to have the same coefficient of expansion) = 48-59775 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 89 



Latitude of Birmingham 

Height of balance-room above sea-level 

Gravity at Birmingham, g^ ... 

Mean radius of earth ... 

Volume of earth 

Equatorial ' centrifugal force '/gravity 

Ellipticity of earth 

f]sin2 52° 28' -41x10-6 



3 , /5 



= 52° 28' 

= 13725 centims. 

= 981-21 centims./sec.2 

= 6-3709 X 108 centims. 

= 1-0832 X 1027 cub. centims, 

= -999161. 



Table II. Vertical and Horizontal Distances. 
Vertical Diameters of Masses taken by the Cathetometer, described p. 65. 

In the tables below p.s. signifies divisions on the scale over which moves 
the pointer, which is attached to the small adjustment-plate, v.s. signifies 
divisions on the vertical millimetre-scale. 



Diameter of Large Attracting Mass M. 

Reading on pointer-scale 
Upper telescope sighting top of mass ... 73-2, 73-4, 73-2 

Lower „ „ bottom „ ... 23-0, 23-0, 23-2 



Mean 
73-27 P.s. 
23-07 p.s. 



Turning round to the Vertical Scale. 

Reading on pointer-scale 
Upper telescope sighting 459 millims. v.s. ... 94-6, 94-9, 94-4, 95-0, 94-0 
458 „ ... 68-8, 68-8, 69-7, 69-0, 68-7, 

70-0, 69-6, 69-4 

Therefore 25-33 p.s. divisions = 1 millim. v.s., 



Mean 
94-58 P.s. 
69-25 p.s. 



and scale-reading for top of mass = 458 
= 458-158 millims. v.s. 



73-27 - 69-25 
25^33 



Lower telescope sighting 153 millims. v.s. 
„ 152 



Reading on pointer-scale Mean 

27-3, 27-4, 27-7, 270 27-35 r.s. 

0-0, 0-3, - 5, 0-0 - 0-05 p.s. 



Therefore 27-40 p.s. divisions = 1 milhm. v.s., 

23-07 -f 0-05 



and scale-reading for bottom of mass = 152 + 
= 152-844 milhms. v.s. 
The difference = 30-5314 centims. 



27-40 



90 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

This IS rather greater than the diameter of the mass, as the cross- wire was 
made to touch the image of the mass in each case. A series of measures of 
1 milUm. on the scale, in which the cross-wire was on the centre of each 
division, and of 1 miUim. between the jaws of a wire-gauge, in which the wire 
touched the images of the jaws, showed that at the distance at which the scale 
was, -005 centim. must be subtracted, leaving 

Diameter of if = 30-526 centims. 

Vertical Diameter of Small Attracting Mass m. 

Reading on pointer-scale Mean 

Upper telescope sighting top of mass ... 75-6, 75-6, 75-0 75-40 p.s. 

Lower „ „ bottom „ ... 26-5, 26-3, 26-8 26-53 p.s. 

Turning round to the Vertical Scale. 

Reading on pointer-scale Mean 

Upper telescope sighting 388 millions, v.s. ... 100, 99-9, 99-7 99-87 p.s. 

387 „ ... 73-9,73-4,74-0 73-77 p.s. 

Therefore 26-10 p.s. divisions = 1 millim. v.s., 

75-40 — 73-77 
and scale-reading for top of mass = 387 + — ^r^^r. 

= 387-062 millims. v.s. 

Reading on pointer-scale Mean 

Lower telescope sighting 146 millims. v.s. ... 45-9, 45-9, 45-0 45-60 p.s. 

145 „ ... 20-4,19-4,20-0 19-93 p.s. 

Therefore 25-67 p.s. divisions = 1 millim. v.s., 

and scale-reading for bottom of mass = 145 + "^'^^ ~ ^^'^^ 

25-67 
= 145-257 milhms. v.s. 

The difference = 24-1805 centims. 

Subtracting the same correction as in the last case for the cross-wire, 

Diameter of m = 24-176 centims. 



Vertical Diameters of Attracted Masses A and B taken between the 
Junctions of the Securing Nuts ivith the Sphere. 

A. 

Reading on pointer-scale Mean 

Upper telescope sighting top of mass ... 82-7, 83-0, 82-9 82-9 p.s. 

^'''''^'' •' » bottom „ ... 31-0,' 31-5,! 31-3 31-3 p.s. 



I 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 91 

• Turning round to the Vertical Scale. 

Reading on pointer-scale Mean 

Upper telescope sighting 429 millims, v.s. ... 95-0, 95-2, 95-3 95-2 p.s. 

428 „ ... 69-0, 69-0, 68-9 69-0 p.s. 

Therefore 26-2 p.s. divisions = 1 millim. v.s., 

and scale-reading for top of mass = 428 H ^r^-^ 

= 428-531 millims. v.s. 

Reading on pointer-scale Mean 

Lower telescope sighting 271 millims. v.s. ... 48-0, 47-8, 48-4 48-1 p.s. 

„ 270 „ ... 23-4, 230, 22-8 23-1 p.s. 

Therefore 25-0 p.s. divisions == 1 millim. v.s., 

31*3 — 23*1 

and scale-reading for bottom of mass = 270 H ^r^^r 

^ 25-0 

= 270-328 millims. v.s. 

The difference gives the diameter since the middle of the cross-wire was 
used, so that 

Diameter of ^ = 15-8203 centims. 

B. 

Reading on pointer-scale Mean 

Upper telescope sighting top of mass ... 72-0, 71-0, 71-0 71-3 p.s. 

Lower „ „ bottom „ ... 24-6, 25-0, 25-2 24-9 p.s. 

Turning round to the Vertical Scale. 

Reading on pointer-scale Mean 

Upper telescope sighting 430 millims. v.s. ... 94-0, 94-6, 940 94-2 p.s. 

429 „ ... 68-0, 68-1, 68-1 68-1 p.s. 

Therefore 26-1 p.s. divisions == 1 milHm. v.s., 

and scale-reading for top of mass = 429 H -^^ ,, 

= 429-123 minims, v.s. 

Reading on pointer- scale Mean 

Lower telescope sighting 272 millims. v.s. ... 43-3, 430, 430 43-1 p.s. 

„ 271 „ ... 171, 17-3, 17-4 17-3 p.s. 

Therefore 25-8 p.s. divisions = 1 milhm. v.s., 

24-9 — 17-3 
and scale-reading for bottom of mass = 271 -f ^„ „ 

= 271-294 milhms. v.s. 

And diameter oi B = 15-7829 centims. 



92 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

Vertical Distances between the Levels of the Centres of the Attracting 
and Attracted Masses Measured by Cathetometer. 

The measurements were made as soon as possible after the completion 
of a set of weighings, usually on the following day. 

It was necessary to fix the attracted masses in the position occupied 
during the weighings, and with the beam of the balance in the same strained 
condition. This was done in some cases by gripping the left suspending wire 
by a pair of jaws ; in others, by adding a small weight to one side, and placing 
a block of the right thickness under the mass on that side. 

The cathetometer was placed in front of the left side of the balance-case, 
from which position all the masses could be viewed by turning the telescope 
round the central pillar (Fig. 2). It was read when sighting the top of each 
attracting mass and the top of each attracted mass when in the lower position, 
the bottom of each attracted mass when in the upper position, the top and 
bottom being taken at the junctions of the securing nuts with the masses. 
It is therefore necessary to add to the distances measured by the cathetometer 
the difference of the radii of attracting and attracted masses in the lower 
position, and their sum in the upper position (see p. 88). The work is shown 
in full for February 5 and May 5. 

Tests were made at various times, showing that there was no change in 
the distances (at least within errors of reading), either through moving the 
turn-table or in the course of a few days (see February 5 and May 5 for 
examples). 

Temverature-Corrtction. The cathetometer-scale is taken as correct at 
18°, and its coefficient of expansion is assumed to be 1/60000. That of the 
masses is probably about 1/40000, but, for simphcity, is taken as equal to 
that of the scale, the difference, 1/120000, never amounting to as much as 
the errors of reading, since the greatest length concerned is 23 centims. 

The temperature was estimated to be about 1° above that observed during 
the immediately preceding weighings, the presence of the observer and the 
lights used tending to raise it. 

The cathetometer rested always on the brick floor of the room. Its vernier 
reads to -002 centim. 

Set I. 

Attracted masses A on the left, B on the right. Attracting mass M 
moving round from left to right in front of the balance-case. 

February 5, 1890. Attracted masses in upper position. Assumed 
temperature 11°. 

Half-way through the measurements the cathetometer was accidentally 
moved, and could not be exactly replaced. Repeating the reading of A it 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 93 

was found that -197 must be added to the previous readings to compare with 
the following ones. This addition is made where the numbers have an asterisk. 



23-448 


A 
64-999* 
65-001* 

^1 

25-895* 
25-889* 


B 
65-284 
65-282 

26-070 
26-064 


mi 
23-947* 


Differences: ^ - m, = 41-552 


A- Mt^ = 39-108 


B - M^ = 39-216 


B - m^ = 41-336 



Table I 


P- 


88, the sums of the radii of the masses are 






' Rm + Ra = 


23-173, 






Rm+ ^B = 


23-154, 






Rm-^RA = 


19-998, 






Rm+RB = 


19-979, 




d 


= 1 {39-108 + 23-173 + 39-216 + 23-154} 






= 62-326, 






h 


= 1 {41-336 + 19-979 + 41-552 + 19-998} 






= 61-433. 





and 



These are in terms of a scale correct at 18°, so that the value is too great 
by about 7/60000. We take as true values 

Corrected d = 62-318, 

h = 61-425. 

Test Experiment. At the conclusion, the distance A — M^ was measured 
again and found to be 39-110. 



May 28, 1890. Attracted masses in upper position. 
Assumed temperature 14°. 







A 
64-674 
64-674 


B 

65-286 
65-288 






mo 
23-422 
23-424 


25-726 
25-724 


25-920 
25-920 


m^ 
23-766 
23-756 


Differences: J. 


- ma = 41-251 


A- M^ = 38-949 


J5 - -¥2 = 39-367 


5 - m.i = 41-526 


whence 




d = 62-312 


5 








h = 61-377 







94 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Subtracting temperature-correction -004, 
Corrected d = 62-308, 

/i- 61-373. 

Mean values in Set I, d = 62-313, 

h = 61-399. 

May 5, 1890. Attracted masses in lower position. 
Assumed temperature 13°. 





A 


B 






50-324 


50-622 






50-324 


50-634 






50-328 


50-630 




7m, 


M, 


M, 


Ml 


23-672 


25-972 


26-138 


23-998 


23-674 


25-970 


26-138 


24-008 




25-972 


26-132 




Differences : A - m.^ = 26-652 


A - Mi = 24-354 


B- M^ = 24-493 


B - m^ = 26-626 


From Table I, p. 88, 


^31 -Ra = 7-353, 
Rjyj -Rb= 7-372, 
R,, -i?^== 4-178, 
R,, -i?5^ 4-197, 





whence 
and 



D 



l_ {24-354 + 7-353 + 24-493 + 7-372} 
= 31-786, 
H = \ {26-626 + 4-197 + 26-652 + 4-178} 
- 30-827. 
Subtracting temperature-correction -0025, 
Corrected values for Set I, 

D = 31-783, 
H = 30-824. 

Test Experiment. The balance was set free at the end of these measures, 
and two days later, on May 7, it was again fixed, and the distance D was 
determined by the cathetometer described on p. 65. The value obtained 
was D = 31-786. 

Note. If the apparatus were perfectly rigid and constant in its dimensions 
we should expect D - H = d~h = constant. The values actually given by 
the above experiments are 

February 5 ... ... -892, 

May 5 .959^ 

May 28 .935. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 95 

There is apparently a slight increase during the course of the spring, 
probably due to the warping of the wood supporting the mass m. But 
there was some uncertainty in sighting the top of the mass m, especially when 
in the distant position on the right. 

Set II. 
Attracted masses A on the right, B on the left. Attracting mass M 
moving round from left to right behind the balance-case. All the masses 
inverted. 

July 29, 1890. Attracted masses in lower position. 
Assumed temperature 16°. 



mi 
22-434 
22-436 


B 
49-014 
49-014 

M^ 
24-584 
24-586 


A 
49-846 
49-844 

M, 

24-788 
24-782 


^2 

22-868 
22-864 


Differences : B - m^ = 26-579 


B- M^ = 24-429 


A- M^ = 25-060 


A - m^^ 26-979 



whence D - 32-107, 

H = 30-967. 

Subtracting temperature-correction -001 , 
Corrected D - 32-106, 

H = 30-966. 



September 18, 1890. Attracted masses in lower position. 
Assumed temperature 16°. 





B 


A 






49-076 


49-768 






49-074 


49-766 




mi 


J/2 


Ml 


^2 


22-467 


24-576 


24-756 


22-840 




24-576 


24-758 




Differences : B - m^ = 26-608 


B- M^ = 24-499 


A - M^ = 25-010 


A - m^ = 26-927 


whence D - 32-117 






H = 30-955 






Subtracting temperature-correction -001, 






Corrected D = 32-116 






H = 30-954 






Mean values in Set II, 






Z) = 32-111 








H = 30-960 







96 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



September 27, 1890. Attracted masses in upper position. 
Assumed temperature 16°. 



) 






B 




A 












63-880 
63-876 




64-540 
64-544 








Ml 

22-450 

22-448 




24-570 
24-572 




24-756 
24-758 




Mo 

22-8^10 
22-816 


Differences: B - 


- Wj = 41-429 


B- 


- M^ = 39-307 


A - 


- M, = 39-785 


A 


- mo = 41-729 



whence d - 62-710, 

h= 61-568. 
Subtracting temperature-correction -002, 

Corrected values for Set II, 

d = 62-708, 

h = 61-566. 

Note. The values oi D — H and d — h, which should be constant, are 
from the above, and from another set of measures (not here recorded, see 
p. 85) on September 15, as follows. (We have no reason to expect the same 
value as in Set I, as the masses M, m have changed sides.) 

July 29 1-140, 

September 15 1-110, 

September 18 1-162, 

September 27 1-142. 

From July 29 to September 15 inclusive, the balance was swinging freely 
without alteration. The values of H should, therefore, be the same on those 
dates. They were 

July 29 30-967, 

September 15 30-978, 

equal almost within errors of reading for the top of m. 
Means of Sets I and II : 

Z) = 1(31-783 + 32-111) 

= 31-947. 
H = \ (30-824 + 30-960) 

- 30-892. 

d=\ (62-313 + 62-708) 

- 62-511. 

h = \ (61-399 -t 61-566) 
= 61-483. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 97 



At 18° 



and 

whence 



Horizontal Distances, 

Set I. 

L = 123-269 centims. 
Zi = 122-915 

^= 61-635 



h + %= 184-550 



h-^= 61-280 „ 

Taking the mean temperature of the Set as 12°, and assuming 1/60000 as 
the coefficient of expansion, on correcting to 12°, 

l^ + ^ = 184-532 centims. 
L 



At 18' 



h 


±J 

~ 2^ 


61-274 
Set XL 


55 




\ = 


122-795 centims. 




L 

2 


61-635 


55 


h 


*\' 


184-430 


" 


h 


L 

2 ~ 


61-160 


55 



Whence 



Taking the mean temperature of the Set as 15°, and correcting to 15°, 
^2 + - = 184-421 centims. 



''2 9 



61-157 



Mean values for the two Sets 

L = 123-260 



l-\-%= 184-477 



l-~= 61-216 



p. c. w. 



98 



ON A DETERMmATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III. Determination of Attraction by the Balance. 
Determinations of the Attraction in terms of the Riders by the Balance. 
In each case four turning-points of three successive swings are recorded 
in tenths of a division, i.e., in divisions on the diagonal lines. In the columns 
headed i the masses and riders are in the initial position, in those headed r 
the riders are moved, and in those headed m the masses are moved. Under 
each set of four readings is the calculated centre of swing (see p. 71). In the 
next hne are the deflections due to movements of riders and masses, each 
placed under the middle one of the three centres of swing from which 
it is calculated. In the next Hne are the values of deflection due to 
mass -^ deflection due to rider, or M/R (see p. 74). 

Set I. 

Attracted Masses in Upper Position. Feb. 4, 1890, 7.59 p.m. to 10.49 p.m. 
Temperature: in Observing Koom, 15°-7-16°-5; in Balance Eoom, 
10°-05. Barometer, 752-2-752-0 miUims. Weather mild and still, after 
shght frost on the two previous nights. Time between successive 
passages of centre about 20 seconds. 



I. 





i 


r 


i 


771 




r 




m 


Scale-readings . . . 


(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


725 


912 


725 


804 


764 


913 


726 


804 




798 


838 


799 


787 


779 


838 


800 


787 




759 


878 


759 


796 


771 


879 


759 


797 




780 


856 


781 


791 


776 


857 


781 


791 


Centre of swing 


772-55 


863-85 


773-00 


792-80 


773-90 


864-60 


773-40 


793-20 


Deflection due to 


















rider or mass... 


... 


91-075 


... 


19-350 




90-950 




19-700 


Mass deflections 


















rider deflection 


... 


... 




-212608 




-214688 


... 


-217110 





/ 


r 


i 


m 


i 


r 


i 


m 




(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


Scale-readings... 


763 


913 


724 


804 


764 


914 


725 


805 




779 


837 


801 


787 


779 


838 


801 


789 




771 


879 


759 


796 


771 


880 


760 


796 


Centre of swing 
Deflection due to 


775 


857 


782 


792 


776 


857 


783 


792 


773-60 


864-25 


773-85 


79305 


773-90 


865-05 


774-50 


793-65 


rider or mass... 
Mass deflections 




90-525 




19-175 




90-850 


... 


18-950 


rider deflection 




-214720 


1 


-211440 


... 


•209824 




•208758 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 99 



Table III (continued). 





i 


r 


i 


m 


i 


r 


i 


m 




(17) 


(18) 


(19) 


(20) 


(21) 


(22) 


(23) 


(24) 


Scale-readings... 


765 


916 


726 


803 


765 


914 


725 


805 




780 


839 


803 


788 


779 


838 


800 


787 




772 


881 


762 


797 


771 


879 


759 


796 




111 


859 


784 


792 


776 


857 


782 


791 


Centre of swing 


774-90 


866-30 


776-30 


793-65 


774-00 


864-50 


773-60 


792-85 


Deflection due to 


















rider or mass... 


... 


90-700 


... 


18-500 




90-700 


... 


19-225 


Mass deflection -^ 


















rider deflection 


... 


•206174 




•203966 


... 


-207966 


... 


•211438 





i 


r 


i 


m 


i 


r 


i 


m 




(25) 


(26) 


(27) 


(28) 


(29) 


(30) 


(31) 


(32) 


Scale-readings... 


763 


914 


121 


805 


764 


913 


725 


804 




780 


838 


800 


789 


779 


838 


800 


786 




770 


880 


760 


797 


771 


879 


760 


796 




776 


857 


782 


792 


115 


857 


781 


791 


Centre of swing 


773-65 


865-05 


774 15 


793-90 


773-65 


864-65 


773-80 


792-50 


Deflection due to 


















rider or mass... 


... 


91-150 


... 


20-000 


... 


90-925 




19-275 i 


Mass deflections 


















rider deflection 


... 


•215167 


... 


-219690 


... 


-215975 




•211494 





i 


r 


i 


m 


^• 


r 


• 


i m 




(33) 


(34) 


(35) 


(36) 


(37) 


(38) 


(39) 


(40) 


Scale-readings... 


763 


913 


724 


803 


763 


912 


726 


802 




778 


838 


801 


789 


778 


837 


799 


787 




770 


879 


759 


796 


769 


879 


759 


797 




774 


857 


782 


792 


774 


857 


780 


792 


Centre of swing 


772^65 


864-60 


773-85 


793-50 


772-30 


864-20 


772-95 


793-30 


Deflection due to 


















rider or mass... 


... 


91-350 


... 


20-425 


... 


91-575 


... 


19-875 


Mass deflections 


















rider deflection 




-217296 


... 


•223316 


... 


-220038 


... 


•216503 



7—2 



100 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III (continued). 





i 


r 


i 


m 


i 


r 


i 


m 




(41) 


(42) 


(43) 


(44) 
803 


(45) 


(46) 


(47) 


(48) 


Scale-readings... 


764 


915 


725 


763 


913 


726 


803 




779 


839 


800 


786 


778 


838 


799 


787 




771 


880 


759 


795 


770 


879 


759 


796 




776 


858 


780 


791 


774 


857 


781 


792 


Centre of swing 


773-90 


86-555 


773-15 


792-25 


772-70 


864-60 


773-15 


792-95 


Deflection due to 


















rider or mass... 




92-025 


... 


19-325 




91-675 


... 


19-200 


Mass deflection ^ 


















rider deflection 




•212986 


... 


-210397 


... 


•210117 


... 


-209693 





* 


r 


i 


m 


i 


r 


i 


m 


1 


(49) 


(50) 


(51) 


(52) 


(53) 


(54) 


(55) 


(56) 


Scale-readings... 


764 


914 


724 


803 


762 


912 


721 


801 




779 


839 


802 


787 


778 


836 


798 


785 




772 


880 


759 


795 


769 


877 


755 


794 




776 


858 


781 


790 


774 


855 


779 


788 


Centre of swing 


774-35 


865-55 


773-85 


792-10 


772-20 


862-55 


770-35 


790-55 


Deflection due to 


















rider or mass... 




91-450 




19-075 




91-275 




20-275 


Mass deflections 


















rider deflection 




-209267 




-208784 




-215557 


... 


-222161 



(57) 



(58) 



(59) 



m 
(60) 



Scale-readings, 



760 


911 


724 


776 


836 


799 


767 1 


877 


758 


772 


855 


779 



803 
785 
795 
789 



(61) 



762 
777 
769 
773 



(62) 



(63)* 



911 

836 
877 
855 



Centre of swing 
Deflection due to 

rider or mass... 
Mass deflection s 

rider deflection 



70-20 



722 
800 
758 
780 



862-50 772-20 791-35 , 771-70 862-50 ; 772-50 
91-250 ... 19-425 | ... 90-400 

•217534 ... -213873 



•217506 



(63 a) 



725 

799 
759 

780 



m 
(64) 



803 
786 
796 
790 



772-90 I 792-30 
19-900 
217873 



* After 63 the riders were moved by mistake instead of the masses, therefore it was necessary 
to return to the initial position, and take the readings in (63 a). 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 101 



Table III (continued). 





i 


r 


i 


m 


i 


r 


i 


m 




(65) 


(66) 


(67) 


(68) 


(69) 


(70) 


(71) 


(72) 


Scale-readings... 


762 


913 


725 


802 


759 


909 


722 


802 




in 


838 


800 


785 


776 


834 


797 


784 




769 


879 


758 


794 


767 


875 


757 


794 




774 


857 


780 


789 


772 


854 


779 


789 


Centre of swing 


771-90 


864-60 


112'lb 


790-80 


770-15 


860-80 


771-05 


790-55 


Deflection due to 


















rider or mass... 




92-275 




19-350 




90-200 




19-450 


Mass deflection -:- 








, 










rider deflection 




•213170 


... 


-212084 


... 


-215078 




-214947 





* 


r 


i 


m 


i 


r 


i 


m 




(73) 


(74) 


(75) 


(76) 


{11) 


(78) 


(79) 


(80) 


Scale-readings... 


760 


911 


724 


803 


762 


911 


721 


800 




777 


835 


798 


785 


111 


835 


797 


784 




768 


877 


151 


795 


769 


877 


756 


793 




773 


854 


779 


790 


773 


854 


778 


788 


Centre of swing 


771-15 


862-05 


771-40 


791-50 


771-70 


862-05 


770-30 


789-75 


Deflection due to 


















rider or mass... 




90-775 




19-950 


... 


91-050 


... 


20150 


Mass deflection -^ 


















rider deflection 


... 


•217020 


... 


-219442 




-220209 


... 


-221064 





i 


r 


. 


m 


i 


r 


i 


m 




(81) 


(82) 


(83) 


(84) 


(85) 


(86) 


(87) 


(88) 


Scale-readings . . . 


759 


910 


722 


801 


759 


910 


723 


802 




774 


833 


796 


783 


115 


834 


797 


783 




766 


876 


757 


793 


767 


876 


757 


793 




771 


854 


778 


787 


771 


854 


778 


787 


Centre of swing 


768-90 


860-95 


770-50 


789-30 


769-60 


861-25 


770-90 


789-35 


Deflection due to 


















rider or mass... 




91-250 




19-250 


... 


91-000 




19-300 


Mass deflection -^ 


















rider deflection 


... 


•215990 


... 


-211248 


... 


•211813 




-212995 



102 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III 





i 


r 


i 


m 


i 


r 


i 


m 




(89) 


(90) 


(91) 


(92) 


(93) 


(94) 


(95) 


(96) 


Scale-readings . . . 


759 


908 


719 


800 


760 


910 


723 


800 




775 


831 


795 


783 


775 


835 


798 


785 




766 


874 


754 


792 


767 


876 


756 


793 




771 


852 


776 


788 


772 


854 


779 


789 


Centre of swing 


769-20 


859-00 


768-35 


789-05 


769-90 


861-60 


770-90 


790-30 


Deflection due to 


















, rider or mass... 


... 


90-225 


... 


19-925 


... 


91-200 




19-350 


Mass deflection -^ 


















rider deflection 




-217373 




•219650 




•215323 


... 


•212462 





i 


r 


i 


m 


i 


r 


* 


m 


i 


Scale-readings . . . 


(97) 


(98) 


(99) 


(100) 


(101) 


(102) 


(103) 


(104) 


(105) 


761 


910 


721 


798 


759 


909 


721 


800 


759 




111 


835 


796 


783 


775 


833 


796 


783 


115 




768 


876 


756 


791 


765 


874 


756 


793 


767 




772 


853 


111 


787 


770 


852 


777 


787 


771 

i 


Centre of swing 


771-00 


861-35 


769-80 


788-35 


768-60 859-65 


769-80 


789-35 


769-60 


Deflection due to 




















rider or mass... 




90-950 




19-150 




90-450 




19-650 




Mass deflections 




















rider deflection 




-211655 




-211136 


... 


-214483 









Feb. 4, 1890. Mean of 50 determinations of M/R = a 
Attracted masses in upper position 



•21422122. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 103 



Table III (continued). 
11. Attracted Masses in Lower Position. April 30, 1890, 7.45 p.m. to 
10.32p.m. Temperature: in Observing Room, 17°-16°-1 ; in Balance 
Room, 11°-1. Barometer, 748-6-749-2 millims. Weather clear; S.E. 
wind; sunny during day. Time between successive passages of centre 
not quite 20 seconds. 





i 


r 


i 


m 


i 


r 


i 


m 




(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


Scale-readings . . . 


1046 


1133 


951 


1127 


955 


1134 


952 


1123 




969 


1055 


1024 


1062 


1025 


1059 


1028 


1069 




1012 


1098 


984 


1099 


986 


1102 


985 


1099 




988 


1075 


1007 


1078 


1007 


1077 


1009 


1082 


Centre of swing 


996-60 


1082-85 


998-35 


1085-50 


999-80 


1086-25 


1000-40 


1088-25 


Deflection due to 


















rider or mass... 


... 


85-375 




86-425 




86-150 




87-350 


Mass deflections 


















rider deflection 


... 




... 


1-00772 


... 


1-00856 


... 


1-01437 





i 


r 


i 


m 


i 


r 


i 


m 




(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


Scale-readings . . . 


962 


1136 


955 


1129 


961 


1136 


956 


1133 




1023 


1060 


1029 


1067 


1027 


1064 


1031 


1069 




989 


1104 


987 


1102 


989 


1105 


989 


1103 




1009 


1079 


1012 


1083 


1012 


1081 


1013 


1085 


Centre of swing 


1001-40 


1088-00 


1002-45 


1089-55 


100315 


1090-00 


1004-15 


1091-25 


Deflection due to 


















rider or mass... 




86-075 




86-750 




86-350 




86-350 


Mass deflection-^ 


















rider deflection 




1-01133 


... 


100623 


... 


1-00232 




1-00101 





i 


r 


i 


m 


i 


r 


i 


m 




(17) 


(18) 


(19) 


(20) 


(21) 


(22) 


(23) 


(24) 
1134 


Scale-readings... 


967 


1141 


957 


1135 


965 


1143 


958 




1027 


1064 


1034 


1070 


1031 


1066 


1036 


1073 




994 


1108 


990 


1106 


994 


1110 


993 


1108 




1012 


1083 


1015 


1086 


1015 


1085 


1017 


1089 


Centre of swing 


1005-65 


1092-00 


1006-00 


1093-20 


1007-40 


1094-00 


1008-35 


1095-40 


Deflection due to 


















rider or mass... 


... 


86-175 


... 


86-500 




86-125 




86-825 


Mass deflection -H 


















rider deflection 


... 


1-00290 


... 


100406 




1-00624 


... 


1-01106 



104 ON^ A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table HI (continued). 





i 


r 


* 


m 


i 


r 


i 


m 




(25) 


(26) 


(27) 


(28) 


(29) 


(30) 


(31) 


(32) 


Scale-readings... 


976 


1143 


963 


1141 


966 


1145 


960 


1141 




1027 


1069 


1037 


1073 


1037 


1070 


1040 


1075 




998 


1110 


996 


1112 


996 


1112 


995 


1113 




1016 


1087 


1019 


1090 


1019 


1089 


1020 


1092 


Centre of swing 


1008-80 1095-35 


1010-65 1097-90 


1010-85 


1097-05 


1011-15 


1099-30 


Deflection due to 


















rider or mass... 


... 


85-625 




87-150 




86-050 




87-575 


Mass deflections 


















rider deflection 


... 


1-01591 




1-01529 




1-01264 




101713 





i 


r 


i 


m 


i 


r 


i 


1 

m 1 




(33) 


(34) 


(35) 


(36) 


(37) 


(38) 


(39) 


(40) 


Scale-readings... 


973 


1147 


962 


1137 


975 


1146 


965 


1138 




1034 


1071 


1041 


1079 


1034 


1075 


1042 


1080 




1000 


1114 


996 


1112 


1002 


1114 


999 


1113 




1020 


1090 


1022 


1094 


1020 


1091 


1021 


1094 


Centre of swing 


1012-30 


1098-55 


1012-50 


1100-20 


1013-40 


1099-80 


101400 


1101-00 


Deflection due to 


















rider or mass... 




86-150 




87-250 




86-100 


... 


86-650 


Mass deflections 


















rider deflection 




1-01465 


... 


1-01306 




1-00987 


... 


1-00858 





i 


r 


i 


m 


i 


r 


i 


m 


' 


(41) 


(42) 


(43) 


(44) 


(45) 


(46) 


(47) 


(48) 


Scale-readings... , 


977 


1150 


964 


1089 


976 


1153 


968 


1 
1148 1 


i 


1035 


1073 


1043 


1 1110 


1038 


1074 


1044 


1078 




1003 


1116 


1000 


1098 


1004 


1118 


1001 


1118 




1022 


1093 


1025 


1104 


1023 


1094 


1025 


1095 


Centre of swing 


1014-70 


1100-80 


1015-451 1102-20 


1016-15 


1102-35 


1016-45 


1103-40 


Deflection due to 


















rider or mass... 


... 


85-725 




86-400 




86-050 




86-475 


Mass deflections 


















rider deflection 


... 


1-00933 




j 100597 




1-00450 




1-00625 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 105 



Table III (continued). 





i 


r 


i 


m 


i 


r 


i 


m 




(49) 


(50) 


(51) 


(52) 


(53) 


(54) 


(55) 


(56) 


Scale-readings... 


978 


1153 


969 


1149 


976 


1153 


968 


1145 




1039 


1076 


1045 


1080 


1043 


1079 


1047 


1085 




1005 


1119 


1002 


1118 


1005 


1121 


1004 


1118 




1025 


1094 


1027 


1097 


1026 


1096 


1028 


1099 


Centre of swing 


1017-40 


1103-35 


1017-65 


1104-45 


1018-60 


1105-55 


1019-25 


1106-15 


Deflection due to 


















rider or mass... 


... 


85-825 


... 


86-325 




86-625 




86-875 


Mass deflection -^- 


















rider deflection 


... 


1-00670 




1-00116 


... 


-99971 


... 


1-00973 





i 


r 


i 


m 


i 


. 


i 


m 




(57) 


(58) 


(59) 


(60) 


(61) 


(62) 


(63) 


(64) 


Scale-readings . . . 


984 


1155 


971 


1151 


977 


1157 


972 


1152 




1039 


1078 


1048 


1083 


1046 


1081 


1051 


1087 




1008 


1120 


1004 


1122 


1007 


1123 


1007 


1123 




1026 


1097 


1029 


1100 


1030 


1100 


1031 


1102 


Centre of swing 


1019-30 


1105-10 


1020-00 


1107-85 


1021-25 


1108-10 


1022-60 


1109-95 


Deflection due to 


















rider or mass... 


... 


85-450 




87-225 


... 


86-175 




86-850 


Mass deflection -^ 


















rider deflection 




1-01872 


... 


1-01646 


... 


1-01011 




1-00798 





i 


r 


I 


m 


i 


r 


i 


m 




(65) 


(66) 


(67) 


(68) 


(69) 


(70) 


(71) 


(72) 


Scale-readings... 


983 


1159 


976 


1153 


983 


1161 


978 


1158 




1046 


1082 


1051 


1088 


1049 


1083 


1053 


1088 




1011 


1125 


1008 


1126 


1012 


1127 


1011 


1127 




1031 


1102 


1033 


1104 


1032 


1102 


1034 


1106 


Centre of swing 


1023-60 


1109-80 


1023-70 


111205 


1025-15 


1111-05 


1025-95 


111315 


Deflection due to 


















rider or mass... 


... 


86-150 




87-625 




85-500 


... 


86-700 


Mass deflections- 


















rider deflection 


... 


101262 


... 


1-02097 


... 


1-01944 


... 


1-01226 



106 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III (continued). 





i 


r 


i 


m 


* 


r 


* 


m 




(73) 


(74) 


(75) 


(76) 


(77) 


(78) 


(79) 


(80) 


Scale-readings... 


985 


1163 


980 


1156 


990 


1163 


982 


1161 




1051 


1086 


1056 


1092 


1051 


1087 


1057 


1093 




1014 


1129 


1013 


1129 


1017 


1131 


1015 


1132 




1033 


1104 


1036 


1108 


1036 


1107 


1039 


1109 


Centre of swing 


1026-95 


1113-40 


1028-25 


1115-50 


1029-20 


1115-20 


1030-15 


1117-65 


Deflection due to 


















rider or mass... 


... 


85-880 


... 


86-775 




85-525 


... 


87-100 


Mass deflection -^ 


















rider deflection 




1-01093 


... 


1-01299 


... 


1-01652 


... 


1-01782 





^ 


r 


i 


m 


i 


r 


i 


m 




(81) 


(82) 


(83) 


(84) 


(85) 


(86) 


(87) 


(88) 


Scale-readings... 


991 


1167 


984 


1158 


992 


1169 


984 


1161 




1054 


1090 


1059 


1098 


1056 


1092 


1063 


1100 




1018 


1133 


1018 


1132 


1021 


1136 


1018 


1135 




1038 


1108 


1041 


1112 


1041 


1111 


1042 


1115 


Centre of swing 


1030-95 


1117-40 


1032-60 


1119-55 


1033-55 


1120-00 


1034-00 


1122-20 


Deflection due to 


















rider or mass... 




85-625 




86-475 




86-225 




87-600 


Mass deflection -^ 


















rider deflection 




1-01358 




1-00640 




1-00942 




1-01890 





i 


r 


i 


m 


i 


r 


i 


m 




(89) 


(90) 


(91) 


(92) 


(93) 


(94) 


(95) 


(96) 


, Scale-readings... 


996 


1171 


987 


1165 


995 


1172 


989 


1169 




1058 


1094 


1064 


1100 


1061 


1097 


1066 


1099 




1022 


1137 


1022 


1137 


1024 


1137 


1023 


1140 




1043 


1114 


1045 


1117 


1045 


■ 1116 


1046 


1117 


Centre of swing 


1035-20 


1121-75 


1036-85 


1123-75 


1037-35 


1123-15 


1038-20 


1125-05 


Deflection due to 


















rider or mass... 




85-725 




86-650 




85-375 




86-575 


Mass deflection ^ 


















rider deflection 




101633 


... 


1-01286 


... 


101450 


... 


1-01065 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 107 



Table III (continued). 





i 


r 


i 


m 


i 


r 


i 


m 


i 




(97) 


(98) 


(99) 


(100) 


(101) 


(102) 


(103) 


(104) 


(105) 


Scale-readings... 


998 


1175 


992 


1174 


995 


1176 


995 


1169 


1001 




1062 


1097 


1066 


1102 


1064 


1098 


1067 


1105 


1066 




1026 


1141 


1025 


1141 


1027 


1143 


1026 


1143 


1029 




1045 


1116 


1047 


, 1119 


1048 


1118 


1049 


1121 


1049 


Centre of swing 


1038-75 


1125-05 


1039-45 


1127-15 


1040-15 


1126-70 


1040-75 


1128-90 


1042-20 


Deflection due to 




















rider or mass... 




85-950 




87-350 




86-250 




87-425 




Mass deflection -^ 




















rider deflection 


... 


1-01178 


... 


1-01452 




101319 









April 30. Mean of 50 determinations of M/R = A 
Attracted masses in lower position 



1-010905. 



May 4, 1890, 11.11 to 11.50 a.m. Temperature: in Observing Koom, 13°-5 
to 13°-8; in Balance Room, ll°-7. Barometer, 742-0 to 741-7 millims. 
Weather inclined to rain; a little cooler than previous day; wind S. 
to S.W. 





* 


r 


i 


m 


^• 


r 


* 


m 




(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


Scale-readings . . . 


875 


1045 


865 


1044 


865 


1045 


861 


1041 




936 


969 


939 


970 


938 


967 


940 


971 




900 


1013 


896 


1014 


897 


1013 


894 


1012 




920 


988 


921 


989 


920 


986 


921 


989 


Centre of swing 


91310 


996-95 


911-80 


997-75 


911-60 


996-00 


910-95 


997-10 


Deflection due to 


















rider or mass... 




84-500 




86-050 


... 


84-725 


... 


86-275 


Mass deflection -h 


















rider deflection 






... 


1-01699 


... 


1-01697 


... 


101950 



108 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III (continued). 





* 


r 


i 


m 


i 


r 


i 


m 




(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


Scale-readings... 


869 


1046 


862 


1036 


867 


1045 


860 


1040 




936 


966 


939 


972 


936 


964 


937 


968 




896 


1012 


894 


1009 


896 


1011 


893 


1009 




919 


985 


920 


988 

t 


918 


984 


919 


986 


Centre of swing 


910-700 


995-10 


910-45 


995-55 


910-40 


993-80 


909-20 


994-20 


Deflection due to 


















rider or mass... 


... 


84-525 


... 


85-125 


... 


84-000 


... 


85-450 


Mass deflections 


















rider deflection 




1-01390 


... 


1-01024 




1-01533 




1-01454 





i 


r 


i 


m 


i 


r 


. 


m 


i 




(17) 


(18) 


(19) 


(20) 


(21) 


(22) 


(23) 


(24) 


(25) 


Scale-readings .-. . 


863 


1043 


859 


1033 


863 


1040 


857 


1035 


862 




934 


964 


936 


971 


932 


963 


936 


966 


930 




894 


1009 


892 


1006 


892 


1007 


889 


1006 


891 




916 


983 


917 


985 


915 


982 


915 


983 


914 


Centre of swing 


908-30 


992-60 


908-00 


993-15 


906-60 


991-00 


906-10 


991-40 


905-25 


Deflection due to 




















rider or mass... 




84-450 




85-850 




84-650 




85-725 




Mass deflections 




















rider deflection 




1-01421 


... 


1-01538 




1-01344 









May 4, morning. Mean of 10 determinations of MjR = A 
Attracted masses in lower position 



1-015050. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 109 

Table III (continued). 

Same Day. 2.40 to 4.54 p.m. Temperature: in Observing Koom, 13°-9- 
14°-1 ; in Balance Room, ll°-7-ll°-75. Barometer, 740-3-739-7 millims. 





i 


r 


i 


m 


i 


r 


i 


m 




(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


Scale-readings... 


847 


1035 


853 


1031 


864 


1035 


853 


1026 




933 


957 


930 


961 


925 


960 


931 


965 




883 


1003 


886 


1002 


890 


1003 


885 


1001 




912 


977 


911 


979 


909 


977 


912 


980 


Centre of swing 


901-40 


986-20 


90200 


987-05 


902-55 


987-10 


90200 


987-65 


Deflection due to 


















rider or mass... 


... 


84-500 


... 


84-775 


... 


84-825 




85-300 


Mass deflection -^ 


















rider deflection 


... 


... 


... 


1-00133 




1-00251 




1-00783 



1 


^• 


r 


i 


m 


i 


r 


. 


m 




(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


Scale-readings... 


866 


1035 


853 


1023 


864 


1036 


854 


1024 




924 


959 


932 


968 


925 


960 


931 


968 




891 


1004 


886 


1000 


889 


1004 


886 


1000 1 




909 


977 


912 


982 


910 


978 


912 


982 


Centre of swing 


902-70 


987-20 


902-80 


988-35 


902-30 


987-75 


902-50 


988-45 


Deflection due to 


















rider or mass... 


... 


84-450 


... 


85-800 


... 


85-350 


... 


85-925 


Mass deflection -^ 


















rider deflection 




1-01303 




1-01060 




1-00601 


... 


1-00940 





i 


r 


i 


m 


i 


r 


i 


m 




(17) 


(18) 


(19) 


(20) 


(21) 


(22) 


(23) 


(24) 


Scale-readings . . . 


864 


1039 


855 


1025 


866 


1040 


855 


1011 




925 


958 


931 


969 


925 


958 


932 


977 




890 


1005 


887 


1001 


891 


1005 


888 


996 




909 


978 


913 


982 


911 


978 


913 


985 


Centre of swing 


902-55 


987-80 


903-25 


989-20 


903-50 


987-90 


904-00 


989-10 


Deflection due to 


















rider or mass... 


... 


84-900 


... 


85-825 


... 


84-150 


... 


85-225 


Mass deflection -f- 


















rider deflection 


... 


1-01148 


... 


1-01538 




1-01634 


... 


101232 



110 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III (continued). 





i 


r 


i 


m 


i 


r 


i 


m 




(25) 


(26) 


(27) 


(28) 


(29) 


(30) 


(31) 


(32) 


Scale-readings... 


864 
927 


1036 
961 


854 
933 


1024 
970 


865 
926 


1037 
962 


854 
934 


1031 
967 




890 


1004 


887 


1001 


892 


1005 


888 


1004 




912 


979 


914 


984 


912 


980 


915 


983 


Centre of swing 


903-75 


988-10 


904-00 


989-85 


904-35 


989-25 


904-90 


990-55 


Deflection due to 


















rider or mass... 




84-225 


... 


85-675 




84-625 




85-625 


Mass deflections 


















rider deflection 




101454 


... 


1-01481 


... 


1-01211 


... 


1-01182 





* 


r 




m 


i 


r 


^ 


m 


, 


(33) 


(34) 


(35) 


(36) 


(37) 


(38) 


(39) 


(40) 


Scale-readings... 


864 


1039 


855 


1024 


864 


1041 


856 


1025 




928 


961 


934 


972 


929 


961 


934 


971 




892 


1006 


888 


1002 


891 


1006 


888 


1002 




912 


980 


914 


985 


913 


980 


915 


984 


Centre of swing 


904-95 


989-50 


904-80 


99M0 


905-00 


989-65 


905-00 


990-65 


Deflection due to 


















rider or mass... 




84-625 




86-200 




84-650 


... 


85-500 


Mass deflections 


















rider deflection 


... 


1-01521 


... 


1-01846 


... 


1-01418 


... 


1-00796 





i 


r 


i 


m 


. 


r 


. 


m 




(41) 


(42) 


(43) 


(44) 


(45) 


(46) 


(47) 


(48) 


Scale-readings... 


866 


1038 


857 


1030 


865 


1043 


858 


1035 




927 


962 


934 


960 


930 


962 


934 


966 




893 


1007 


889 


1004 


893 


1008 


891 


1006 




913 


981 


915 


984 


915 


981 


915 


984 


Centre of swing 
Deflection due to 


905-30 


990-40 


905-50 


991-30 


906-60 


991-15 


906-55 


991-55 


rider or mass... 
Mass deflections 


... 


85-000 


... 


85-250 




84-575 


... 


84-95 


rider deflection 


... 


1-00441 




1-00546 


... 


1-00621 




1-00741 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 111 



Table III (continued). 





i 


r 


i 


m 


i 


r 


i 


m 




(49) 


(50) 


(51) 


(52) 


(53) 


(54) 


(55) 


(56) 


Scale-readings... 


869 


1041 


858 


1037 


867 


1041 


856 


1029 


i 


927 


961 


935 


965 


929 


963 


936 


972 




895 


1008 


891 


1007 


894 


1007 


890 


1004 




914 


982 


916 


984 


914 


982 


916 


986 


Centre of swing 


906-65 


990-90 


907-00 


991-80 


906-60 


991-05 


906-70 


992-50 


Deflection due to 


















rider or mass... 




84-075 


... 


85-000 




84-440 




85-750 


Mass deflections 


















rider deflection 




1-01070 


... 


1-00905 


... 


1-01155 


... 


1-0] 509 





i 


r 


i 


m 


^ 


r 


i 


m 




(57) 


(58) 


(59) 


(60) 


(61) 


(62) 


(63) 


(64) 


Scale-readings... 


871 


1041 


859 


1035 


869 


1044 


860 


1030 




928 


963 


936 


969 


931 


963 


937 


973 




895" 


1008 


890 


1008 


895 


1010 


892 


1005 




913 


982 


917 


985 


916 


983 


917 


986 


Centre of swing 


906-80 


991-50 


907-10 


993-50 


908-20 


992-85 


908-25 


993-30 


Deflection due to 


















rider or mass... 




84-550 




85-850 




84-625 


... 


84-775 


Mass deflections 


















rider deflection 




1-01478 




1-01493 




1-00812 


... 


1-00162 





i 


r 


i 


m 


i 


r 


i 


m 




(65) 


(66) 


(67) 


(68) 


(69) 


(70) 


(71) 


(72) 


Scale-readings . . . 


863 


1042 


863 


1039 


840 


1042 


861 


1037 




935 


965 


935 


969 


949 


965 


937 


971 




894 


1010 


894 


1008 


886 


1009 


893 


1009 




917 


984 


917 


985 


923 


985 


918 


987 


Centre of swing 


908-80 


993-45 


908-80 


993-75 


909-15 


993-25 


909-05 


995-05 


Deflection due to 


















rider or mass... 




84-650 


... 


84-775 




84-150 




85-750 


Mass deflection s 


















rider deflection 


... 


1-00148 




100444 


... 


1-01322 




101449 



112 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III (continued). 





i 


r 


i 


m 


i 


r 


i 


m 




(73) 


(74) 


(75) 


(76) 


.(77) 


(78) 


(79) 


(80) 


Scale-readings... 


865 


1045 


860 


1038 


868 


1045 


863 


1041 




935 


965 


938 


969 


934 


965 


937 


969 




895 


1011 


893 


1010 


895 


1012 


894 


1011 




918 


985 


919 


987 


918 


985 


919 


988 


Centre of swing 


909-55 


994-30 


909-25 


995-00 


909-50 


994-75 


909-80 


995-80 


Deflection due to 


















rider or mass... 




84-900 


... 


85-625 


... 


85-100 




85-625 


Mass deflection-^ 


















rider deflection 


... 


1-00928 


... 


100735 


... 


1-00617 




1-00765 





i 


r 


i 


m 


i 




(81) 


(82) 


(83) 


(84) 


(85) 


Scale-readings... 


864 


1044 


860 


1036 


867 




938 


967 


940 


974 


936 




895 


1012 


894 


1010 


896 




919 


986 


920 


989 


919 


Centre of swing 


910-55 


995-45 


910-65 


996-80 


910-60 


Deflection due to 












rider or mass... 




84-850 




86-175 




Mass deflections 












rider deflection 


... 


1-01238 









May -1, afternoon. Mean of 40 determinations of MIR = A] 

r 1 '01 on'?7ft 
Attracted masses in lower position J ^ * 

April 30 and May 4. Mean of 100 determinations of MIR = A] 

\4.. . 1 . . . r 1-0109685. 

Attracted masses m upper position j 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 113 

Table III (continued). 

III. Attracted Masses in Upper Position. May 25, 1890, 11.20 to 12.53 
noon. Temperature : in Observing Room, 15°-4-16° ; in Balance Room, 
13°-3. Barometer, 748-5-748-1 millims. Weather, E. wind, warm, very- 
bright. Time of swing not recorded. 





i 


r 


i 


m 


i 


r 


i 


m 




(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


Scale-readings... 


1071 


1175 


960 


1049 


1003 


1173 


956 


1049 




986 


1085 


1047 


1028 


1021 


1085 


1047 


1028 




1033 


1134 


998 


1041 


1010 


1134 


996 


1040 




1005 


1108 


1025 


1034 


1017 


1107 


1024 


1034 


Centre of swing 


1015-90 


1116-90 


1015-50 


1036-20 


1014-25 


1116-55 


1014-25 


1035-80 


Deflection due to 


















rider or mass... 




101-200 


... 


21-325 


... 


102-300 




21-500 


Mass deflection ^ 


















rider deflection 


... 


... 


... 


-209582 




-209311 




•210320 





i 


r 


i 


m 


i 


r 


i 


m 




(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


Scale-readings... 


1001 


1173 


958 


1049 


1002 


1173 


957 


1049 




1021 


1085 


1046 


1028 


1020 


1084 


1046 


1028 




1011 


1134 


996 


1038 


1010 


1134 


995 


1040 




1016 


1106 


1024 


1033 


1015 


1105 


1023 


1032 


Centre of swing 


1014-35 


1116-35 


1014-05 


1034-70 


1013-50 


1115-80 


1013-25 


1035-90 


Deflection due to 


















rider or mass... 




102-150 




20-925 




102-425 


... 


22-850 


Mass deflection ^ 


















rider deflection 


... 


-207660 


... 


-204571 


... 


-213688 


... 


-223297 





i 


r 


i 


m 


i 


r 


i 


m 




(17) 


(18) 


(19) 


(20) 


(21) 


(22) 


(23) 


(24) 


Scale-readings... 


1003 


1173 


956 


1048 


1001 


1172 


958 


1048 




1019 


1082 


1043 


1025 


1019 


1081 


1042 


1026 




1009 


1133 


994 


1038 


1008 


1131 


994 


1038 




1015 


1104 


1023 


1032 


1014 


1103 


1021 


1030 


Centre of swing 


1012-85 


1114-60 


1011-90 


1033-60 


1012-05 


1113-10 


1011-35 


1033-50 


Deflection due to 


















rider or mass... 




102-225 




21-625 


... 


101-400 


... 


22-375 


Mass deflections 


















rider deflection 


... 


-217535 




-212400 




-216962 


... 


•220172 



p. c. w. 



114 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III (continued). 





i 


r 


i 


m 


i 


r 


I 


m 




(25) 


(26) 


(27) 


(28) 


(29) 


(30) 


(31) 


(32) 


Scale-readings... 


1000 


1171 


953 


1047 


1000 


1171 


955 


1046 




1017 


1081 


1044 


1025 


1016 


1080 


1041 


1025 




1007 


1130 


992 


1037 


1007 


1130 


992 


1036 




1014 


1103 


1021 


1031 


1013 


1103 


1021 


1030 


Centre of swing 


1010-90 


1112-70 


1010-80 


1032-90 


1010-45 


1112-40 


1010-00 


1032-15 


Deflection due to 


















rider or mass... 




101-850 


... 


22-275 


... 


102-175 




22050 


Mass deflection -^ 


















rider deflection 




-219195 




•218356 


... 


•216907 


... 


•215885 





i 


r 


; 


m 


i 


r 


i 


m 




(33) 


(34) 


(35) 


(36) 


(37) 


(38) 


(39) 


(40) 


Scale-readings... 


999 


1168 


952 


1046 


999 


1170 


955 


1046 




1017 


1080 


1043 


1024 


1018 


1082 


1043 


1026 




1006 


1131 


992 


1037 


1007 


1130 


993 


1039 




1013 


1102 


1021 


1030 


1012 


1102 


1021 


1030 


Centre of swing 


1010-20 


1112-40 


1010-40 


1032-35 


1010-70 


1112-65 


1011-05 


1033-75 


Deflection due to 


















rider or mass... 


... 


102-100 




21-800 




101-775 




22-100 


Mass deflection -^ 


















j rider deflection 


... 


•214740 




•213857 


... 


•215672 




-216858 





i 


r 


i 


m 


i 


r 


i 


m 




(41) 


(42) 


(43) 


(44) 


(45) 


(46) 


(47) 


(48) 


Scale-readings... 


998 


1171 


955 


1046 


1000 


1173 


956 


1048 




1019 


1082 


1043 


1027 


1019 


1082 


1046 


1028 




1009 


1132 


994 


1038 


1009 


1131 


995 


1039 




1014 


1104 


1022 


1031 


1014 


1104 


1023 


1032 


Centre of swing 
Deflection due to 


1012-25 


1114-00 


1011-65 


1033-85 


1012-35 


1113-80 


1013-20 


1034-85 


rider or mass... 
Mass deflections 




102050 


... 


21-850 


... 


101-025 




21-900 


rider deflection 


... 


•215388 


... 


-215197 


... 


•216531 




-216350 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 115 

Table III (continued). 





i 


r 


i 


m 


i 


r 


i 




(49) 


(50) 


(51) 


(52) 


(53) 


(54) 


(55) 


Scale-readings . . . 


1001 


1172 


956 


1048 


1000 


1173 


956 




1019 


1083 


1046 


1027 


1020 


1082 


1044 




1009 


1132 


996 


1039 


1009 


1133 


995 




1015 


1105 


1023 


1032 


1016 


1105 


1023 


Centre of swing 


1012-70 


1114-60 


1013-65 


1034-60 


1013-15 


1114-70 


1012-65 


Deflection due to 
















rider or mass... 


... 


101-425 


... 


21-200 




101-800 




Mass deflection -^- 
















rider deflection 


... 


•212472 


... 


-208636 









May 25, morning. Mean of 25 determinations of MjR = a 
Attracted masses in upper position 



•21446168. 



Same Day. 3.15 to 4.50 p.m. Temperature: in Observing Room, 16°-0 to 
16°-25; in Balance Room, 13°-3 to 13°-35. Barometer, 747-7-747-4millims. 





i 


r 


i 


m 


i 


r 


i 


m 




(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


Scale-readings . . . 


1001 


1205 


990 


1081 


1034 


1207 


991 


1083 




1069 


1116 


1077 


1061 


1055 


1120 


1080 


1062 




1031 


1165 


1029 


1073 


1044 


1168 


1030 


1076 




1052 


1138 


1057 


1066 


1049 


1139 


1059 


1068 


Centre of swing 


1044-55 


1147-60 


1046-30 


1068-55 


1047-60 


1150-50 


1048-35 


1070-65 


Deflection due to 


















rider or mass... 


... 


102-175 


... 


21-600 




102-525 




21-675 


Mass deflection -^ 


















rider deflection 




... 




-211041 


... 


•211046 




-212162 





i 


r 


i 


m 


i 


r 


i 


m 




(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


Scale-readings . . . 


1037 


1209 


995 


1086 


1039 


1213 


994 


1088 




1056 


1119 


1082 


1066 


1058 


1121 


1086 


1069 




1046 


1169 


1030 


1078 


1048 


1172 


1034 


1078 




1052 


1141 


1059 


1071 


1054 


1145 


1064 


1073 


Centre of swing 


1049-60 


1151-10 


1049^00 


1073-50 


1051-60 


1154-10 


1052^90 


1074-90 


Deflection due to 


















rider or mass... 


... 


101-800 


... 


23-200 


... 


101-850 


... 


21-675 


Mass deflection ^ 


















rider deflection 


... 


•220408 


... 


-227842 


... 


•220299 


... 


•216738 



8—2 



116 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III {continued). 





i 


r 


* 


m 


i 


r 


i 


m 




(17) 


(18) 


(19) 


(20) 


(21) 


(22) 


(23) 


(24) 


Scale-readings... 


1045 


1212 


996 


1088 


1043 


1215 


1002 


1094 




1059 


1126 


1088 


1071 


1063 


1126 


1088 


1072 




1050 


1175 


1037 


1080 


1053 


1177 


1039 


1083 




1056 


1147 


1066 


1076 


1058 


1148 


1066 


1077 


Centre of swing 


1053-55 


1157-20 


1055-30 


1077-10 


1056-30 


1158-50 


1056-55 


1079-25 


Deflection due to 


















rider or mass... 


... 


102-775 




21-300 


... 


102075 


... 


22-350 


Mass deflection ^ 


















rider deflection 


... 


•209073 


... 


•207957 


... 


•213813 


... 


•219575 





i 


r 


^ 


m 


i 


r 


i 


m 




(25) 


(26) 


(27) 


(28) 


(29) 


(30) 


(31) 


(32) 


Scale-readings . . . 


1044 


1217 


999 


1095 


1048 


1221 


1002 


1096 




1064 


1127 


1093 


1073 


1067 


1131 


1094 


1076 




1053 


1178 


1041 


1086 


1056 


1180 


1042 


1088 




1061 


1150 


1069 


1079 


1061 


1152 


1071 


1081 


Centre of swing 


1057-25 


1159-80 


1059-35 


1081-35 


1059^70 


1162-50 


1060^70 


1083-55 


Deflection due to 


















rider or mass... 




101-500 




21-825 




102-300 




22-800 


Mass deflection ^ 


















rider deflection 


... 


•217611 




•214181 




•218112 




•223147 



(33) 


r 

(34) 


(35) 


m 

(36) 


(37) 


(38) 


(39) 


m 

(40) 


Scale-readings... 1049 
1067 
1057 
1064 


1221 
1131 
1182 
1154 


1004 
1095 
1045 
1072 


1097 
1079 
1089 
1082 


1053 
1069 
1059 
1066 


1224 
1134 
1183 
1156 


1007 
1096 
1047 
1075 


1101 
1079 
1091 
1085 


Centre of swing 1060-80 
Deflection due to 

rider or mass... 
Mass deflections 

rider deflection 


1163-75 
102-050 
-221583 


1062-60 


1085-20 

22-425 
-219907 


1062-95 


1165-70 
101-900 
-217983 


1064-65 


1086-90 
22-000 
-215898 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 117 







Table III 


{continued). 










* 


r 


i 


m 


i 


r 


i 


m 




(41) 


(42) 


(43) 


(44) 


(45) 


(46) 


(47) 


(48) 


Scale-readings . . . 


1054 


1226 


1009 


1101 


1054 


1225 


1008 


1102 




1072 


1135 


1096 


1081 


1073 


1135 


1099 


1081 




1061 


1185 


1048 


1093 


1064 


1187 


1048 


1094 




1068 


1157 


1076 


1086 


1068 


1159 


1080 


1089 


Centre of swing 


1065-15 


1167-15 


1065-35 


1088-55 


1066-85 


1168-40 


1067-00 


1089-70 


Deflection due to 


















rider or mass... 


... 


101-90 




22-450 




101-475 




21-625 


Mass deflection -^ 


















rider deflection 




•218106 


... 


-220774 




-217172 


... 


•213607 





i 


r 


i 


m 


i 


r 


i 




(49) 


(50) 


(51) 


(52) 


(53) 


(54) 


(55) 


Scale-readings . . . 


1058 


1228 


1014 


1104 


1059 


1229 


1019 




1075 


1138 


1102 


1086 


1076 


1141 


1103 




1066 


1189 


1053 


1097 


1068 


1192 


1055 




1072 


1161 


1080 


1091 


1074 


1163 


1081 


Centre of swing 


1069-15 


1170-80 


1070^45 


1093-00 


1070-95 


1173-40 


107215 


Deflection due to 
















rider or mass... 


... 


101-000 




22-300 


... 


101-850 




Mass deflection -f 
















rider deflection 




•217458 


... 


•219867 









May 25, afternoon. Mean of 25 determinations of MjR = a 



Attracted masses in upper position 
Mean of 50 determinations, morning and afternoon, '2157379 



21701412. 



Summary of Set I. 

February 4 ... a= -2142212 

May 25 

Mean value of 

April 30 
May 4 

Mean value of 



a = -2157379 
a = -2149791 

A = 1-010905 
A = 1-011032 
A = 1-0109685 



therefore 



a = -7959894. 



118 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III (continued). 

Set II. 

All Attracting and Attracted Masses inverted and changed over, each to the other 
side. The Suspending Rods also reversed and Riders interchanged. The 
initial position always the higher reading on the scale. 

I. Attracted Masses in Lower Position. July 28, 1890, 8.10 to 9.43 p.m. 
Temperature: in Observing Room, 17°-16°-9; in Balance Room, 15°4. 
Barometer, 747-6-748 millims. Weather fine and calm; wind W. 







r 


i 


m 


i 


r 


i 


m 




(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


Scale-readings... 


1099 


912 


1130 


917 


1126 


914 


1131 


922 




1051 


1007 


1034 


1005 


1036 


1008 


1035 


1005 




1081 


951 


1093 


952 


1091 


951 


1093 


954 




1063 


985 


1057 


985 


1058 


986 


1057 


984 


Centre of swing 


1069-65 


971-95 


1070-55 


97210 


1070-20 


972-60 


1070-95 


973-15 


Deflection due to 


















rider or mass... 


... 


98-150 




98-275 




97-975 




97-575 


Mass deflections 


















rider deflection 






... 


1-00217 




-99949 




-99541 





i 


r 


i 


m 


i 


r 


i 


m 




(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


Scale-readings... 


1128 


913 


1134 


924 


1130 


915 


1137 


919 




1035 


1010 


1035 


1006 


1038 


1013 


1034 


1012 




1092 


951 


1095 


956 


1094 


953 


1098 


955 




1058 


987 


1058 


987 


1061 


989 


1060 


989 


Centre of swing 


1070-50 


973-30 


1072-25 


975-00 


1073-05 


975-65 


1073-80 


976-40 


Deflection due to 


















rider or 7nass... 




98-075 




97-650 




91-775 




97-700 


Mass deflections 


















rider deflection 


... 


•99528 


... 


-99719 


... 


-99898 


... 


-99719 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 119 



Table III (continued). 





i 


r 


i 


m 


i 


r 


i 


m 




(17) 


(18) 


(19) 


(20) 


(21) 


(22) 


(23) 


(24) 


Scale-readings... 


1132 


917 


1140 


924 


1134 


916 


1136 


960 




1040 


1014 


1036 


1009 


1042 


1016 


1041 


991 




1095 


954 


1099 


958 


1098 


955 


1098 


972 




1062 


989 


1060 


990 


1064 


993 


1064 


983 


Centre of swing 


1074-40 


976-55 


1075-05 


977-40 


1076-90 


978-20 


1076-70 


979-05 


Deflection due to 


















rider or mass... 


... 


98-175 




98-575 


... 


98-600 


... 


98-100 


Mass deflections- 


















rider deflection 


... 


•99962 


... 


100191 


... 


-99734 


... 


-99506 





i 


r 


i 


m 


i 


r 


i 


m 




(25) 


(26) 


(27) 


(28) 


(29) 


(30) 


(31) 


(32) 


Scale-readings... 


1133 


916 


1142 


925 


1134 


918 


1143 


925 




1044 


1018 


1039 


1013 


1045 


1019 


1042 


1018 




1098 


956 


1103 


960 


1101 


957 


1103 


961 




1065 


994 


1064 


994 


1068 


997 


1066 


996 


Centre of swing 


1077-60 


979-55 


1078-65 


980-30 


1079-80 


981-00 


1080-00 


982-65 


Deflection due to 


















rider or mass... 


... 


98-575 


... 


98-925 




98-900 




97-075 


Mass deflection -^ 


















rider deflection 


... 


-99937 




1-00190 


... 


•99090 




-99031 





i 


r 


i 


m 


i 


r 


i 


m 




(33) 


(34) 


(35) 


(36) 


(37) 


(38) 


(39) 


(40) 


Scale-readings... 


1136 


924 


1145 


930 


1140 


918 


1143 


928 




1046 


1019 


1042 


1016 


1046 


1022 


1045 


1018 




1099 


961 


1104 


964 


1104 


959 


1104 


962 




1068 


996 


1067 


995 


1069 


997 


1068 


996 


Centre of swing 


1079-45 


982-95 


1080-75 


983-50 


1082-00 


982-80 


1081-80 


983-30 


Deflection due to 


















rider or mass... 




97-150 




97-875 


... 


99-100 


... 


98-875 


Mass deflections 


















rider deflection 


... 


1-00335 


... 


-99745 


... 


•99268 


... 


100051 



120 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III (continued). 





i 


r 


i 


m 


i 


r 


* 


m 




(41) 


(42) 


(43) 


(44) 


(45) 


(46) 


(47) 


(48) 


Scale-readings... 


1138 


926 


1146 


928 


1142 


927 


1144 


937 




1047 


1020 


1045 


1022 


1047 


1021 


1047 


1015 




1104 


963 


1107 


964 


1106 


963 


1107 


967 




1071 


997 


1069 


998 


1071 


999 


1070 


996 


Centre of swing 


1082-55 


984-45 


1083-45 


985-75 


1083-70 


985-10 


1084-05 


985-10 


Deflection due to 


















rider or mass... 


... 


98-550 


... 


97-825 


... 


98-775 




98-900 


Mass deflections 


















rider deflection 


... 


•99797 




-99151 




-99582 




1-00139 





i 


r 


i 


m 


i 


r 


i 




(49) 


(50) 


(51) 


(52) 


(53) 


(54) 


(55) 


Scale-readings... 


1140 


923 


1148 


932 


1141 


924 


1144 




1049 


1024 


1045 


1021 


1050 


1024 


1048 




1105 


962 


1108 


966 


1106 


963 


1107 




1072 


999 


1071 


998 


1072 


1001 


1072 


Centre of swing 


1083-95 


985-40 


1084-35 


986-60 


1084-80 


986-25 


1084-75 


Deflection due to 
















rider or mass... 


... 


98-750 




97-975 




98-525 




Mass deflection s 
















rider deflection 


... 


-99684 


... 


-99328 









July 28, 1890. Mean of 25 determinations of MjR 
Attracted masses in lower position 



•9973168. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 121 

Table III (continued). 

September 17, 1890, 8.0 to 9.31 p.m. Temperature: in Observing Room, 
17°-17°-5; in Balance Room, 15°-8. Barometer, 746-2-746-4 millims. 
Weather warm, cloudy. 





i 


r 


i 


m 


i 


r 


i 


m 




(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


Scale-readings . . . 


1085 


908 


1118 


921 


1109 


905 


1126 


921 




1051 


1004 


1029 


995 


1036 


1006 


1026 


996 




1073 


945 


1085 


949 


1081 


944 


1087 


951 




1058 


981 


1050 


978 


1053 


981 


1050 


978 


Centre of swing 


1064-20 


967-35 


1063-40 


966-70 


1063-75 


967-35 


1063-95 


967-90 


Deflection due to 


















rider or mass... 




96-450 




96-875 




96-500 




96-450 


Mass deflection -^ 


















rider deflection 


... 


... 


... 


1-00415 


... 


1-00168 




•99613 





i 


r 


i 


m 


i 


r 


i 


m 




(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


Scale-readings . . . 


1113 


907 


1126 


929 


1110 


910 


1131 


934 




1034 


1006 


1027 


993 


1038 


1007 


1027 


993 




1084 


944 


1088 


953 


1083 


947 


1092 


956 




1053 


982 


1052 


978 


1056 


984 


1052 


979 


Centre of swing 


1064-75 


967-75 


1065-05 


968-40 


1065-90 


969-90 


1067-10 


970-20 


Deflection due to 


















rider or mass... 




97-150 




97-075 




96-600 




96-850 


Mass deflection -;- 


















rider deflection 


... 


-99601 


... 


1-00206 


... 


100375 


... 


1-00026 





i 


r 


i 


m 


i 


r 


i 


m 




(17) 


(18) 


(19) 


(20) 


(21) 


(22) 


(23) 


(24) 


Scale-readings . . . 


1104 


910 


1129 


924 


1116 


909 


1121 


927 




1044 


1008 


1030 


1001 


1040 


1009 


1036 


1000 




1081 


947 


1091 


953 


1086 


947 


1088 


956 




1059 


985 


1054 


983 


1057 


986 


1056 


984 


Centre of swing 


1067-00 


970-44 


1067-90 


971-45 


1066-70 


970-85 


1067-05 


972-80 


Deflection due to 


















rider or mass... 




97-050 




958-50 




96-025 


... 


95-550 


Mass deflections 


















rider deflection 


... 


•99279 




-99288 




-99662 


... 


•99105 



122 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III (continued). 





i 


r 


* 


m 


i 


r 


* 


m 




(25) 


(26) 


(27) 


(28) 


(29) 


(30) 


(31) 


(32) 


Scale-readings... 


1112 


914 


1131 


929 


1114 


916 


1132 


934 




1043 


1009 


1033 


1002 


1044 


1011 


1035 


1002 




1086 


951 


1093 


957 


1087 


952 


1094 


960 




1060 


987 


1056 


984 


1061 


988 


1058 


986 


Centre of swing 


1069-65 


973-10 


1070-15 


974-00 


1070-70 


974-50 


1071-70 


976-00 


Deflection due to 


















rider or mass... 




96-800 


... 


96-425 




96-700 




96-550 


Mass deflection -^ 


















rider deflection 




-99161 


... 


•99664 




-99780 




-99690 



[ - 


i 


r 


i 


m 


^• 


r 


. 


m 




(33) 


(34) 


(35) 


(36) 


(37) 


(38) 


(39) 


(40) 


Scale-readings... 


1097 


916 


1135 


942 


1119 


919 


1136 


935 




1058 


1015 


1037 


999 


1048 


1017 


1039 


1006 




1083 


954 


1098 


965 


1093 


957 


1099 


963 




1067 


991 


1061 


986 


1066 


993 


1063 


989 


Centre of swing 


1073-40 


977-10 


1074-80 


977-85 


1075-80 


979-70 


1076-25 


979-10 


Deflection due to 


















rider or mass... 




97-000 




97-450 




96-325 




97-025 


Mass deflections 


















rider deflection 




1-00000 


... 


1-00815 




1-00947 




1-00362 





(41) 


(42) 


(43) 


m 

(44) 


i 
(45) 


(46) 


(47) 


m 
(48) 


Scale-readings . . . 


1122 
1048 
1093 
1065 


917 

1018 

956 

994 


1141 
1038 
1101 
1062 


929 

1011 

962 

993 


1118 
1053 
1093 

1068 


921 
1019 

958 
996 


1141 
1041 
1103 
1065 


941 

1009 

966 

993 


Centre of swing 
Deflection due to 

rider or mass... 
M;iss deflections 

rider deflection 


1076-00 


979-45 
97-025 
-99948 


1076-95 


980-65 
96-925 
•99704 


1078-20 


981-40 
97-400 
-99538 


1079-40 


982-65 
96-975 
•99628 



THE GBAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 123 



Table III {continued). 





i 


r 


i 


m 


i 


r 


* 




(49) 


(50) 


(51) 


(52) 


(53) 


(54) 


(55) 


Scale-readings . . . 


1134 


920 


1143 


932 


1134 


925 


1140 




1047 


1022 


1041 


1016 


1048 


1021 


1045 




1100 


958 


1104 


964 


1102 


962 


1104 




1067 


998 


1065 


996 


1068 


997 


1068 


Centre of swing 


1079-85 


982-65 


108000 


983-85 


1081-15 


984-20 


1081-55 


Deflection due to 
















rider or mass... 


... 


97-275 




96-725 




97150 




Mass deflection -h 
















rider deflection 


... 


•99563 


... 


-99499 









September 17, 1890. Mean of 25 determinations of M/R = A] 
Attracted masses in lower position J 

July 28 and September 17. Mean of 50 determinations of M/R = A, -9978658. 



II. Attracted Masses in Upper Position. September 23, 1890, 7.52 to 
9.30 p.m. Temperature: in Observing Eoom, 15°-3-15°-4; in Balance 
Room, 15°-05. Barometer, 749-8-750-2 millims. Weather, light S.W. 
wind and clear after heavy showers. Scale-readings between about 1100 
and 1300; 1000 omitted. 





i 


r 


i 


m 


^• 


r 


^• 


?/i 




(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


Scale-readings . . . 


307 


113 


329 


235 


281 


112 


326 


233 




248 


210 


235 


257 


261 


208 


232 


256 




285 


151 


293 


243 


273 


149 


290 


241 




263 


186 


257 


251 


265 


185 


256 


249 


Centre of swing 


271-00 


173-10 


270-95 


248-25 


268-35 


171-45 


268-25 


246-60 


Deflection due to 


















rider or mass... 




97-875 




21-400 




96-850 




21-175 


Mass deflection h- 


















rider deflection 


... 


... 


... 


-219797 




-219799 


... 


•218581 



124 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III {continued). 





i 


r 


i 


m 


i 


r 


i 


m 




(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


Scale-readings . . . 


279 


110 


331 


232 


277 


110 


324 


230 




260 


207 


228 


255 


258 


205 


229 


254 




272 


148 


290 


239 


271 


147 


288 


239 




264 


183 


253 


248 


262 


182 


252 


247 


Centre of swing 


267-30 


170-15 


266-80 


245-15 


265-80 


168-90 


265-55 


244-50 


Deflection due to 


















rider or mass... 


... 


96-900 




21-150 


... 


96-775 


... 


20-225 


Mass deflection -^ 


















rider deflection 




•218395 


... 


-218407 


... 


-213769 




-209179 





i 


r 


^ 


m 


^• 


r 


^■ 


m 




(17) 


(18) 


(19) 


(20) 


(21) 


(22) 


(23) 


(24) 


Scale-readings... 


275 


108 


323 


228 


276 


107 


328 


226 




256 


204 


228 


253 


255 


203 


224 


252 




269 


145 


286 


237 


268 


145 


287 


236 




261 


181 


251 


247 


260 


179 


249 


245 


Centre of swing 


263-90 


167-40 


264-10 


243-15 


263-00 


166-65 


263-^5 


241-85 


Deflection due to 


















rider or mass... 




96-600 




20-400 




96-475 




20-225 


Mass deflections 


















rider deflection 


... 


•210274 




-211317 




•210547 


... 


-209652 





i 


r 


i 


m 


i 


r 


i 


m 




(25) 


(26) 


(27) 


(28) 


(29) 


(30) 


(31) 


(32) 


Scale-readings... 


274 


106 


320 


232 


271 


100 


317 


222 




254 


199 


224 


245 


252 


197 


221 


247 




265 


143 


283 


237 


262 


138 


281 


232 




258 


176 


246 


241 


255 


175 


243 


241 


Centre of swing 


260-90 


163-90 


260-40 


239-85 


258-25 


160-50 


257-90 


237-60 


Deflection due to 


















rider or mass... 




96-750 




19-475 




97-575 




19-100 


Mass deflection -^ 


















rider deflection 




-205323 




-200437 




-197668 




-196730 



THE GBAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 125 



Table III (continued). 





i 


r 


i 


m 


i 


r 


i 


m 




(33) 


(34) 


(35) 


(36) 


(37) 


(38) 


(39) 


(40) 


Scale-readings . . . 


266 


98 


317 


221 


265 


97 


311 


218 




249 


197 


219 


244 


246 


193 


218 


242 




259 


136 


279 


228 


257 


135 


275 


226 




254 


173 


241 


237 


251 


171 


240 


235 


Centre of swing 


255-50 


159-10 


255-90 


234-20 


253-05 


157-00 


253-30 


232-10 


Deflection due to 


















rider or mass... 


... 


96-600 


... 


20-275 




96-175 




20-150 


Mass deflection -^ 


















rider deflection 


... 


-203804 




-210351 


... 


-210164 


... 


-209271 





i 


r 


i 


m 


i 


r 


i 


m 




(41) 


(42) 


(43) 


(44) 


(45) 


(46) 


(47) 


(48) 


Scale-readings... 


264 


95 


310 


215 


261 


91 


311 


212 




243 


191 


216 


239 


241 


188 


210 


236 




256 


133 


273 


223 


253 


128 


272 


220 




249 


168 


238 


232 


246 


166 


234 


229 


Centre of swing 


251-20 


154-90 


251-40 


229-10 


248-55 


151-10 


248-40 


226-10 


Deflection due to 


















rider or mass... 




96-400 


... 


20-875 


... 


97-375 




20-675 


Mass deflection -^ 


















rider deflection 


... 


-212785 


... 


-215456 


... 


-213350 


... 


-213585 





i 


r 


i 


m 


i 


r 


^• 


m 




(49) 


(50) 


(51) 


(52) 


(53) 


(54) 


(55) 


(56) 


Scale-readings . . . 


257 


90 


306 


211 


256 


88 


303 


208 




237 


186 


208 


234 


236 


184 


209 


232 




250 


127 


269 


218 


248 


125 


265 


216 




243 


162 


232 


227 


242 


160 


231 


225 


Centre of swing 


245-15 


149-25 


245-80 


224-10 


244-25 


147-20 


243-95 


222-10 


Deflection due to 


















rider or mass... 




96-225 




20-925 




96-900 




20-350 


Mass deflections 


















rider deflection 




-216160 




-216699 




•212977 


... 


•209956 



126 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



Table III (continued). 





(57) 


(58) 


(59) 


m 
(60) 


(61) 


Scale-readings... 


253 
233 

246 
238 


86 
180 
122 
157 


301 
205 
263 

227 


203 

228 
213 

222 


293 
204 

280* 
245 


Centre of swing 
Deflection due to 
rider or mass... 


240-95 


144-00 
96-950 


240-95 







September 23, 1890. Mean of 27 determinations of M/R = a) .2112753 
Attracted masses in upper position J 



September 25, 1890, 7.10-8.43 p.m. Temperature: in Observing Koom, 
15°-15°-2; in Balance Room, 15°. Barometer, 760-8 millims., steady. 
Weather cloudy, with westerly airs. Time of swing 21 seconds. 1000 
omitted in scale-readings. 





I 


r 


i 


m 


i 


r 


*■ 


m 




(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


Scale-readings... 


246 


84 


301 


206 


248 


82 


297 


202 




238 


179 


205 


229 


233 


178 


204 


228 




243 


121 


263 


215 


243 


119 


260 


212 




239 


156 


228 


224 


236 


156 


226 


222 


Centre of swing 


240-90 


142-90 


240-95 


220-40 


238-95 


141-60 


238-95 


218-10 


Deflection due to 


















rider or mass... 


... 


98-025 




19-550 




97-350 




20-850 


Mass deflections 


















rider deflection 








•200128 




•207499 




•213163 



* This is a considerable rise, showing either a sudden disturbance or a displacement of the 
apparatus; possibly the telescope was touched. The rise was maintained and therefore the 
observations were discontinued. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 127 



Table III (continued). 





i 


r 


i 


m 


* 


r 


i 


m 




(9) 


(10) 


(11) 


(12) 


(13) 


(14) 


(15) 


(16) 


Scale-readings... 


248 


83 


300 


204 


252 


84 


303 


207 




233 


176 


203 


228 


233 


182 


206 


232 




243 


119 


261 


214 


245 


122 


265 


217 




236 


155 


226 


224 


239 


158 


228 


226 


Centre of swing 


238-95 


140-80 


239-20 


•219-50 


240-70 


144-60 


242-45 


222-60 


Deflection due to 


















rider or mass... 


... 


98-275 




20-450 


... 


96-975 




20-825 


Mass deflection -^ 


















rider deflection 


... 


-210125 


... 


-209475 


... 


-212813 


... 


•214718 





i 


r 


. 


m 


*■ 


r 


i 


m 




(17) 


(18) 


(19) 


(20) 


(21) 


(22) 


(23) 


(24) 


Scale-readings... 


255 


87 


307 


210 


257 


90 


271 


215 




237 


184 


207 


234 


238 


184 


233 


233 




249 


125 


267 


219 


251 


127 


255 


222 




242 


162 


232 


228 


244 


163 


241 


229 


Centre of swing 


244-40 


147-55 


244-70 


224-65 


24605 


148-75 


246-70 


226-15 


Deflection due to 


















rider or mass... 




97-000 




20-725 




97-625 


... 


20-725 


Mass deflection-:- 


















rider deflection 


... 


-214175 




-212974 


... 


-212292 


... 


-212564 





i 


r 


• 


m 


i 


r 


^• 


m 




(25) 


(26) 


(27) 


(28) 


(29) 


(30) 


(31) 


(32) 


Scale-readings... 


258 


90 


307 


213 


262 


93 


307 


215 




241 


186 


213 


237 


242 


189 


215 


239 




251 


129 


270 


223 


253 


131 


272 


225 




244 


164 


236 


232 


246 


167 


237 


233 


Centre of swing 


247-05 


150-45 


248-60 


228-30 


248-85 


153-00 


250-25 


23005 


Deflection due to 


















rider or mass... 




97-375 




20-425 


... 


96-550 


... 


19-750 


Mass deflection-^ 


















rider deflection 




-211297 




•210649 


... 


•208053 




-204425 



128 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 

Table III (continued). 





i 


r 


i 


m 


i 


r 


* 


m 




(33) 


(34) 


(35) 


(36) 


(37) 


(38) 


(39) 


(40) 


Scale-readings... 


261 
243 


93 

189 


312 
214 


215 
241 


263 
245 


96 
192 


312 

217 


217 
243 




253 


132 


273 


225 


257 


135 


275 


227 




247 


167 


237 


235 


250 


168 


240 


237 


Centre of swing 


249-35 


153-40 


250-80 


231-10 


252-40 


156-05 


253-05 


23305 


Deflection due to 


















rider or mass... 


... 


96-675 




20-500 




96-675 


... 


20-525 


Mass deflection -f 


















rider deflection 


... 


•208172 




•212051 


... 


•212180 


... 


•212254 





i 


r 


. 


m 


i 


r 


i 


m 




(41) 


(42) 


(43) 


(44) 


(45) 


(46) 


(47) 


(48) 


Scale-readings... 


264 


98 


314 


220 


267 


100 


321 


224 




247 


194 


219 


243 


249 


197 


223 


246 




259 


136 


277 


231 


260 


138 


281 


233 




251 


171 


242 


238 


255 


174 


246 


242 


Centre of swing 


254-10 


157-85 


25505 


235-20 


256-25 


160-35 


259-30 


238-05 


Deflection due to 


















rider or mass... 


... 


96-725 




20-450 


... 


97-425 


... 


21-250 


Mass deflection -^ 


















rider deflection 




•211812 


... 


-210662 


... 


•214011 




•218650 





i 


r 


i 


m 


i 


r 


i 




(49) 


(50) 


(51) 


(52) 


(53) 


(54) 


(55) 


Scale-readings... 


271 


102 


321 


224 


271 


102 


314 




252 


200 


221 


247 


251 


198 


226 




264 


139 


282 


233 


264 


140 


280 


1 


256 


176 


245 


242 


256 


174 


247 


' Centre of swing 


259-30 


162-20 


259-00 


238-40 


258^95 


161-60 


259-50 


Deflection due to 
















rider or mass... 




96-950 




20-575 


... 


97-625 




Mass deflections 
















rider deflection 


... 


-215704 


... 


•211487 









September 25, 1890. Mean of 25 determinations of MIR = a\ .01125332 
Attracted masses in upper position J 

' ^September 25^ } ^^^^ ^^ ^^ determinations of MjR = a, -2112647. 



THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 129 



Table III (continued). 
Summa/ry of Set II. 



July 28 
September 17 

Mean value of 
September 23 
25 

Mean value of 



A = -9973168 
A = -9984148 
A = -9978658 
a - -2112753 
a = -2112533 
a =-2112647 



Therefore A-a= -7866011. 

Mean value, giving equal weights to Sets I and II, 

A-a = -791295. 



[The experimental data in these tables have been verified from the 
original MS. Certain slips in calculations from the data occur in Table III, 
but it was not considered advisable to correct these. Ed.1 



p. c. w. 



310 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 



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132 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH AND 




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THE GRAVITATION CONSTANT BY MEANS OF THE COMMON BALANCE 133 









































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136 ON A DETERMINATION OF THE MEAN DENSITY OF THE EARTH 




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Diagram VII. Distribution of results about the means assumed cor- 
rect for each set of distances. The numbers along the base are in 
percentages of the distance a A. The numbers on the side-line 
show the numbers in each interval 0-25 per cent, from the mean. 
The distance aA should be 40 inches to show both on the same 
diagram. 



J; 60 


TT 


' 1 


rjT 


1 


_,,... 








- 


— r 


T 


- 


j 1 






T7"^ 


r 




T- 


- 




— 


TT 




.^ 


. |. 


I" 


















1 












Ml 


! 








1 






t 1 


j 


1 


"T"*"^ 
















1 




























J 


1 


! 


1 














1 




1 






















" 






- 








1 


'• 
























1 


















? 


■* 1 ' 






1 


' 










1 




1 




























t 1 ♦ 1 
























i 










i 






1 












•5 






















1 


( 




1 




















1 


1 




5 


! ' ' ' 


1 


i 








; 




























-T" 






: ' : i *■! 


1 






i 


' 


1 






1 




■«• ' 










! 






: , : ' 1 ■ 1 , i 1 






t 


t 




1 




! 


, 4 1 1 




I 






■ 








fi 


1 






• 






• V 


■Ml 




t 






1 




5 ^ 


I ' ' ' 


! i ^ 














\^\ 




t 








1 






t ^ 


\ ', \ \ 




♦ xl ! 










1 


a; , 1 


Mm 












1 


1 


-i ? 






1 1 ' i 


; Hx 


X 








Xl^: ; r 


1 ! 










; 


1 




^ ^ 


-r-j-^ 








♦x 


, 


^ •* 


•• 












1 




1 


i 


1 


4 .- 


































^^ 


^ 






! 1 




! ! 


■ !_;_[_ 


! 




I'M 


1 i ' 










' 






c^i"^ 








i 


' 1 ' 1 ( 1 w 






MM 












1 








^ M 1 






_L ' J N 1 








! ' 
















to io 50 - 

_>. Norrx2yers of 3ux:c««sii^e ^^e^hjings vUh, A ojrA D 






- 


; ; . . . -J 


Mil 




' 1 ■ i 1 i 


j 1 i M 1 t : 


■^-r 


-rr 






; : , , , 








1 I M ^ 


i ' 1 


! 


i ' ! 




1 1 


M ' ' 




1 : 1 : i !-m 




^_ 


_ 


! 1 i h 




1 


- M- 


1 1 


--U- 


i 1 




J'l j!' 


Ti- 


1 1 1 . j 


— 


- 


i 1 1 1 j ; 


t r 


T,, 


^TXSV^ 


^-t 


! T 


Idjv 














X: 


1 




6 T^A 


\k 




^ J M ! 




! 1 i 1 1 


■ 










1 ' 


' 1 


r 1 









- i ,1 


. 


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• ' ' 1 ' ' 


-i — U- 




1 


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M i i 




-J"p 


^-^^ 






- 






-- 


1 """ 




^ 





=91 


t 
5 


X- 


a 


53 


— ■ 


i 


: 


- 


- 




I 


i 




-H 


\ \ \ \ 

#M 


5 

1 


^. 




\ 


W- 


fe 



Diagram VIII. Comparison of Riders A and i). 
Successive values of i) - ^ in terms of 1 scale-division = -0035 mgm. 





1 
















1 


J 




T 






■ 


1 1 '' 










' 


















\ 














]" 




TM^" 


















\ 


















' 1 














~1 










r ! 




















1 








1 














i 


















» 1 




















1 










1 










I 




■ 














































1 


' 






























: 












































... 




MM 


; j 


























1 




1 


' 




























i i 








i 




MM: 








1 ! 












i 




1 ' 




Mm 










1 






i 








Mi M 




i 




11 










\ ' 














1 ■ , 




1 ' 1 


: ; 


















' i 


4 


1 


, 1 ' 




I i M '■ i 














1 i i 




1 ' 




j 1 
















1 






1 








! M M ! ' ' 








' 






\ ' ' 






1 1 










i 










1 i i' 




< 
























: : 1 1 


! _i 








i ^ : : 














' 1 




Ij M 


" ' 1 1 « 1 
























1 




T ; 






I j 1 j 1 














Mm 




' t 








' ' ! ' 




-^ 










M ' i 










i 1 ' ' 1 






1 








1 


' 


! 






M 










1 






' ^" 




' 1 ! 


1 


t M M 






M 




1 














1 




L ^_ 


-X 


1 M ■ M 


' 






























~'^ 










1 ' 1 ■ 1 






1 












1 










1 






1 


H : 










! 






i 






1 




1 




■ 




: 












i 1 






1 1 




1 i 






1 








: I 












' 










1 1 


J 




I 




"" * 






„ 
























1 








700 
























1 : 


1 










































! 




■ ■ MM , 




















1 




















































"^ 


MM 
































1 










































1 








! 












1 














1 


X 


! 






1 


t 


























~[ 


4I 


1 






' M 






















' 










1 










Tin 


1 
























1 












ISO 


























1 


^ 


i 1 


' 


































, 


' ' 


U ' 1 
































1 












































! 1 




















~ 




















"~ 






1 




























































! 
















































I 




































~" 






















































~i 






































































































































































" "^ 








































































































































































J 


















































































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130- 





l_ 


i_ 


_ 


_ 


- 


-. 




-- 





































Diagram IX. Relation between temperature 
and scale-ieading. May 9 — 22. 



4. 

AN EXPERIMENT IN SEARCH OF A DIRECTIVE ACTION 
OF ONE QUARTZ CRYSTAL ON ANOTHER. 

By J. H. PoYNTiNG and P. L. Gray, B.Sc. 

[Phil Trans. A, 192, 1899, pp. 245-256.] 
[Received September 27. Read November 17, 1898.] 

Since so many of the physical properties of crystals differ along the 
different axes, our ignorance of the nature and origin of gravitation allows 
us to imagine that the gravitative field of crystals may also differ along those 
axes. Dr. A. S. Mackenzie {Phys. Rev., vol. 2, 1895, p. 321) has described 
an experiment in which he failed to find any such difierence. Using Boys's 
form of the Cavendish apparatus, he showed that the attraction of calc-spar 
crystals on lead and on other calc-spar crystals was independent of the 
orientation of the crystalline axes, within the limits of experimental error — 
about one-half per cent, of the total attraction. He further showed that 
the inverse-square law holds in the neighbourhood of a crystal, the attractions 
at distances 3-714 centims., 5-565 centims., and 7-421 centims. agreeing with. 
the law" to one-fifth per cent. 

One of the authors of this paper had already pointed out (The Mean 
Density of the Earth, 1894, p. 7) that if the attraction between two crystal 
spheres were different for a given distance, according as their like axes were 
parallel or crossed, such difference should show itself by a directive action 
on one sphere in the field of the other. This directive action is suggested 
by the growth of a crystal from solution, where the successive parts are laid 
down in parallel arrangement — a fact which we might perhaps interpret on 
the molecular hypothesis as showing that, within molecular range at least, 
there is directive action. 

The experiment now to be described is a modification of one indicated in 
the work above referred to, carried out for two quartz spheres, and we may 
say at once that we have certainly not succeeded in proving the existence of 
a directive action of the kind sought for. 

To bring out the principle of the method, let us suppose that the law of 
the attraction between two spheres w^ith their hke axes parallel, as in Fig. 1 (a), 



138 



AN EXPERIMENT IN SEARCH OF A DIRECTIVE ACTION 




is GMM'jr^, where M, M' are the masses, r the distance between the centres, 
and G a constant for this arrangement. Let us further suppose that the law 
of attraction when the axes are crossed, as in Fig. 1 (6), is G'MM'/r^, where 
G' is a constant for this arrangement, and different from G. 

Let us start with the spheres r apart, as 
in Fig. 1 {a). The work done in removing 
M' to an infinite distance, in a Hne perpen- 
dicular to the parallel axes, is GMM'jr. 
Now turn M' through 90° to cross the axes, 
and bring it back to the original position, but 
with the axes crossed. 

The force will do work G'MM'jr. Then 
turn M' through 90° into its original orienta- ^^S- ^■ 

tion. Assuming that the forces are conserva- 
tive, the total work vanishes, so that there must be a couple acting during 
the last rotation, which does work equal to the difference between the works 
done on withdrawal and approach. 

If we take the average value of the couple as L, then 

Our suppositions as to the law of force are doubtless arbitrary, but they 
serve to show the probability of the existence of a directive couple accom- 
panying any axial difference in the gravitative field. 

In the absence of any distinction between the ends of an axis we may 
assume that the couple is 'quadrantal,' that is, that it goes through its 
range of values with the rotation of the sphere through 180° and vanishes 
in every quadrant, and we shall suppose that it is zero when the crystals are 
in the positions shown in Fig. 1 (a), and Fig. 1 (h). 

Taking the couple as a sine-function of amplitude F, we have 



F sin 26 cW 



whence 



F=^{G-G' 



MM' 



But it is conceivable that the two ends of an axis are different, having 
polarity of the magnetic type. The couple would then be 'semicircular,' 
going through its range of values once and vanishing twice in the revolution. 
We shall suppose that the couple is zero when the axes are parallel. We 
should now have G and G' constants for the axes parallel, the one when Hke 



OF ONE QUARTZ CRYSTAL ON ANOTHER 139 

ends are in the same direction, the other when they are in opposite directions, 
and we have 

MM' 
r 
But if F is the amphtude of the couple 



ttL = [" J sin QdB = 2F, 

Jo 



MM' 

and 2F=(G-G')^^. 

r 

To seek for the directive action we have made use of the principle of 
forced oscillations, thereby obtaining to some extent a cumulative effect, 
and at the same time largely eliminating the errors due to accidental dis- 
turbances. 

Briefly the method was as follows : A small quartz sphere, about 0-9 
centim. in diameter, was carried in a frame to which a light mirror was 
attached, and suspended by a quartz fibre inside a brass case, the position 
being determined by the reflection of a scale in the usual way. The complete 
time of torsional vibration was about 120 seconds. 

Outside the case was a larger quartz sphere, about 6-6 centims. in diameter, 
its centre being level with that of the suspended sphere, and 5-9 centims. 
from it. The larger sphere could be rotated about a vertical axis through 
its centre at any desired rate. The crystalline axes of both were horizontal, 
that of the smaller sphere being perpendicular to the hne joining the centres. 

To test for the quadrantal couple, the larger sphere was rotated once in 
230 seconds — a period nearly double that of the smaller sphere. To test for 
the semicircular couple, the larger sphere was rotated once in 115 seconds, or 
nearly the period of the smaller sphere. 

Assuming that a couple exists, a continuous rotation of the larger sphere 
would set up a forced oscillation in the smaller sphere of the same period as 
the couple, and since the damping was very considerable, this forced oscillation 
would soon rise to approximately its full value. Meanwhile, any natural 
vibrations of the suspended system would be rapidly damped out. Though 
continually renewed by disturbances due to convection-currents and tremors, 
they would be irregularly distributed, and there was no reason to suspect 
that their maximum amplitude would recur at any particular phase of the 
period of the apphed couple. To secure the distribution of successive maxima 
of natural vibrations of the smaller sphere over all phases of the forced period, 
the latter was made sensibly different from the natural period in the ratio 
23 : 24 ; and though the cumulative effect of the forced oscillations was 
reduced by the largeness of this difference, we did not think it advisable 
to make the periods more nearly coincident, lest the distribution of the 
disturbances, which were sometimes large, should not be sufficient. This 



140 AN EXPERIMENT IN SEARCH OE A DIRECTIVE ACTION 

conclusion was arrived at from the results of preliminary experiments with 
more nearly equal periods. 

During each complete period of the supposed appHed couple, the position 
of the smaller sphere was read ten times at equi-distant intervals of time, 
and the scale-readings were entered in ten parallel columns, one horizontal 
line for each period. The observations were continued usually for 70 or 80 
periods. Adding up the columns and dividing by the number of periods, 
any forced oscillation would be indicated by a periodicity in the quotients. 
The periodicities found were too irregular to be taken as evidence of the 
existence of a couple. 

Bescriftion of the Afj)aratus. 

The quartz spheres were placed in a cellar at Mason College, Birmingham, 
below the room in which the observing telescope and rotating-apparatus 
were fixed. 

The smaller sphere, 0-9 centim. diameter and weighing 1-004 grams, was 
held in an aluminium wire cage, and was suspended by a long, fine quartz 
fibre in a brass case from a torsion-head at the top of the case. 

A light plane mirror was fixed to the cage, and opposite this mirror was 
a glass window in the case; in front of the window was a plane mirror at 
45°, by means of which the light from the scale was reflected into the case 
and back again to the telescope, as shown in Fig. 2. 

The case was surroimded by a double-sided wooden box, lined within and 
without with tin-foil, and with cotton-wool between its inner and outer walls. 
The box was supported on indiarubber blocks to lessen tremors. 

The larger sphere, 6-6 centims. diameter and weighing 399-9 grams, was 
held at the lower end of a vertical brass tube which terminated in a very 
carefully turned shallow brass bell, in which the sphere was held by tapes. 
The tube passed upwards through the top of the wooden casing without 
contact, a kind of air stuffing-box indicated in the figure serving to prevent 
currents through the hole. The tube came into the room above, and was 
there connected with a train of wheels, driven by an electromotor, the 
rotation of the motor being geared down from 1000 to 1. The observing 
telescope was fixed to a heavy stone slab resting on indiarubber blocks, 
standing on a brick-pillar, which was built on the brick arches forming the 
cellar-roof. A diagonal scale (of half-milUmetre graduations, divided into 
tenths by the diagonal ruhng) was clamped to the telescope-tube and 
illuminated by an incandescent lamp, aided by a concave mirror. A tenth 
of a division could be read with certainty, and as the distance from scale 
to mirror was 358 centims., the position of the suspended sphere could be 
determined within a Httle more than one second of arc. 

The steady rotation of the larger sphere was maintained by a regulator, 



OF ONE QUARTZ CRYSTAL ON ANOTHER 



141 



for which we are indebted to Mr. R. H. Housman. It consisted of two parts : 
(1) the governor proper, which automatically maintained approximate 
steadiness, and (2) a fine hand-adjustment, by which the motion could be 
accelerated or retarded when it got 'out of time.' 

One lead to the motor went through two mercury-cups, and the circuit 
was completed by a fork of platinum- wire dipping into the cups. This wire 




7b Accumf 



ReguUa.U>f 



Inc Lined Mi 



•9 






[ ni 



rm it,n"inr 



xr 



Fig. 2. Diagrammatic sketch of the apparatus. 



was fastened to one end of a wooden lever, the other end of which was attached 
to a sliding collar on the axle of the motor. To this collar were fastened the 
upper ends of the loaded springs of the governor, as shown in the figure. If 
the speed increased, the loads flying out pulled the collar down and so raised 
the wire out of the mercury-cups, and broke the circuit. As the speed 
diminished, the wire again dipped into the mercury and re-estabhshed the 
current. To diminish sparking the mercury was covered with alcohol, and 
the two cups were permanently connected by a high resistance shunt. 



142 AN EXPERIMENT IN SEARCH OF A DIRECTIVE ACTION 

The fine hand-adjustment consisted of a small wooden plunger working 
in a tube connected with one of the mercury-cups ; by means of a screw the 
plunger could be raised or lowered, and the level of the mercury in the cup 
varied accordingly. 

If the revolving sphere was found to be gaining or losing, it was quite 
easy to bring it 'up to time' again by working the screw of the plunger. 

The last of the train of driving-wheels was fixed on the tube supporting 
the larger sphere ; its rim was divided into equal parts by numbered marks, 
the use of which will be explained directly. There were 20 numbered marks, 
at intervals of 18° ; of these only 10 alternate ones were used for the quicker 
rotation, while the whole 20 were used for the slower speed. 



The Observations. 

Two observers were required, one at the telescope to note the position of 
the smaller sphere, the other to regulate the speed of rotation of the larger 
sphere, and to notify when readings were to be taken by the first observer. 
The motion having been started, and brought to about the right speed, a 
time-table was rapidly prepared, showing the times, on the chronometer used, 
at which each of the numbered marks above mentioned should pass a fixed 
mark throughout the whole set of observations for one occasion. A signal 
was given at each passage of a mark past the fixed point, the observer at the 
telescope putting down the simultaneous scale-reading in a manner which 
will be understood from Table I, which may serve as a typical record. It 
does not appear to be necessary to give the full details in other cases. If 
the motion did not keep to the time-table, it was easily corrected by the 
hand-adjustment already described. 

Every reading in the same column is taken at the same phase in the 
rotation of the larger sphere, and therefore the mean readings of the columns 
should preserve any periodicity in the motion of the smaller sphere equal to 
that of the larger sphere, and more or less ehminate all others. These mean 
readings are given at the foot of Table I, and appear to indicate a sKght 
periodic vibration, but this might be due to a want of symmetry in the larger 
sphere and its attachments about its axis of rotation, since the system 
supporting the smaller sphere and mirror was necessarily not symmetrical. 
The observations for each couple were on this account divided into two sets : 
for the semicircular couple the larger sphere was, in the second set, turned 
through 180° about a vertical axis from its position in the first set ; for the 
quadrantal couple the rotation was 90°. For the final results the means of 
the results of the two sets were taken, in each case after the second set had 
been advanced by an amount corresponding to the change of position of the 
sphere. 



OF ONE QUARTZ CRYSTAL ON ANOTHER 143 

Table II contains all the mean results obtained in the same way as the 
figures at the foot of Table I, the greatest range being given in the last column 
as an indication of the magnitude of the disturbances. 

In Table III are given the means for each azimuth of the larger sphere in 
its support, the B and D series being advanced as mentioned above. 

In combining the results it appeared useless to attempt to weight them 
according to the number of periods taken, since no accurate conclusion could 
be expected. It will be seen that in each case there is an outstanding 
periodicity, but the amplitude is less when the disturbances (as indicated 
by the greatest range during a period) are less, and it diminishes when the 
results are combined so as to lessen the effect of want of symmetry. 

In the 'quadrantal' observations (Series C, D), where the effect of want 
of symmetry of the apparatus should almost be eliminated, since it is 
approximately semicircular, the mean range is much smaller than in Series 
A and B. 

For these reasons we do not think that our observations can be taken as 
indicating the existence of a couple of the kind sought, but only as giving 
a superior limit to its value, should it exist. 

We now proceed to the Calculation of the Superior Limit of the Couple. 



Equation of Motion of the Smaller Sphere. 

Let / be the moment of inertia of sphere and cage. 

„ jjL „ torsion-couple per radian. 

,, A ,, damping couple per unit angular velocity. 

,, F cos 'pt be the supposed couple due to the larger sphere, having 
period Stt/^. 

Then Id + XO + yi^ = F cos ft. 

Putting K^XjI; n^ = ^II', E = F/I 

we have . S + k6 + n^d = E cos ft (1) 

The solution of this is 

d = — ^^ cos (pt -€) + Ae-^'^ cos {Vin^ - ^k^) t - a}, (2) 

where tan e = -ij^ — ^ and A, a are constants. 
n^ — f^ 

The first term in the value of 6 in (2) gives the forced, and the second 

term the natural vibrations, the period of the latter being 

= T, say. 



Vin^ - \k^) 



144 AN EXPERIMENT IN SEARCH OF A DIRECTIVE ACTION 

The value of T was always very near to 120 sees., and the mean of various 
determinations during the observations gave 

r=/v-^-i^, = 120-8 sees (3) 

Value of K. When there are only natural vibrations 
any complete swing ^ i^ ly 
next complete swing 
The value of this ratio was usually near 14. The mean of a number of 
determinations taken at various times was 1-3953. Putting 

g3o-2. _ 1-3953, 

,veget K' = 0-011033. 

Value of n. Substituting for k in the value of T in (3) we get 

^2 - 0-0027359, 
and n = 0-052306. 

Value of e. The forced period 27r/p was always 115 sees., whence 

tan 6 =-,-^-^ = 2-420, 

and e ^ 67° 33', 

sin e = 0-9242. 
From equation (1) it will be seen that the steady deflection due to F is 

n 

while from (2) the amphtude of the forced oscillations is 

E sin c n^ sin e E 
or . -^ . 

Using the values found for uk and e we have 

or the forced oscillations give a cumulative effect, about four times the steady 
deflection due to the couple at its maximum value. 

Value of Moment of Inertia, I. This was found by vibrating the cage 
hung by a short quartz fibre, (1) when empty, (2) when containing the sphere, 
the times of vibration being respectively 8-38 sees, and 11-22 sees. The 
sphere weighs 1-004 grams, and its radius is 0-45 centim., so that its moment 
of inertia jMr^ - -08132. 

From this, and the times of vibration, we get 

7 = 0-1821. 
Value of F. The vibrations were observed in scale-divisions, each 
0-05 centim., the distance between mirror and scale being 358 centims. If 



OF ONE QUARTZ CRYSTAL ON ANOTHER 145 

N is the number of scale -divisions in the ampHtude of vibration, i.e., in half 
the range, we have from (2) 

Eainc _ 6N 
pK " 2 X 35800' 
whence F = EI = 0-8293i\^ x lO-^, 

using the values already found for e, k, I. 

Taking the Hmiting values of the amplitudes as half the mean ranges 
given in Table III, the vibration due to the quadrantal couple has amplitude 
not greater than 0-033 div., and that due to the semicircular couple, amplitude 
not greater than 0-095 div. Whence 

F (quadrantal) is not greater than 2-737 x 10-^°, 
and F (semicircular) is not greater than 7-878 x 10"^°. 

Perhaps some idea of these values may be obtained by noticing that the 
times of vibration of the small sphere under couple F per radian would be 
respectively 32 hours and 25 hours. But it is probably best to interpret the 
value in terms of the assumptions we made as to the force in the introduction. 
We found for the quadrantal couple 

F = (G- G') MM'jf, 
G-G' GMM' 
'^ G ' r ' 
where M, M' are the masses of the spheres, r the distance between their centres, 
G, G' the parallel and crossed gravitation-constants. 

Now M, the mass of the larger sphere, is 399-9, say 400 grams, 
M' „ ,, smaller „ 1-004 grams, 

r is 5-9 centims., 
G and G' are exceedingly near 6-66 x 10"^, 

G-G' Ft 

whence -^ = GMM'^^^^' 

On the assumed law of force this imphes that the attractions between the 
two spheres, with distance 5-9 centims. between their centres, do not differ in 
the parallel and crossed positions by as much as yeioo ^^ ^^^ whole attraction. 

We may compare this result with Rudberg's values of the refractive indices 
of quartz for the mean D line 

^,-^„ 1-55328 - 1-54418 _ , 
^/x— r54418 -TT^^bout. 

For the semicircular couple 

^^ G-G' GMM' 

2F = —^ , 

G r 

whence — __- = 3^j^^. 

p. c. w. ^^ 



146 



AN EXPERIMENT IN SEARCH OF A DIRECTIVE ACTION 



On the assumed law of force, this imphes that the attractions between 
the two spheres, with distance 5-9 centims. between their centres, with their 
axes parallel and respectively in Hke and unhke directions, do not differ by 
as much as -^-^-^-^ of the whole attraction. 

This hmit is large, undoubtedly owing to the want of axial symmetry in 
the apparatus which produced a semicircular couple as already pointed out. 
This couple was large, and though we attempted to ehminate it by the two 
sets of observations with the different azimuths of the larger sphere, in all 
probability w^e failed. 

Table I. Showing Scale- Readings in Tenths of a Division at Phases at 
Heads of Columns. Time of Revolution of Larger Sfhere 115 sees. 






1 


2 


3 


4 


5 


6 


7 


8 


9 










55 


61 


61 


64 


61 


42 


25 


26 


31 


40 


50 


55 


60 


53 


52 


45 


44 


40 


45 


50 


54 


51 


49 


49 


48 


49 


52 


57 


52 


54 


57 


52 


40 


33 


28 


25 


30 


44 


57 


66 


70 


64 


52 


40 


38 


36 


39 


46 


55 


60 


63 


61 


52 


44 


44 


45 


44 


43 


49 


50 


52 


48 


42 


30 


32 


37 


45 


50 


62 


71 


69 


62 


52 


42 


39 


38 


44 


55 


58 


65 


65 


66 


61 


51 


45 


45 


41 


38 


40 


49 


56 


61 


62 


60 


56 


50 


48 


42 


39 


37 


40 


42 


58 


69 


69 


68 


58 


48 


41 


38 


38 


42 


48 


54 


60 


57 


55 


50 


43 


41 


41 


42 


47 


49 


55 


57 


63 


60 


58 


49 


46 


47 


46 


44 


51 


52 


50 


54 


48 


45 


44 


40 


36 


40 


50 


60 


67 


67 


62 


54 


44 


33 


35 


35 


38 


50 


57 


62 


68 


62 


52 


45 


36 


36 


39 


44 


51 


56 


59 


53 


47 


48 


53 


51 


50 


49 


50 


49 


52 


50 


50 


51 


53 


52 


54 


55 


48 


47 


44 


41 


44 


52 


55 


58 


60 


56 


49 


41 


41 


42 


43 


47 


50 


55 


60 


60 


60 


56 


58 


47 


43 


47 


49 


50 


50 


50 


54 


54 


54 


48 


50 


51 


49 


52 


52 


45 


42 


43 


48 


49 


bQ 


56 


52 


52 


52 


57 


56 


51 


46 


42 


42 


43 


49 


51 


55 


55 


55 


52 


49 


57 


50 


50 


50 


44 


43 


50 


50 


50 


43 


43 


46 


50 


58 


54 


55 


50 


49 


49 


49 


48 


50 


51 


54 


53 


5.6 


56 


57 


58 


56 


56 


51 


43 


40 


38 


41 


51 


60 


60 


60 


58 


52 


48 


48 


48 


52 


57 


58 


60 


57 


47 


41 


41 


51 


62 


63 


59 


53 


46 


40 


40 


40 


43 


49 


51 


61 


60 


60 


56 


51 


48 


42 


42 


43 


51 


59 


63 


62 


61 


55 


52 


51 


50 


51 


51 


52 


56 


58 


58 


53 


45 


40 


41 


49 


60 


70 


70 


60 


52 


48 


48 


50 


50 


54 


55 


53 


51 


50 


50 


1 47 


50 


50 


52 


53 


53 


50 


48 


48 


46 


48 


50 


51 


51 


50 


50 


52 


52 


49 


46 


44 


44 


49 


50 


55 


59 


57 


58 


51 


49 


46 


43 


44 


51 


59 


68 


64 


56 


50 


42 


40 


49 


57 


68 


71 


70 


59 


50 



OF ONE QUARTZ CRYSTAL ON ANOTHER 



147 



Table I (continued). 



Mean of 80 in 
divisions . . . 






1 


2 


3 


4 


5 


6 


7 


8 


9 


47 


41 


43 


56 


65 


66 


56 


45 


39 


35 


33 


40 


48 


59 


68 


70 


64 


51 


43 


42 


48 


51 


60 


64 


70 


67 


56 


42 


39 


38 


40 


47 


52 


65 


60 


61 


59 


51 


51 


50 


48 


47 


50 


54 


53 


60 


52 


50 


41 


39 


40 


44 


51 


58 


66 


70 


71 


63 


50 


38 


35 


38 


41 


43 


50 


56 


70 


75 


70 


59 


50 


46 


45 


51 


61 


70 


71 


70 


62 


52 


41 


40 


40 


40 


47 


54 


60 


71 


71 


68 


60 


50 


48 


45 


39 


39 


42 


49 


51 


60 


64 


61 


50 


46 


47 


49 


52 


60 


72 


75 


70 


62 


57 


32 


23 


21 


30 


42 


57 


77 


84 


79 


63 


51 


42 


33 


34 


38 


49 


60 


66 


64 


57 


51 


49 


44 


47 


49 


52 


52 


55 


55 


52 


58 


59 


56 


57 


51 


42 


40 


43 


47 


55 


61 


66 


64 


60 


52 


50 


45 


41 


45 


49 


59 


67 


67 


56 


50 


49 


43 


38 


45 


48 


53 


55 


56 


57 


55 


54 


56 


53 


49 


42 


42 


51 


61 


69 


70 


65 


54 


45 


41 


40 


47 


51 


56 


61 


59 


55 


49 


48 


52 


60 


60 


60 


58 


50 


46 


44 


43 


45 


51 


53 


60 


63 


67 


62 


60 


55 


48 


44 


46 


49 


50 


52 


54 


53 


50 


50 


52 


60 


62 


63 


61 


51 


41 


39 


38 


42 


50 


55 


59 


54 


51 


48 


47 


42 


47 


48 


55 


58 


61 


62 


60 


59 


54 


52 


52 


50 


50 


58 


54 


55 


55 


58 


56 


56 


50 


51 


51 


56 


58 


51 


52 


48 


48 


54 


55 


50 


51 


52 


51 


51 


50 


45 


44 


42 


46 


51 


55 


56 


53 


56 


59 


59 


60 


58 


59 


59 


54 


49 


46 


46 


49 


50 


52 


58 


56 


57 


53 


51 


50 


50 


46 


49 


51 


58 


66 


67 


69 


65 


62 


51 


46 


39 


39 


39 


45 


51 


56 


62 


61 


53 


48 


40 


38 


47 


62 


67 


63 


55 


52 


57 


56 


56 


53 


49 


42 


38 


41 


51 


60 


65 


71 


73 


72 


60 


52 


50 


40 


42 


49 


52 


62 


71 


73 


73 


65 


59 


44 


39 


38 


40 


43 


51 


51 


59 


61 


61 


50 


49 


41 


49 


51 


52 


58 


58 


52 


52 


50 


53 


56 


57 


51 


50 


49 


49 


49 


51 


52 


49 


52 


53 


52 




... 


... 


... 


... 




5175 


5163 


5143 
min. 


5-186 


5-246 


5-294 


5-355 
max. 


5-284 


5-300 


5-216 



Mean range 5-355 - 5-143 - 0-212 division. 
Greatest range in one period 7-5 — 3-5 =^ 4-0 divisions. 



10—2 



148 DIRECTIVE ACTION OF ONE QUARTZ CRYSTAL ON ANOTHER 



Table II. 



1 

Series 


Azimuth of large 
sphere 






Mean readings at phases (whole numbers omitted) 




Greatest range in 

a period in 

Scale-divisions 





1 


2 


3 \ 4 


5 


6 


7 


8 


9 


A 1 


o 




sees. 
115 


80 


•175 


•163 


•143 


•186 ^246 


•294 


•355 


•284 


•300 


•216 


•212 


40 


A2 





115 


80 


•653 


•558 


•566 


•653 I-813 


•950 


1030 


1-008 


•929 


•769 


•472 


31 


Bl 


180 


115 


80 


•485 


•590 


•624 


•648 -556 


•464 


•379 


-328 


•284 


•364 


•364 


30 


B2 


180 


115 


70 


•423 


■503 


•650 


•836 -941 


•961 


•843 


•714 


•540 


•464 


•538 


75 


B3 


180 


115 


54 


•650 


•632 


•619 


•648 ^656 


•680 


•717 


•739 


785 


•739 


•166 


2^7 


CI 


°0 


sees. 
230 


72 


•708 


•731 


•708 


•739 -717 


•711 


•676 


•678 


-642 


•688 


-097 


1^3 


(72 





230 


80 


•370 


•400 


•358 


•326 


•271 


•214 


•173 


•158 


•253 


•310 


-242 


3-4 


C3 





230 


80 


•616 


•654 


•686 


•673 


-663 


•627 


•571 


•560 


•566 


-584 


-126 


2^0 


D\ 


90 


230 


50 


b024 


1-042 


1-031 


b004 


-988 


•920 


•926 


•954 


-994 


1-010 


•122 


' 2-2 


D2 


90 


230 


70 


•031 


•090 


-150 


•210 


-220 


•230 


•223 


•176 


•126 


•096 


•199 


31 



Table III. 



Series 




Mean readings at phases 






Mean 





1 


2 


3 4 ' 5 6 7 


8 


9 
•493 


range 
•338 


A 


•414 


-361 


-355 


-420 -530 -622 ' -693 -646 


•615 


5 (advanced 180°) 


•702 


•646 ^594 -536 -522 -519 


•575 ^631 


•711 
•663 


•718 


•199 


Means of A and B 


•558 


-503 -474 -478 -526 -570 


•634 ^638 


•605 ^189 


C 


•565 


1 ' 
•595 -584 -579 -550 -517 


•473 ^465 


•487 


-527 


•130 


D (advanced 90°) 


•575 


-575 -565 -560 -553 -528 


•566 ^592 


•607 


•604 


-079 


Means of C and D 


-570 


1 i 
-585 -575 -570 -552 -523 


•520 ^529 


•547 


•566 


-065 i 

1 



AN EXPERIMENT WITH THE BALANCE TO FIND IF CHANGE OF 
TEMPERATURE HAS ANY EFFECT UPON WEIGHT. 

By J. H. PoYNTiNG and Percy Phillips, M.Sc. 

[Roy. Soc. Proc. A, 76, 1905, pp. 445-457.] 
[Received July 12, 1905.] 

In all the experiments hitherto made to determine the gravitative attrac- 
tion between two masses, the temperature has not varied more than a few 
degrees, and there are no results which would enable us to detect with 
certainty any dependence of attraction upon temperature even if such 
dependence exists. It is true, as Professor Hicks has pointed out*, that 
Baily's results for the Mean Density of the Earth, if arranged in the order 
of the temperature of the apparatus when they were obtained, show a fall in 
value as the temperature rises. But this is almost certainly some secondary 
effect, due to errors in the measurements of the apparatus, or to the seasons 
at which different attracted masses were usedf . 

The ideal experiment to find if temperature has an effect on gravitation 
would consist in one determination of the gravitative attraction between two 
masses at, say 15° C, and another determination at, say, the temperature of 
boiling liquid air. But the difficulties of exact determination at ordinary 
temperatures are not yet overcome, and at any very high or very low tempera- 
tures, they would be so much increased that the research seems at present 
hopeless. 

The question can, however, be attacked in a somewhat less direct method 
by examining whether the weight of a body— the gravitative attraction of 
the earth upon it — varies when the temperature of the body varies. The 
various parts of one of the attracting masses — the Earth — remain, each 
part, at the same temperature throughout, and this is, no doubt, a weak- 
ness of the method. For it is perhaps conceivable that in the expression 
for the attraction a temperature factor might exist of some such form as 
1 + AC {Mt + mt')l(M + m), where M and m are the two masses, and t and 
t' are their temperatures. If m/M is negligible, this reduces to 1 + kI, and 

* Proc. Camb. Phil. Soc, vol. 5, p. 156. 

t Poynting, Mean Density of the Earth, p. 56. 



150 AN EXPERIMENT WITH THE BALANCE TO FIND IF 

is independent of the temperature t' of the smaller mass. But it seems more 
hkely that each mass would have a separate temperature factor. If such 
a factor exists, and if its variation is appreciable, then we ought to be able 
to detect a change of weight with change of temperature. 

Observations on pendulums suffice to show that at the most any such 
effect must be small. The nearly constant period of vibration with the 
nearly constant length of a compensated or an 'invar' pendulum shows 
constancy of weight of the bob to a considerable degree of exactness. Again, 
the agreement of weight-methods and volume-methods of measuring the 
expansion of liquids with rise of temperature shows, though less conclusively, 
that there is no great variation. 

It appeared to us that it would be possible to go much further in testing 
constancy of weight by a direct weighing experiment, in which the weight 
on one side of a balance should be subjected to great changes of temperature 
while the counterpoise should remain at a uniform temperature. We give 
an account in this paper of a series of experiments carried out on the following 
principle. A brass cylinder weighing 266 grammes was hung by a wire from 
one arm of a balance so as to be near the bottom of a tube depending from 
the floor of the balance-case, the tube being closed at the bottom and opening 
at the top into the case, the wire passing down through the opening. The 
brass cylinder was counterpoised by an equal cylinder hung by a short wire 
from the other arm inside the case. To this short wire was attached a finely 
divided scale on which the swings of the balance could be read by a microscope 
looking through a window in the case. The balance was released and left 
free to swing. Then the case was exhausted till the pressure was not more 
than a small fraction of a milHmetre of mercury. Steam was passed round 
the lower part of the tube where the weight hung, and after a time the weight 
was allowed to cool again. In other experiments the lower part of the tube 
was cooled by liquid air and again brought up to the temperature of the room. 

While the changes of temperature were in progress there were considerable 
apparent variations in weight. But ultimately, when the temperature became 
steady, the weight, too, became steady. At 100° C. it was slightly less than at 
the temperature of the room. This difference was partly due to a rise in the 
temperature of the case, such a rise being always accompanied by an apparent 
diminution of the weight in the tube, whether steam was apphed or the 
balance was merely left to follow the temperature of the room. Probably this 
effect was due to some change in the balance-beam. But the difference was 
partly due to convection-currents, or at any rate to the residual air in the 
case, for it varied with the disposition of diaphragms in the tube. There 
were no doubt convection-currents, as there was always a tendency for the 
case to rise in temperature when steam was apphed, and this could hardly 
be accounted for by conduction or radiation, under the conditions of the 



CHANGE OF TEMPERATURE HAS ANY EFFECT UPON WEIGHT 151 

apparatus. As effects due to residual air should depend upon surface and 
not upon volume, similar experiments were made with hollow weights, each 
about 58 grammes, and of the same size and form as the solid weights. There 
was again an apparent diminution in weight when steam was applied. Any- 
true diminution due to change of temperature should be shown by a difference 
in the diminution with the solid and with the hollow weights, the surface 
effects being eliminated, and this diminution should be that of 266 — 58 = 208 
grammes. 

The net result of all the experiments was that there was not a greater 
change in 208 grammes between 15° C. and 100° C, than 0-003 milligramme. 
But an inspection of the detailed account given later shows that this result 
is probably accidentally small — within the limits of experimental error. It 
would imply that there is not a change greater than 1 in 6 x 10^ per 1° C. 
But the experiments hardly justify us in saying more than that there is not 
a change greater than 1 in 10^ per 1°. 

When liquid air was used, air-currents were absent, and the temperature 
variations of the case were much less. The net result of these experiments 
was that there is not a change of weight in 208 grammes between 16° C. and 
— 186° C. greater than 0-002 milHgramme. This would imply that there is 
not a change greater than 1 in 1-3 x 10^^ per 1° change of temperature. We 
may probably assert that the change is not greater than 1 in 10^^ per 1° C. 

We now proceed to a detailed account of the apparatus and of the mode 
of using it. 

The Balance, 

The balance has a 6-inch beam and was specially constructed for the 
experiment by Mr. Oertling. The general arrangement will be seen from 
Fig. 1. The base-plate is of gun-metal, as are also two sides and the top of the 
case. The front and back of the case are of thick plate-glass fixed to the 
metal by marine glue. In the experiments the base-plate was supported on 
levelling-screws on a slate slab, and between it and the slab was a gas-pipe 
with pinhole burners so that it could be warmed. When the case was to be 
fixed in position the jets were lighted and seahng-wax was smeared on to the 
area of contact of plate and case. When the wax was quite liquid the case 
was put down on the plate and the gas was turned off. When the metal was 
cool the joint was perfectly air-tight. 

The tube T in which the weight W hung was of brass, 4-1 cm. internal 
diameter and 62-5 cm. long. It consisted of three parts. The topmost was 
brazed to the base-plate and the two lower parts were attached to it and to 
each other by flanged joints ff. Between the flanges was placed a circular 
lead washer of diamond-shaped section. When the flanges were pressed 
together by bolts the joint was quite air-tight. Round the middle section of 



152 



AN EXPERIMENT WITH THE BALANCE TO FIND IF 



the tube was a water-jacket wj through which water flowed while an experi- 
ment was in progress, and round the lowest section was a steam-jacket sj 
through which either water or steam could be passed. This jacket could be 
removed and could be replaced by a vacuum- vessel 30 cm. long containing 
liquid air. 




Fig. 1. 

W, weight of which the temperature is to be raised, Tf counterpoise. 

T, tube in which it hangs, with a number of diaphragms with |-inch holes. 

sj, steam-jacket, replaced by hquid-air jacket. 

//, flanged joint with lead washer. 
wj, water-jacket. 

p, pipe to the exhausting pump. 

sc, scale read by a microscope not shown. 

rr, rider-rod passing through stuffing-boxes, sb, enlarged in Fig. 2. 
GG, gas-burners to heat the base-plate before seahng up. 

The weights W, W were turned from the same gun-metal bar. The length 
of each was 4-45 cm., the diameter 3 cm., and the solid weights were each 
266-17 grammes, while the hollow ones were 57-86 grammes. They were hung 
directly from the end-plates of the balance by platinum wires, and any 
residual mequality was compensated by moving a centigramme-rider along 



CHANGE OF TEMPERATURE HAS ANY EFFECT UPON WEIGHT 



153 



the beam by the rider-rod rr. This rod passed through stuffing-boxes sh, 
designed for us by Mr. G. 0. Harrison, the mechanical assistant in the labora- 
tory, to whom we are much indebted for this and many other valuable 
suggestions, and for the careful construction of all the apparatus except the 
balance. These stuffing-boxes were perfectly air-tight when screwed up and 
the rod could still be rotated without any leakage. But to draw it in or out 
it was necessary to loosen the screws slightly, and in one case when this was 
done some leakage occurred. As the construction appears to give an efficient 
mode of moving apparatus inside a vacuum from without, we give in Fig. 2 
a section of a stuffing-box. 




Fig. 2. 

rr, rider-rod. 
CO, side of case. 
ivw, two or three circular washers punched out of soft leather and soaked in oil. 

p, plunger driven in by screws, ss. 
oh, oil-hole through which valvoline, a thick lubricating oil, was inserted. 



The position of the balance-beam was read by a microscope viewing a glass 
scale sc, Fig. 1, interposed in the suspension of W . The scale was divided to 
0-1 mm. and numbered in millimetres. The objective of the microscope was 
placed inside the case and the eye-piece with cross-hairs was fixed outside it. 
The axis of the microscope was horizontal and a lamp at the back illuminated 
the scale. The case was surrounded by felt and a tin cover was placed over 
the whole, small windows through the felt then allowing the scale to be seen. 
A thermometer placed between the felt and the case was taken to give the 
temperature of the case. 

A brass pipe f from the floor of the case led to the pumping apparatus. 
This pipe was connected to a branched glass tube, one branch going to a 
Fleuss pump and the other to a 4-fall Sprengel, made continuous in its action 



154 AN EXPERIMENT WITH THE BALANCE TO FIND IF 

by a steel pump which was worked by a motor, and which raised the mercury 
again from the cistern at the base to the reservoir at the top. When the case 
was to be exhausted the Fleuss pump was first used and then sealed off and 
the exhaustion was carried on by the Sprengel. The degree of exhaustion 
was estimated by sending a discharge through a vacuum-bulb 10 cm. diameter 
connected with the tube to the pump, and usually the pumping was continued 
till the negative dark space was of the order of 4 cm. As a rule the vacuum 
held without serious change for days or even for weeks. 



Mode of Experiment. 

A large number of preliminary experiments was made with a pair of brass 
weights each about 187 grammes. These were only useful in bringing to the 
front the difficulties in obtaining good results and in suggesting means for 
overcoming them. We shall only record the final results with the 266 
grammes and 58 grammes weights. 

The weights and the lower section of the tube were first cleaned by boiling 
in caustic potash solution and washing in distilled water. They were then 
suspended, being handled with gloves only, and the lowest section of the tube 
was screwed on. 

Stea?n- Heating. 

The jacket sj (Fig. 1) was fixed on the lower section of the tube and the 
balance was set free to vibrate, being left free during a whole series of 
experiments. The case was then sealed on and the value of a scale-division 
was determined by the rider. Any change in the value during a series could 
be determined from the change in period of the swing. The time of swing in 
different series ranged from 24 to 42 seconds. After the stuffing-boxes were 
tightened the case was exhausted till the pressure was estimated to be not 
more than j^jj mm. of mercury. The weight of air displaced by a weight was 
then of the order 0-001 milHgramme and the change in this with change of 
temperature was quite neghgible. 

Cold water was passed through the water-jacket tvj, and sometimes, while 
steam was being got up in a boiler at some distance well screened from the 
balance, through sj also. The centre of swing and the temperature of the case 
were observed, and before any heating occurred the balance was usually quite 
steady. Steam was then blown through sj, water still flowing through wj. 
After considerable changes, which will be described later, the centre of swing 
in the course of five or six hours settled down to a steady march which 
appeared to correspond to change in temperature of the case. KSometimes 
steam was turned off after eight or nine hours, but in some cases it was kept 
on for 24 and 48 hours and even longer. Then it was turned off and the 



CHANGE OF TEMPERATURE HAS ANY EFFECT UPON WEIGHT 155 

jacket was allowed to cool. The centre of swing was observed when steady, 
several hours later or next day. 

The results in the first few heatings and cooHngs, after an exhaustion of the 
case, were rejected, as there was evidence that the earlier heatings drove gas 
from the weight. Only after successive heatings gave fairly consistent values 
were these taken into account. 

One effect of the steam-heating was always to raise the temperature of the 
case, probably through convection of the residual air. A rise of temperature 
in the case was always accompanied by a lowering of the scale-reading, 
corresponding to a diminution in weight. The effect was somewhat irregular, 
but an average value of the lowering per 1° rise was determined by observing 
the centre of swing of the balance at intervals through several days, when 
the balance was left to follow the varying temperature of the room and no 
steam was flowing. The value thus obtained was used to correct all the 
readings to 15° C. 

As an example of the method pursued, we give in Table I the series of 
readings used to obtain the temperature-correction for the hollow weights. 



Table I. Change of Centre of Swing with Change of Temperature of Case. 



Date 


Time 


Centre of swing in 


Temperature 


millimetres of scale 


of case 


6.12.04 


1.0 p.m. 


14-34 


14-75 


,, 


3.0 „ 


14-355 


14-55 


,, 


5.40 „ 


14-35 


14-65 


7.12.04 


12.35 „ 


14-ai 


12-70 


jj 


5.5 „ 


14-465 


13-60 


8.12.04 


11.5 a.m. 


14-43 


14-70 


,, 


12.55 p.m. 


14-20 


14-95 


9.12.04 


■ 9.51 a.m. 


14-555 


12-80 


>> 


2.27 p.m. 


14-525 


13-3 


10.12.04 


9.37 a.m. 


14-46 


15-0 



The temperature-correction deduced from these numbers by the method of 
least squares is a decrease of 0-13 division per 1° C. rise, and as the sensibility 
was 1 division for 0-248 milligramme, there was an apparent decrease of 
weight of 0-032 miUigramme per 1° rise. 

Two similar series with the soHd weights gave a decrease of 0-044 division 
per 1° C. rise, and as the sensibility was now 1 division per 0-803 milligramme, 
there was an apparent decrease of weight of 0-035 milhgramme per 1° rise. 

Another series with the solid weights when steam was passing all the time 
for several days, gave a decrease of 0-052 division per 1° rise, but as the 



156 



AN EXPERIMENT WITH THE BALANCE TO FIND IF 



values were more irregular, the series giving 0-044 division were used. This 
series with steam sufficed to show that very nearly the same temperature- 
correction apphed when the weight was hot as when it was cold. 

The irregularity of the observations is only to be expected when it is 
remembered that the balance was subjected to some considerable vibration 
at times through machinery running in the same building, and that the 
observations extended over several days. Indeed it is remarkable that there 
was not more irregularity, and the fair consistency of the observations illus- 
trates once more the marvellous accuracy of a well-made balance. 

The following Table II will serve as an example of a complete experiment 
in which one of the hollow weights was cold initially, was then surrounded 
with steam for 24 hours, and was then allowed to get cold again. The obser- 
vations recorded are at about hourly intervals, but intermediate ones, not 
used, were frequently taken to be sure that there were no sudden changes. 



Table II. Experiment with Hollow Weight raised to 100° C. and then 
cooled, 1 mm. = 0-248 milligramme. 

Correction for temperature of case — 0-13 division per 1°. 







Condition 


Centre of 


Tempera- 


Centre of 




Date 


Time 


of 
weight 


swing, 
1 = 1 mm. 


ture 
of case 


corrected 
to 15° C. 


Remarks 


17.11.04 


9.25 a.m. 


Cold 


14-905 


14°-75 


14-872 


Steam put on just after 9.25 


,, 


4.0 p.m. 


Hot 


14-40 


15 -0 


14-400 


and kept on till 10 a.m. 


,, 


5.20 „ 


,, 


14-39 


15 -0 


14-390 


next day 


>? 


6.12 „ 


,, 


14-375 


15 -0 


14-375 




^, 


7.8 „ 


?j 


14-365 


15 -05 


14-372 




,, 


7.56 „ 




14-36 


15 -1 


14-373 




18.11.04 


9.5 a.m. 


,, 


14-30 


15 -05 


14-307 




„ 


9.55 „ 


., 


14-295 


15 -1 


14-308 


Steam turned off just after 


,, 


5.36 p.m. 


Cold 


14-55 


16 -00 


14-680 


9.55 


,, 


6.39 „ 


" 


14-56 


15-95 


14-684 




,, 


7.56 „ 


j» 


14-58 


I 15 -75 


14-678 




19.11.04 


9.40 a.m. 




14-67 


14 -30 


14-579 




1 


11.38 „ 


" 


14-665 


14-20 


14-561 





Initial reading, cold at 15° 14-872 divisions 

Final mean reading, cold at 15° 14-636 „ 

Mean reading, cold 14-754 „ 

hot 14-360 

Cold-hot 0-394 division 

The following Table III gives the results of the various experiments with 
the hollow weight, treated as in Table II, the readings of the centre of swing 
being at about hourly intervals when on the same day. 



CHANGE OF TEMPERATURE HAS ANY EFFECT UPON WEIGHT 



157 



Table III. Exferiments with Hollow Weight raised to 100° C. and then 
cooled, 1 mm. = 0-248 milligramme. 

Correction for temperature of case — 0-044 division per 1°. 



Date 


Condi- 
tion of 
weight 


Centre 
of swing 
corrected 

to 15° 


Number 
of readings 
from which 

centre 
of swing is 

found 


Greatest 
deviation 

from 
the mean 


Excess 

of cold 

above 

hot 


Remarks 


16.11.04 ... 
17.11.04 ... 


Hot 
Cold 


14-749 
14-872 


4 

1 


0-0201 


0123 


Temperatures 14°-75 to 16°-3 
The initial cold reading was 
rendered useless by a subse- 
quent shift of scale-reading, 
probably due to slight dis- 
placement of the eye- piece 


17.11.04 ... 
17-18.11.04 
18-19.11.04 


Cold 
Hot 
Cold 


14-872 
14-360 
14-636 


1 

7 
5 


0-058 i 
0-075J 


0-394 


Temperatures 14°-2 to 16° 
The set given in full in Table 
II. The last of the preceding 
used as the first of this 


21.11.04 ... 
22-23.11.04 
23.11.04 ... 


Cold 
Hot 
Cold 


14-218 
14-091 
14-294 


1 
6 
3 


0-095 i 
0-040j 


0-165 


Temperatures 10° -45 to 14° -8 


25.11.04 ... 

1 

26.11.04 ... 


Cold 
Hot 
Cold 


14-421 
14-029 
14-010 


1 
2 

2 


0-001 
0016 


0-187 


Temperatures 10°-95 to 14°-5 


1.12.04 ... 
2.12.04 '.'.'. 


Cold 
Hot 
Cold 


14-500 
14100 
14-367 


1 
3 
1 


0-007 


0-334 


Temperatures 16° to 16° -9 


2.12.04 ... 
2-3.12.04 
3.12.04 ... 


Cold 
Hot 
Cold 


14-367 
14-218 
14-400 


1 
3 
1 


0-048 


0-166 


Temperatures 16°-9 to 17°-45 


12.12.04 ... 
13.12.04 ... 


Cold 
Hot 
Cold 


14-379 
14-106 
14-390 


1 
3 
1 


0-018 


0-279 


Temperatures 13°-65 to 15°-75 



Mean value cold - hot = 0-235 division = 0-058 milligramme. 

The following Table IV gives the results with the sohd weight. They are 
not so consistent as those with the hollow weight, probably because they were 
spread over a longer time on the average. This was done to secure that the 
weight should be more nearly at the temperature of its surroundings. A 
rough estimate shows that if heat be gained by radiation alone and the brass 
is taken as a full radiator, three hours will be required to bring it within 
1° of the temperature of the steam. The last two experiments were incom- 
plete in that no final cold weighing was taken, but the results obtained were 
regarded as probably sufficient. 



158 



AN EXPERIMENT WITH THE BALANCE TO FIND IF 



Table IV. Exferiments with Solid Weight raised to 100° C. and then 

cooled, 1 mm. = 0-803 milligramme. 

Correction for temperature of case — 0-044 division per 1°. 



Date 


Condi- 
tion of 
weight 


Centre 
of swing 
corrected 

to 15° 


Number 
of readings Greatest 
from which deviation 

centre from 
of swing is ; the mean 

found 


j 

Excess 

of cold T> 1 

above I Remarks 

hot 

1 


26.12.04 ... 
27-28.12.04 
29.12.04 ... 


Cold 
Hot 
Cold 


16-295 
16-032 
16-046 


1 

2 
1 


0-017 


0-139 


Temperatures 9°-9 to ll°-0 


30.12.04 ... 

30-31.12.04 

2.1.05 


Cold 
Hot 
Cold 


16-016 
15-914 
16-035 


1 
4 

1 


0-021 


0-112 


Temperatures 9°-l to 12°-05 


2.1.05 

3.1.05 

4.1.05 


Cold 
Hot 
Cold 


16-035 
16-022 
16-047 


1 

2 

1 


0-018 


0019 


Temperatures 9°-l to 12°- 1 


5.1.05 

>9 


Cold 
Hot 


15-987 
15-919 


1 
3 


0-002 


Temperatures ll°-95 to 12°-65 
0-068 Experiment interrupted by 
stoppage of steam tubes 


9.1.05 

10.1.05 


Cold 
Hot 


16-048 
16-039 


1 
4 


0-020 


0-009 


Temperatures 13°-7 to 15°-65 
Heating continued several days 
after this to obtain tempera- 
ture-correction. No final cold 
reading taken 



Mean value cold -hot = 0-069 division = 0-055 milligramme. 
From Tables III and IV we have : 

Solid weight, 266 grammes: cold — hot ... = 0-055 milligramme. 
Hollow weight, 58 grammes: cold — hot ... = 0-058 ,, 
For the difference, 208 grammes : hot — cold = 0-003 ,, 
Taking the rise in temperature as 85°, this gives a change of the order 
of 1 in 6 X 10^ per 1° rise. But evidently the smallness of the result is 
accidental, and probably all we can assert from the work is that any change 
of weight with change of temperature between 15° C. and 100° C. is not 
greater than 1 in 10^. 

Cooling with Liquid Air. 
Experiments were made in which heating by steam was replaced by 
coohng with liquid air. This was supphed to us by Sir Wilham Ramsay, 
and we desire to express our hearty thanks to him for his ready kindness 
in helping us to increase the temperature range so considerably. In these 
experiments the steam-jacket was removed and replaced by a vacuum- vessel 
30 cm. deep and 6 cm. inside diameter, kept full of Hquid air. After the 



CHANGE OF TEMPERATURE HAS ANY EFFECT UPON WEIGHT 159 



steady state was reached the hquid air was removed, the jacket was replaced 
and cold water was again passed round the tube. 

Owing to the evaporation of the air the experiments had to be carried out 
more rapidly than those with steam, but through the absence of convection- 
currents a steady state was more rapidly reached, and the variation in the 
temperature of the case was very small. 

The temperature-correction was not observed, but as in the subsequent 
observations with both sohd and hollow weights it was found to be about 
0-03 milHgramme per 1°, this value was assumed to hold here. In any case 
its effect is very small, as the temperature varied so little. 

The centre of swing was observed nearly continuously from the time when 
the hquid air was apphed and again after it was removed. After a time in 
each case it became steady and only these steady values are recorded in the 
following Tables. 

Table V. Experiment with Solid Weight cooled by Liquid Air, 

1 mm. = 0'315 milligramme. 

Correction for temperature of case — 0-1 division per 1°. 



Date 


Time 


Condition 

of 

weight 


Centre of 
swing 


Tempera 

ture 

of case 


Centre of 

swing 

corrected 

to 16°-6 


Remarks 


28.7.04 


3.25 p.m. 
5.50 „ 
6.0 „ 
6.10 „ 
8.45 „ 
9.15 „ 


Normal 
Cold 

Normal 


11-085 

11-07 

11-07 

11-07 

11-095 

11-095 


16°-6 
16 -65^ 
16 -65 \ 
16 •65j 
16 -4 \ 
16 -4 j 


11-085 
11-075 

11-075 


Liquid air applied just after 
3.25 

Liquid air removed and 
water applied just after 
6.10 



Normal -cold = 0-005 division = 0-0016 milligramme. 

Table VI. Experiment with Hollow Weight cooled by Liquid Air, 
1 mm. = 0-343 milligramme. 
Correction for temperature of case — 0-1 division per 1°. 



Date 


Time 


Condition 

of 

weight 


Centre of 
swing 


Tempera- 
ture 
of case 


Centre of 

swing 
corrected 
to 16°-6 


Remarks 


9.9.04 


9.40 a.m. 
11.40 „ 
11.50 „ 

5.25 p.m. 


Normal 
Cold 

Normal 


14-485 
14-480 
14-480 
14-480 


16°-3 
16 -4 1 
16 -4 / 
16 -4 


14-455 
14-460 
14-460 


Liquid air applied at 9.43 

Removed at 11.52 
Steady 

The balance next morning 
read 14-48 at 16°-3 



Normal - cold = - 0-002 division = - 0-0007 milligramme. 



160 AN EXPERIMENT WITH THE BALANCE TO FIND IF 

From Tables V and VI we have : 
Solid weight, 266 grammes: normal - cold ... = 0-0016 milhgramme. 
Hollow weight, 58 grammes: normal — cold... = — 0-0007 „ 

For the difference, 208 grammes : normal — cold = 0-002 „ 

Taking the fall in temperature as 200°, this gives a change of the order 
of 1 in 2 X 1010 per 1° fall. 

These hquid-air experiments were not repeated. But the conditions are 
probably much less disturbed than with the steam experiments, and we may 
safely say that if there is any change of weight with change of temperature 
between 16°-6 C. and - 186° C, it is not so great as 1 in lO^^ per 1° C. 

Note on the Change of Af parent Weight on First Heating or Cooling. 

We have mentioned that while the changes in the temperature of the 
weight were in progress there were considerable apparent variations in 
weif^ht. These, in a few cases, amounted to as much as 0-6 milligramme. 
They were almost certainly due to radiometric forces or to other gas-action, 
for they were very dependent on the disposition of the diaphragms in the 
tube T (Fig. 1), and also on the way in which the steam was blown through 
the jacket. 

In the preliminary experiments with solid weights the lowest diaphragm 
was 5 to 6 inches above the weight, and the steam was blown into the top 
of the jacket. Under these circumstances the following variations occurred 
when the steam was turned on : 

At first the weight apparently increased, until in 15 to 20 minutes it 
reached a maximum, which was in some cases as much as 0-6 milligramme 
above the real weight. After reaching this maximum the weight apparently 
decreased, till in four hours it had reached a nearly steady value, which was 
a little less than the value at the temperature of the laboratory. 

If, now, the jacket was filled with cold water, the apparent weight first 
increased for about one minute and then decreased for about two hours to 
a minimum, which was a little lower than the final weight at 100°. After 
this the weight very slowly increased, till in five to six hours it had recovered 
the value which it had before the experiment. 

These changes did not vary very much with the pressure, but at lower 
pressures they took place more rapidly than at higher ones. 

On coohng the weight with liquid air, changes occurred exactly similar 
to those which occurred when the weight was cooled from 100° to the 
temperature of the laboratory ; and when the weight was warmed up from 
the temperature of liquid air to the temperature of the laboratory, the 
changes were similar to those when the weight was warmed from the 
temperature of the laboratory to 100° C. 



CHANGE OF TEMPERATURE HAS ANY EFFECT UPON WEIGHT 161 

So long as the arrangement of the diaphragms and the weight remained 
the same, and so long as the steam was blown through in the same way, 
bhese changes were exactly similai:, but as soon as any alteration was made 
in these arrangements the character of the changes altered. 

In one series of experiments a sealed glass bulb containing mercury was 
used in place of the brass weight. In this case, immediately after the steam 
was turned on there was a rapid decrease in weight, and a minimum was 
reached in less than one minute. After this the changes were very similar 
to those occurring with the brass weight. On cooling, however, the changes 
were almost exactly the reverse of the changes on heating, and were not at 
all like the changes with the brass weight. 

In the final experiments, those recorded, the lowest diaphragm was within 
J-inch of the top of the brass weight. With this arrangement and with the 
steam blown into the top of the jacket, the following changes occurred : 

The apparent weight first increased rapidly, reaching a maximum in about 
one minute, then it rapidly decreased, reaching a minimum in about four 
minutes, and again increased to another and lower maximum in 8 to 10 
minutes. After this it slowly decreased to a nearly steady value a little 
lower than the original value. 

On passing cold water through the jacket the apparent weight rapidly 
increased for about one minute, and then slowly decreased to its original 
value. 

Still another variation was arranged by blowing the steam in at the 
bottom of the jacket instead of at the top, all the other things remaining as 
in the last experiment. 

In this case, on turning on the steam, the apparent weight first decreased 
to a minimum in about one minute, then increased to a maximum in about 
six minutes, and finally decreased slowly to a nearly steady value a little 
below the original value. 

The cooling and the consequent changes were exactly similar to those in 
the last experiment. 

It is somewhat difficult to follow out exactly the changes which w^ould be 
caused by radiometer-action and by convection-currents in these different 
arrangements of the apparatus, but the fact that these changes depend 
entirely on the arrangement is sufficient evidence that they are caused by 
gas-action, and, as we have before said, we have some reason to believe that 
even the small final difference is due to air-currents. 



p. c w. 11 



6. 

ON A METHOD OF DETERMINING THE SENSIBILITY 
OF A BALANCE. 

By J. H. PoYNTiNG and G. W. Todd, M.Sc. 

[Phil. Mag. 18, 1909, pp. 132-135.] 

[Read June 25, 1909.] 

In the method, as we have arranged it, a small frame (Fig. 1, end-view) 
is fixed at the centre of the beam of a 16-inch Oerthng balance. This carries 
two Vs about 2 cm. apart, and in the Vs hes a straight wire or fibre about 
3J cm. long, parallel to the beam and level with the central knife-edge. This 
wire takes the place of the ordinary rider, and we shall call it 'the rider.' 
Its weight is determined before use as accurately as possible by weighing on 
an assay-balance. The sensibility is determined by moving the rider either 
to right or left a measured distance. If this distance is d, if the half-length 
of beam is b, and if the weight of the rider is R, the movement is equivalent 
to an addition of weight to one pan, Rd/b. 

In order to move the rider a definite distance a stout horizontal rod 
(Fig. 2) passes through the balance-case from side to side without contact 
with the case, and is supported at its ends outside, and independent of, the 
case. It is parallel to the beam and a little lower than the V frame. On 
the rod are fixed horizontally two Brown and Sharp micrometer-screws 
divided to 0-01 mm. and allowing an estimate of 0-001 mm. Their axes are 
in one fine coinciding with the axis of the rider, and they are fixed so that 
one can bear against one end and the other against the other end of the rider. 
Their ends are plane and the ends of the rider are bluntly pointed. Each 
micrometer-screw head has a cross-piece fixed on it ; and a fork, which can be 
rotated about an axis in the continuation of the axis of the screw by a pulley 
outside the case, can engage with the cross-piece and so advance or withdraw 
the screw. The pulley is worked by an endless string passing to a pulley at 
the side of the observer, who is about 2 metres in front of the balance. The 
micrometer-divisions are illuminated and each micrometer is viewed by its 
own telescope. The position of the balance-beam is read by a double- 
suspension mirror, telescope and scale. The scale is divided to miUimetres 



ON A METHOD OF DETERMINING THE SENSIBILITY OF A BALANCE 163 

and is about 3 metres from the mirror. The double-suspension mirror is fully 
described in the Phil. Trans. A, vol. 182, p. 572*. It is of course not essential 
to the method, but was chosen because of the great magnification of the 
deflection which it gives. 

Let us suppose that the value of the scale-divisions of the deflection is to 
be determined by a movement of the rider from right to left. The two 
micrometer-screws are withdrawn so that neither is in contact with the rider, 
that on the left so far that the rider will not touch it in its subsequent travel. 
The beam is lowered and allowed to swing. Then the right-hand screw is 
advanced till it bears against the end of the rider and pushes it some small 
distance. The contact is seen to have occurred by the interference with 
freedom of swing, as watched in the telescope. The micrometer is then 



y/ qULJi> " I 



Fig. I. End-view of V frame fixed to balance-beam. 




Fig. 2. Arrangement of right-hand micrometer-screw. 

read. Let its reading be m^. It is then withdrawn a httle so as to leave 
the rider free, and the centre of swing C-^ is determined in the usual way 
from three successive turning-points. Then the micrometer is advanced 
again so as to push the rod a little further, and its reading m^ is taken. It 
is then withdrawn and the new centre of swing C^ is taken. If m^ — m^ = d, 
Ci — C2 divisions of deflection are due to an addition of Rdjh to the left pan. 

The right-hand micrometer may then be withdrawn and the left-hand 
micrometer may be brought into action in a similar manner, and so on, the 
two screws being used alternately. 

The balance-case was fixed on a shelf and was enclosed in a tin-foiled 
wooden box with wool loosely packed between box and case. The case and 

* [Collected Papers, Art. 3.] 

11—2 



164 ON A METHOD OF DETERMINING THE SENSIBILITY OF A BALANCE 

box were provided with plate-glass windows to view the mirror and the 
micrometer-divisions. The following abstract of some determinations of 
sensibihty will serve to show what accuracy may be attained: 

I. Rider German silver wire, 7-35 mgm. 
Half-length of beam, 20-272 cm. 
10 determinations alternately left and right. 
Mean travel of rider ... ... 24850 mm. 

Mean deflection ... ... ... 21-26 divisions. 

Mean value for 20 divisions . . . 0-0848 mgm. 
The separate determinations range between 
20 divisions - 0-0877 
and 20 „ = 0-0824. 

II. The same rider. 

10 determinations alternately left and right. 
Mean travel of rider ... ... 5-2713 mm. 

Mean deflection 45-47 divisions. 

Mean value for 40 divisions ... 0-1681 mgm. 
The separate determinations range between 
40 divisions =-- 0-1722 
and 40 „ = 0-1632. 

III. Rider German silver wire, 189-05 mgm. 

7 determinations alternately left and right. 

Mean travel of rider 0-1764 mm. 

Mean deflection 38-70 divisions. 

Mean value for 40 divisions ... 0-1691 mgm. 
The separate determinations range between 
40 divisions = 0-1709 
and 40 „ = 0-1654. 

IV. The same rider. 

7 determinations alternately left and right. 

Mean travel of rider 0-3004 mm. 

Mean deflection 64-84 divisions. 

Mean value for 60 divisions ... 0-2578 mgm. 
The separate determinations range between 
60 divisions = 0-2612 
and 60 „ = 0-2518. 



PAET II. 

ELECTRICITY. 



ON THE LAW OF FOKCE WHEN A THIN, HOMOGENEOUS, 
SPHEKICAL SHELL EXEKTS NO ATTRACTION ON A 
PARTICLE WITHIN IT. 

[Manchester Lit. Phil. Soc. Proc. 16, 1877, pp. 168-17L] 
[Read March 6, 1877.] 

If a homogeneous, thin, spherical shell of uniform thickness exert no 
attraction on a particle within it, then the law of the force is the law of 
nature. 

Professor Maxwell uses this proposition {Electricity, vol. 1, § 74) to deduce 
the law of the force between electrified bodies, and shows that it proves, far 
more conclusively than any direct measurements of electrical forces, that 
the law is that of the inverse square. It would therefore be an advantage 
to have a simpler proof of such an important proposition than that given 
by Laplace {Mec. Celeste, liv. ii, ch. 2) and followed by Maxwell. The following 
seems more simple, as it requires neither integration nor the solution of a 
functional equation : 

Let P be any point inside the spherical shell, 
C the centre of the sphere, DPCE the diameter 
through P, and APB perpendicular to CD. In 
Newton's proof of the proposition that, if the law 
of attraction be that of the inverse square, the 
force at P is zero, the surface is divided into an 
indefinitely great number of opposite elements by 
small cones having their vertices at P, and the 
attractions of each of these pairs of elements are 
shown to balance each other. We shall first show 




Fig. 1. 



that if the attraction at P is zero, then it follows inversely that, for at least 



166 ON THE LAW OF FORCE WHEN A THIN, HOMOGENEOUS, SPHERICAL 

one position (if not for all positions) of the cone MPm besides the position 
APB, the attractions of the opposite elements balance each other ; and we 
shall thence prove that the law of attraction must be that of the inverse 
square. 

Let us suppose the cone, with vertex at P, to move round from the position 
where AB is its axis to any other position MPm. At AB the attractions of 
the opposite sections on P are e({ual, whatever the law of the force. As the 
cone leaves AB let us suppose the resultant attraction of the two opposite 
elements to be no longer zero, but to act, say, towards the centre side of 
APB. Then it will either continue towards that side as the cone moves all 
the way round from APB to BPA, or it will vanish at some position, and then 
act in the opposite direction. In the first case we should have a number of 
forces all acting from P towards the same side of APB, whose resultant is 
zero ; then each separate force must be zero. In the second case the resultant 
attraction of the opposite sections vanishes somewhere between AP and 
EP\ then for at least one position of the cone, besides the position APB, 
the resultant attraction of the opposite sections vanishes. 

Since this is true for any position of P, we can show that the law of the 
force must be that of the inverse square. 

In the position where the two opposite sections exert equal attractions, 
two sections of the same thickness perpendicular to the axis of the cone 
would also exert equal attractions ; for they would bear to each other the 
same ratio as the two oblique sections made by the sphere, since these two 
obhque sections make equal angles with the axis of the cone. Then what 
we have proved is, that for every position of P there are two different distances 
for which the attractions of the sections of a small cone of equal thickness 
on a point at its vertex are equal. 

N 



Fig. 2. 

Let us take VMX to represent the axis of a cone of very small angle of 
which F is the vertex. 

At any point M draw an ordinate MN to represent the attraction of a 
section of the cone at M of small given thickness on a point at the vertex. 
Then N will trace out a curve as M moves along VX. 

Now take a spherical shell of thickness equal to the thickness of the 
sections of the cone, and of radius nearly equal to VM, where M is any 
arbitrary point in VX. Take a point near the centre of this sphere. As 
a cone moves round with this as vertex, its sections by the sphere must 
be always at distances very nearly equal to the radius from the vertex; 



SHELL EXERTS NO ATTRACTION ON A PARTICLE WITHIN IT 167 

and, by what we have proved above, for some position of the cone the 
attractions of the opposite sections must be equal. Therefore (in Fig. 2), 
for two distances very nearly equal to VM the ordinates must be equal to 
one another. Then the tangent to the curve near N must be parallel to VX. 
But M is arbitrary, for we can take the sphere of any size. Therefore at 
all points the tangent to the curve is parallel to VX ; and therefore the curve 
must be a straight line parallel to VX; or, the attractions by sections of 
the cone of equal thickness are constant, wherever the sections be taken. 
But the sections are proportional to the direct square of the distance; and 
therefore the law of the attraction must be that of the inverse square of the 
distance. 



8. 



ARRANGEMENT OF A TANGENT GALVANOMETER FOR LECTURE 
ROOM PURPOSES TO ILLUSTRATE THE LAWS OF THE 
ACTION OF CURRENTS ON MAGNETS, AND OF THE RESIS- 
TANCE OF WIRES. 

[Manchester Lit. Phil Soc, Proc. 18, 1879, pp. 85-88.] 
[Read April 1, 1879.] 

Three coils of similar wire are arranged round the circumference of a circle 
with a compass-needle at the centre, each wire going only once round the 
circle. The six ends of the three wires are connected by thick copper w'ltes 
Avith the six binding screws A, B, C, D, E, F. 

On a concentric circle of twice the 
radius is arranged a coil of the same 
wire going twice round the circle and 
having its ends connected by thick 
wires with the binding screws G, H. 
On the sam.e circle is a single wire of 
twice the diameter, making only one 
turn round the circle, and having its 
ends connected with the binding screws 
K, L. 

The coils are denoted respectively 
by the numbers 1-5. 

The null method is adopted in each 
case. That is two forces acting on 
the needle and arising from different 
arrangements of the circuit are shown 
to balance each other, and from the 
arrangements necessary to produce this 
equihbrium the desired laws are deduced. 

I. If the current be reversed the force is reversed. Introduce the current 
at A. Join BD and lead away at C. Then the same current goes in opposite 




B A 



GALVANOMETER TO ILLUSTRATE ACTION OF CURRENTS ON MAGNETS 169 

directions round the coils (1) and (2), and since the needle is not deflected the 
reversal of the force when the current is reversed is proved. 

II. The force is frofortional to the length of current acting. This is proved 
by the last, for the two coils (1) and (2) exert equal forces on the needle. 
If the current went round them in the same direction we should have twice 
the force which each exerted singly, with twice the length of current. This 
assumes that the current in different parts of the circuit is the same, which 
might be shown by sHghtly modifying I, thus : introduce between B and B 
various resistances and the equilibrium is not disturbed. 

III. The force is proportional to the strength of the current. Introduce at 
A, and connect A with C, and B with D; and connect D with F, and lead off 
from E. There will be two equal currents in the same directions in coils 
(1) and (2), for they are exactly similar to each other and similarly situated. 
These two currents unite to give a double current in the opposite direction 
in coil (3), and the double current in a single wire exerts twice the force 
exerted by each single current, since there is no deflection. 

IV. The force is inversely proportional to the square of the distance. Intro- 
duce at A, connect B with H, and lead off at G. Then since we have two 
turns to the coil (4) we have a current of four times the length at twice the 
distance acting in opposition to the same current through (1). Since there 
is no deflection, the two exert equal forces, and therefore the force is inversely 
proportional to the square of the distance. 



Resistance. 

I. The resistance is proportional to the length. Connect B with C, Z) with 
E, A with F. Introduce at A and lead off at E. We have then a divided 
circuit joining A and E, one branch consisting of the two coils (1) and (2), 
and the other the coil (3) only one-half the length and going in the opposite 
direction. But as there is no deflection the current through the first circuit 
of twice the length must be only one-half that through the other to exert 
an equal force, i.e., the resistance is doubled when the length is doubled. 

II. The resistance is inversely proportional to the cross-section. Introduce 
at A, connect A with L, connect B with C and D with K, and lead off at A'. 
Then we have two circuits connecting A with K, the first consisting of the 
two coils (1) and (2), the second of the coil (5) of the same length but of four 
times the cross-section and going round in the opposite direction. Since the 
needle is not deflected the two currents exert equal forces. But the coil (5) 
is at twice the distance and must therefore have four times as great a current 
through it as that through (1) and (2). That is, with equal lengths of wire 
of the same material connecting two points the currents conveyed are pro- 
portional to the cross-sections. 



9. 

ON THE GKADUATION OF THE SONOMETER. 

[Phil Mag. 9, 1880, pp. 59-64.] 

[Read before the Physical Society, December 13, 1879.] 

It seems likely that such valuable results will be obtained by means of 
Professor Hughes's sonometer, that it is desirable that some method should 
be employed to turn its at present arbitrary readings into absolute measure, 
so that, for instance, the induced currents caused by different metals in the 
induction-balance may be measured and compared with each other. 

In Maxwell's Electricity, vol. 2, chap, xiv, the general formula is given 
for the coefficient of induction of one circular circuit on another. Adapting 
this to the case where two equal circular circuits are on the same axis at a 
distance apart greater than the radius of the coils, the following formula is 
obtained. 

Let a = distance between centres, 
h = radius of either circle, 

c = distance of either circumference from centre of other, 
M = coefficient of induction. 

a^ (2 2a^^l6a^ 32 a^^ 256 a^ etc.| . ...^^j 

Of these the latter uses directly the distance between the centres, the observed 
quantity — but is not nearly so convergent as the former, in which c may be 
at once deduced from c = Va^^b^ 

To obtain formulae which might be strictly applied to the sonometer, we 
should have to consider the more general case of two coils of unequal radii 
6 and ^, for which I have found the formula corresponding to (2), viz. 

^-^ _ 47762^2 ji ^ 3 52^_|_ ^2 15 54 ^ 352^2 _!_ ^4 



+ 



16 a4 

_35 6e+66_^^66^^_^^^^^ (3) 



ON THE GRADUATION OF THE SONOMETER 171 

We should then have to take the finite integrals of each term between the 
limiting values of b and ^. But this would be exceedingly complicated and 
would require a knowledge of all the details of construction; and we may 
at least get a first approximation to the true result by replacing the coils by 
a single one of a radius intermediate between the greatest and least radii. 

In Prof. Hughes's paper {Phil. Mag. July 1879) he gives the internal and 
external radii of his coils as 15 miUims. and 27-5 millims. respectively. I have 
considered, then, that 25 millims. will give results not very far from the truth ; 
and as it makes the calculations considerably easier, I have taken that as the 
value of b and applied the formulae to the numbers given in the paper. The 
resultant current in the middle coil was zero when it was distant 47 millims. 
from one end and 200 from the other. This enables us to find the ratio between 
the number of turns in the two ends at least sufficiently nearly to apply to 
some of the results. 

Let M^ be the coefficient of induction of the larger coil on the moveable 
one, ilf 2 that of the smaller, the former having m turns, the latter n. When 
the moveable coil was 200 millims. from the large and 47 millims. from the 
small coil, since there was no induced current, 

mMi = nM^ . 
Applying formula (1), we have for the larger coil 

c = V2002+ 252 = 201-5, 
and for the smaller coil 

c = \/472+ 252 = 53-2, 
b being the same for both. Then 

_jri__ jl _ 3 / 25 y 15 / 25 y _ 
(201-5)3 [2 4 1201-5/ ^ 8 V201-5y ~ ^*^' 

1 3/25 \2 15 / 25 \4 35 / 25 \6 2835 / 25 



(53-2)3 (2 4 



/ 25 \2 15 / 25 y 35 / 25 \« 2835 / 25 y 
153-2/ + 8 V53-2J 8 V53-2/ "^ 256^ 153-2; 



Multiplying each side by 2 and finding the successive terms, 

122 

m X ^3 {1 - -02308 + -00088 - etc.} 

= nx ^ {1 - -33123 + -18286 - -09422 + -02633 - etc.}, 

^ = 43-6. 

n 

I have applied the formula to the results for various metals given by 
Prof. Hughes in a table in his paper. In the table below, in the second column 
are Prof. Hughes's numbers, i.e. distances from the point of no induction. 
In the third are numbers proportional to mM^ — nM^ ; where M^ , M2, are 
the coeiB&cients of induction of two simple coils calculated on the above 



172 



ON" THE GRADUATION^ OF THE SONOMETER 



hypothesis, m and n the number of turns in the two respectively. In the 
fourth column are the resistances for bars of the metal 100 miUims. long and 
1 millim. in diameter (Jenkin, p. 249). In the last column are the products 
of the numbers in the two preceding columns. 



Metal 


Distance 
from point of 
no induction 


7nMj^ - nM., , 
proportional to 


E 


(mif 1 - nM^) R 


Silver 


125 


178 


•21 


37-4 


Gold 


117 


135 


-27 


36-5 


Aluminium 


112 


116 


•375 


43-5 


Copper 


100 


84 


-21 


17-6 


Zinc 


80 


501 


•72 


36-1 


Tin 


74 


44-6 


1-70 


75-8 


Iron ... 


45 


22-46 


1-25 


281 


Lead 


38 


18-87 


2-5 


47-2 


Antimony 


35 


17-35 


4-5 


78-1 


Bismuth 


10 


5-75 


16-8 


96-6 



Mercury has been omitted, as it gives a very much higher value than any 
of the others. Were the induced currents in the induction-balance pro- 
portional to the resistances given in the table, the numbers in the last column 
would of course be all the same. The deviations from equality are far greater 
than could be accounted for by errors in the approximations I have adopted, 
especially for the metals not at the beginning or end of the list. Hence we 
are driven to conclude, either that the resistances of the metals given in the 
tables are not the same as the resistances of the metals used by Prof. Hughes, 
or that the induced current is not proportional to the conductivity of the 
metal. 

It should be noticed that the method of measuring currents by the 
sonometer assumes that the telephone integrates, as it were, the current; 
i.e. the loudness of the sound depends only on the total current, not on the 
time during which the current is passing, provided that the time be very 
short. I do not know whether this point has been investigated ; but if not, 
it would probably be easy to examine it by means of the sonometer. It 
would be advisable to modify the instrument in such a way that the formulae 
might be more easily employed, and that the approximations might be nearer 
to the truth. 



The formulae used in this paper may be obtained as follows, the method 
being adapted from that given by Maxwell. 

The potential of a circular unit current at any point is the same as that 
of a magnetic shell of unit strength bounded by the circuit. This, again, 
is the same as the attraction of a thin plate of matter of unit surface-density 



ON THE GRADUATION OF THE SONOMETER 173 

in a direction perpendicular to the plane of the plate. If co be the attraction 
of a plate of radius b, at a point distant c from the plate along its axis, 



= 277 



Cv'-i) 



^ (162 1.3 6* 1.3.5 6« ) 

= ^" 12 ^-271 0-^ + 27176 c«-"*"t 

If we introduce zonal harmonics as coefficients, this becomes 

^ fl 62 1 . 3 6* ^ 1 . 3 . 5 6« ^ 

" = ^" 12 ^^^"2-74 0^^3+ 27476 -B^5-etc. 

This is now the potential at any point in space where 6 < c. 

If there be a second circular circuit of radius j8 on the same axis, we may 
suppose it replaced by a magnetic shell bounded by the current and lying on 
the sphere, with centre at the centre of the first current, the radius of the 
sphere being c. 

This shell may be considered to consist of two layers of matter of equal 
and opposite densities, fi and — /x, at distances c and c + dc from the centre. 
The potential on the second layer is 



II' 



II 



fJLO) dS, 

where the integration is taken over the shell. The potential on the first 
layer is 

jLt f CO + -,- dc) dS, 

the sum being — j I [jl -^ dcdS ; 

but since the strength = 1, fjidc = 1, and we have the mutual potential 

Replacing the element dS by cH^zdcj), the Umits will be for ^ from to 277, 
and for jjl from 1 to fx. 

Integrating with respect to (f>, and remembering that c is constant in 
integrating for jjl, we have 

But we have the relation for zonal harmonics, 



174 ON THE GRADUATION OF THE SONOMETER 

Substituting, we obtain 

The following are the values for the coefficients (Ferrers's Spherical Har- 
monies : p. 23), both in terms of /jl and when we substitute /x^ = 1 — ^ : 

djji 



dl^- 


4 „,._.,. l(4_4:). 


dp, 

'dl^- 


.i?|2V'-lV+l)-f(8-28f + ='«'.)^ 


dp, 


3003/x« - 3465^4 + 945^^ _ 35 ^^^ " ^^^4 ^ + etc. 


di. 


16 16 


dPs 


109395/x8 - 180180/x6 + 90090^* - 13860/x2 + 315 


d^ 


128 
5760 + etc. 



128 
Substituting these values and putting c^ = a^ + ^^^ ^g obtain 

, 62/3M1 3 62 + R2 15 6*+ 362^2 _^ ^4 

M = — 477 — ^ - — I - - - - - 

a3 ^^2 4 a2 -+-16 ^4 

_ 35 66jf664^2 _!_ 652^4 _|_ ^6 I 

32 a« +etc.|. 

The more useful form is obtained by retaining c. If we take the two circles 
of equal radius (i.e. 6 = ^), we obtain 

,. , o &M1 3 62 15 64 35 ¥ 2835 6^ ] 

^^-^""c3]2-4c2 + ¥c4-¥c«+^5^cS-^'4' 



10. 

ON THE TRANSFER OP ENERGY IN THE 
ELECTROMAGNETIC FIELD. 

[Phil. Trans. 175, 1884, pp. 343-361.] 

[Received December 17, 1883. Read January 10, 1884.] 

A space containing electric currents may be regarded as a field where 
energy is transformed at certain points into the electric and magnetic kinds 
by means of batteries, dynamos, thermoelectric actions, and so on, while in 
other parts of the field this energy is again transformed into heat, work done 
by electromagnetic forces, or any form of energy yielded by currents. 
Formerly a current was regarded as something travelling along a conductor, 
attention being chiefly directed to the conductor, and the energy which 
appeared at any part of the circuit, if considered at all, was supposed to be 
conveyed thither through the conductor by the current. But the existence 
of induced currents and of electromagnetic actions at a distance from a 
primary circuit from which they draw their energy has led us, under the 
guidance of Faraday and Maxwell, to look upon the medium surrounding 
the conductor as playing a very important part in the development of the 
phenomena. If we believe in the continuity of the motion of energy, that is, 
if we believe that when it disappears at one point and reappears at another 
it must have passed through the intervening space, we are forced to conclude 
that the surrounding medium contains at least a part of the energy, and 
that it is capable of transferring it from point to point. 

Upon this basis Maxwell has investigated what energy is contained in the 
medium, and he has given expressions which assign to each part of the field 
a quantity of energy depending on the electromotive and magnetic intensities 
and on the nature of the matter at that part in regard to its specific inductive 
capacity and magnetic permeabihty. These expressions account, as far as 
we know, for the whole energy. According to Maxwell's theory, currents 
consist essentially in a certain distribution of energy in and around a con- 
ductor, accompanied by transformation and consequent movement of energy 
through the field. 

Starting with Maxwell's theory, we are naturally led to consider the 
problem : How does the energy about an electric current pass from point to 



176 ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 

point — that is, by what paths and according to what law does it travel from 
the part of the circuit where it is first recognisable as electric and magnetic 
to the parts where it is changed into heat or other forms ? 

The aim of this paper is to prove that there is a general law for the transfer 
of energy, according to which it moves at any point perpendicularly to the 
plane containing the Hnes of electric force and magnetic force, and that the 
amount crossing unit of area per second of this plane is equal to the product 
of the intensities of the two forces, multiphed by the sine of the angle between 
them, divided by 477 ; while the direction of flow of energy is that in which 
a right-handed screw would move if turned round from the positive direction 
of the electromotive to the positive direction of the magnetic intensity. 
After the investigation of the general law several appHcations will be given 
to show how the energy moves in the neighbourhood of various current- 
bearing circuits. 

The following is a general account of the method by which the law is 
obtained. 

If we denote the electromotive intensity at a point (that is, the force per 
unit of positive electrification which would act upon a small charged body 
placed at the point) by @, and the specific inductive capacity of the medium 
at that point by K, the magnetic intensity (that is, the force per unit pole 
which would act on a small north-seeking pole placed at the point) by ^ and 
the magnetic permeability by yi, Maxwell's expression for the electric and 
magnetic energies per unit volume of the field is 

K^^I^TT + ilSy-I^TT (1) 

If any change is going on in the supply or distribution of energy the 
change in this quantity per second will be 

/.ef/4. + ,^f/4. (2) 

According to Maxwell the true electric current is in general made up of 
two parts, one the conduction-current ^T, and the other due to change of 
electric displacement in the dielectric, this latter being called the displace- 
ment-current. Now, the displacement is proportional to the electromotive 
intensity, and is represented by /i(S/47r, so that when change of displacement 
takes place, due to change in the electromotive intensity, the rate of change, 

that is, the displacement-current, is K~ Utt, and this is equal to the difference 

between the true current (5 and the conduction-current >T. Multiplying this 
difierence by the electromotive intensity ik the first term in (2) becomes 

— ^^=m-^.^ (3) 



ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 177 

The first term of the right side of (3) may be transformed by substituting 
for the components of the total current their values in terms of the com- 
ponents of the magnetic intensity, while the second term, the product of the 
conduction-current and the electromotive intensity, by Ohm's law, which 
states that ^ = 6'@, becomes ^^/C, where C is the specific conductivity. 
But this is the energy appearing as heat in the circuit per unit volume according 
to Joule's law. If we sum up the quantity in (3) thus transformed, for the 
whole space within a closed surface, the integral of the first term can be integrated 
by parts, and we find that it consists of two terms — one an expression 
depending on the surface alone to which each part of the surface contributes 
a share depending on the values of the electromotive and magnetic intensities 
at that part, the other term being the change per second in the magnetic 
energy (that is, the second term of (2)) with a negative sign. The integral 
of the second term of (3) is the total amount of heat developed in the con- 
ductors within the surface per second. We have then the following result. 

The change per second in the electric energy within a surface = (a 
quantity depending on the surface) — (the change per second in the magnetic 
energy) — (the heat developed in the circuit). 

Or rearranging : 

The change per second in the sum of the electric and magnetic energies 
within a surface together with the heat developed by currents is equal to 
a quantity to which each element of the surface contributes a share depending 
on the values of the electric and magnetic intensities at the element. That 
is, the total change in the energy is accounted for by supposing that the 
energy passes in through the surface according to the law given by this 
expression. 

On interpreting the expression it is found that it implies that the energy 
flows as stated before, that is, perpendicularly to the plane containing the 
lines of electric and magnetic force, that the amount crossing unit area per 
second of this plane is equal to the product 

electromotive intensity x magnetic intensity x sine included angle 

while the direction of flow is given by the three quantities, electromotive 
intensity, magnetic intensity, flow of energy, being in right-handed order. 

It follows at once that the energy flows perpendicularly to the fines of 
electric force, and so along the equipotential surfaces where these exist. It 
also flows perpendicularly to the lines of magnetic force, and so along the 
magnetic equipotential surfaces where these exist. If both sets of surfaces 
exist their lines of intersection are the lines of flow of energy. 

The following is the full mathematical proof of the law : 

V. a. w. 12 



178 ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 

The energy of the field may be expressed in the form (Maxwell's Electricity, 
vol. 2, 2nd ed., p. 253) 



1 1 j(P/+ Qg + Ith) dxdydz + g- | U(aa + 6^ + cy) dxdydz, 



the first term the electrostatic, the second the electromagnetic energy. 

But since f = — P, with corresponding values for g and h, and a = fia, 
h = /x^, c ^ /xy, substituting, the energy becomes 

^ \\\{P^ + Q^ + i?2) ^:r%6?0 + £- [J|(a2 + ^^ + ^2) dxdydz. ...(1) 

Let us consider the space within any fixed closed surface. The energy 
within this surface will be found by taking the triple integrals throughout 
the space. 

If any changes are taking place the rate of increase of energy of the electric 
and magnetic kinds per second is 

Now Maxwell's equations for the components of the true current are 
, df dg dh 

where f, q, r are components of the conduction-current. 

But we may substitute for ^ its value y- , and so for the other two, 

and we obtain 

KdP_ 

477 dt ~ 



K dQ 

4^dt^'-^ 
K dR 

in dt 
Taking the first term in (2) and substituting from (3) we obtain 



,(3) 



477 



= i i |{^ i^* -~P)^Q{v-q) + R {w - r)} dxdydz 

= \\\(Pu + Qv + Rw) dxdydz - 1 1 j(P^ + §^ + Er) dxdydz. ...(4) 



Now the equations for the components of electromotive force are (Maxwell 
vol. 2, p. 222) : 



ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 179 

Q = az-cx-^^ -^r = az-cx + Q'^, (5) 

„ , . . dH dih J . . -,,\ 
R = hx — ay — -J- — -^ = bx — ay + R ] 

where P', Q', R' are put for the parts of P, Q, Ji which do not contain the 
velocities. 

Then 
Pu + Qv + Rw = (cij — h'z) u -h {az — ex) v + (bx — ay) iv + P'u + Q'v + R' w 

= — {{vc — wb) X + {wa — uc)y -{- (ub — va) z) + P'u + Q'v + R'w 

= -(Xx+ Yy + Z'z) + P'u + Q'v + R'w, 

where X, Y, Z are the components of the electromagnetic force per unit of 
volume (Maxwell, vol. 2, p. 227). 

Now substituting in (4) and putting for u, v, w their values in terms of 
the magnetic force (Maxwell, vol. 2, p. 233) and transposing we obtain 



+ I \j{{Xx +Yy + Z'z) + [Pv + Qq + Rr)} dxdydz 
= \\\(P'u + Q'v + R'w) dxdydz 



(Integrating each term by parts) 

= ^ \\(R'^ - Q'y) dydz + ^ [[(P^ - R'a) dzdx + ^ \\(Q'a - P'^) dxdy 
I [(({^dR' dQ' ^ dP' dR' ^ dQ' r,dP'\. . . 

(The double integral being taken over the surface) 

1 



^^ ^{l (R'p - Q'y) + m (P'y - R'a) + n (Q'a - P'^)} dS 

- Mk S - f ) - * (f - f ) - r (f - SI "'"■■■■^" 

where 2, m, n are the direction- cosines of the normal to the surface outwards. 

12—2 



180 ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 

But from the values of P' , Q\ R' in (5) we see that 



dQ' 
dz 


dR' d^G d^ dm ^ d^ 
dy ~ dtdz dxdz dtdy dzdx 

d /dH dG\ 

dt \ dy dz ) 




"Tt^^Tt (^^^^^11' ^^^- 2> P- 216) 




dR' dP' db dp 
dx dz dt ^ dt' 




dP' dQ' _dc _ dy 
dy dx dt ^ dt' 



similarly 



Whence the triple integral in (6) becomes 
Transposing it to the other side we obtain 

(Xx + Yy + Zz) dxdydz + I \(Pf + Qq+ Rr) dxdydz 



= ~ j |{/ (R'P - Q'y) + m {P'y - R'a) + n (Q'a - P'^)} dS. ...(7) 

477.' .' 

The first two terms of this express the gain per second in electric and 
magnetic energies as in (2). The third term expresses the work done per 
second by the electromagnetic forces, that is, the energy transformed by the 
motion of the matter in which currents exist. The fourth term expresses 
the energy transformed by the conductor into heat, chemical energy, and so 
on ; for P, Q, R are by definition the components of the force acting at a point 
per unit of positive electricity, so that Pp dxdydz or Pdxpdydz is the work 
done per second by the current flowing parallel to the axis of x through the 
element of volume dxdydz. So for the other two components. This is in 
general transformed into other forms of energy, heat due to resistance, 
thermal effects at thermoelectric surfaces, and so on. 

The left side of (7) thus expresses the total gain in energy per second 
within the closed surface, and the equation asserts that this energy comes 
through the bounding surface, each element contributing the amount ex- 
pressed by the right side. 

This may be put in another form, for if (S' be the resultant of P', Q\ R\ 
and 9 the angle between its direction and that of ^, the magnetic intensity, 



ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 181 

the direction-cosines L, M, N of the Une perpendicular to the plane containing 
@' and Ǥ are given by 

@'§sin<9' r§sin6>' ($'^ sin ^ ' 

so that the surface-integral becomes 

^ [fd'^ sin ^ (Z? + Mm + iV^n) cZ/Sf. 

If at a given point (?/S be drawn to coincide with the plane containing 
(S' and t§, it then contributes the greatest amount of energy to the space ; 
or in other words the energy flows perpendicularly to the plane containing 
($' and ^, the amount crossing unit area per second being ^'^ sin ^/477-. To 
determine in which way it crosses the plane take ^' along Oz, § along Oy. 
Then 

P' = 0, g' = o, |!=1, 

a=0, 1 = 1, y = 0, 

and if sin ^ = 1 X - 1, M = 0, iV = 0. 

If now the axis Ox be the normal to the surface outwards, ? = 1, m = 0, 
n = 0, so that this element of the integral contributes a positive term to the 
energy within the surface on the negative side of the yz plane ; that is, the 
energy moves along xO, or in the direction in which a screw would move if 
its head were turned round from the positive direction of the electromotive 
to the positive direction of the magnetic intensity. If the surface be taken 
where the matter has no velocity, (S' becomes equal to d, and the amount 
of energy crossing unit area perpendicular to the flow per second is 

electromotive intensity x magnetic intensity x sine included angle 

477 

Since the surface may be drawn anywhere we please, then wherever there is 
both magnetic and electromotive intensity there is flow of energy. 

Since the energy flows perpendicularly to the plane containing the two 
intensities, it must flow along the electric and magnetic level surfaces, when 
these exist, so that the lines of flow are the intersections of the two surfaces. 

We shall now consider the appHcations of this law in several cases. 

Applications of the Law of Transfer of Energy. 

(1) A straight wire conveying a current. 

In this case very near the wire, and within it, the lines of magnetic force 
are circles round the axis of the wire. The Hnes of electric force are along 
the wire, if we take it as proved that the flow across equal areas of the cross- 



182 ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 



section is the same at all parts of the section. If AB, Fig. 1, represents the 

wire, and the current is from A to B, then a tangent plane to the surface 

at any point contains the directions of both the electromotive 

and magnetic intensities (we shall write e.m.i. and m.i. for 

these respectively in what follows), and energy is therefore 

flowing in perpendicularly through the surface, that is, along 

the radius towards the axis. Let us take a portion of the 

wire bounded by two plane sections perpendicular to the axis. 

Across the ends no energy is flow^ing, for they contain no 

component of the e.m.i. The whole of the energy then enters 

in through the external surface of the wire, and by the general 

theorem the amount entering in must just account for the heat 

developed owing to the resistance, since if the current is steady 

there is no other alteration of energy. It is, perhaps, worth 

while to show independently in this case that the energy moving 

in, in accordance with the general law, will just account for 

the heat developed. 

Let r be the radius of the wire, i the current along it, a 
the magnetic intensity at the surface, P the electromotive 
intensity at any point within the wire, and V the difference of potential 
between the two ends. Then the area of a length I of the wire is 27Trl, 
and the energy entering from the outside per second is 



c. 



_) 



Fig. 1. 



area x e.m.i. x m.i. ^Trrl.P.a 



Att 



477 

27rm . PI 



4i7T 

for the hne-integral of the magnetic intensity 277m round the wire is 47r x current 
through it, and PI = V. 

But by Ohm's law 7 = iR and iV =-- i^R, or the heat developed according 
to Joule's laAv. 

It seems then that none of the energy of a current travels along the wire, 
but that it comes in from the non-conducting medium surrounding the wire, 
that as soon as it enters it begins to be transformed into heat, the amount 
crossing successive layers of the wire decreasing till by the time the centre 
is reached, where there is no magnetic force, and therefore no energy passing, 
it has all been transformed into heat. A conduction-current then may be 
said to consist of this inward flow of energy with its accompanying magnetic 



ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 183 

and electromotive forces, and the transformation of the energy into heat within 
the conductor. 

We have now to inquire how the energy travels through the medium on 
its way to the wire. 

(2) Discharge of a condenser through a wire. 

We shall first consider the case of the slow discharge of a simple condenser 
consisting of two charged parallel plates when connected by a wire of very 
great resistance, as in this case we can form an approximate idea of the actual 
path of the energy. 




Fig. 2. 

Let A and B, Fig. 2, be the two plates of the condenser, A being positively 
and B negatively electrified. Then before discharge the sections of the equi- 
potential surfaces will be somewhat as sketched. The chief part of the energy 
resides in the part of the dielectric between the two plates, but there will 
be some energy wherever there is electromotive intensity. Between A and B 
the E.M.I, will be from A to B, and everywhere it is perpendicular to the level 
surfaces. Now connect A and 5 by a fine wire LMN of very great resistance, 
following a line of force and with the resistance so adjusted that it is the same 
for the same fall of potential throughout. We have supposed this arrange- 
ment of the resistance so that the level surfaces shall not be disturbed by the 
flow of the current. The wire is to be supposed so fine that the discharge 
takes place very slowly. 

While the discharge goes on a current flows round LMN in the direction 
indicated by the arrow, and there is also an equal displacement-current from 
B to A due to the yielding of the displacement there. The current will be 



184 ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 

encircled by lines of magnetic force, which will in general form closed curves 
embracing the circuit. The direction of these round the wire will be from 
right to left in front, and round the space between A and B from left to right 
in front. The e.m.i. is always from the higher level surfaces — those nearer 
A, to the lower— those nearer B, both near the wire and in the space between 
A and B. 

Now, since the energy always moves perpendicularly to the hues of e.m.i. 
it must travel along the equipotential surfaces. Since it also moves perpen- 
dicularly to the hnes of m.i. it moves, as we have seen in case No. (1), inwards 
on all sides to the wire, and is there all converted into heat — if we suppose the 
discharge so slow that the current is steady during the time considered. But 
between A and B the e.m.i. is opposed to the current, being downwards, 
while the m.i. bears the same relation to the current as in the wire. Eemem- 
bering that e.m.i., m.i., and direction of flow of energy are connected by the 
right-handed screw relation, we see that the energy moves outwards from the 
space between A and B. As then the strain of the dielectric between A and 
B is gradually released by what we call a discharge current along the wire 
LMN, the energy thus given up travels outwards through the dielectric, 
following always the equipotential surfaces, and gradually converges once 
more on the circuit where the surfaces are cut by the wire. There the energy 
is transformed into heat. It is to be noticed that if the current may be con- 
sidered steady the energy moves along at the same level throughout. 

(3) A circuit containing a voltaic cell. 

When a circuit contains a voltaic cell we do not know with certainty what 
is the distribution of potential, but most probably it is somewhat as follows* : 
— Suppose we have a simple copper, zinc, and acid cell producing a steady 
current. There is probably a considerable sudden rise in passing from the 
zinc to the acid, the place where the chemical energy is given up, a fall through 
the acid depending on the resistance, a sudden fall on passing from the acid 
to the copper, where some energy is absorbed with evolution of hydrogen, 

* It seems probable that the only legitimate mode of measuring the difference of potential 
between two points in a circuit consisting of dissimilar conductors carrying a steady current, 
consists in finding the total quantity of energy given out in the part of the circuit between the 
two points while unit quantity of electricity passes either point. If this is the case, it seems 
impossible that the surface of contact of dissimilar metals can be the chief seat of the electro- 
motive force, for we have only the very slight evolution or absorption of energy there due to the 
Peltier effect. I have therefore adopted the theory of the voltaic circuit in which the seat of 
at least the chief part of the electromotive force is at the contact of the acid and metals. The 
large differences of potential found by electrometer methods between the air near two different 
metals in contact are, in this theory, to be accounted for by the supposition that the air acts in 
a similar manner to an oxidising electrolyte. A short statement of the theory is given in a letter 
by Professor Maxwell in the Electrician for April 26th, 1879, quoted in a note on page 149 of his 
Elementary Treatise on Electriciiy. (See also § 249, vol. 1, Maxwell's Electricity and Magnetism.) 
June 19, 1884. 



ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 185 

and then a gradual fall through the wire of the circuit round to the zinc 
again. There will be a sHght change of potential in passing from copper to 
zinc, but this we shall neglect for simphcity. The equipotential surfaces will 
probably then be somewhat as sketched in Fig. 3*, all the surfaces starting 
from where the acid comes in contact with the zinc, some of the highest 
potential passing through the acid, others passing between the acid and 
copper, and crowding in there, the rest lower than these cutting the circuit 
at right angles in points at intervals representing equal falls of potential. 




Fig. 3. 

If this be the actual arrangement, then it is seen that the current, which 
travels round the circuit from zinc through acid to copper, is in opposition 
to the E.M.I, between the zinc and acid, while the m.i. is related to the current 
in the ordinary way. The energy will therefore pass outwards from there 
along the level surfaces. In fact, the medium between the zinc and acid 
behaves Kke the medium between the plates of the condenser in case No. (2j, 
and it seems possible that the chemical action produces continually fresh 
* electric displacement' from acid towards zinc which yields as rapidly as it 
is formed, the energy of the displacement moving out sideways. 

Some of this energy which travels along the highest level surfaces will 
converge on the acid, and there be, at any rate ultimately, converted into 
heat. Some of it will move along those surfaces which crowd in between 
the acid and copper and there converge to supply the energy taken up by the 
escaping hydrogen. The rest spreads out to converge at last at different 
parts of the circuit, and there to be transformed into heat according to 
Joule's law. 

It may be noticed that if the level surfaces be drawn with equal differences 

* In this and the succeeding cases the circuit is alone supposed to cause the distribution of 
potential. In actual cases the surfaces would probably be very much deflected from their normal 
positions in the dielectric through the presence of conductors, electrified matter, and so on. 



186 ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 

of potential, equal amounts of energy travel out per second between successive 
pairs of surfaces. For the amount transformed in the circuit in a length 
having a given difference of potential V between its ends will be 7 x current, 
and therefore the amount transformed between each pair of surfaces drawn 
with the same potential difference will be the same. But since the current 
and the field are steady, the energy transformed will be equal to the energy 
moving out from the cell between the same surfaces — the energy never 
crossing level surfaces. This admits of a very easy direct proof, but the 
above seems quite sufficient. 

This result has a consequence which, though already well known, is worth 
mentioning here. Let F^ be the difference of potential between the zinc and 
acid, V2 that between the acid and copper. If i be the current, V-^i is the 
total energy travelling out per second from the zinc surface. Of this Fg^ 
is absorbed at the copper surface, the rest, viz., (Fi — Fg) ^, being trans- 
formed in the circuit. The fraction, therefore, of the whole energy sent out 

which is transformed in the circuit is - ^w — -, a result analogous to the 

expression for the amount of heat which can be transformed into work in 
a reversible heat-engine. 

One or two interesting illustrations of this movement of energy may be 
mentioned here in connection with the voltaic circuit. 

Suppose that we are sending a current through a submarine cable by a 
battery with, say, the zinc to earth, and suppose that the sheath is every- 
where at zero potential. Then the wire will everywhere be at higher potential 
than the sheath, and the level surfaces will pass from the battery through 
the insulating material to the points where they cut the wire. The energy 
then which maintains the current, and which works the needle at the further 
end, travels through the insulating material, the core serving as a means to 
allow the energy to get in motion. 

Again, when the only effect in a circuit is the generation of heat, we have 
energy moving in upon the wire, there undergoing some sort of transformation, 
and then moving out again as heat or Ught. If Maxwell's theory of light be 
true, it moves out again still as electric and magnetic energy, but with a 
definite velocity and intermittent in type. We have in the electric Hght, 
for instance, the curious result that energy moves in upon the arc or filament 
from the surrounding medium, there to be converted into a form which is 
sent out again, and which, though still the same in kind, is now able to affect 
our senses. 

(4) Thermoelectric circuits. 
Let us first take the case of a circuit composed of two metals, neither of 
which has any Thomson effect. Let us suppose the current at the hot junction 
flows from the metal A to the metal B, Fig. 4. According to Professor Tait's 



ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 187 

theory it would appear that the e.m.i. at the hot junction is to that at the cold 
as the absolute temperature at the hot is to that at the cold junction. If 
the current is steady there is probably then a sudden rise in potential from 
A to B at the hot junction, a gradual fall along B, a sudden fall at the cold 
junction — less, however, than the sudden rise at the other — and a gradual 
fall along A. The level surfaces will then all start from the hot junction, 
the higher ones cutting the circuit at successive points along B, several con- 
verging at the cold junction, and the rest cutting the circuit at successive 
points along A. The heat at the hot junction is converted into electric and 
magnetic energy, which here moves outwards, since the current is against 
the E.M.I. Some of this energy converges upon B and A, to be converted 




Fig. 4. 

into heat, according to Joule's law, and some on the cold junction, there 
producing the Peltier heating effect. 

Let us now suppose that we have a circuit of the same two metals, now all 
at the same temperature, but with a battery interposed in B, which sends 
a current in the same direction as before (Fig. 5). Then if C be the junction 
which was hot, and D that which was cold in the last case, we know that the 
current will tend to cool C and to heat Z). In going from A to B at C there 
will be a sudden rise of potential, and in going from B to A at D there will 
be a sudden fall. Then, since the potential falls, as we go with the current 
along A, there will be a point on A near C which has the same potential as 
B at the junction. From this point to C, A will have lower potentials, and 
points with the same potentials will exist on B between C and the battery. 
Then either the level surfaces passing through C are closed surfaces, cutting 
A or B, and not passing through the battery at all, or, as seems much more 



188 ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 

probable, the surfaces from the battery which pass through C cut the circuit 
in three points in all outside the battery: once somewhere along A, once at 
C, and once somewhere along B. I have drawn and numbered the surfaces 
in the figure on this supposition. The heat developed in the parts of the 
circuit near C will thus be partly supphed from the junction C, where the 




Fig. 6. 

current is against the e.m.i. The energy therefore moves out thence, giving 
a cooling effect. 

The Thomson effect may be considered in somewhat the same way. Let 
us suppose that a metal BC of the iron type, and with temperature falhng 
from B to C, forms part of a circuit between two neutral metals of the lead 
type AB and CD, Fig. 6, and let us further, for simphcity, suppose that these 



1 



ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 189 

metals are each at the neutral temperatures with respect to BC, so that there 
is no E.M.I, at the junction. If we drive a current from A to D hj means 
of some external e.m.i., say at a junction elsewhere in the circuit, the potential 
will tend to fall from A to D. But a current in iron from hot to cold cools 
the metal, that is, the e.m.i. appears to be in opposition to the current, so 
that the energy moves outwards. The potential, therefore, tends to rise 
from B to C, and actually will do so if the resistance of BC is negligible 
compared with that of the rest of the circuit. In this case the level 
surfaces will probably be somewhat as indicated in Fig. 6, where they are 
numbered in order, each surface which cuts BC also cutting AB and CD, 
and the energy moving outwards will come into the circuit again at the 
parts of AB and CD near the junctions, where it will be transformed once 
more into heat. If the resistance of BC be gradually increased the fall of 
potential, according to Ohm's law, will tend to lessen the rise, and fewer 
surfaces will cut BC. It would seem possible so to adjust matters that the 
two exactly neutralised each other so that no energy either entered or left 
BC. In this case we should only have hues of magnetic force round BC, 
and no other characteristic of a current in that part of the circuit*. 

If this is the true account of the Thomson effect it would appear that it 
should be described not as an absorption of heat or development of heat by 
the current but rather as a movement of energy outwards or inwards, 
according as the e.m.i. in the unequally heated metal opposes or agrees with 
the direction of the current. 

(5) A circuit containing a motor. 

This case closely resembles the third case of a circuit containing a copper- 
zinc cell, the motor playing a part analogous to that of the surface of contact 
of the acid with the copper. Let us, for simplicity, suppose that the motor 
has no internal resistance. When it has no velocity all the level surfaces 
cut the circuit, and the energy leaving the dynamo or battery is all transformed 
into heat due to resistance. But if the motor is being worked the current 
diminishes, the level surfaces begin to converge on the motor and fewer cut 
the circuit. Some of the energy therefore passes into the motor, and is there 
transformed into work. As the velocity increases the number cutting the 
rest of the circuit decreases, for the current diminishes, and, therefore, by 
Ohm's law, the fall of potential along the circuit is less ; and ultimately when 
the velocity of the motor becomes very great the current becomes very small. 
In the limit no level surface cuts the circuit, all converging on the motor. 

* Perhaps this is only true of the wire as a whole. If we could study the effects in minute 
portions it is possible that we should find the seat of the e.m.i. due to difference of temperature 
not the same as that which neutrahses it, which is according to Ohm's law. One, for instance, 
might be between the molecules, the other in their interior, so that there might be an interchange 
of energy still going on, though no balance remained over to pass out of the wire. 



190 ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 

That is, all the energy passes into the motor when it is transformed into work, 
and the efficiency of the arrangement is perfect, though the rate of doing 
work is infinitely slow. 

(6) Induced currents. 
It is not so easy to form a mental picture of the movement of energy 
which takes place when the field is changing and induced currents are created. 
But we can see in a general way how these currents are accounted for. When 
there is a steady current in a field there is corresponding to it a definite dis- 
tribution of energy. If there is a secondary circuit present, so long as the 
primary current is constant, there is no e.m.i. in the secondary circuit for it 
is all at the same potential. The energy neither moves into nor out of it, 
but streams round it somewhat as a current of liquid would stream round 
a solid obstacle. But if the primary current changes there is a redistribution 
of the energy in the field. While this takes place there will be a temporary 
E.M.I, set up in the conducting matter of the secondary circuit, energy will 
move through it, and some of the energy will there be transformed into heat 
or work, that is, a current will be induced in the secondary circuit. 

(7) The electromagnetic theory of light. 

The velocity of plane waves of polarised light on the electromagnetic 
theory may be deduced from the consideration of the flow of energy. If 
the waves pass on unchanged in form with uniform velocity the energy in 
any part of the system due to the disturbance also passes on unchanged in 
amount with the same velocity. If this velocity be v, then the energy con- 
tained in unit volume of cubical form with one face in a wave-front will all 
pass out through that face in l/t'th of a second. Let us suppose that the 
direction of propagation is straightforward, while the displacements are up 
and down; then the magnetic intensity will be right and left. If (S be the 
E.M.I, and «5 the m.i. within the volume, supposed so small that the intensities 
may be taken as uniform through the cube, then the energy within it is 
/i(£'-/87r -f- [mS^^/Stt. The rate at which energy crosses the face in the wave- 
front is l^"Ǥ/477 per second, while it takes 1/vth of a second for the energy in 
the cube to pass out. 

Then p>^m^^f^^ 

4.7TV 877 ^ 877 ^ ^ 

Now, if we take a face of the cube perpendicular to the direction of dis- 
placement, and therefore containing the m.i., the fine-integral of the m.i. 
round this face is equal to 477 x current through the face. If we denote 
distance in the direction of propagation from some fixed plane by z, the fine- 
integral of the M.I. is — -J- , while the current, being an alteration of dis- 

, ^ . K di^ 

placement, is j" "T~ • 



ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 191 

Therefore -f = ^f (2) 

But since the displacement is propagated unchanged with velocity v, 
the displacement now at a given point will alter in time dt to the displace- 
ment now a distance dz behind, where dz == vdt. 

Therefore W^'^Tz (^^) 

Substituting in (2) -^ = Kv -^ , 

whence § = Kv^, (4) 

the function of the time being zero, since S^ and (S* are zero together in the 
parts which the wave has not yet reached. 

If we take the line-integral of the e.m.i. round a face perpendicular to 
the M.I. and equate this to the decrease of magnetic induction through the 
face, we obtain similarly 

^ = iJ^v^ (5) 

It may be noticed that the product of (4) and (5) at once gives the value 
of V, for dividing out ($'*& we obtain 

1 = ixKv'' 

1 

or _ v= -=^ . 

But using one of these equations alone, say (4), and substituting in (1) 
K for <^ and dividing by (E^, we have 

K^K^ IJiKV 

47r " Stt Stt 

or 1 --= p.Kv'^, 

whence v = -^=^ . - 

This at once gives us the magnetic energy equal to the electric energy, for 

877 877 877 

It may be noted that the velocity is the greatest velocity with which 

the two energies can be propagated together, and that they must be equal 
when travelhng with this velocity. For if v be the velocity of propagation 
and 6 the angle between the two intensities, we have 

^^ sin^ _ m^ fj.^ 

4:7TV StT 877 ' 

2 sin d 

43 (i- 



192 ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 

The greatest value of the numerator is 2 when ^ is a right angle, and the 
least value of the denominator is 2 V]JiK, when the two terms are equal to 
each other and to VijlK. 

1 TT 

The maximum value of v therefore is . — , and occurs when 6 = ^ and 

The preceding examples will suffice to show that it is easy to arrange 
some of the known experimental facts in accordance with the general law of 
the flow of energy. I am not sure that there has hitherto been any distinct 
theory of the way in which the energy developed in various parts of the 
circuit has found its way thither, but there is, I believe, a prevaihng and 
somewhat vague opinion that in some way it has been carried along the 
conductor by the current. Probably Maxwell's use of the term 'displace- 
ment' to describe one of the factors of the electric energy of the medium 
has tended to support this notion. It is very difficult to keep clearly in mind 
that this 'displacement' is, as far as we are yet warranted in describing it, 
merely a something with direction which has some of the properties of an 
actual displacement in incompressible fluids or solids. When we learn that 
the ' displacement ' in a conductor having a current in it increases continually 
with the time, it is almost impossible to avoid picturing something moving 
along the conductor, and it then seems only natural to endow this something 
with energy-carrying power. Of course it may turn out that there is an 
actual displacement along the lines of electromotive intensity. But it is 
quite as likely that the electric 'displacement' is only a function of the true 
displacement, and it is conceivable that many theories may be formed in 
which this is the case, while they may all account for the observed facts. 
Mr Glazebrook has already worked out one such theory in which the com- 
ponent of the electric displacement at any point in the direction of x is 

^- V^^, where | is the component of the true displacement {VMl. Mag. June 

1881). It seems to me then that our use of the term is somewhat unfortunate, 
as suggesting to our minds so much that is unverified or false, while it is so 
difficult to bear in mind how little it really means. 

I have therefore given several cases in considerable detail of the apphcation 
of the mode of transfer of energy in current-bearing circuits according to the 
law given above, as I think it is necessary that we should reaUse thoroughly 
that if we accept Maxwell's theory of energy residing in the medium, we 
must no longer consider a current as something conveying energy along the 
conductor. A current in a conductor is rather to be regarded as consisting 
essentially of a convergence of electric and magnetic energy from the medium 
upon the conductor and its transformation there into other forms. The 
current through a seat of so-called electromotive force consists essentially 



ON THE TRANSFER OF ENERGY IN THE ELECTROMAGNETIC FIELD 193 

of a divergence of energy from the conductor into the medium. The magnetic 
Hnes of force are related to the circuit in the same way throughout, while 
the lines of electric force are in opposite directions in the two parts of the 
circuit — with the so-called current in the conductor, against it in the seat 
of electromotive force. It follows that the total e.m.i. round the circuit 
with a steady current is zero, or the work done in carrying a unit of positive 
electricity round the circuit with the current is zero. For work is required 
to move it against the e.m.i. in the seat of energy, this work sending energy 
out into the medium, while an equal amount of energy comes in in the rest 
of the circuit where it is moving with the e.m.i. This mode of regarding the 
relations of the various parts of the circuit is, I am aware, very different 
from that usually given, but it seems to me to give us a better account of 
the known facts. 

It may seem at first sight that we ought to have new experimental indica- 
tions of this sort of movement of energy, if it really takes place. We should 
look for proofs at points where the energy is transformed into other modifi- 
cations, that is, in conductors. Now in a conductor, when the field is in 
a steady state, there is no electromotive intensity, and therefore no motion 
and no transformation of energy. The energy merely streams round the 
outside of the conductor, if in motion at all in its neighbourhood. If the 
field is changing, energy can pass into the conductor, as there may be 
temporary e.m.i. set up within it, and there will be transformation. But 
we already know the nature of this transformation, for it constitutes the 
induced current. Indeed, the fundamental equation describing the motion 
of energy is only a deduction from Maxwell's equations, which are formed 
so as to express the experimental facts as far as yet known. Among these 
are the laws of induction in secondary circuits, and they must therefore 
agree with the law of transfer. We can hardly hope, then, for any further 
proof of the law beyond its agreement with the experiments already known 
until some method is discovered of testing what goes on in the dielectric 
independently of the secondary circuit. 



p. c. w. 



13 



11. 

ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND 
THE ELECTRIC AND MAGNETIC INDUCTIONS IN THE 
SURROUNDING FIELD*. 

[Phil Trans. 176, 1885, pp. 277-306.] 
[Received January 31. Read February 12, 1885.] 

In a paper published in the Philosophical Transactions for 1884 (Part ii, 
pp. 343-361 )t, I have deduced from Maxwell's equations for the electromagnetic 
field the mode in which the energy moves in the field. The result there 
obtained is that the energy moves at any point perpendicularly to the plane 
containing the directions of the electric and magnetic intensities, and in the 
direction in which a right-handed screw would move if turned round from 
the positive direction of the electric intensity to the positive direction of the 
magnetic intensity. The quantity crossing the plane per unit area per 
second is equal to the product of the two intensities, multipHed by the sine 
of the included angle, divided by 47r t. 

Hence it follows that the energy moves along the intersections of the two 
sets of level surfaces, electric and magnetic, where they both exist, their 
intersections giving, as it were, the lines of flow. In the particular case of 
a steady current in a wire where the electrical level surfaces cut the wire 

* [Added July 15. Since the reading of the paper I have found a remarkable passage in 
Faraday's Experimental Researches, vol, 1, p. 529, § 1659, which I give below. The words I have 
put in italics might be regarded as the starting-point of the views which I have attempted to 
develop in this paper. '§ 1659. According to the beautiful theory of Ampere, the transverse 
force of a current may be represented by its attraction for a similar current and its repulsion of 
a contrary current. May not then the equivalent transverse force of static electricity be repre- 
sented by that lateral tension or repulsion which the lines of inductive action appear to possess 
(1304)? Then, again, when current or discharge occurs between two bodies, previously under 
inductrical relations to each other, the lines of inductive force will weaken and fade away, and, as 
their lateral repulsive tension diminishes, will contract and ultimately disappear in the live of 
discharge. May not this be an effect identical with the attractions of similar currents, i.e., may 
not the passage of static electricity into current electricity, and that of the lateral tension of the 
lines of inductive force into the lateral attraction of Hnes of similar discharge, have the same 
relation and dependence, and run parallel to each other?'] 

t [Collected Papers, Art. 10.] 

J I here adopt the simpler term 'Electric Intensity,' denoted by e.i., instead of 'Electro- 
motive Intensity,' for the force which would act on a small body charged with unit of positive 
electrification. The magnetic intensity, i.e., the force which would act on a unit north-seeking 
Pole, will be denoted by m.i. 



ON THE CONNECTION BETWEEN ELECTRIC CURRENT, ETC. 195 

perpendicularly to the axis, it appears that the energy dissipated in the wire 
as heat comes in from the surrounding medium, entering perpendicularly to 
the surface. 

In that paper I made no assumption as to the transfer of the electric and 
magnetic inductions — the electric and magnetic conditions — through the 
medium, merely considering the movement of energy. I now propose to 
develop a hypothesis as to the transfer of the inductive condition in the 
medium, and its movement inwards upon current-bearing wires. 

The value of the electric induction at any point in an isotropic medium is 
equal to K x E.I./477, and the direction of the induction coincides with that 
of the intensity. Maxwell terms this electric induction 'displacement,' but 
I think that 'induction' is preferable, as it impUes no hypothesis beyond 
that of some alteration in the medium, which can be described by a vector. 
The value of the magnetic induction is equal to /x x m.i., and its direction 
coincides with that of the magnetic intensity. 

If we symbolise the electric and magnetic conditions of the field by 
induction-tubes running in the directions of the intensities, the tubes being 
supposed drawn in each case so that the total induction over a cross-section 
is unity, then we have reason to suppose that the electric tubes are con- 
tinuous except where there are electric charges, while the magnetic tubes 
are probably in all cases continuous and re-entrant. 

In the neighbourhood of a wire containing a current, the electric tubes 
may in general be taken as parallel to the wire while the magnetic tubes 
encircle it. The hypothesis I propose is that the tubes move in upon the 
wire, their places being supplied by fresh tubes sent out from the seat of the 
so-called electromotive force. The change in the point of view involved in 
this hypothesis consists chiefly in this, that induction is regarded as being 
propagated sideways rather than along the tubes or lines of induction. This 
seems natural if we are correct in supposing that the energy is so propagated, 
and if we therefore cease to look upon current as merely something 
traveUing along the conductor carrying it, and in its passage affecting the 
surrounding medium. As we have no means of examining the medium, to 
observe what goes on there, but have to be content with studying what takes 
place in conductors bounded by the medium, the hypothesis is at present 
incapable of verification. Its use, then, can only be justified if it accounts 
for known facts better than any other hypothesis. 

The basis of MaxwelVs Electromagnetic Theory. 
Maxwell's Electromagnetic Theory rests on three general principles. 
I. The first principle consists in the assumption that energy has position, 
i.e., that it occupies space. The electric and magnetic energies of an electro- 
magnetic system reside therefore somewhere in the field. It is an inevitable 

13—2 * 



196 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

conclusion that they are present wherever the electric and magnetic intensities 
can be shown to exist. For instance, suppose a small electrified body placed 
in a field where there is electric intensity ; then the body will be acted on by 
force and will receive energy which appears as the energy of motion, the 
electric energy at the same time decreasing. If energy has position, that 
which is now in the body must have come into it through the surrounding 
space, or it was present in that space before the body took it up. The 
alternative that it appeared in the body without passing through the space 
immediately surrounding the body need not be discussed. Hence the 
existence of electric intensity impUes the existence of electric energy in the 
place where the electric intensity is capable of manifestation. Similarly 
magnetic energy accompanies magnetic intensity. The inductive condition 
of the medium imagined by Faraday is due then to its modification when 
containing energy. Maxwell has shown that all the energy is accounted for 
on the supposition that the electric energy per unit volume at any point is 
K{B.i.yj87T, and that the magnetic energy is ju, (m.i.)2/877. He has given in 
his Elementary Treatise on Electricity, p. 47, another way of describing the 
distribution of energy which will be more useful for my purpose. If the 
field be mapped out by unit induction-tubes — either electric or magnetic — 
i.e., tubes drawn so that the total induction over every cross-section of a tube 
is unity, and if these tubes be divided into cells of length such that the 
difference of potential or the line-integral of the intensity between the two 
ends of each cell is unity, then each cell contains, if electric, half a unit of 

enero^v, if magnetic 5^ of a unit, the divisor 47r being introduced bv the 

077 ''^ " 

difference in definition of the two inductions. Maxwell terms these unit 
cells. 

II. The second principle is in part experimental, viz. : — that the line- 
integral of the electric intensity round any closed curve is equal to the rate 
of decrease of the total magnetic induction through the curve. This is 
verified by experiment when the curve is drawn through conducting material. 
Maxwell supposes it to be true in all cases, that is, he supposes that electric 
induction can be produced in insulators by means of magnetic changes, 
without the presence of charges on conductors, and is therefore led to identify 
the growth and decrease of electric induction with current. 

III. The third principle is also in part experimental, viz. : — that the 
hne-integral of the magnetic intensity round any closed curve is equal to 
iv X current through the curve. This is verified by experiment when the 
current is in a wire, and Maxwell supposes it to be also true in the case where 
there is change of electric induction in an insulator. The supposition is 
justified by Prof. Rowland's well-known experiment. 

From these three principles Maxwell deduces his general equations of the 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 197 

Electromagnetic Field. I have stated them in full as I propose to modify 
the second and third principles, and I wish to make quite clear the nature 
of the proposed changes. 

Modification of the Second Principle. 

I propose to replace the second principle by the following : Whenever 
electromotive force is produced by change in the magnetic field, or by motion of 
matter through the field, the e.m.f. per unit length or the electric intensity is equal 
to the number of tubes of magnetic induction cutting or cut by the unit length per 
second, the e.m.f. tending to produce induction in the direction in which a right- 
handed screw would move if turned round from the direction of motion relatively 
to the tubes towards the direction of the magnetic induction*. 

In order that the results obtained from this should agree with those 
obtained from Maxwell's statement of the principle, it is necessary that 
change in the total quantity of magnetic induction passing through a closed 
y curve should always be produced by the passage of induction-tubes through 

" the curve inwards or outwards. In some instances this is undoubtedly the 

case, as, for instance, where a part of a circuit moves so as to cut a fixed 
magnetic field, or where a magnet moves in the neighbourhood of a circuit. 
Here the e.m.f. is equal to the number of tubes cut by the wire per second, 
and its seat is that part of the wire cutting the tubes. In other cases, as, 
for instance, where the wire is between the poles of an electromagnet whose 
magnetising current is changing, we have no direct experimental evidence 
of the movement of the induction in or out. But the induction- tubes are 
closed, and to make them thread a circuit we might expect that they would 
have to cut through the boundary. The alternative seems to be that they 
should grow or diminish from within, the change in intensity being propagated 
along the tubes. This would be inconsistent with their closed nature, unless 
the energy were instantaneously propagated along the whole length, and is 
further negatived by the theory of the transfer of energy, which implies that 
the energy flows transversely to the direction of the tubes. I shall suppose, 
then, that alteration in the quantity of magnetic induction through a closed 
curve is always produced by motion of induction-tubes inwards or outwards 
through the bounding curve. 

* Taking the electric intensity as always perpendicular to the plane of motion of the magnetic 

tubes through a point, and equal to the number cut per second by unit length of the normal 

to the plane of motion, we can easily show that the component of the intensity in any other 

direction will be equal to the number of tubes cut by a Hne of unit length in that direction. For 

let OA represent a small length drawn perpendicular to the plane of motion, and let OP represent 

a line drawn in any direction making an angle 6 with OA. Draw AP perpendicular to OA, and 

meeting OP in P. Then the same number of tubes will cut both OA and OP, since AP is parallel to 

their plane of motion. If the number cutting OA he Ex OA, where E is the number cutting unit 

length, and therefore equal to the resulting intensity, the number cutting unit length of OP will 

OA 
be E . Yyp = E cos d, or the component of the intensity along OP. 



198 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

Modification of the Third Principle. 

The third principle admits of similar analysis, according to which we may 
regard the magnetic intensity along a closed curve as due to the cutting of 
the curve by tubes of electric induction. If we regard the line-integral of 
the magnetic intensity round a tube of induction as measuring the magneto- 
motive force— employing a useful term suggested by Mr. Bosanquet — we may 
put the modification in the following form : 

Whenever magnetomotive force is produced by change in the electric field, or 
by motion of matter through the field, the magnetomotive force per unit length is 
equal to iir x the number of tubes of electric induction cutting or cut by unit 
length per second, the magnetomotive force tending to produce induction in the 
direction in which a right-handed screw would move if turned round from the 
direction of the electric induction towards the direction of motion of the unit 
length relatively to the tubes of induction. 

This is the most general form of the principle, but we shall only require 
the more special statement which immediately follows from it : that the 
line-integral of the m.i. round any curve is equal to 477 x the number of tubes 
passing in or out through the curve per second. 

We have reasons exactly similar to those given in the last case for supposing 
that any change in the total electric induction through a curve is caused by 
the passage of induction-tubes in or out across the boundary. The alternative, 
that change takes place by propagation from the ends, seems inconsistent 
with the theory of the transverse flow of energy. 

I shall postpone the discussion of the modifications of the general equations 
of the electromagnetic field following from these changes in the fundamental 
principles, and proceed to discuss the bearing which they have upon the nature 
of currents in conductors. 

A straight wire carrying a steady current. 

Let AB represent a wire in which is a steady current from A to B. The 
direction of the electric induction in the surrounding field near the wire, if 
the field be homogeneous, is parallel to AB. 

Let E be the value of the electric intensity, or the difference of potential 
per unit length perpendicular to the level surfaces, and let R be the resistance 

E 

of the wire per unit length. Then = ^ where C is the current, and C is 

uniform throughout the circuit. The magnetic intensity in the immediate 

2C 
neighbourhood of the wire at a distance r from the axis of the wire is — . 

The hypothesis proposed as to the nature of the current is that C electric 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 199 



c^) 



induction-tubes close in upon the wire per second. The wire is not capable 
of bearing a continually-increasing induction, and breaks the tubes up, as it 
were, their energy appearing finally as heat*. 

Let us see how this hypothesis accounts for known facts, 
when aided by the two principles just laid down. 

It accounts at once for the constancy of the current at all 
parts of the wire in the steady state, in so far as it reduces 
this constancy to a particular case of the law according to 
which there is the same total induction over all cross-sections 
of a tube. If, for instance, there were more induction entering 
at A than at B, then more tubes must be entering at A, and 
so there would be an increase in the number of tubes left in 
the medium about B, or the field would not be steady. 

Further, if we draw any closed curve embracing the wire 
once, we may apply the third principle to give us the fine- 
integral of the magnetic intensity round the curve. For this ^ . 

is a case where change is certainly going on in the electric field. ^jg, i, 

and the magnetomotive force is due to this change. The field 
being steady, if C tubes enter the wire and are there broken up, C tubes 
must cross through any encirchng curve to supply their place, or the line- 
integral of the magnetic intensity round the curve is equal to iir x number of 
tubes passing through the boundary per second, i.e., iirC. If the curve be 
a circle of radius r, with its centre in the axis and plane perpendicular thereto, 
the intensity at any point of this circle will be tangential to it, and equal to 

477(7 ^ 2C 
277r r 

The known constancy of the line-integral of the magnetic intensity round 
the wire, which the hypothesis thus accounts for, almost seems to force the 
hypothesis upon us, if we regard the field as caused by the inward flowing of 
the energy rather than by something propagated out from the wire. 

Assuming that the induction-tubes bring in their energy, the quantity is 
easily found. The number of unit cells per unit length is equal to the difference 
of potential per unit length, or E. Hence the energy per unit length of each 

E . 

tube is ^ , since each cell contains a half unit. If C tubes disappear in the 

CE 



Now the total 
energy dissipated per unit length is CE per second. Or the movement 



wire per second, they yield up -^ of energy per unit length. 



* May we not say that the tubes are dissolved ? The term seems to suggest that the induction 
is not destroyed, but only loses its continuity. Probably this is the case; for on the electro- 
magnetic theory of radiant energy, when the wire is heated, it sends out the energy it received, 
again in the electromagnetic form. 



200 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

inwards of the electric induction will only account for half of the energy. 
The other half must be accounted for by the movement inwards of the 
magnetic induction. This movement of the magnetic induction is suggested 
by the existence of electric induction, which cannot be ascribed to statical 
charges. 

The electric intensity is E. Hence E tubes of magnetic induction must 
move in per second, cutting unit length parallel to the axis of the wire, in 
accordance with the second principle, and it will easily be seen that the inward 
motion gives the right direction of the electric intensity. The hne-integral 
of the magnetic intensity round a tube is ^ttO, the tubes being closed rings. 
Hence there are 4:7rC unit cells in the length. Since each of these contains 

5— of energv, the quantitv per tube = -^ — = o • ^ tubes entering the wire 

07T " " 077" Z 

OF' 

per second will carry in -^ of energy, the other half to be accounted for. 

We can in a similar manner trace the dissipation of the energy, which we 
must suppose taking place within the wire. The line-integral of the magnetic 
intensity round a circle, with its centre in the axis of the wire, is constant 
up to the wire, and equal to irrC. Within the wire it gradually diminishes 

as the circle contracts. At a distance r from the centre it is ^ttC -^ where 

a is the radius of the wire. If we assume this intensity to be still due to the 

passage inwards of the tubes of electric induction only, — ^ cross inwards 

per second at a distance r, the difference between this number and the C tubes 
entering the outer boundary being destroyed and their energy dissipated. 
The energy thus dissipated per unit length between the outer boundary and 

a coaxal cylinder of radius r will be -^ [l -A per second. H r = the 

whole of the electric energy is dissipated. It would appear, then, that we 
may represent the dissipation of the electric energy by the total destruction 
of the tubes all through their length. 

The value of the electric intensity being E throughout the wire the number 
of tubes of magnetic induction cutting unit length parallel to the axis is the 
same at all parts, viz., E per second. Hence the magnetic tubes are not 
destroyed as the electric tubes are. But the Hne-integral of the magnetic 

mtensity round the tubes diminishes as they approach the axis, being ^tt — ^ 

round that at distance r. The number of unit cells diminishes, and, therefore, 
the energy per^tube is less, the decrease being due to that dissipated. Thus 

4- OF OF 

the energy entering in the E tubes at the outer boundary is "^ or -^ . 

877 2i 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 201 

That crossing in E tubes at a distance ri&^ E = -^ — - . The difference 

877 a^ 2 a^ 

^(1 



-gj has been dissipated. 



Hence it appears that the energy dissipated per second may be repre- 
sented as half electric half magnetic, the electric energy being dissipated by 
the breaking up of the tubes and their disappearance, while the magnetic 
energy is dissipated by the shortening of the tubes and their final disappear- 
ance by contraction to infinitely small dimensions of the diameters of the 
rings by which we may represent them. At all points therefore outside and 
inside the energy crossing any surface may be represented as equally divided 
between the two kinds. 

As we know the value of the induction at any point, or the number of 
tubes passing through unit area, and as we also know the number of tubes 
cutting the boundary it is easy, on the assumption that the tubes move on 
unchanged, to calculate their velocity. Of course this velocity is purely 
hypothetical, as we cannot examine minutely into the medium and observe 
what goes on there. Probably, if we could observe with sufficient minuteness 
we should find unevennesses in the induction. If the velocity of the tubes 
has any physical meaning it is that these unevennesses are carried forward 
with that velocity. To illustrate this let us suppose that we have water 
flowing through a glass tube at a steady rate. We have nothing to show 
that the water is moving past any point in the tube beyond its disappearance 
at the entrance and its appearance at the exit, but knowing the cross-section 
of the tube, i.e., the quantity of water in any part of it, and the quantity 
entering and leaving, it is easy to assign a velocity to the water in the tube 
which shall account for the observed amount entering and leaving. This 
velocity is to a certain extent hypothetical. But if we examine the tube 
with a sufficient magnifying power to show particles of dust in the water 
the existence of the velocity receives a more direct proof. I do not know 
whether we should have any right to expect a similar proof of the motion of 
induction even if we had the means of observation. 

To find the hypothetical velocity of the electric induction-tubes let us 
calculate the number of tubes passing through a circular band with radii 
r and r -\- dr and centre in the axis of the wire, and lying in a plane perpen- 

KE 

dicular to the axis. The intensity being E the induction is -r— , and theretore 

4:7T . .... 

the area of cross-section of each tube is -fv-^, since area x induction is unity. 

The number passing through the circular band is therefore 

^ , KE KErdr 
ZTvrdr . -j— = — ^ — . 

4:77 2 



202 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

Since C tubes move in through the inner circle per second, — ^ — tubes 

move in in of a second, i.e., all the tubes passing through the band will 

have just moved in in this time. The outermost tubes therefore describe 
the space dr in time — ^r^:^ , or the velocity is -^-^ . Now we know that if 

E 

R be the resistance per unit length, C =- ^. Hence we may put the velocity 

in the form 

2 1 
KR' r' 

which is independent of the current. 

To take a special case, let us calculate the velocity just outside the 
boundary of a copper wire, the specific resistance of copper being 1642 in 
electromagnetic measure. Then if a be the radius of the wire 



,2 



and K = ^ where v is the ratio of the units, which in air may be taken as 
3 X IQio. 

2'U^77tt^ 

Then the velocity = v/> /^~ 

-^ 1642a 

2 X 9 X IO^Ott^ 



1642 
- 345 X lO^S. 

At greater distances the velocity will be less, diminishing according to the 
inverse distance. 

The hypothetical velocity of propagation of the magnetic induction may 

be calculated in a similar manner. The intensity at a distance r from the 

2C ^llC 

axis is' — and the induction is ^— . The area of each tube is therefore 
r r 

T 

^— ^ , and the number lying in a ring of rectangular section with depth unity 

and interna] and external radii r and r + dr, will he 1 x dr -^ ^^ = — . 

2/xC r 

But E tubes move in per second through the inner face of the ring, so that 

"^yiCdr . . . 2uiCdr 

—~ — tubes move m m time ^^ - , or this is the time taken by the outer- 
r Er ^ 

most tubes to move across the ring describing a distance dr. The velocity is 

therefore 

Er Rr 

'2iiC~~ 2jLt' 

which is again independent of the current. 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 203 

If the current-bearing wire is copper, R = , and with ft = 1 the 



7Ta' 



velocity becomes 



1642r 

277^2 



We cannot assign a velocity to the electric tubes within the wire since 
the number is diminishing as their energy dissipates. But the magnetic 
tubes crossing unit length parallel to the axis are still unchanged in number, 
so that we may assign a velocity to them. This velocity means that with 
the known value of the magnetic induction this velocity will give the number 
crossing inwards required to produce electric intensity E, 

The velocity will be found equal to 



2/xCr ^^ 2/xr' 



In the case of a copper wire this becomes 

1642 



2/x77r * 



Discharge of a condenser through a fine wire. 

Let us suppose that we have a condenser consisting of two parallel plates 
A and B and charged with equal and opposite charges. Then we know that 
there will be electric induction between the two plates, and that according 
to Maxwell's theory the energy of the system is stored there. We may form 
an idea of the distribution of the energy by drawing the unit induction-tubes, 
each starting from and ending in unit quantity of electricity, and dividing 




these into unit cells by the level surfaces, drawn at unit difference of potential 
(Fig. 2). If the dimensions of the plates be great compared with their distance 
apart, then nearly all the cells will be between the two plates, and since each 
cell contains half a unit of energy, nearly all the energy is there. There will, 



204 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

however, be slight induction, and therefore some small quantity of energy in 
the surrounding space. 

Now let the two plates be connected by a wire. Discharge takes place, 
and we are fairly justified, from the heat in the wire and the transient magnetic 
effects, in saying that a current has been in the wire from the positive to the 
negative plate, or the wire was for the time being in the same relation to the 
surrounding medium as the wire in the case just considered, the condition of 
affairs, however, not being steady. 

Let us suppose the wire to have a very great resistance, in order that, at 
least in imagination, we may lengthen out the time of discharge. On the 
ordinary current- theory, combined with Maxwell's 'displacement' theory, 
the medium between the plates has returned from the strained condition, 
denoted by 'displacement' from the positive to the negative plate, causing 
displacement through the plates and along the wire, the displacement being 
in the same direction all round the circuit. This is generally, I think, 
supposed to take place by the recovery of the medium between the plates 
causing displacement in the metal immediately in front of it, the displacement 
being analogous to the forcing of water along a pipe corresponding to the 
plates and wire, by the recovery from strain of some substance placed in 
a chamber corresponding to the space between the plates. 

According to the hypothesis here advanced we must suppose the lessening 
of the induction between the plates — induction being used with the same 
physical meaning as Maxwell's displacement — to take place by the divergence 
outwards of the induction-tubes. We may picture them as taking up the 
positions of successive Hues of induction further and further away from the 
space between the plates, their ends always remaining on the plates. They 
finally converge on the wire, and are then broken up and their energy dissi- 
pated as heat. At the same time some of the energy becomes magnetic, this 
occurring as the difference of potential between the plates lowers, so that the 
tubes contain fewer unit cells. 

The magnetic energy will be contained in ring-shaped tubes which will 
expand from between the plates and then contract upon some other part of 
the circuit. To illustrate the movement of the electric induction-tubes let 
us suppose them to be represented by elastic strings stretched between the 
two plates. Then the motion of the tubes outwards would be roughly repre- 
sented by pulhng the elastic strings outwards and doubhng them back close 
against the wire, their ends being still attached to the plates. It is evident 
that if any ring surround the wire each of the strings must break through it 
in order to reach the wire. Hence the total number of strings cutting any 
ring surrounding the wire is the same wherever the ring be placed. Similarly 
the total number of tubes of electric induction cutting any curve encirchng 
the wire is the same, and therefore the fine-integral of the magnetic intensity 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 205 

round the curve integrated throughout the time of discharge is the same, or 
the total magnetic effect is the same at all parts of the circuit. It is not 
necessary to suppose that a tube enters the wire at the same moment through- 
out its whole length ; indeed, the experiments of Wheatstone on the so-called 
velocity of electricity prove clearly that this is not the case, for in those 
experiments the tubes reached air-breaks near the two ends of the wire before 
they reached a break in the middle. 

We cannot by this general reasoning show that the energy entering any 
length of the wire will be proportional to the resistance of that length — the 
result obtained by Kiess. Indeed, this cannot always be the case. For 
instance, imagine a condenser discharged by two wires connected to the two 
plates of another condenser of greater capacity, whose plates are again 
connected by a fine wire of enormous resistance, through which the discharge 
can only take place slowly. Then the energy dissipated in the wires will not 
to a first approximation depend on their resistances but on the ratios of the 
capacities, that in the wire of high resistance bearing to that in the other 
wires the ratio of the less capacity to the greater. Probably Riess's results 
only hold when the discharge takes place in such a way that it may be looked 
upon at any one moment as approximately in the steady state. 

We have shown that the magnetic measure of the total current is the 
same all along the wire. Probably also the chemical measure is the same — 
meaning by the chemical measure whatever interchanging or turning round 
of molecules may occur when induction takes place in a conductor. For 
even if a tube does not enter the wire at the same time throughout its length, 
an end part, say, entering first, the point of attachment of the tube to the 
conductor being transferred from the plate to somewhere along the wire, this 
transference of the point of attachment from molecule to molecule imphes 
the same amount of chemical change within the wire as if the tube entered 
all at the same moment. It will not, however, take place equally throughout 
the cross-section as it does in the steady state. 

Probably we only have the simultaneous disappearance of all parts of 
a tube when the wire follows a line of electric induction, and has its resistance 
per unit length proportional to the intensity which would exist there if the 
wire were removed. 

The hypothesis here advanced is in accordance with Maxwell's doctrine 
of closed currents. For the induction dissipated at one part of the circuit 
has come there from another part where relatively to the circuit it ran in 
the opposite direction. The total result is equivalent to the addition of so 
many closed induction-tubes to the circuit, the induction running the same 
way relatively to the circuit throughout. 

If the two plates of the condenser are not connected by a wire but are 
discharged gradually by the imperfect insulation of the dielectric, then we 



206 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

must suppose that the tubes of induction in this case are dissipated in situ, 
the induction simply decaying at a rate depending on its amount and upon 
the conductivity of the dielectric. We may still represent this process by 

a closed current by regarding the loss of induction (Maxwell's — j.) and the 

quantity of induction dissipated (Maxwell's f) as two different quantities. 

We have then p-{- -J- = ov we have two equal and opposite currents. But 

this seems artificial. It is more natural to look upon the process merely as 
a decay of electric induction without movement inwards of fresh induction- 
tubes, and therefore without the formation of magnetic induction. 

I have discussed the case of discharge of a condenser at some length, as 
we can here reahse more easily what goes on at the source of energy. The 
results obtained suggest that a similar action occurs at the source of energy 
or seat of the electromotive force in other cases where we do not know the 
distribution of induction, and are obhged to guess at the action. 



A circuit containing a voltaic cell. 

We may pass on from the discharge of a condenser to the consideration 
of the current in a circuit containing a voltaic cell. The chemical theory of 
the cell will be here adopted — in fact, the hypothesis I am endeavouring to 
set forth has no meaning on the voltaic metal-contact theory. 

Let us suppose the cell to consist of zinc and copper plates, a vessel of 
dilute sulphuric acid, and copper wires attached to each plate which on 
junction complete the circuit. For simplicity I shall disregard the effect 
of the air and suppose that it is a neutral gas causing no induction. 

We shall begin by supposing the circuit open. Then we know that on 
immersion there will be temporary currents in the wires, the quantities of 
these currents depending on the electrostatic capacity of the system composed 
of the wires. The currents last till the wires have received charges such that 
they are, say, at difference of potential V. If the terminals are connected 
to a condenser the temporary currents may be easily detected by a galvano- 
meter in the circuit. They are in no way to be distinguished in kind from the 
permanent current which will be estabhshed when the circuit is complete, 
except that they are of short duration and in general very small. There is 
no reason then to suppose that the action in the cell is different from that 
which takes place when the current is permanent, and I think we may safely 
assume that Faraday's law of electrolysis holds according to which the 
quantity of electricity flowing along either wire is proportional to the quantity 
of chemical action — or, in the form appropriate here, the number of tubes of 
induction produced is proportional to the quantity of chemical action. 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 207 

Let Q be the total quantity of electricity upon the positive terminal ; then 

QV 

^r- is the total energy thrown out into the dielectric. 

Let z be the quantity of zinc consumed per unit of electricity, then Qz is 
the total quantity consumed in the charging of the terminals. Let E be the 
energy set free by each quantity z of zinc consumed, after all actions in the 
cell have been provided for. E then is the e.m.f. which the cell will have 
on the closure of the circuit, as long as the chemical actions remain the same, 
for z corresponds to the passage of a unit of electricity or the production of 
one tube, and we know that the energy set free by C units is CE. 

Now while the charges are gathering and while the potential difference 
of the terminals is gradually increasing, the energy required to add equal 
increments of charge will also increase, and the charging will cease when the 
amount of energy given up by a given amount of chemical action in the cell 
is equal to the amount required to add the corresponding charge to the 
terminals. For to suppose the action to go beyond this is to suppose that 
the energy thrown out into the space between the terminals is greater than 
that yielded by the battery. 

Let dQ be the last quantity of charge added to the terminals. This 
requires energy VdQ. 

The corresponding quantity of zinc consumed is zdQ, giving up energy EdQ. 

The condition of equilibrium is that 

VdQ = EdQ 

or V^E, 

which agrees with the result of experiment that the difference of potential 
of the terminals in open circuit is equal to the e.m.f. of the cell immediately 
after closure. 

It may be noticed that the total quantity of energy extracted from the 
batterv is 

QE = QV, 

while the electric energy left in the medium is 

QV 

2 ' 

or half the energy has been converted into heat in the wires. 

We will now consider the distribution of level surfaces in the field while 
the circuit is still open. There will be F — 1 surfaces between the terminals, 
dividing each tube into V cells. None of these will cut the homogeneous 
parts of the circuit, since the whole of each of these must be at one and the 
same potential. 

They can only cut the circuit by passing through the regions where there 



208 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

is contact of dissimilar bodies. We will neglect the contact of the zinc and 
copper, as the difference of potential there is insignificant compared with 
that at the two surfaces, zinc-acid and copper-acid. 

Now we know that the energy of the cell is put out at the zinc-acid contact, 
but the amount is greater than that obtained from a consideration of the 
E.M.F. of the cell, for some energy is absorbed again, probably, at the copper- 
acid contact in the evolution of hydrogen. There is probably, then, induction 
between the acid and the zinc, and between the acid and the copper, these 
resembUng the spaces between the plates of two condensers, the acid being 
at a higher potential than either. But if a given amount of induction dis- 
appears from the zinc-acid contact and appears at the terminals, more energy 
is lost at the former than appears at the latter. Hence all the cells have not 
been transferred from one to the other, or the difference of potential zinc-acid 




Fig. 3. 

is greater than 7. Then more than V — I level surfaces pass between the 
zinc and the acid, the excess over 7—1 going round and passing between 
the copper and the acid, somewhat as in Fig. 3, where A, B are the metal 
plates. The surfaces are roughly sketched and numbered, on the supposition 
that the zinc terminal is at 0, the copper at 5, and the acid at 8. They have 
probably the same shape as those which would be produced by condensers 
at A and B with the wires attached, respectively, to one terminal of each, 
the other terminals being connected together and the charges adjusted so 
that the difference of potential of the two terminals at A was 3, while that 
at B was 8. 

Let us now suppose the circuit closed. Then the level surface will 'cut 
the circuit at various points, somewhat as in Fig. 4. 

The energy being dissipated in the wire, the cell will continually send out 
fresh energy, the induction-tubes, which proceed from the acid to the zinc. 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 209 

diverging outwards in the same way as described in the discharge of a con- 
denser. They bend round, and finally go into the circuit, the energy they 
carry being used for the necessary molecular changes, and finally appearing 
as heat in the circuit — except at the copper-acid contact where there is a 
crowding in of level surfaces, and therefore a convergence of more energy, 
which is required to set the hydrogen free. 




Fig. 4. 

At the same time magnetic ring-shaped tubes will be continually sent out 
from the zinc-acid contact, expanding for a time and then contracting again 
on various parts of the circuit and also giving up their energy. 

There is, therefore, a convergence of tubes of electric induction on the 
circuit, running in the same direction throughout, viz., from copper to zinc 
outside the cell, and from zinc to copper inside, except between the zinc 
and acid, where there is a divergence of tubes in which the induction runs 
in the opposite way. But a divergence of negative tubes causes magnetic 
intensity in the same direction as, and may therefore be considered as equiva- 
lent to, a convergence of positive tubes. The current may therefore be said 
to go round the circuit in the same way throughout. 

The tendency to a steady state in which the current or the number of 
induction-tubes broken up per second is the same at all parts of the circuit, 
admits of simple explanation. We know, as the result of experiment given 

P 

by Ohm's law, that C' = p where R is the resistance per unit length and E 

the electric intensity. Until we can explain the molecular working of the 
current, i.e., the mode in which the induction-tubes are broken up, we must 
accept Ohm's law as a simple fact. Let us suppose that we have not yet 
arrived at the steady state, so that in some part of the circuit the electric 
intensity is less than in the steady state, while in another part it is equal 
p. c. w. 14 



210 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

to it or greater. Let the steady value of the intensity be E, the actual value 
in the former part E' , and in the latter E" , By Ohm's law the number of 
tubes absorbed by the wire per second is given by C = E' jR, and C" = E" jR, 
in the two parts respectively, so that C"< C" since E' < E" or less tubes are 
being destroyed in the first than in the second part. But all the tubes are 
sent out from the source of the energy, and are only destroyed in the circuit, 
being otherwise continuous and with their two ends in the circuit. Hence, 
if more tubes are destroyed at one part than another, the parts of the tubes 
not yet destroyed will gather in the medium surrounding the part where 
fewer are destroyed, increasing the induction there, and so raising the intensity 
in the wire and therefore the number of tubes destroyed. The field can 
evidently only be steady when the number of tubes destroyed in all parts 
of the circuit is the same. 

But it does not follow that in the steady state each tube enters the wire 
along its whole length at the same moment. This would imply that the axis 
of the wire is a line of electric induction perpendicular everywhere to the 
level surfaces. If we draw the level surfaces due to the seats of induction 
at the contacts of acid and metal, they will probably be somewhat as drawn 
in Fig. 4. If now the wire is not so arranged as to follow with properly 
adjusted resistances a line of induction for these surfaces, but pursues an 
irregular course, then the level surfaces will be much distorted, and the 
distribution of the induction will be greatly altered. 

We may ascribe this alteration to a distribution of electricity along the 
wire, the quantity in any small area on the surface of the wire being equal 
to the difference between the number of tubes which have entered and the 
number which have left that area since the beginning of the system. We 
have a famihar example of this in the charging of deep-sea cables. Another 
example is afforded by a condenser with terminals connected to two points 
in the circuit. The plates of the condenser are then virtually parts of the 
circuit. 

The effect of a junction of two wires, say of the same diameter, but of 
different specific resistances, upon the level surface will resemble that of a 
charge upon the separating surface. This can be seen in a general way from 
the fact that the level surfaces must cut the wire with the higher specific 
resistance at intervals shorter than those at which it cuts the other wire. 

If there be an insulated conducting body, say a metal sphere, near the 
circuit, we know that in the steady state there is no electric intensity, and 
therefore no current within it ; consequently there is no movement of energy 
and no movement of induction through it. We can see how this condition 
is arrived at. As the first tubes of electric and magnetic induction come 
up to the sphere they will enter it, and the parts of the electric induction- 
tubes thus entering will be broken up, causing a transient current in the 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 211 

sphere. The parts of the tubes left in the medium will end on the sphere 
giving a negative charge on the end nearer the regions of higher potential, 
and a positive charge on the end nearer the regions of lower potential. This 
will go on until such charges have accumulated that the sphere becomes 
itself a level surface. When this point is reached no more energy can enter 
the sphere, and the parts of the magnetic tubes within it cease to move. 

The charges formed on the wire or on neighbouring conductors are to 
be distinguished from ordinary statical charges in this : that their existence 
depends on the existence of the current, and therefore on the motion of 
magnetic induction. If the current is stopped by a break in the circuit, so 
that the motion of the magnetic induction ceases, the electric induction 
ceases and the charges are all lost. We should expect, therefore, to find 
that these charges can be described in terms of the magnetic motions which 
have occurred and are occurring in the system. 

Current produced by motion of a conductor iyi a magnetic field. 
We may explain by general reasoning the production of a current by 
motion of a part of a circuit so as to cut the tubes of magnetic induction. 
We will consider the simple case of a sHder 

AB, Fig. 5, running on two parallel rails, ^ ^ 

AC, BD, with a fixed cross-piece CD, the 
tubes of magnetic induction running from 
above downwards through the paper. Let 
AB move so as to enlarge the circuit. We 
know from experiment that this tends to Fig. 5. 
cause a current in the direction ACDB. 

As AB moves through the field its motion tends to cause electric intensity 
in the direction BA. At the same time its kinetic energy is being continually 
converted into electric and magnetic energy which travels to the rest of the 
circuit there to be dissipated, that is, there must be a divergence of energy 
from AB. Instead then of a convergence of positive tubes running from 
B to A, we shall have what is magnetically equivalent — a divergence of 
negative tubes or tubes running from A to B, their motion outwards being 
accompanied by tubes of magnetic induction running round in the same 
way as if there were an ordinary current from B to A. These magnetic 
tubes must be supposed to move outwards in order to account for the direction 
of the electric intensity*. 

When these electric and magnetic tubes converge upon the rest of the 
circuit they will evidently form a current running in the direction ACDB. 

* Note added July 15: The above must not be regarded as an attempt to explain the 
production of electric induction by the motion of a conductor in a magnetic field, but merely as 
an attempt to show how the induction arising in the moving part of a circuit finds its way into 
the rest of the circuit. 

14—2 



212 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

We have here taken, just as in the case of the condenser and the voltaic cell, 
the lessening of negative induction by its motion outwards, as equivalent to 
the increase of positive induction by its motion inwards, and we have con- 
sidered both of them to indicate the apphcation of electric intensity in the 
same direction in the conductor. 

If instead of considering AB as a whole we break it up into elements, 
each element will be a source of diverging negative tubes, and the remainder 
of AB will, to that element, be a part of the rest of the circuit. Hence some 
of the energy sent out from the element will converge on and be dissipated 
in AB, or AB will be heated just as the rest of the circuit. 



The general equations of the electromagnetic field. 

We can easily obtain equations corresponding to and closely resembling 
those of Maxwell by means of the principles upon which this paper is founded. 

The assumption that if we take any closed curve the number of tubes of 
magnetic induction passing through it is equal to the excess of the number 
which have moved in over the number which have moved out through the 
boundary since the beginning of the formation of the field, suggests a historical 
mode of describing the state of the field at any moment. 

Let a, b, c be the components of magnetic induction at any point 0. 
Consider a small area dy dz close to the point, then the number of tubes 
passing through the area dy dz will be adydz. This will be equal to the 
difference between those which have come in and those which have gone out. 

Let Ldx, Mdy, Ndz denote the numbers of tubes which have cut the 
lengths dx, dy, dz since the beginning of the system, those being positive 
which have tended to produce electric intensity in the positive direction 
along the axes, and those being negative, and therefore subtracted, which 
have tended to produce intensity in the opposite direction. 
Let us consider the number which has come into the area 
OB DC = dydz (Fig. 6). The number which has come in 
across OB is - Mdy (- because the movement of tubes 
passing through dydz in the positive direction must be 
outwards to produce e.i. along OB). The number which 

has passed out across CD is - (m + -j- dz) dy. The differ- ^ig- ^■ 

dM 

^nce is -^ dydz. The number which has come in across OC is + Ndz 

{+ because the movement of tubes passing through dydz in the positive 
direction must be inwards to produce e.i. along OC). The number which 

has passed out across BD is (n + ^ dy] dz. The difference is - ^ 




ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 213 



The number still passing through dydz is therefore (-^ -^-^ dydz. 



dy. 
Equating this to the actual induction through the area, viz., 

adydz 

and performing the same process for the corresponding areas dzdx, dxdy, we 
obtain 

dM dN\ 
dz dy 

dN_dL 
dx dz 



a = 



c = 



dL 
dy 



dM 
dx 



(1) 



Comparing these with Maxwell's equations (vol. 2, p. 216) we see that 

dM_dN^dH_dG 
dz dy dy dz ' 

with two similar equations, F, G, H being the components of the vector- 
potential. We should obtain Maxwell's equations if we defined F, G, H to 
be the number of tubes which would cut the axes per unit length if the system 
were to be allowed to return to its original unmagnetic condition, the tubes 
now moving in the opposite direction. According to Maxwell, the vector 
whose components are F, G, and H 'represents the time-integral of the 
electromotive force which a particle placed at the point {x, y, z) would 
experience if the primary current were suddenly stopped' (vol. 2, 2nd ed., 
p. 215). If the electric intensity is produced by the motion of magnetic 
induction, then our definition of F, G, H will by the second fundamental 
principle agree with Maxwell's statement. 

If u, V, w be the components of current — including, of course, under 
currents, growth of induction — we have from the third principle Maxwell's 
equations E (vol. 2, p. 233), which on multiplying by /x become when /x is 
constant 

, dc dh\ 



dy 



da do 

4:7TIJLV = ^ -^ 



4:7TfJLW 



db 
dx 



dx 

da 
dy 



>• 



(2) 



Combining these with equations (1) (as in Maxwell, vol. 2, pp. 236-7), 

d^ 



and writing 



V^ for i^ + i 
dx^ dy 



2 + ^^2' we obtain 



214 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 



d /dL dM , dN\\ 



dx \dx 
d /dL 



„,,^ d (dL , dM , dJS\ 



dM^ 

dy 

dM 



dz J 

dN^ 



y. 



dN\ 



4^ ^ = _ vw - - r + "^ + -- 1 

^^ dz \dx dy dz J 



(3) 



These equations only differ in sign from Maxwell's, and are therefore to be 
solved in the same way. 

It is easy to see by substitution that if we assume 



L' = — ^\\\- dxdydz 
M' = — ii\\\- dxdydz 
iV' = — jLt I - dxdydz 



dx dy 
then the following will be solutions 

L = U - 

M = M'- 

N = N' ■ 



dz 



dH 

dx 

dH 

dy 

dH 

dz , 



J - dxdydz 



(4) 



,(5) 



It is evident that we may add to the right-hand side of equations (5) 

7 ' 77 ' 77 respectively, where </> is any function of x, y, z, since these 

will disappear from (3) and also from (1). 

The electric intensity, in so far as it depends upon magnetic motions, will 
consist of two terms, one depending upon the motion of the material at the 
point (its components being found as in Maxwell, vol. 2, p. 227, note), the 
other upon the motion of magnetic induction about the point. We may add 
a third term, arising from any electrical distribution with a potential ijj. 
If there is no material motion we shall have 

d^\ 
dx 

dM di/j 
dt dy 

^_# 
dt dz / 



dL 

dt 



Q 



y. 



R 



(6) 



dx dy 


dR , 

dz = '^ 




- dxdydz, 


'ffdu 


- dxdydz 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 215 

Substituting from (4) and (5) we get 

r> (ddu 1 ■, 7 7 Id fff/ d dL , d dM , d dN\ 1 ^ , , # 

^^-^\\\dt'-r^''^y^'-^Tx\j\[Txlt+ryW^dz^^^^ 

\du \ . . . Id {[[(dP dQ dR\l ^ ^ ^ 
_ . _ dxdydz - ^^^j\\ (-^- + ^- + -^) - dxdydz 

substituting for -^ , etc., from (6). 

The last two terms cancel each other, and we get 

du 1 . , . Id [[{(dP dQ dR\l , -, . ,„, 

or if we put 

and 

jj.fr " ' 

with similar equations for Q and' 7?. 

If the system is steady 777^ jT^ ;7r are all zero, and then 

dx' dy' dz' 

The quantity />, of which 7 is the potential, will be zero within non- 
conducting homogeneous parts of the field, for there 

,_ZP _KQ ._KR 

^ 477 ' ^ " 477 ' '^ ~ 477 ' 

nd ^ ^ dR_i7Tfdl dg dJi\ _ 

dx dy dz K \dx dy dz) ' 

since no charges can reside within a homogeneous non-conducting medium. 
Or, stating it in another way, all the induction-tubes brought into any part 
of such a medium remain there without dissipation, a charge in a non-homo- 
geneous medium being due to unequal amounts of dissipation of induction in 
different parts of the medium. 

But p will have value at surfaces separating dissimilar substances either 
in the insulating or conducting parts of the medium. For in the former the 
induction is continuous, while the intensity is discontinuous, and in the latter 
the current or rate of destruction of induction may be continuous, but the 
relation between intensity and current changes discontinuously with the 



216 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

conductivity. At surfaces separating insulators from conductors p may have 
value, as, for instance, at the surfaces of the plates of a condenser with its 
terminals connected with two points in a circuit, or at the surface of an 
insulated conductor near the circuit. It is also to be noted that p will have 
value at the seat of electromotive force. 

The values of the components of magnetic induction a, b, c are not in 
any way dependent on p. For taking the first of equations (1) and sub- 
stituting from (5) we have 

_dM _dN ^dM^_dN^_ d^ d^^dM;__dN^ 

~ dz dy ~ dz dy dydz dzdy dz dy ' '"^ ' 

where M' and N' depend on the currents in the system and not on the charges. 

Comparing our equations with Maxwell's we see that the important point 

of difference is that we can no longer put the quantity corresponding to his 

7 1 . 7 K • • . dF ^dG ^dH 

J equal to zero, J bemg given by -, — h -7 — ^ ~J~ ' 

This does not affect the determination of velocity of propagation of dis- 
turbance in a homogeneous non-conducting medium. 

For in such a medium we shall have 

df K d^ 

dt 477 dt ' 

with corresponding values for v and w. 

Substituting in (3) the first equation becomes 

dt dx\ dx dy dz 

differentiating with respect to t 

^ dt^ dt dx [dx dt dy dt dz dt . 

and putting -^7 = ? + ^, 

since d^^dQ dR^i^^df dg dj.^ 

dx^ dy^ dz K \dx ^ dy^ dz)~ 

within a homogeneous non-conductor. 

This gives the velocity of propagation of electric induction equal to 

We can also obtain the corresponding equation for the magnetic induction. 



(10) 



ELECTBIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 217 

Substituting in (3) for u, v, and w in terms of P, Q, and R, as above, 
differentiating the second with respect to z, and the third with respect to y, 
and subtracting 

dfdQ_dR\^_ fdM _ dN\ 
^dt\dz dy)~ ^ \dz dyl' 
then from (6) 

^ d/d^dMd^ddN d^ \_ fdM dN\ 

dt\dt dz dzdy dt dy dydzj~ \dz dyj' 

^dt^Kdz dy) \dz dyj' 

or from (1) Kyi'^^ = -V^a, (]]) 

whence the velocity of propagation of magnetic induction is also equal to 

It would seem that in some cases, such as that of the field surrounding 

a straight wire with a steady current, the electric intensity may be regarded 

as entirely due to the motion of magnetic induction, and its components will 

^. , . dL dM dN 
thereiore be -^ , -^r- , -^r - 
dt' dt ' dt 

But in other cases it would seem that the electric induction cannot be 
wholly due to the motion of magnetic induction, and we must therefore 
introduce the terms involving ifj. If, for instance, the electric and magnetic 
intensities were inchned at an angle 6, we should have to suppose the electric 
intensity E to be produced by the motion of the component of magnetic 
induction I perpendicular to E, viz., /xZsin^, the other component [jlI cos 6 
being at rest. To produce intensity E, E tubes must cut unit length in the 
direction of E per second ; and since the value of the magnetic induction is 
^I sin 6, this requires a velocity v, given hj v . [jlI sind = E oi v = E/fil sin 6. 
Now we can easily imagine a case where E and I coincide, as, for instance, 
a condenser with its planes parallel to the axis of a wire carrying a current, 
and its terminals connected with two points in the wire. Here 7 sin ^ = 0, 
and V is infinite. Or we have to suppose the electric intensity to be produced 
by the movement of tubes of induction of no intensity with infinite velocity, 
a statement without physical meaning. 

But it is, perhaps, worth noting that if we suppose that the electric 
intensity is produced by the motion of magnetic induction, and that the 
magnetic intensity is produced by the motion of the electric induction, each 
carrying its energy with it, the right quantity of energy crosses the unit 
area. 



218 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 

For E magnetic tubes, with / sin d unit cells per unit length, will carry 

p -n IT ^., EI sind 1 ,£ j_-, 
across unit area m the plane of E and / a quantity — ^ , or halt the energy 

which actually crosses the plane. If I sin 9 is due to the motion of electric 
tubes, then I sin d/i-n- tubes must cut unit length in the direction of I sin 6 
per second. The number of unit cells per unit length is E, and therefore 

the motion of the tubes will carry a quantity ot energy — ^ , or the other 

half actually crossing. 

The equations which have been obtained in the foregoing manner by the 
aid of the hypothesis of movement of magnetic induction may also be 
obtained without any special hypothesis as to the motion of the induction- 
tubes, merely assuming that growth of induction through a curve is accom- 
panied by electric intensity round the curve. Instead of connecting L, M, N 
with the number of tubes which have cut the axes, we start with the following 
definitions : 

Let L, M, N denote the time-integrals of the components of the electric 
intensity parallel to the axes since the origin of the system, so that 

L = \pdt, M = JQdt, N = ^Rdt, 

,. J. dL ^ dM -^ dN 

then p = Q = -,^ , R = ^^ , 

dt ^ dt ' dt 

If a, b, c be components of magnetic induction, since the growth of in- 
duction through a curve is equal to the Hne-integral of the electric intensity 
round a curve in the negative direction, we have 

(h^dQ_dR^d /dM _ ^\ 
dt dz dy dt \ dz dy j 

with corresponding equations for -j and -j . 

Integrating with respect to t from the origin of the system, when all the 
quantities were zero 

^dM_dN 
dz dy 

'-S-S^ <>■) 

_ dL dM 
dy dx 

equations the same in form as equations (1). 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 219 



As before we obtain equations (3), (4), and (5), while instead of (6) we have 
the simple equations P = -^rr and the two others. 



dt 



dL 



Substituting for -j- we obtain an equation of the same form as (7), which 

may also be put into the form (8). Equations (9) and (10) will also follow. 

Just as we have obtained equations by considering the growth of the 
magnetic induction to its present state so we may obtain corresponding 
equations by considering the growth of the electric induction. 



Let 



Adx 



Bdy 

477 = 



Cdz 



be the algebraic sum of the number of electric induc- 



477 ' 477 ' 477 

tion-tubes which have cut dx, dy, dz drawn from a point in such a way as to 
create magnetic intensities in the positive direction along dx, dy, dz. 

The excess of the number of tubes which have passed in over those which 
have passed out through the boundary of any area will be equal to the time- 
integral of the total current through the area. 

The components of the total current are 



V + 



de 



dg 
dt' 



w 



r + 



dh 
dt' 



f, q, and r being the components of the conduction-current or the number of 
tubes dissipated per second, and/, g, h the components of the induction actually 
existing. 

As in the last case, if we put/'== \udt, etc., we at once obtain the equations 



477/ 



dC 
dy 



dJB 

dz 



^ dz dx 



^h' = 



dB 

dx 



dA 

dy \ 



,(12) 



Corresponding to the current-equations (2) we have three equations obtained 
from the condition that the rate of increase of magnetic induction through 
an area is equal to the integral of the electric intensity round it in the negative 
direction. These are 

da _ dQ dR 

dt dz dy 

dh _dR _dP 
dt dx dz 
dc_dP_dQ 
dt dy dx 



.(13) 



220 



ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 



If C^ is the specific conductivity we may by Ohm's law put the current- 
equations after integrating in the form 



KQ 

4:77 ■ 

KR 



g'= C.JQdt 

J 47r 

whence in media where K is constant 

dg' dh' _ ^(dQ dR 



dz dy ' }\dz dy 



dt 



K(dQ_ 
477 \dz 



dR 

dy. 



K da 



with two similar equations. 

Finding the values of the left-hand side from (12) we obtain 



47rC,a + iiC^ = -VM 
' dt 



d fdA 
dx \dx 



dB dC\\ 
dy dz) 



' dt dy\dx dy dz) 



irrCn + K 



dc 
dt 



-V^C 



dy 
d fdA 



dy 
dB 



+ 



dC 



If we assume 



477 



/// 



dz\dx ' dy ' dz 
da\ 1 



iirC .a + X^j- dxdyd 



with corresponding values for B' and C and 

J^ [[[fdA dB dC\\ 
4:7TjJJ\dx dy dz 



M 



1 - dxdydz, 



then 



A' 
B' 

C 



dL^ 

dx 
dM 

dy 

dN 
dz 



(14) 



(15) 



are solutions of (13). 

We may obtain by substitution from (15) in (12) values for/, g', h' 
corresponding to the values of the magnetic induction in (9), viz. : 

dC _ dR 
dy dz ' 



w 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 221 

and two others ; where A', B\ C are given in terms of the magnetic induction 
as above. 

It is only in special cases, such as that of a straight wire with a steady- 
current, that the magnetic intensity will be equal to 47r times the number of 
electric induction- tubes passing through unit length per second. In all cases 
the Une-integral of the magnetic intensity round a closed curve is equal to 
47r times the number of electric tubes passing through the boundary, but the 
electric tubes may be more crowded in some parts than in others, while the 
magnetic intensity is not altered in a corresponding manner. For instance, 
the magnetic tubes will be continued through an insulated conductor in the 
field, while in the steady state no electric tubes pass through it. But each 
element adds to the hne-integral the quantity which, after Mr. Bosanquet, 
I have called the magnetomotive force, this being equal to hr times the 
number of electric tubes passing through the element. But it only adds it 
on integrating round the whole of the closed curve. 

The intensity at any point will therefore be the resultant of the intensities 
produced by the magnetomotive forces in the various elements. Perhaps 
the simplest mode of finding it is as follows. 

The components of the magnetomotive force produced in a cube dx, dy, dz 
parallel to the three edges will be 

dA . dB . dC . 

W^^' W^^' Tt^'^ 

for T- , -. T— , -r- -T- are by definition the rates at which electric tubes 

47r dt ' i^ dt ' 477- dt -^ 

are cutting unit lengths parallel to the axes. 

But these magnetomotive forces would be produced by currents round the 
cube in planes perpendicular to the axes respectively, and equal to 

I dA , ^ dB . 1 ^C , 

4.rW^^' SrW^^' i^Tt^'^ 

for the Hne-integral of the intensity round a curve threading a current is 
47r X current. But the magnetic intensity at any point due to a current is 
equal to that of a magnetic shell of strength (i.e., intensity x thickness) 
equal numerically to the current bounding the shell. 

If we suppose the thickness of the shell equal to that of the cube, the 
effect is the same as if the cube were magnetised with intensity having com- 
ponents 

]^dA l^dB \_dC 

47r dt ' ^ dt' 47r dt ' 



222 ON THE CONNECTION BETWEEN ELECTRIC CURRENT AND THE 



^-m- 



The potential of such a distribution of magnetisation is (Maxwell, vol. 2, 
p. 29, equation (23)) 

dA dp dBdjp dCdjp\ 
Wdx^dtdy^ dt dz) ^''^^^''' 

where p = -, and the magnetic intensity is given by 

""' dx' ^ dy' '^ dz' 

It may be noticed that in a steady field ~j-, -^, rr are all zero, so that 



47rjj. 



d dM dp d^dM dp d^dM dp\, 

dx dt dx dy dt dy dz dt dz) ^ ' 



We may obtain equations of the same form as those given in (14) without 
any hypothesis as to the movement of electric induction- tubes, merely assuming 
that the total current through a curve is equal to 477 x line-integral of magnetic 
intensity round the curve. 

We start with the following definitions. Let A, B,ChQ the time-integrals 
of the components of magnetic intensity since the origin of the system. 



Then 



A=^jadt, B=l^dt, C^jydt, 



and 



a 



dA. n_dB _dC 

dt' ^~lt' '^~Tt' 



We have the equation ^ttu = ^ ^ 

dy dz 
and two others. 

Integrating with respect to t we have 



. r ,, , ., dC dBs 
J -^ dy dz \ 



also 4^.' = ^ _ ^ I 

dz dx I 

4 y ._ ^-^ ^^ 
dx dy 
which are of the same form as (12). 

Hence exactly as before we obtain equations (14) and their solutions (15). 
The equations for the magnetic intensity are now 
^_dA dB dC 

""- dt^ p = iu' y^it' 



ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD 223 

If we differentiate (14) with respect to t, and substitute from these equations 
for magnetic intensity, we obtain 

with corresponding equations for ^ and y. 

Differentiating the second of these with respect to z, and the third with 
respect to y, and subtracting, we obtain 

with corresponding equations for v and w. 

These correspond to Maxwell's equations (7), p. 395. 

In conclusion it may be remarked that the equations found in this paper 
give the same expression for the rate of Transfer of Energy as that in my 
previous paper derived from Maxwell's equations involving F, G, and H. 



12. 

DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSULATOR. 

[Birmingham Phil. Soc. Proc. 5, 1885, pp. 68-82.] 

[Read December 10, 1885.] 

Maxwell has shown that the phenomenon known as the Residual Discharge 
may be accounted for on the supposition that the dielectric is an imperfect 
insulator in which the conductivity varies in different parts. His theory is 
really quite simple and straightforward and free from any hypothesis beyond 
the fundamental one of electric displacement. But its very generality makes 
it, I beheve, difficult to grasp. The idea of a yielding of displacement in the 
dielectric, accompanied by a conduction-current in the opposite direction, 
gives us no help in forming a mental picture of the process actually going on 
in the dielectric. A hypothesis as to the nature of electric current, which 
will shortly be pubHshed in the Philosophical Transactions, seems to me to 
render the theory easier to follow, and I propose in this paper to arrange 
Maxwell's account of the Residual Discharge in accordance with it. 

I shall first give some account of the hypothesis referred to in the special 
case of the discharge of a condenser. Let us imagine that we have two 
conductors, A and B, which we may suppose to be the two plates of a con- 
denser, charged with equal and opposite amounts of electricity, that of A 
being positive. Then the lines of force will run from A to B through the 
medium, the condition of the medium being described by saying that there 
is 'electric displacement' from A to B. Or we may describe it without 
introducing the confusing term 'displacement' by returning to Faraday's 
term 'induction.' We may then say that tubes of electric induction pass 
through the medium, each tube starting from + 1 of electricity on A, and 
ending in — 1 on B. The total induction across any section of a tube is then 
always equal to L If we draw the level surfaces at unit differences of 
potential the tubes will be divided up into cells, and if we suppose each cell 
to contain half a unit of energy then the whole energy of the electrified system 
is accounted for. Maxwell has called these unit cells {Elementary Treatise on 
Electricity, p. 47). According to the views of Faraday and Maxwell, the 
charges on the conductors bounding the dielectric are to be regarded as the 
surface-manifestations of the altered state of the dielectric corresponding to 



DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSULATOR 



225 



the energy put into it, somewhat as the pressure on a piston in the wall of 
a closed vessel of compressed water might be regarded as the surface-mani- 
festation of the strained condition of the water. 

In order to follow out the process of discharge in the medium, i.e., the 
mode in which it is relieved from its strained condition, we will first take 
a simpler ease in which we connect the two plates, A and B (Fig. 1), of 
one condenser to the two plates, C and D, of another condenser previously- 
uncharged, and so far from A and B that there is no appreciable direct induc- 
tive action on C and D. When equilibrium is again restored the + charge 
is shared between A and C, the — charge between B and D, while the difference 
of level has decreased. There is the same total number of tubes of induction, 
but each contains fewer unit cells than before, the energy corresponding to 
the decrease having been transferred to the wires, where it has been dissipated 
as heat. I shall use the term energy-length to indicate the line-integral of 
the electric intensity along its axis, this being the same as the difference of 

WIRE 



lllllllllimiTTTT 



""™=n 




■■""■ i""iijij f 



imimiijiiiiimirn 



potential when there is equilibrium. We may say then that the energy- 
length of the tubes has decreased. During the change some of the electric 
energy was converted into magnetic energy in the medium. This might be 
observed if sufficiently delicate means were used. 

If we confine our attention to the charges on the conductors we must say 
that equal quantities of + and — have moved respectively from ^ to C and 
from B to D along the wires. 

But taking into account the condition of induction in the medium, 
described by the induction-tubes, we must say that the induction- tubes move 
sideways out from the space between A and B into the space between C and 
D, the motion of the charges along the wires being really the motion of the 
ends of the induction- tubes. (See Fig. 1, where 1-6 may be taken as 
successive positions of a tube.) During the motion of the tubes some of 
their energy was converted into the magnetic form, the co-existence of the 
two forms, electric and magnetic, being a necessary condition of motion. 
We may illustrate this from the analogous case of a strained incompressible 
solid which can be sheared. If there is any mode of escape given to the strain- 

15 



p.c.w 



226 DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSULATOR 

energy by a slipping of the surface against the constraint, then the state of 
strain will be propagated outwards from the interior of the sohd, but some 
of the strain- energy will be converted into kinetic energy, and the presence 
of the two is a necessary condition for the propagation of the strain. 

Since the energy-length of a tube diminishes as its ends move along the 
connecting wires, we may represent this by supposing that parts of the tubes 
move into the wire. If a similar motion of electric induction took place into 
a dielectric it would remain, and the dielectric would become electrically 
strained, but in the wire the strain breaks down rapidly, the energy being 
converted into heat. I think there is good reason tO suppose that it is the 
electric energy which thus breaks down, the magnetic only being dissipated 
after it has been reconverted into the electric form. 

We may now consider the case in which total discharge of a condenser 
takes place through a connecting wire. Considering merely the conducting 
plates and the wire, we say that the charges move along them towards each 
other and finally unite, neutralising each other and producing heat in the 
wire. Regarding the medium we must suppose the tubes of induction to 
move sideways towards the wire, shortening as their ends, which are repre- 
sented by the charges, approach each other, and finally disappearing into the 
wire. Faraday describes the process by saying that 'when current or dis- 
charge occurs between two bodies, previously under inductrical relations to 
each other, the lines of inductive force will weaken and fade away, and, as 
their lateral repulsive tension diminishes, will contract and ultimately dis- 
appear in the line of discharge.' [Exf. Res. vol. 1, p. 529, § 1659.) The 
so-called velocity of electricity is merely the velocity of the ends of the tubes, 
and this may evidently vary according to the nature of the circuit. It is 
quite conceivable that if the wire be in a neutral medium, i.e., one in which 
there is no surface-difference of potential, say gold in air, and if it follow the 
direction of a tube of induction, then a tube may move into the wire throughout 
its whole length at once. In this case the 'velocity of electricity' would be 
infinite. 

We know from experiment that if a galvanometer be inserted in the 
connecting wire then the same magnetic impulse is observed wherever in 
the circuit the galvanometer be placed, the impulse depending on the 
galvanometer-constant and on the total discharge. The same experimental 
result may be stated in an equivalent form, viz., that the line-integral of the 
magnetic intensity round a closed curve encircHng the wire if integrated for 
the time of discharge is the same for all positions of the curve. On the 
hypothesis here described all the electric induction-tubes of the system 
finally pass sideways from the medium into the wire. They must, therefore, 
on their way pass inwards across any curve encircling the wire, so that the 
total number of induction-tubes cutting such a curve is the same for all 



DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSULATOR 227 

positions of the curve. In the paper above referred to I have sought to 
connect these two constants by supposing that the magnetic effect is due to, 
or more correctly accompanies, the motion inwards of the condition of electric 
induction. As soon as motion commences some of the electric energy is 
converted into magnetic, and the magnetic induction may be represented by 
ring-shaped closed tubes surrounding the wire. The two inductions, electric 
and magnetic, co-existing, will propagate the energy onwards till it finally 
arrives in the wire and is dissipated as heat, the induction there losing its 
directed condition. 

The flowing of electric charges along the wire, which is usually considered 
as the essential part of the phenomenon, or at least that to which attention 
is to be chiefly directed, becomes on this hypothesis merely the last stage in 
the process, which consists of a propagation from the surrounding dielectric 
towards the wire of electric and magnetic induction, which we may symbolise 
by the motion inwards of two sets of tubes, the electric tubes being, on the 
whole, more or less in the direction of the wire, the magnetic tubes being 
closed rings surrounding it. The wire plays the part of the refrigerator in 
a heat-engine, turning the energy it receives into heat — a necessary condition 
for the working of the machinery. 

Let us now take the case of a condenser in which the dielectric, though 
homogeneous, is imperfectly insulating, so that the charge gradually dis- 
appears. According to Maxwell, in this case 'induction and conduction are 
going on at the same time.' Though Maxwell gave no precise account of 
the process of discharge, his theory and the mechanical illustration accom- 
panying it are based on the supposition that two processes are going on at 
the same time in every part of the medium, viz. : (1) a yielding of the electric 
strain or 'displacement' in the dielectric, equivalent to a displacement- 
current from the negative towards the positive plate, and (2) a conduction- 
current from the positive plate to the negative equal to (1) in amount. This 
latter is accompanied by dissipation of energy. The two equal and opposite 
currents being superposed have no external magnetic effect. 

But it seems to me that we may equally well and more simply represent 
the facts by considering the first process alone, viz., the yielding of the electric 
strain, the medium being incapable of bearing it permanently. The electric 
energy is gradually converted into heat in the same part of the dielectric 
where it was previously electric, i.e., there is here no transfer of energy. The 
decrease of induction in the medium is accompanied by a corresponding 
decrease of charge on the plates, not by conduction of + or — electricity 
either way through the medium, but simply because there is a decrease of 
the induction in the medium of which the charges on the plates are the surface- 
manifestations. The induction decreases equally through the whole length 
of a tube, so that the tube 'weakens' at the same rate throughout its length. 

15—2 



228 DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSULATOR 

There will be no magnetic effect in the surrounding space for there is no 
movement inwards of electric induction-tubes to supply the place of those 
which decay. 

Perhaps we may take the following as illustrating the two modes of 
regarding the process. Suppose that a solid is submitted to some strain 
and kept in the strained position, but that the energy of the strain gradually 
dissipates ; then we may confine ourselves simply to the statement that owing 
to some rearrangement of the molecules they cease to have molecular strain- 
energy, the energy in each portion of the mass being transformed into heat 
in that portion, or we may imagine that there is a continual return from the 
strained towards the original position, accompanied by an equal reverse flow 
of the matter towards the strained position, this latter not storing up energy 
but dissipating the energy given up by the yielding of the strain. The 
ultimate result according to each is the same, but the latter account is purely 
hypothetical. 

We may at once obtain the equation giving the value of the charge at any 
time in terms of the initial charge when the condenser is left insulated. 

Let a be the charge per unit area, this being equal to the electric induction 
across unit area in the dielectric. 

Let K be the specific inductive capacity. 

Let X be the electric intensity in the dielectric, i.e., force per unit electricitv 
on a small electrified body. 

We have ^ "^ ^ (1) 

Now we know that the rate of decrease of charge on the ends is proportional 
to the charge a and therefore to X. 

The decrease of charge or of induction in the medium is therefore 

(it r' ^-^ 

where r is a constant, which we may term the specific resistance. 

Hence from (1) ^ + ^ ^^ 0, (3) 

or CT = GToC ^^'^ (4) 

If we use p to denote the decrease of induction per second, 

_ da _ _KdX 

KX^ 

The energy per unit volume is -^— ; its rate of decrease is therefore 

KX dX 
"l^TW (^> 



DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSLTLATOR 229 

Substituting from (2) and (5) we get the expression which here corresponds 
to Joule's law for the heating effect, viz., rate of decrease of electric energy 
per unit volume = pV. 

If at any moment the two end-plates be connected by a wire, transfer 
of induction will at once take place into the wire, and the whole system will 
be completely discharged. During this discharge there will be magnetic 
energy accompanying the motion of electric induction. 

We will now investigate the more complicated case of a stratified dielectric 
in which the different layers have different specific resistances. Before 
proceeding to the mathematical account we shall consider the process generally, 
taking the simple case in which K is the same throughout. Let the con- 
denser be charged very rapidly and then insulated. At the first moment 
there will be equal and opposite charges on the two end- plates, and the number 
of induction-tubes running through unit area parallel to the plates will be 
the same in each layer. But decay of induction, and dissipation of energy, 
at once sets in, the rate of decay varying in different layers, so that after 
a time the number of induction-tubes in contiguous layers will differ and there 
will be charges on the separating surfaces. In those layers where the rate 
of decay is most rapid there will be negative charges on the surface nearer 
the + plate, and + charges on the surface nearer the — plate. But still the 
induction in all is in the same direction. 

Now let the two end-plates be connected by a wire. At once induction 
is propagated into the wire and transference takes place from the space between 
the plates until they are at the same potential, i.e., until the line-integral of 
the electric intensity, or, since K is constant, that of the induction, from plate 
to plate is zero. The same number of tubes must have entered all parts of 
the wire, otherwise there would be charges at points along its length. Hence 
the same number of tubes running in the positive direction must have passed 
out from each of the layers. The result must be a reversal of the induction 
in some of the layers, viz., in those in which the induction decayed most 
rapidly. This, of course, means that after their positive induction has all 
flowed out and they are quite discharged, tubes from the other layers have 
bent round and entered them, now charging them in the opposite direction. 
We may imagine the process to be somewhat as in Figs. 2 and 3, representing 
a condenser with three layers, A, B, C, the decay having been most rapid 
in the middle one, so that it has become completely discharged, while there 
is still positive induction in A and C. 1, 2, 3, 4 (Fig. 2) represent successive 
positions of a tube moving out from A towards the wire ; T, 2', 3', 4', successive 
positions of a tube moving out from C. When they have taken up the positions 
4, 4' they come in contact, and where they overlap they will neutraHse each 
other and break up into two portions, the outer part of each forming one 



230 



DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSULATOR 



positive tube, as 5, Fig. 3, which will move off to the wire, inner parts uniting 
to form a negative tube 6 in B. 

When the difference of potential between the end-plates is zero, suppose 
the wire to be removed. The induction still remaining decays. If it decayed 
in the same proportion throughout, the difference of potential would always 
remain zero. But it decays in greater proportion in the negative layers, 
since in these the dissipation is, by hypothesis, most rapid. Hence in the line- 
integral of the induction from plate to plate the negative terms decrease more 
rapidly than the positive, and so the total value becomes positive. Then on 




WIRE 




Fig. 2. 



Fig. 3. 



again connecting with a wire another positive discharge occurs, 
may evidently be repeated, the discharge 
always being positive, until finally it 
becomes insensible. 



The process 



p r r A r r 































































r B 



r r 

Fig. 4. 



The analogy between the residual dis- 
charge and the phenomenon of elastic 
recovery in strained solids, pointed out 
by Kohlrausch, suggests a simple illus- 
tration. 

Suppose that we build up a cube with 
successive layers of substances with the 

same instantaneous rigidity but with different viscosities. Let this be 
placed between two plates, A, B, Fig. 4, the lower plate being fixed. Let 
rigid transverse partitions, r, r, be passed through the layers and attached 
by hinges to the two plates, and then let the upper plate be acted on by 
a force in a direction perpendicular to the partitions, so that a shearing strain 
is given to the whole cube. The partitions, r, r, are merely put so that the 
distortion from the original position shall always be the same throughout. 
When a given strain has been produced let the upper plate be also fixed. 
Now if the rate of dissipation of strain- energy were the same throughout 
the layers the stress would also be the same throughout, though gradually 



DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSULATOR 231 

decreasing, and on removing the constraint the upper plate would return by 
a certain amount and then remain in its new position. But the dissipation 
is not uniform and after a time the stress in some of the layers is greater 
than in others. Hence, on removing the constraint from A and allowing it 
to return, when those in which dissipation has been most rapid have become 
entirely free from strain-energy, there is still some remaining in the other 
layers. These latter will, therefore, strain the former, and we shall have 
a reverse stress in some of the layers. Thus A will come to a new position 
of equilibrium, not so far, however, as its first position. Suppose that it 
is now again fixed. At first no force is necessary to keep it in position, but 
the stress exerted by the negative layers decays more rapidly than that 
exerted by the positive, and soon, on being released, A will return still further 
towards its original position. The process may be repeated, the successive 
discharges of momentum imparted to A being always in the same direction. 

(Added April 16th, 1886. The supposition of stratification made by 
Maxwell is, no doubt, very artificial, and was made for the sake of simplicity 
in the mathematical treatment. He states that 'an investigation of the 
cases in which materials are arranged otherwise than in strata would lead 
to similar results, though the calculations would be more complicated, so 
that we may conclude that the phenomena of electric absorption may be 
expected in the case of substances composed of parts of different kinds, even 
though these individual parts should be microscopically small. 

' It by no means follows that every substance which exhibits this pheno- 
menon is so composed...' (Electricity and Magnetism, 2nd ed., vol. 1, p. 419). 

Probably in the case of blown glass or any dielectric made up of hetero- 
geneous parts, which has been flattened by rolling, there is more or less 
approach to the stratified condition, but in other cases, such as shellac or 
paraffin, we might fairly expect the dielectric to be similarly constituted in 
all directions. We can only, therefore, take Maxwell's investigation as 
showing in a general way that heterogeneity would introduce absorption 
phenomena, and we cannot expect the results obtained on the supposition 
of such a special arrangement to agree with those of experiment. We may 
regard the stratified arrangement as giving a superior limit, as it were, this 
being the constitution most favourable to the production of the phenomena 
in the way supposed. The inferior limit would be given by an arrangement 
in which each portion of the substance of the same kind stretched from plate 
to plate with the same cross-section throughout. In this case there would 
be no residual discharge produced. Using Maxwell's notation (see below), 
the resistance per unit cross-section may be shown to be 

^ + -'+ ... 



232 DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSULATOR 

instead of R = a^r^ + a^rz + ... 

and R' is always less than R. In any intermediate composition in which 
portions of more conducting matter are insulated from each other by less 
conducting matter we shall have residual discharge. 

It appears probable from experiments of Dr. Schulze-Berge {Nature, 
March 4th, 1886, p. 432) that the resistance of certain dielectrics is not pro- 
portional to the thickness, but is much less for thin layers than might be 
expected. May this not possibly arise from the size of the heterogeneous 
portions being comparable with the thickness of the dielectric, so that the 
more easily conducting portions may stretch in some parts from plate to 
plate? If so, we approximate more nearly to the inferior limit.) 

The mathematical account of the residual discharge on this hypothesis is 
practically the same as Maxwell's, but it may, perhaps, be worth while to 
give it with the necessary alterations, as these seem to make it somewhat 
more straightforward and evident. 

We shall suppose with Maxwell, ' for the sake of simplicity, that the 
dielectric consists of a number of plane strata of different materials and of area 
unity,' and that the induction is in the direction of the normal to the strata. 

Let a^, a^, etc. be the thicknesses of the different strata. 

Let Zi , Z2 , etc. be the electric intensity within each stratum. 

Let 2^1, 2^2 5 ®tc. be the amount of decay of induction per second in each 
stratum. 

Let/1,/2, etc. be the induction in each stratum. 

Let %i-^, ^^2 5 ^^^- b^ ^tie total number of tubes of electric induction entering 
each layer sideways, i.e., crossing in through its boundary, per second. 

Let r^, r^, etc. be the specific resistance referred to unit of volume. 

Let K^, K2, etc. be the specific inductive capacity. 

Let ^1, ^2 5 ^tc. be the reciprocal of the specific inductive capacity. 

Let E be the electromotive force due to a voltaic battery placed in the 
part of the circuit leading from the last stratum towards the first, which we 
shall suppose good conductors. 

Let Q be the total number of induction-tubes which have left the battery 
and entered the wires and dielectric up to the time t. 

Then since the same number of tubes enter all parts of the circuit in 
a given time, 

u^ = u^^ u^= ... ^ u say (1) 

These tubes tend to increase the induction in the layers. But at the same 
time decay is going on so that we have 

% = Vi + §, ^h = V2 + 5, etc., (2) 



DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSULATOR 233 

whence 2?i + ^ = 2^2 + -^^ = etc (3) 

We also have by Ohm's law 

Vi = ^> :P2=v^> etc., (4) 

and by the relation between induction and intensity 

X,= ^7rKh, (5) 

whence '^=-^+ir^ tt (6) 

Let us suppose that at j&rst there is no charge and that suddenly the 
E.M.F. E is made to act. Then if at once Q tubes enter the dielectric, 

Zi= iTTk^Q, etc., (7) 

and since £■ = a^Zi + 0^2X3 + ..., (8) 

E ^ 4:77 (^1% +^52^2+ ...)Q. 

The instantaneous capacity C which is equal to ^ is given by 

C= . .. J. — -. (9) 

i7r{k^ai + k^a^-i- ...) 

But dissipation at once sets in, and if the electromotive force E be continued 
uniform a steady state will ultimately be reached in which the dissipation 
in each layer is equal to the number of fresh tubes reaching that layer. The 
number of tubes entering being the same throughout, the dissipation p is 
also the same throughout. 

We have then p = ^ = ^ = etc., (10) 

and substituting in (8) E = (/!% + r2^2 + •-•) V- 
Hence if R = r^^a^-^- ..., 

P-R <^^' 

In this state we have the induction given by 

'^^^ ^7Tk^~ ^irkj ^irk^R ^ ^ 

If we now suddenly connect the extreme strata by means of a conductor 
of small resistance, E will be suddenly changed from the value Eq to zero 
and Q' tubes will pass out from each layer of the dielectric into the wire. 
If then X' be the new value of the intensity, 

O' = ^1 _ ^1 

^ iTTk^ iirk^' 

whence * Z/ = Zi - 477^^1^' (13) 



234 DISCHARGE OF ELECTRICITY IN AN IMPERFECT INSULATOR 

Since then the difference of potential is zero 

a^X^ ^a^X^ -V .,. = 0, 
substituting from (13) we get 

^iZi + a^X^ + ... = 477 («!% + ajk^ + ...)Q' 
or ^ %Zi + a,Z, + ... Q^^Q (14) 

from (8) and (9). 

Hence the instantaneous discharge is equal to the instantaneous charge. 
By (10) and (11) we may put (13) in the form 

Zi' = fV^ - iTT^Q 

= ^^-i^k,Q (15) 

Let us next suppose the connection broken immediately after the dis- 
charge. No fresh tubes enter any layer, so that putting t^ = we have 
from (6) 



= 



Zi 1 dX^ 
^1 477A;i dt ' 



4TrA;i , 



or Zi = Z/6 

where Zi is now the value of the electric intensity at any time t after the 
connection is broken. 

Substituting from (15) and putting Eq for the initial value of E, 

X,==E,(^^-i7Tk,Cy~'^ (16) 

The value of E at any time is 
E = ^iZi + a^X^-^ ... 

= Eo (f"^'^ - ^^a,k,c) e-'-f' + (^^^^- - 477^2^1^26') 



e 

i-rrko 



E, {^-^ - 477;l^iCJ a,e r. + Q - 477y^2C) a,e -^ ' + ..\ 
EM-.^CnY-^^e-"^^' ^\ (17) 



A J R 

The instantaneous discharge obtained at any time t will be, as before, CE. 

If the terms be arranged in descending order of magnitude of ~\ then the 

exponentials are also in descending order of magnitude, or the negative terms 
decrease more rapidly than the positive, and E is positive. 



13. 

ON THE PKOOF BY CAVENDISH'S METHOD THAT ELECTRICAL 
ACTION VARIES INVERSELY AS THE SQUARE OF THE 
DISTANCE. 

[British Association Report, 1886, pp. 523-524.] 

The proof of the law of electrical action depending on the fact that there 
is no electrification within a charged conductor was first given by Cavendish. 
His proof was made more general by Laplace, who has been followed by other 
writers, including Maxwell. Maxwell and MacAlister have also verified the 
experimental fact, repeating an investigation of Cavendish only recently 
pubHshed in Maxwell's edition of the Cavendish papers. The proof may be 
analysed in the following way : Take the case of a uniformly charged sphere. 
The action at a point within it may be considered as the resultant of the actions 
of the pairs of sections of the surface by all the elementary cones, with the 
point as vertex. If, then, the resultant action is zero for all points and for 
all sizes of the sphere, it follows that the action of the pair of sections by each 
elementary cone is zero ; and, since the sections of the surfaces are directly 
as the squares of the distances, the two sections neutraHsing each other, the 
force per unit area must be inversely as the squares of the distances. There 
appear to be two objections to this proof. (1) That it takes no account of 
the always existing opposite charges. When the sphere, for instance, is 
positively charged, an equal and opposite negative charge is on the walls of 
the room, and the action of this should be considered. Probably this objection 
could be removed. (2) There is a solution still simpler than the inverse 
square law — viz., that no element of the surface has any action within the 
closed conductor. If we suppose that a conductor is a complete screen to 
electrical action, then, whatever the law of the force exerted across an insulator, 
there will be no action within the conductor. In any null proof it is not 
sufficient merely to show that there is no action in the null arrangement, but 
it is also necessary to show that on disturbing the null arrangement some 
action is manifested. Now, in the case here considered it is impossible to 
obtain any action within the conductor in any statical arrangement; it is 
only during changes of the system while charging or discharging that we can 
get a disturbance of the null arrangement. But here new phenomena come 



236 ELECTRICAL ACTION VARIES INVERSELY AS THE SQUARE OF DISTANCE 

in, for we have currents, and therefore electromagnetic action. But, even 
disregarding the different kind of action occurring, the only experiment which 
I know of on this point was that of Faraday with his electrified cube. While 
the most violent charges and discharges were taking place on the outside of 
the cube, so that the null arrangement was probably disturbed, he found no 
action on his electroscope within. Possibly the actions were alternating, and 
so rapid that no electroscope of ordinary construction would reveal them. 
But he himself went into the cube, and he would probably be sensitive to 
rapidly alternating electromotive forces. It appears to me, then, that we 
cannot accept this proof, and must fall back upon the more direct proof of 
Coulomb*. I do not know whether Maxwell was aware of this objection ; but 
it is worthy of note that in the remarkable fragment published since his 
death, as An Elementary Treatise on Electricity, he returned to Coulomb's 
proof, and was apparently building up the mathematical theory of electricity 
in a way quite different from that followed in his larger work. 

* [In his lectures on electrostatics Poynting used to give an experimental proof somewhat 
differing from that of Coulomb, and simpler. A brief account of Poynting's apparatus will be 
found in Electricity and Magnetism by Poynting and Thomson, vol. 1, pp. 65 and 66. Ed.] 



14. 

ON A FORM OF SOLENOID-GALVANOMETER. 

[Birmingham Phil Soc. Proc. 6, (1888), pp. 162-167.] 

[Read May 10, 1888.] 

The instrument described in this paper is a form of solenoid-galvanometer 
in which the iron core is still far from saturation, so that the attraction of 
the core by the coil is nearly proportional to the square of the current. The 
peculiarity consists in an arrangement by which a pointer moves over a scale 
a distance not very far from proportional to the current. 

The moveable core of the solenoid consists of an iron rod or bundle of 
wires, and is suspended by a silk fibre, which is wrapped on to the circumference 
of a small wheel with a horizontal axis turning in bearings as free from friction 
as possible. The wheel has an arm (Fig. 1) with a moveable bob on it, and 
the bob is so adjusted that the weight of the iron core just balances it when 
the arm is horizontal. The equilibrium is of course unstable, and a stop S 
is necessary jiist above the arm when in the horizontal position. The arm 
ends in a pointer moving over a divided quadrant. 

The solenoid is placed above the iron core so as to act against its weight, 
and in the position of maximum pull. The coil is moveable up and down by 
means of a screw, so that it may always be put in this position of maximum 
pull. The current passing through the coil does not saturate the iron, and 
the upward attraction is therefore nearly proportional to the square of the 
current. Let it be equal to KC^ where K is a constant for the particular 
instrument. 

If W is the weight of the core, a the radius of the wheel, w the weight of 
the wheel and bob, and b the distance of its centre of gravity from the axis, 
the condition for equilibrium when no current passes is 

Wa = wb (1) 

If now a current C passes, the down pull of the core is lessened and the 
arm falls into a position in which the bob has a less moment. If it moves 
through an angle 9, 

(W -KC^) a = wb cos (2) 



238 ON A FORM OF SOLENOID-GAL VANOMETEE 

Substituting from (1) for Wa, 



KC^a = wb{l- cos 6) = 2wb sin^ 



e 



or 



From this 



^ / 2wb . 6 



de = 2 






Ka dC 



cos 



which only very gradually increases with 6, and when 6 = 90° it has a value 
'\/2 or 141 times its value at 0°. 




Fig. L 



ON A FORM OF SOLENOID-GALVANOMETER 239 

It is very easy to construct an arc divided to give readings proportional to 

sin ^ as follows : — Describe a quadrant, and mark off points on it with 

ordinates increasing by equal amounts. On the radius from which these 
ordinates are measured describe a semicircle. Drawing the radii of the 
quadrant to the successive points marked, they will intersect the semicircle 

in points with equally increasing values of sin ^, 6 being the angle subtended 

at the centre of the semicircle. 

In practice it would no doubt be better to graduate by trial, having a 
standard instrument in the circuit. 

The instrument shown, though faulty in several points and far from 
frictionless, works fairly well. The range is limited by the fact that unless 
the adjustment is very perfect, the readings cannot be trusted below 10° or 
20°, but I think the principle might be usefully adopted for voltmeters of 
small range, or for ammeters, to give a correct value for a current within 
a small range. It might be useful to extend the range by a counterpoise 
to part of the weight of the core, on the other side of the wheel. The instru- 
ment has the advantage that, when the current is passing, the pointer very 
rapidly comes to rest. 



A Suggestion for a Wattmeter. 

The above instrument has suggested to me a possible form of wattmeter 
which I have not seen described before. I have not yet constructed an 
instrument on this plan. 

A soft iron core is fixed vertically at one end of a steelyard, with a moveable 
counterpoise as usual on the arm beyond the knife-edge. Two co-axial 
solenoids, one of high and the other of low resistance, are fixed in the position 
of maximum pull on the core when the arm is horizontal. For stability they 
should be above the core. The ends of the high-resistance-coil are connected 
to the two ends of the circuit in which the rate of working is to be measured, 
a commutator being interposed so as to reverse the current in the coil. The 
low-resistance-coil forms part of the main circuit. When the currents pass 
in the same way through the two coils, the pull on the core will be 

{aC + hEf, 

where a and h are constants for the solenoids. The counterpoise is to be 
adjusted for equilibrium. The current now being reversed in the high- 
resistance-coil, and the counterpoise being again adjusted, the pull on the 
core will be 

(aC - hE)\ 



240 



ON A FORM OF SOLENOID-GALVANOMETER 



The distance through which the counterpoise has been moved will be pro- 
portional to the difference between these two pulls, or to 

4:abCE, 
i.e., to the rate of working CE. 



A Square-Root Steelyard. 

Some years since another arrangement occurred to me for obtaining an 
equally divided scale, giving directly the square root of the pull on a soft 
iron core or on a moveable coil. After recently constructing a model, I found 
it was only a particular case of the very remarkable machine for solving 
equations, devised and constructed by Mr. Boys (Philosofhical Magazine, 
vol. 21, 1886, p. 241). Being, however, a very special case, it is less com- 
plicated than the general instrument, and as the model works easily and 
correctly, it may be worth while to describe it. 

ABC (Fig. 2) is a lever balancing on a knife-edge at B, and the pull W, 
of which the square root is to be measured, is applied at the end A. GE is 

C D B A 



H G F 



y^^^.y^yy.^y^yf/yZ'yyy/^y^^yyy^/y^.^^ 



Dp 



^ 



^ w 



Fig. 2. 

another lever balancing on a knife-edge at G, the arm GE being about equal 
to the arm BC. The plane upon which G rests is, in the model, a plate of glass, 
about equal in length to BC, and so arranged that GE may be moved until 
E is under any point of BC. E is connected by a link DE with BC, and from 
E , exactly under C, hangs a fixed weight P. 

If the down pull of the link at I) is T, and w is its weight, the up pull 
dXEi^T — w. 

The equations of equilibrium of the two levers are 



and 
or 



if GR be made equal to ^ GE. 



W . AB ^ T . BD 

(T-w)GE = P. GF, 
T .GE^P.GF 

= p(gf 

= P.HF 

w 



(1) 



.GE 

ge] 



(2) 



ON A FORM OF SOLENOID-GALVANOMETER 241 

Multiplying (1) and (2) together, T is eliminated and 

W .AB.GE = P .BD.HF, 

Making HE equal to BC, and keeping P always exactly under 0, BD is 
equal to HF, and 

W .AB.GE = P,BD^: 



BD 



y 



AB,GE 



Vw, 



If then BC is equally divided, the equilibrium position of D gives a reading 
proportional to the square root of W. 

In the model a lever, not shown in the figure, fixes ABC, and at the same 
time Hfts P up so as to release GE. GE and the link DE can then be moved 
along to a new position. On moving back the lever, ABC is released and 
P is dropped again into position on GE, exactly under 0. 



p.o.w. 



[6 



15. 

ON A MECHANICAL MODEL, ILLUSTRATING THE RESIDUAL 
CHARGE IN A DIELECTRIC. 

[Birmingham Phil. Soc. Proc. 6, 1888, pp. 314-317.] 

[Read November 8, 1888.] 

The model is designed to exhibit a phenomenon analogous to the residual 
charge which gathers in a condenser after it has been charged and then 
discharged, when the dielectric is not a perfect insulator. Its mode of action 
is similar to that which Maxwell supposes to occur in the dielectric. According 
to his theory the residual charge is due to the breaking down of the state of 
strain (or, perhaps, more correctly, of the stress) in the dielectric corresponding 
to the original charge, but in an uneven manner in different parts of the 
dielectric, so that just before the discharge the stress is greater in some parts 
than in others. On discharging, it is impossible, from the nature of electric 
discharge, to remove all the strain by connecting the two plates of the con- 
denser, and the condition of equilibrium which is arrived at consists in an 
actual reversal of the strain in the parts where the breaking down has been 
most rapid, the reversed stress in these parts balancing the remnant of the 
original stress in the other parts. On insulation, the strain breaks down 
again, and at the greatest rate in the same layers, now reversed. Consequently 
the reversed stress is no longer able to balance the direct stress, and, on the 
whole, there is a preponderance of strain in the original direction, or a gathering 
of charge the same in kind as the original charge. 

The model consists of a trough (see figure) of semicircular cross-section, 
24 ins. long, 6 ins. diameter, and divided into eight equal compartments by 
a middle partition along the axis and three cross-partitions. It is supported 
at the two ends, so that it can rotate about its axis 00, a pointer P attached 
to one end moving in front of a scale S. Four pipes, with taps t, t, t, t, connect 
the opposite compartments when the taps are turned on. The trough is 
balanced by the weights w, w, so that when empty it is in neutral equihbrium. 
Turning the taps off, and pouring in water to the same depth in all the com- 
partments, the equilibrium at once becomes stable, and the trough, if displaced, 
stores up energy. It may be considered as analogous to a 'tube of force,' 
connecting charges ± q on the surfaces of two opposite conductors, the axis 



A MECHANICAL MODEL, ILLUSTRATING THE RESIDUAL CHARGE 243 

of the trough representing the axis of the tube of force, the angle of displace- 
ment the charge at either end, or the induction along the tube. A clockwise 
rotation at the pointer-end might signify a positive charge at that end. As 
long as the taps are off, the trough represents a perfect insulator, a displacement 
through a given angle, and fixture at- that angle, corresponding to the com- 
munication of a charge and subsequent insulation. The energy remains in 
the trough undissipated. Discharge, of course, corresponds to release of the 
trough, and we have oscillations corresponding to the electrical oscillations 
brought recently into such prominence. It may be noted that a decrease 
in the quantity of water corresponds to an increase in specific inductive 
capacity, while a decrease ,in the weight of the trough corresponds to an 
increase in magnetic permeability. We might, perhaps, obtain an analogy 
to the spark- discharge by completing the cylinder, of which the trough forms 
half, and carrying the partitions up through the added half. On turning 
the trough through anything more than a right angle it would fall over and 
oscillate about a new position 180° from the original one, the discharge of 




energy occurring now with an increase of strain, not with a return to the 
unstrained condition. If the taps are turned on, but all to the same extent, 
the trough corresponds to a 'leaky' dielectric in which the conductivity is 
uniform. Turning the trough through a given angle and holding it, the 
water begins to flow back from the higher to the lower compartments, thus 
dissipating the energy, and if after a short time the trough is released it 
returns to a position short of the original position and remains there, the 
level of the water in the two sides of the middle partition being the same. 
But if the taps are turned on by different amounts — if, for example, the two 
end-taps are turned off while the two middle ones are turned on, — then on 
turning the trough through a given angle and holding it, the energy of the two 
middle pairs of compartments is gradually lessened, and on release the trough 
moves part way back. But now it is only the mean level which is the same on 
the two sides. In the two pairs of compartments with no communication there 
is still a positive difference of level, while in the other two there is now a 
negative difference. Holding the trough in its new position for a short time, 
the negative difference is reduced by leakage from one side to the other, and 
on release the trough returns by another amount towards its original position 

16—2 



244 A MECHANICAL MODEL, ILLUSTRATING THE RESIDUAL CHARGE 

—and this may be repeated several times, until finally the original position is 
sensibly regained. 

The first model I made, for ease of construction and without sufiicient 
consideration, with rectangular instead of circular cross-section; and with 
this the phenomenon of residual charge is obtained, even though all the taps 
are turned on equally. For consider what happens if the trough is turned 
through an angle and held. The water comes to a level after a time, but 
still its centre of gravity is not in the lowest possible position, and on release 
the trough returns part way, making a negative difference of level between 
the two sides. Again holding it, this difference is reduced, and on release 
there is another return, and so on. This suggests that possibly residual 
charge may occur not only when the substance is heterogeneous, but also 
when it is homogeneous, if with electric induction or strain there is both 
energy of the molecules as a whole and internal energy between the parts 
of each molecule. If the latter dies away after the bounding conductors 
are charged, the former may still remain, and on discharge it is possible 
that it may not all be dissipated, but may partly go to renew the internal 
energy. If this renewal accompanies a reversal of the direction of electric 
strain, we shall have the phenomena of residual charge. 

It is hardly necessary to point out that the model serves equally as an 
illustration of a possible explanation of elastic after-action. It is evident 
that the phenomenon of residual charge will always occur when a body 
strained is such that the stress dies away unequally in different parts, while 
at the same time its constitution is such that on release from strain an equal 
amount of strain is taken from each part. 

From this illustration of residual charge we may pass to a possible analogue 
of conduction in a metal wire. Let us suppose the trough replaced by a hollow 
cylinder, with its axis horizontal, ends closed, and without partitions. If the 
cylinder is only partly filled with water, a small couple applied to it will 
produce continuous rotation, but with a limiting angular velocity, attained 
when the water is dragged up in one side so far that the moment of its weight 
about the axis is equal to that of the applied couple. The energy put in by 
the couple is all ultimately converted into heat in the water. Thus the 
angular displacement increases indefinitely, though the stress always remains 
small. Similarly, as I believe, the 'electric strain,' or 'induction,' or 
' displacement ' in a wire carrying a current increases indefinitely, as induction 
IS continually coming into it from the outside, although the stress always 
remains small. 



16 



ELECTRICAL THEORY. LETTERS TO DR. LODGE. 

[Electrician, 21, 1888, pp. 829-831.] 

to the editor of ' the electrician.* 
Sir: 

I have prevailed on Prof. Poynting to let me send you the enclosed 
two letters, wherein he continues the discussion of electrical theory begun 
in Section A at Bath. It must be understood that the letters are merely 
hasty epistles, not intended for publicity ; but Prof. Poynting's ideas are so 
original and weighty that one is glad to extract from him, when possible, 
a casual contribution to a discussion, as well as one of his sledge-hammer 
communications to the Royal Society. I hope that this may be the means 
of extracting a reply or a criticism more competent than anything of mine 
would be. 

Yours, etc. 

Oliver J. Lodge. 



Dear Lodge: 

I thank you very much for the copy of your exceedingly interesting 
account of Electrical A. Perhaps my gratitude would be best shown by 
silence, but I am tempted to show my appreciation by asking you to help 
me with some difficulties. 

My first difficulty is as to the interpretation of Hertz. You say — though, 
I think, FitzGerald is responsible for the statement — that ether is a demon- 
strated fact. I do not see how Hertz adds to our certainty. Is not our 
belief in ether due to the fact that light takes time to travel in interplanetary 
spaces, where we cannot put enough matter for it to travel by. so that we 
have to imagine something else for it to use. The fact that the velocity of 
light is nearly the same in vacuo and in gases, and not widely different in 
denser substances, of course supports the view that on the earth it also uses 
ether. Hertz shows that there is an 'interference' in electromagnetic 
disturbance which we can only (at least, with our present knowledge) put 
down to wave-motion traveUing with a definite velocity which he finds equal 
to that of light. Hence these disturbances probably make use of the same 
ether. Does this prove its existence any more? I should expect a sceptic 



246 



ELECTRICAL THEORY. LETTERS TO DR. LODGE 



to ask why may not electromagnetic disturbance make use of air, since 
Hertz carried out his experiments in air. I could only reply to the sceptic 
that he was a very disagreeable person. 

Secondly, Thomson's [Kelvin's] 'Simple Hypothesis' Paper* appears to be 
a very serious attack on Maxwell's theory ; in fact, on reading it over carefully, 
I can only come to the conclusion that it would lop off not only Maxwell's 
excrescences but his whole theory. According to the concluding sentence 
of § 4, (/ each component of electric current at any point is equal to the electric 
conductivity multiplied into the sum of the corresponding component of 
electrostatic force and the rate of decrease per unit of time of the corresponding 
component of velocity of liquid in our primary') the current 



pf_d^_ duj\ 
\ dx dtj' 



which = if C the conductivity = 0, so that Maxwell's / (his ' displacement- 
current ') goes altogether. The x component of Maxwell's e.m.f. will contain 

a term 



^ ' ' ' -r-^ dxdydz, since Maxwell's u 



477 



Thomson's u-{- -. — ^ , where 

477 dt 



P = E.M.F. along X. This has no representative in Thomson. Thus with 
a homogeneous but leaky condenser with no connecting wire, we have, 
according to Maxwell, total current = 0, for leak is made up for by yield of 
displacement ; 

•• dx' 

where V is potential due to electrification on the plates. 

According to Thomson, u is to be taken as rate of leak, and is positive ; 



P = 



du 

dt , , , dV 

— axaydz r- • 

r dx 



According to Maxwell there is no magnetic effect, since total current = 0. 
According to Thomson^ — using his notation — x component of magnetic 
dtVi dv-i _ ^_2 fdw dv^ 

dy dz \dy dz, 

d 



force u 



if V-2 and 
But 



dz \dy 

etc., are transposable 
u = KP, V 

:. u, = - 477V-2 



from Thomson's equation (7), i.e.. 



dy 



KQ, 



w 



KB; 



dR dQ 



dy 



477V- 



[dy\ 



dz) 

' dw 

di 



ff, 



dxdydz 



T " dz 

* Reprinted in The Electrician, vol. 2L Sept. 14, 1888, p. 605. 



— dxdydzi 



ELECTRICAL THEORY. LETTERS TO DR. LODGE 247 

the term in V disappearing. This is awful ; but I see no reason to suppose 
that it vanishes. If it does not vanish, then the existence of magnetic effect 
would decide against Maxwell. 

Thirdly, I note on p. 10 of your sketch that Rowland and FitzGerald 
consider that electrostatic potential is not propagated by end-thrust, and you 
remark that it is the magnetic potential which travels, generating the electro- 
static potential as it goes along. I think I remember that you have expressed 
the view that potential energy must undergo a kind of conversion, and that 
unless it be born again as kinetic energy it can in no wise go forward on its 
journey*. With strained solid waves it looks as if it were so, though it is, 
I think, possible to regard both energies as going forward linked together, 
yet retaining their individuality. And if potential energy is, after all, kinetic, 
but of another kind, it is conceivable that they should keep their separate 
identities. But with electrostatic and electromagnetic strains, which is 
potential and which kinetic? I know it is usual to call the magnetic kinetic ; 
but if we had started with permanent magnets, and travelled by means of 
magnetoelectric machines to our present knowledge of electric phenomena, 
I expect we should now^ be discussing the propagation of magnetostatic 
potential, and magnetoelectric potential, and we should, perhaps, consider 
the former generated by the latter. This is really the view I take, or, rather, 
I think both are true. It seems to me that the sideway propagation of 
electric induction is accompanied by (let us drop 'generated by') magnetic 
induction, and equally the sideway propagation of magnetic induction is 
accompanied by electric induction. The two go together when a disturbance 
is propagated. In a steady state they do not, i.e., if we can separate them 
from each other ; at least I do not see how otherwise to interpret the results 
I have obtained (see pp. 284-5 of paper referred to below) f. This brings us 
back to the old point whereon we have differed before. It would be better 
to give in like the unjust judge for the sake of peace and quietness, and to 
ward off any more such letters as this. 

I have been going again through a paper ' On Connection between Electric 
Current and Electric and Magnetic Induction' (Phil. Trans. 1885) {, in which 
I tried to work out the equations to the magnetic field on the supposition of 
this sideway propagation. "^ ceases to be troublesome, and both electric and 
magnetic inductions are propagated at the same rate; indeed, they are by 
Maxwell's equations, though I do not think he ever definitely worked this 
out. I cannot see any point, in the assumption to begin with, or in the 
subsequent reasoning, where I have gone astray. If you have any time to 

* Phil. Mag. October, 1879, p. 281, § 11; June, 1881, p. 534; and June, 1885, p. 486. I do 
not regard this as a 'view,' however, but as a proved truth, — 0. J. L. 

t [Collected Papers, pp. 201-2.] J [Collected Papers, Art. 11.] 



248 ELECTRICAL THEORY. LETTERS TO DR. LODGE 

spare, would you look at the Paper, pp. 277-281, and 294-300? The rest 
is not essential, though on 301 I show that Maxwell, pure and simple, gives 

the same results*. 

Yours, etc., 

J. H. POYNTING. 

Mason College, Birmingham, 
October 12, 1888. 

[The following extract from my note in answer may be inserted, in order 
to make the next letter clear : 

' As regards the proof of the ether, I confess I did not quite see FitzGerald's 
point as to why Hertz's experiments rendered the existence of ether any more 
certain; but knowing that I had felt it thoroughly estabhshed long ago, 
T supposed I was not a good judge. Of course he must appreciate all the 
stock arguments about air, etc., not transmitting transverse disturbances, 
and about neither it nor glass transmitting anything at the speed 3 x lO^^, 
etc. And Hertz's experiments only seemed to me to prove that electro- 
magnetic waves existed and travelled at the same speed, thus practically 
proving that Ught is electromagnetic waves, and estabhshing Maxwell's 
theory. 

' This seems to me far more important than proving once more the existence 
of ether. At the same time I feel sure FitzGerald has some point. It may 
be only that an electromagnetic ether has been proved, and thus the action- 
at-a-distance-Germans confounded. He spoke as if he meant more than this. 
Perhaps I have to that extent misrepresented him in my " sketch." How 
does it strike you? 

'With regard to Thomson's Paper, it is certainly very anti-Maxwellian, 
but I believe it only represents a transition stage through which he was 
somewhat rapidly passing, and through which he may now have almost 
passed. I certainly do not know now where he is. 

' Is it not that, finding that displacement-currents have no magnetic effect, 
therefore he ignores them? But, then, have they no magnetic effect? In 
some cases they cannot have, for electrostatic displacement is of the nature 
of an '' expansion," as Clifford called it ; there is no '' spin " about it. 

' I cannot find Thomson's Paper this minute to refer to, but I have it some- 
where. I will look it up again in the light of your remarks.' 0. J. L. 

After referring to my reply, Prof. Poynting writes, in a second letter :] 

About the ether, I should entirely accept your interpretation of Hertz. 
I should think, as you say, that FitzGerald was aiming at believers in action- 
at-a-distance, and probably he knew where to have them in showing that 
there is an electromagnetic medium. 

You say, with regard to Thomson's Paper, 'Is it not that, finding that dis- 
placement-currents have no magnetic effect, therefore he ignores them? 
But, then, have they no magnetic effect? In some cases they cannot have.' 

* [Collected Papers, pp. 194-198, 212-217, and 218 respectively.] 



ELECTRICAL THEORY. LETTERS TO DR. LODGE 



249 



It is just here that I find the supposition of transverse propagation of 
electric strain accompanied by magnetic induction so clarifying to my ideas. 
If the magnetic effect is the whirling of the machinery which is sending 
electric strain energy onwards, the machinery cannot know whether the 



U + + + + 4) Ai 




"C_ ni\\\ _J>" 


' -• ] r- , V 

' , ' ' ' ' \ 


/ 1 . * - . J \A 


.' / / / I 




HiW 



energy is going to be dissipated in a Prony-brake-like wire, or whether it is 
going to increase the electric strain, so producing a 'displacement-current.' 
To use an illustration, I regard a re-entrant line of magnetic force as a kind 
of ring of Custom House officers registering the amount of electric strain sent 
in per second (they are porters as well), and they do not know what will happen 
to the energy. They will register just the same whether the imports are for 
immediate consumption or go to add to stock. Of course, they will take no 
account of shooting stars, balloons, or destruction of stock already within 
the ring ; which, being interpreted, is that the line-integral of the magnetic 
force (47^^■, isn't it?) will not be affected by Knes of electric force other than 
those coming in or going out through the boundary. For instance, if A moves 
through the ring RR to A^ (see figure) conveying a charge, hardly any Unes 
of electric force will cut the ring RR, yet the number of lines through it is 
increased. And it will not be affected by induction which dissipates itself 
in situ as in a leaky Leyden jar, where the energy changes to heat without 
moving. I do not know of any other case, but, being utterly ignorant of 
pyro-electricity, I imagine it might supply a case of establishment of electric 
induction without motion of the energy and without magnetic effect. 



Yours, etc., 

J. H. POYNTING. 



October 18, 1888. 



17. 

AN EXAMINATION OF PROF. LODGE'S ELECTRO- 
MAGNETIC HYPOTHESIS. 

[Electrician, 31, 1893, pp. 575-577, 606-608, 635-636.] 

The leaders in Physical Science, impressed perhaps with the responsibility 
of their position, and fearing that their weaker followers will distort their 
views, are, as a rule, very cautious in giving us their vaguer speculations as 
distinguished from the more exact hypotheses which can at once be put into 
working shape. Yet these less-formed speculations are often helpful, even 
if they only arouse our minds to attempt to disprove them. Still more are 
they helpful if they aid us in thinking of facts in a more connected way until 
the finished working hypothesis is ready to take their place. 

We owe a special debt of gratitude on this ground to Dr. Lodge for his 
well-known book on the Modern Views of Electricity, in which he describes 
not only the more definite beliefs which he firmly holds, but gives us also 
any suggestions rising in his mind which seem to give promise of light to 
guide us in the dark ways of the science. He talks as it were confidentially 
to us, and though the speculative character of the book makes it by no means 
the easy reading which the absence of mathematical treatment might lead 
us to expect, and even perhaps unfits it for beginners, the bold attempts at 
explanation give it great value to more advanced students. Such students 
will be brought face to face with many difficulties which they may hitherto 
not have recognised through haziness of thought. And even where they are 
unconvinced by Dr. Lodge's attempts to solve the difficulties they will be 
gainers by the orderly review of their knowledge necessary before they can 
form a judgment. 

The book is built round a central hypothesis of the nature of electric 
action, which I propose to examine. I shall first give an account of the 
hypothesis as it appears to me to stand, freed from the details and the wealth 
of illustration, which, though appropriate, and even necessary, in the original 
work, make the argument at times rather difficult to follow. I shall then 
examine the evidence for or against the hypothesis. 

We start with that which everyone accepts as the result of experiment 
and observation, that there are two kinds of electrification with oppositely- 



AN EXAMINATION OF PROF. LODGE's ELECTROMAGNETIC HYPOTHESIS 251 

directed qualities, and that they make their appearance always in equal 
amounts. Hence, on their union, the net electrification is zero. Compare 
this with the case of momentum. According to our experience, as summed 
up in the third law of motion, we may regard the mutual stress between two 
bodies as consisting of a transfer of momentum from one to the other. A gun 
at rest is fired. The momentum gained by the bullet and powder may con- 
veniently be regarded as derived from the gun, which, having none to begin 
with, now has an amount of negative momentum equal to the positive possessed 
by the charge. In other words, positive momentum is transferred from gun 
to charge. Or compare with a case of material transfer, as when A lends B 
a sum of money. Then A and B, after the transfer, are oppositely affected, 
so that if they both assign their share in the transaction to a third person 
C the effect on C is zero ; or if 5 re-transfers the sum borrowed to A the net 
result is zero. 

Such cases as these suggest that positive and negative electrification are 
merely the creditor and debtor sides of a single transaction, the sending out 
and the reception of something transferred (p. 9). What kind of transfer 
we must imagine is best gathered from the ' ice-pail ' experiment, a particular 
case of the general principle that when induction occurs + and — always /ace 
each other in equal quantities with an insulating medium between. We may 
suppose that we have a nearly closed hollow insulated conductor, and that 
through an orifice we introduce a body having on it a charge + Q. Immedi- 
ately — Q gathers opposite to it on the inside surface, and + Q is on the 
outside surface, facing — Q on the walls of the room in which the experiment 
is made. If, instead of carrying out this electrical experiment, we imagine 
an indefinitely extended incompressible liquid, and think of merely mathe- 
matical surfaces occupying the positions of the surfaces of the conductors, 
the introduction of Q of liquid within the inner surface would force Q of hquid 
through each of the two surfaces, and relative to the space between the 
surfaces, inwards towards the inner surface, and outwards from the outer. 

Now, suppose that such an incompressible hquid, to which for the 
present we need not ascribe gravitation, has an actual existence, that it fills 
all space with which we are concerned, and that it permeates matter. Let 
us suppose that in bodies which we term electrical conductors it is free to 
flow with nothing worse than frictional loss of energy, but that in insulators 
it has some kind of attachment to the matter, so that in the displacement 
of one relative to the other — that is, in the strain — energy is stored, and in 
such a way that it can be regained when the strain is relaxed. We shall 
call this liquid Electricity. When a displacement occurs, so that some of 
the liquid is pushed into or out of any conductor, the quantity flowing through 
the surface from or to the insulator is that which we have hitherto called the 
charge of electricity on the surface. We must now term it Electrification to 



252 AN EXAMINATION OF PROF. LODGE'S ELECTROMAGNETIC HYPOTHESIS 

distinguisli it from the general body of the hquid, which is, according to 
Lodge, all Electricity. It is important to notice this distinction in studying 
Lodge's hypothesis. Electricity is the generic name for the fluid all through 
space, Electrification the specific name for that part of it which happens to 
have flowed in any given disturbance through the surface of a conductor. 
Thus, if we imagine a conducting sphere A within a hollow conducting sphere 
B, B having an orifice through which we can introduce a wire to charge the 
inner sphere, say, positively, the gathering of a charge Q on ^ is to be regarded 
as a flowing of some of the all-extensive electricity along the wire into the 
sphere, which is, however, already full of electricity. Hence Q must be 
pushed through the surface of the sphere out into the insulator or dielectric. 
This outward displacement manifests itself as the + electrification of ^. As 
the dielectric is also initially full of electricity, Q must be pushed out through 
every surface in it completely enclosing A. The displacement relative to the 
air-particles stores energy — the energy of the charge. When we come to the 
inner surface of B, Q is pushed into the substance of B, an inward displacement 
which we term a negative charge. It is also pushed through the substance 
of B, but as this substance is conducting, no energy is stored, and only a little 
is dissipated by the frictional rub, or, perhaps better, the viscous cling. At 
the outer surface of B there is another displacement of Q outwards into the 
dielectric — i.e., another positive charge, and energy-storing begins again. 
The pushing out will take place through the second dielectric till we come to 
the walls of the room in which the action is occurring. Here it will probably 
end, for the generator of the original charge has probably sucked in the fluid 
from the walls. There is therefore a confined circulation, and not an infinitely 
extended pushing-out. 

Sources of electrification with their connecting wires are evidently to be 
regarded as turbines working in pipes or channels laid in space, incompressible 
and fluid electricity filHng both the pipes and the space outside them. When 
the turbines work, the fluid runs along the pipes, forming what we call an 
electric current. When a pipe ends in a reservoir bounded by a dielectric, 
the fluid presses out into the dielectric, and there stores the energy put into 
it by the turbine, minus that dissipated by the viscous resistance in the pipes 
and conducting channels. 

Prof. Lodge illustrates the connection between the electric incompressible 
fluid (why not the electric Hquid?) and the molecules of matter by a series of 
ingenious and suggestive models either with cord running through beads or with 
Hquid to represent the electricity. It is hardly necessary here to describe these. 
Any reader who is not yet acquainted with them should study the original 
account. It is enough to say that Prof. Lodge can make his models behave Hke 
Leyden jars and give the phenomena of charge, residual charge, and oscillating 
discharge as perfectly as if they were thorough beHevers in his hypothesis. 



AN EXAMINATION OF PROF. LODGE's ELECTROMAGNETIC HYPOTHESIS 253 

I find it somewhat easier to form a picture of the hypothesis by supposing 
matter to have a sponge-Hke constitution, i.e., to be permeated in every 
direction by passages, the pores so dear at one time to the writers on elemen- 
tary science. These passages are filled with Hquid electricity. In every 
passage or tube, however short, we must imagine a Httle turbine turning 
round with the flow through it and never letting any fluid pass without 
duly turning. In dielectrics there is a spring, Hke the mainspring of a watch, 
attached to the spindle of each turbine so that when the wheel turns energy 
is stored. In conductors this spring is wanting, and there is only a viscous 
resistance to rotation. We may think of the source of electrification, machine 
battery or induction coil, as a large turbine somewhere in the system with 
a supply of energy behind it dealt out by a motor of some kind or other. 
When the large turbine works, a flow takes place in the system dissipating 
energy in the conductors and storing it in the dielectrics. When the charging 
turbine is removed or disconnected from the motor, so that the way is clear 
for a return, all the wound-up mainsprings return and drive the liquid back 
through the sponge. We have only to make the turbines with different 
moments of inertia, with different qualities of lubricator in the conductors, 
and with different strengths of spring, and different firmness of attachment 
in the dielectrics, to get varying permeability, electric resistance, specific 
inductive capacity, and residual charge. We may,- perhaps, simphfy the 
arrangement of affairs by supposing that the molecules themselves are the 
turbines, and then we get a kind of inversion of the hypothesis described 
hereafter, which Dr. Lodge develops to account for electromagnetism. 

In the displacement- hypothesis, with a single electric Hquid flowing past 
matter, there is a very serious difficulty. The energy being stored by the 
flow past the molecules of matter, we might reasonably expect the electricity 
to pull, or to tend to pull, the molecules with it, and there should, therefore, 
be a motion of matter along the lines of force in one direction. But instead 
of this we have a tension, both ways as it were, along the lines of force or flow. 
There is displacement of matter only in the case of electrolytes, and here it 
is both ways along the lines of flow, one set of atoms going one way and another 
set the opposite way. A modification of the hypothesis is suggested by 
Dr. Lodge to meet this difficulty and to account for the double electrolytic 
procession. He supposes that there are two constituents of the electric fluid, 
each in general filling half any space, intimately mixed and evenly distributed. 
The molecules of matter are made up each of two constituents in accordance 
with the usual view, and one of these is attached to one kind of electric fluid 
the other to the other. When electric displacement occurs it is really a double 
flow, the two constituents of the fluid travelling equal distances in opposite 
directions past each other. The atomic constituents of each molecule move 
in opposite directions, but not in general very far, so that there is in dielectrics 
no displacement of the matter as a whole. In electrolytes the displacement 



254 AN EXAMINATION OF PROF. LODGE's ELECTROMAGNETIC HYPOTHESIS 

continues till separation and re- pairing occur, and thus we get a double 
procession. 

We see that in dielectrics, the pulls of the two constituents of electricity in 
the molecules, one on one atom, the other on the other, balance each other. 
These electric pulls lead to internal stresses within the molecule about which 
Dr. Lodge does not say anything very definite, but it seems to me that we have 
to introduce chemical forces here to account for the pull of the atoms on each 
other, distinct from the electric forces or the pulls of electricity on matter. 
This dualism is hardly in accordance with the late exposition of Dr. Lodge's 
ideas, where he appears to identify electrical and chemical forces (p. 84). 
Perhaps we might as well, while we are inventing a constitution for the ether, 
make a third or neutral constituent to which all the atoms of matter are 
attached. We will suppose this neutral electricity to resist extension and 
compression. We then have electro-positive atoms attached to positive 
electricity, electro-negative atoms to negative electricity, and all of them to 
neutral electricity. When an electric displacement occurs, positive ether 
tugs at one set of atoms, negative ether at the other set, and neutral ether 
prevents their separation, so that all our forces are of one kind, insomuch 
as they are forces between atoms and ether. I rather Hke this neutral ether, 
but I am afraid Dr. Lodge will not adopt a strange infant into a family already 
sufficiently large. 

But taking the hypothesis as set forth by its author, the mere duahty 
does not much affect the general notion of the nature of electric charge. We 
must remember that motion of negative fluid inwards equally with that of 
positive charge outwards gives a positive electrification, so that the explana- 
tion of the electrification of a sphere within a conductor already given has 
only to be amplified by supposing that there is another ethereal fluid displaced 
in the opposite direction at the same time and throughout the system. 

I can imagine the agnostic in ethereal matters protesting here against 
the multiplication of unknowns and unknowables. I can imagine him saying 
that his sense-organs are only excited by material motions and affections, 
that his instruments are all material, and only appear to undergo changes 
of shape, colour, sound — i.e., affections of matter, and that he cannot with 
any certainty get beyond these material affections. He will argue that, as 
we have no sense affected by ether alone, we can form no adequate conception 
of the ether ; we can only suppose it endowed with material properties, and 
conceive of it as some form of matter. And it would appear possible to 
imagine various material ethereal constitutions or connecting machineries 
between the different portions of matter evident to our senses, all equally 
accounting for all known facts. When a new fact turned up he would own 
that probably some of the ethereal machinery would fail to account for it, 
and so would have to be taken off to the lumber-room for worn-out hypotheses ; 



AN EXAMINATION OF PROF. LODGE'S ELECTROMAGNETIC HYPOTHESIS 255 

but the new fact, he would argue, would very Ukely enable us to imagine new 
types of machinery to replace some, at least, of the old rejected ones. And 
probably, till the whole range of physical phenomena was known, it would 
always be possible to imagine more than one kind of machinery to account 
for the phenomena known. Probably only when nothing remained to be 
discovered would there be a single solution, and only then would it be possible 
to give a single answer to the question. What is ether? And even then we 
might be wrong, for the ether might have properties in its action on matter 
quite different from any of which we have material types. 

Though our agnostic, when following this train of thought, may be un- 
convinced by Dr. Lodge's preface, and may urge that the ether is and will 
probably remain a hypothetical medium, he will, no doubt, adopt some form 
of hypothesis for working purposes. If he is an ordinary human being, when 
he studies such actions as we term actions-at-a-distance, he will prefer to 
think of the different parts of the acting matter as connected by something 
continuous, with material properties. Following Boscovich and Faraday, 
he may extend the atoms throughout space, and give this extension material 
properties (a special case of this type of hypothesis is presented to us in the 
ring- vortex theory of the Universe) ; or he may limit the atoms and put in 
some new connecting machinery to fill up the vacuum he abhors, and this 
he may as well call 'ether.' While, therefore, he may protest against 
Dr. Lodge's ' cocksureness ' about any particular constitution for the ether, 
he is bound to examine any hypothesis reasonably presented to see if it is 
likely to form a good working hypothesis to account provisionally for the 
observed facts. He would, no doubt, admit that Dr. Lodge's hypothesis is 
reasonably presented, and it would only be a question with him whether so 
complicated a constitution for the ether enables him to think sufficiently 
easily of the phenomena for which it is to account. Leaving him to consider 
this, we may pass on to the further development of the hypothesis. 

So far, we have only been thinking of the properties of electricity at rest. 
It is true that we have thought of the electricity as being pumped along 
conductors and as wasting energy in the passage, but this was only a step 
onwards to a final statical distribution. We are now to concentrate our 
attention on the pumping stage. 

When electricity is in motion in sufficient quantity and for sufficient time, 
a new set of phenomena come into prominence. Among these are the heating 
of the conductor, the heating or cooHng of junctions, the opposite ionic pro- 
cessions in electrolytes, and the creation of a magnetic field. The heating 
of the conductor is to be explained, according to Dr. Lodge, as something 
analogous to frictional, or rather viscous, dissipation of energy. We may 
think of the two streams of electricity flowing past the atoms of matter, and 
continually catching hold of them and letting them go again, as a fiddle-bow 



256 AN- EXAMINATION OF PROF. LODGE'S ELECTROMAGNETIC HYPOTHESIS 

catches hold of and lets go a fiddle-string. Thus, some of the energy of flow 
is converted into vibrational energy of the atoms, that is into heat. The 
junctional heat- phenomena still wait for complete explanation. We may 
consider that the facts imply that at a junction of dissimilar metals there is 
a tendency for positive electricity to move more easily in one direction than 
the other, and, of course, the reverse with negative electricity. A compound 
bar with free ends will thus tend to be positively electrified at one end and 
negatively at the other. Suppose, further, that the tendency to separation 
of electricities varies with the temperature, and we have at once the thermo- 
electric current in a closed circuit. Returning to the fiddle-bow and string 
used to illustrate the development of heat by conduction, we can see how it 
ought to work to illustrate thermo-electricity. Suppose a set of parallel 
strings in a horizontal plane, one half tuned to one note and the other half 
to another. The one set may represent one metal with its atoms vibrating 
in given modes, and the other set another metal in contact with it and with 
its atoms vibrating in other given modes. Now, laying the fiddle-bow across 
the strings after they are set in vibration, if the motion of the bow is always 
in one direction it illustrates the pushing of one of the electricities from one 
metal to the other. I have found that when light bits of paper are laid across 
two vibrating strings of different pitch there is frequently a movement of 
translation. Unfortunately for the illustration it is sometimes in one 
direction, sometimes in the other. The paper was not part of Prof. Lodge's 
book, and knew nothing of the hypothesis it was expected to support. The 
opposite procession of ions we may think of as the transport of the atoms 
by the positive and negative electric streams respectively, the connections 
between the pairs of atoms being broken down and renewed with fresh partners 
all along the line and continually. 

The most evident phenomenon characterising the electric current is the 
magnetic field around it. This we may regard as manifesting the existence 
of so much magnetic energy in the neighbourhood of the conducting wire. 
Let us see how the hypothesis will account for this energy. The first step 
is to reduce permanent magnetism and current-magnetism to one species by 
adopting Ampere's theory. If we consider a small closed current-bearing 
circuit, observation tells us that, at a distance from the circuit, the field is 
indistinguishable from that due to a small steel magnet, with centre in the 
plane of the circuit and axis perpendicular to it. Starting from this, we know 
that we can deduce an arrangement of permanent magnets, equivalent, as 
regards the outside field, to any current-bearing circuit. The circuit differs 
magnetically from the steel in two respects only, viz., that it requires a con- 
tinual supply of energy to maintain it, and that we can get into its inside. 
This last difference may be merely due to the large size of any apparatus at 
our command, and it is quite thinkable that, if we could make ourselves or 
our apparatus smaller than molecules, we could explore the inside of the steel 



AN EXAMINATION OF PROF. LODGE'S ELECTROMAGNETIC HYPOTHESIS 257 

molecules. The other difierence is possibly not one of the kind ; for suppose 
the resistance to be diminished till it disappears, the rate of energy-supply, 
C^R, disappears also, and we have a current-circuit — never mind how the 
current was started — which is as permanent a magnet as a steel bar. It is 
a short and inevitable step from this to Ampere's hypothesis that a magnetic 
molecule — a molecule of steel, say — is essentially a small closed perfectly- 
conducting circuit with a current of electricity in it ; or, in terms of Dr. Lodge's 
hypothesis, either two equal and opposite currents whirhng round at equal 
speeds in opposite directions in each molecule, or a positive whirl in one 
direction in one molecule, accompanied by a negative whirl in the other 
direction in the next molecule. We may dismiss this duaUty for the present, 
on condition that it comes up for sentence when called upon, and return to 
the single circuit. Such a circuit, when placed in a magnetic field, behaves, 
doubtless, Hke finite circuits, and tends to set itself perpendicular to the lines 
of force, and with its own lines parallel to the lines of the field, and in the 
same direction through the circuit. It tends to move from weaker to stronger 
parts of the field — tends, in fact, to include as many positive and exclude 
as many negative lines as possible. 

But the disappearance of resistance has a pecuHar effect. Even a circuit 
of the resisting kind, with which alone we have practical acquaintance, would 
protest against the inclusion of foreign fines of force in addition to its own, 
and the current would diminish while they were being included. An Amperean 
circuit would not merely protest, but would absolutely prevent any change 
in the total number included, for any increase would be accompanied by a 
finite negative e.m.f. proportional to the rate of increase, and therefore by 
an infinite current, since R is zero. This is not to be accepted as possible, 
so that all that can happen is that some of the current's own fines of force 
shall be replaced by those of the field, and there is a consequent weakening 
of the current. 

When a mass of iron consisting, we suppose, of such Amperean currents is 
brought into a magnetic field, all the circuits tend to turn round to include 
the fines of the field, and at the same time the Amperean currents decrease in 
strength. The circuits thread themselves like beads on to the fines of force 
of the external field, their currents falling as they thread on. But their own 
surviving lines of force are added to those of the field, so that the total field 
is greater, and the iron has greater permeabifity than a vacuum ; or perhaps 
it will be better here to say that the iron conducts the lines of force better 
than a vacuum would, for permeabifity has an exact significance, not quite 
describing the property now under discussion. As new fines of force are 
added to the field the iron will conduct them better than a vacuum, until 
every molecule has all its lines brought into service, and its current, therefore, 
reduced to zero.' After this point is reached, since the molecules either will 

p. c w. 17 



258 AN EXAMINATION OF PROF. LODGE's ELECTROMAGNETIC HYPOTHESIS 

not take any more lines of force, or if they take any more will establish 
negative currents to neutrahse them, they are worse conductors than a 
vacuum, for they fill up part of the space uselessly or injuriously. They will 
therefore tend to move from stronger to weaker parts of the field, as if 
diamagnetic. Some indication of such a change in conducting power has 
been detected by Ewing in the magnetisation of iron in exceedingly strong 
fields, for though the permeabihty or induction produced -f- the field producing 
it — both reckoned from zero — was always greater than 1, the value of increase 
in induction -^ increase in field producing it, or the conducting power, as I have 
called it, fell ultimately below 1. 

For some reason, not yet explicable, the magnetic chains in iron become 
unstable, and break up when the temperature is raised to the neighbourhood 
of 800° C. The iron then above this temperature is practically equivalent 
magnetically to any other substance. 

Assuming then that we have some notion of what we mean by currents 
of electricity whirhng in channels of no resistance. Ampere's hypothesis gives 
us a fair explanation of iron and steel magnetism. If we accept the view 
that the interior of a magnet does not differ from the exterior in kind but 
only in degree and in permanence, we may attempt to extend the hypothesis 
to explain the magnetic quahties of all other substances, i.e., their power of 
carrying the Hnes of force, and of carrying them in sHghtly different degrees. 

Let us think of the lines of force in air circling round a current or passing 
from pole to pole of a magnet. We may think of these as passing through 
a number of electric whirls, or at any rate through perfectly conducting rings 
ready to exist as whirls. Before the passage of the lines of force these rings 
are turned in all directions. After the passage they tend to set perpendicular 
to the lines of force. If the medium is paramagnetic we may suppose the 
rings to have initial currents in them; if it is diamagnetic we may with 
Weber suppose that they have no currents initially, and when no Hnes of 
force pass through. On the estabhshment of the field negative currents are 
excited of such value as to make negative Hnes of force thread each ring equal 
in number to the positive Hnes sent through by the field. Thus each ring 
acts as a part of the field through which no lines of force pass, and the per- 
meabihty is thereby diminished. At the same time the diamagnetic substance 
will tend to weaker parts of the field, and we have the main facts of dia- 
magnetism explained. There is a serious difficulty in the nearly constant 
permeabihty, differing only by a very small amount for a diamagnetic soHd 
Hke bismuth and a magnetic gas like oxygen, the one with its molecules 
crowded together, the other with its molecules comparatively wide apart. 
Perhaps we can strengthen this weak point by supposing the conducting 
rings of very different diameters in the two substances, or we may think 
of the electric channels as different altogether from the molecules and the 



AN EXAMINATION OF PROF. LODGE's ELECTROMAGNETIC HYPOTHESIS 259 

same in number per c.c. in bismuth and in oxygen. Another difficulty Hes in 
the non-existence of permanent magnetism except in iron, nickel, and perhaps 
cobalt, but it is no greater than the difficulty with iron above 800° C. 
Perhaps both will ultimately find the same explanation, and until this is 
forthcoming we need only say that Ampere's hypothesis is merely silent on 
the point and is not necessarily unable to explain it. 

Now as to magnetic energy. Dr. Lodge points out a number of facts 
which suggest that magnetic disturbance is of the nature of spin round the 
Hues of force. The Amperean circuits at once present themselves as being 
the seat of this spinning, and the electric fluid or fluids in the channels as the 
spinning material. These whirHngs of electricity, either in themselves or in 
the accompanying motion of the entangled matter (Maxwell, by the way, 
thought it was the matter), possess, according to Dr. Lodge, the magnetic 
energy of the system. In fact, they are themselves magnetism. 

So far we have been dealing with magnetism and its relation to electricity, 
the space-filling fluid, and we have come to the conclusion that magnetism 
consists of vortices in this fluid. It remains to explain the nature of the 
ordinary electric current, and the way in which its accompanying magnetic 
field is maintained. Dr. Lodge uses for the purpose a mechanical analogue 
or mechanical model, a modification of Maxwell's well-known model, which 
is described in his Scientific Papers, vol. 1, page 451, and I believe by 
Dr. Garnett in Maxwell's Life. 

Let us imagine the two fluids, which are to be regarded as jointly filling 
space, to have a cellular construction, each consisting of spheres or little 
india-rubber bags, or what you like, in contact with each other. In dielectrics 
we think of contiguous cells as gearing in some way. Let these cells, when 
in a magnetic field, be spinning round the fines of force, the positive in one 
direction, the negative in the opposite; and let positive and negative be 
alternated so that the opposite motions may be possible without sHp of 
gearing. 

Fig. 37 from Dr. Lodge's book illustrates this idea when the fines of force 
pass into the paper from above, the axis of spin, therefore, being perpendicular 
to the paper. We may further materiafise the conception by inserting teeth 
round the edges of the cells; and here the family likeness to the parent 
(Maxwell's model) becomes stronger. We then have Fig. 36. If we suppose 
Fig. 37 to represent the unstrained state in a dielectric, then electrostatic 
strain will be presented by some such deformation as that represented in 
Fig. A ; not, I think, in the manner represented by Fig. 46 in Dr. Lodge's 
book. 

If we take Fig. 36 as our type, it is easier to think of the wheels as being 
attached to some kind of framework. We may think of all the positive 

17—2 



260 AN EXAMINATION OF PROF. LODGE'S ELECTROMAGNETIC HYPOTHESIS 

wheels in Fig. 36 as arranged flat against one series of parallel rods going 
down the page, and the negative wheels on another intermediate and parallel 
series. We may think of the wheels, or their teeth, as not quite rigid, so that 
when the positive rods are pulled down and the negative up, there is a slight 





Dr. Lodge's Fig. 36. Rows of cells alternately Dr. Lodge's Fig. 37. Section of a magnetic 
positive and negative, geared together, field, perpendicular to the lines of force; 

and free to turn about fixed axes. alternate cells rotating oppositely. 

(Another mode of drawing Fig. 36.) 




Fig. A. Electrostatic strain. The cells displaced slightly in opposite directions. 

C 




+ 1 +1 



2 



Dr. Lodge's Fig. 46. 



Fig. B. Cells shpping in a wire carrying 
a uniformly distributed current. 



relative displacement and energy is stored, and this will represent an electro- 
static strain. Probably, if we arranged all the forces properly, we should 
be able to do without the framework of rods. Meanwhile I find I cannot 
think of the model clearly without it. 



AN EXAMINATION OF PROF. LODGE's ELECTROMAGNETIC HYPOTHESIS 261 

The distinction between a dielectric and a conductor is to be represented 
by the abohtion of the teeth on the wheels in the conductor. The surfaces 
of the wheels are in contact, and are only more or less rough — viscously 
rough, rather than frictionally rough. It appears rather difficult to think 
of the layer of wheels separating a dielectric and a conductor, for on the 
dielectric-side they must have toothed gearing, and on the conductor-side 
they must only rub ; but we may get over the difficulty by supposing these 
wheels double, one toothed to gear on the dielectric-side, and the other 
untoothed to rub on the conducting side, and both keyed on the same axis. 

Dr. Lodge regards a current as represented by a slip of one row of wheels 
against the next. He says (p. 206), 'Notice that in a medium so constituted 
and magnetised — that is, with all the wheel-work revolving properly — there 
is nothing of the nature of an electric current proceeding in any direction 
whatever. For, at every point of contact of two wheels, the positive and 
negative electricities are going at the same rate in the same direction; and 
this is no current at all.... A current is nevertheless easily able to be repre- 
sented by mechanism such as that of Fig. 36 or 37 ; for it only needs the 
wheels to gear imperfectly and to work with shp. At any such shpping- 
place the positive is going faster than the negative, or vice versa, and so there 
is current there. A line of sHp among the wheels corresponds therefore to 
a linear current.... Understand: one is not here thinking of a current as 
analogous to a locomotion of the wheels — their axes may be quite stationary. 
The shp contemplated is that of one rim on another.' 

Thus in Fig. B, let the row of wheels represent the cells across the dia- 
meter of a wire carrying a steady current, ABCD representing the section of 
the wire. Let the speeds of rotation of the wheels be as marked on each. 
The resultant positive shp is + 1 at each surface of contact, a total of 6. 

It is perhaps presumptuous to quarrel with a parent such as Dr. Lodge 
as to his mode of developing the faculties of his own offspring, but I must 
venture here to differ from him entirely in the way in which he seeks to 
make his model represent current. It appears to me that he has grafted on 
to his own model the representation of current in the entirely different model 
of FitzGerald, and so obtains something quite inconsistent with his previous 
ideas. It is only by stopping short at the centres of the bounding wheels 
that he can obtain a resultant flow in one direction. Obviously, if he took 
in a whole number of wheels, each entire, he could get no resultant flow, for 
the flow on the opposite sides of each wheel is equal and opposite. Thus, 
in the figure, if he took into account the outer sides of the two last wheels 
in the wire, he would have — 6 neutralising the previously obtained -{- 6. 
Or, to put it in another way, if we draw a plane, say, above the line of centres, 
a tangent to all the wheels, there is evidently no resultant flow across that 
plane. And we can think of cases of shp when we have no reason to suppose 



262 AN EXAMINATION OF PROF. LODGE'S ELECTROMAGNETIC HYPOTHESIS 



I 



ABCDOdcba 



there is current. Thus, if a bar of soft iron is placed axially in a magnetic 
field, near the centre of the bar the hues of force are parallel to its length 
within and without the bar. According to Lodge, the electric whirls are 
very rapid within the iron, and comparatively slow in the air outside. There 
must, therefore, be slip at the boundary, and yet we have no dissipation of 
energy to indicate the existence of the current. 

Another objection is that the model worked thus would make a diflterence 
in kind between the process of displacing in a dielectric, the equal and opposite 
motions of the two fluids which are leading to an electrostatic strain, and the 
current in a conductor. This is rather setting back the clock. Perhaps we 
have gone on too fast, and the difference may exist, but I think we should 
hardly accept the evidence of the model on the point. 

What Prof. Lodge calls current appears to me then 
to be merely sudden change of magnetic intensity. If 
this is just criticism let us see if the model can be 
made to represent a true current. I shall take the 
case of a steady current in which the condition of 
affairs is not altering. It is always better to begin 
with such a case, just as it is better to begin with 
statics, hydrostatics, electrostatics, than with dynamics, 
hydrodynamics, and electrodynamics. When matters, 
or ethers, get into changing motion they are, like the 
celebrated pig of the Irishman, difficult to count. 

In the model we shall suppose that the current in 
the wire is represented by two equal and opposite 
processions of cells or wheels along the wire, and to 
help us in thinking of these processions we shall, as 
before, suppose two sets of rods parallel to the axis of 
the wire, the positive wheels on one set with their 
spindles perpendicular to the rods, and the negative 
wheels on the other set. 

In Fig. C the rods alone are represented, directions of motion of the 
wheels on each being shown by arrows. The + rods are to be regarded as 
moving down the page, and the — rods up. The wheels on the bounding 
rods, B, b, gear with the wheels on A, a in the dielectric, but sRp on those 
on C, c in the conductor. Now, if the conductor extended through all space 
I do not see that the motions of the rods would involve rotation of the 
wheels. For any rod would be as it were surrounded by a symmetrical 
system, its wheels would not know which way to turn, and so would merely 
rub against their neighbours. But here the conductor is bounded by an 
insulating medium in which the rods can only move a very httle way, and 
that only by straining the teeth of the wheels. 



Fig. C. Rods on which 
the positive and nega- 
tive wheels are sup- 
posed to be fixed in 
the model of a wire 
carrying a current. 



AN EXAMINATION OF PROF. LODGE'S ELECTROMAGNETIC HYPOTHESIS 263 

The result is that as B moves up past A its wheels behave as pinions 
moving on a rack in the place of A. They tend to turn the wheels of C in 
the opposite direction. These in turn tend to send round the wheels of B, 
and so on to 0, the middle rod. Now, starting from the other side, a is at 
rest, and as h moves past it its wheels must rotate. This rotation is trans- 
mitted inwards, as before, to 0. The result is that the wheels on are 
urged in opposite directions from the two sides, and so remain at rest, while 
those of D and d rub against them. Those of C and c rotate faster than those 
of D and d, the friction due to the sUp balancing that due to the sHp of D 
and d on 0, and so on, the rotation increasing outwards. When we come 
to B, b, their wheels have the frictional resistance against the C, c wheels on 
the inside, which must be balanced — since the motion is steady — by pressure 
on the outside against the A, a wheels. Hence the B, b wheels are not merely 
rolHng on the A, a wheels, but are pressing against them, and so turning them 
round. In other words, the rotation extends out into the dielectric. Thus, 
at the middle of the wire, there is no rotation — no magnetic intensity. It 
increases as we go outwards to the boundary, being in opposite directions 
on the two sides, and it exists in the surrounding medium. The rubbing of 
the surfaces, perhaps of the rods, perhaps of the wheels, dissipates energy 
which represents the heat appearing in the wire. 

The model is probably indeterminate as to the way in which the energy 
finds its way into the various parts of the conductor, where it appears as 
heat. We might imagine the rods moved by end- thrusts along the wire, 
their wheels setting the outside wheels spinning, or we might imagine a spin 
propagated along the outside wheels to the wire, and there setting the rods 
in longitudinal motion. 

I have no doubt that the model would be able to represent the phenomena 
of the current-induction when the motion is accelerating or unsteady, for 
Maxwell's model does this, and the differences of construction would hardly 
affect the point. 

I have now given an account of the main features of Dr. Lodge's hypothesis 
as to the nature of electric charge, magnetism, and current, and I have done 
this at considerable length because I propose to discuss the foundations for 
the hypothesis, and to see if they are sufficiently firm to bear the superstructure. 
To recapitulate the main features, we have a double ether consisting of equal 
-f and — portions attached in some sort of way to matter as a framework — 
elastically in dielectrics — loosely, with a kind of viscous connection in con- 
ductors. The two ethers always move in opposite directions along the fines 
of electric force, and in dielectrics they store energy by their displacement past 
the material framework, while in conductors they only dissipate it during the 
displacement. The ethers are cellular in construction, and the cells are capable 
of rotation, this rotation constituting magnetism, and the accompanying 



264 AN EXAMINATION OF PROF. LODGE'S ELECTROMAGNETIC HYPOTHESIS 

rotational energy being magnetic energy. The axes of rotation are along 
the hnes of magnetic force. The cells roll on one another as if quite rough in 
dielectrics, and as if lubricated with viscous oil in conductors, and they are 
alternately + and — , and moving in opposite directions, a clockwise moving 
+ cell giving the same direction to the magnetic force as a counter-clockwise 
— cell. Current is, according to Lodge, shp of these cells against each other, 
but according to the interpretation I have given of his theory, current is 
a double and continuous procession of the cells in a conductor, with the 
consequent rotations. 

The hypothesis is avowed by its author to be an attempt to obtain a 
mechanical explanation of electric and magnetic phenomena. He uses the 
main idea of Maxwell's well-known model, but replaces Maxwell's duality 
of magnetic wheels and electric 'idle' wheels by a duahty of electric wheels. 
It is, perhaps, open to question whether this is really a simplification, but 
the attempt was very well worth making, for it is only by variation and 
natural selection that the mechanical model will be suited to its environment 
in the electric world. 

It is, I suppose, useless to look for any other than a mechanical hypothesis 
as final. Probably because we are able to picture mechanical processes, able 
to think of ourselves as seeing what goes on, seeing kinetic energy manifested 
in the moving parts, able to think of ourselves as part of the connecting 
machinery, feehng the stresses, and helping to make the strains, we have 
come to regard mechanical explanations as the inevitable and ultimate ones. 

Thus, though the old mechanical hypothesis of light is for the present 
discarded in favour of an electromagnetic one, I suppose no one is content 
with the present position ; but we are all looking forward to the time when, 
by mechanical explanation of electromagnetism, Ught shall once more become 
mechanical. Nevertheless, ifis well to bear in mind that all such explanations 
are merely hypothetical, and may at any time have to be discarded, as the 
solid-ether Hght-hypothesis has been discarded. Indeed, they are solely of 
value as a scaffolding enabling us to build up a permanent structure of facts, 
i.e., of phenomena affecting our senses. And inasmuch as we may at any 
time have to replace the old scaffolding by new, more suitable for new parts 
of the building, it is a mistake to make the scaffolding too solid, and to regard 
it as permanent and of equal value with the building itself. It is on this 
point that I find most cause for disagreement with Prof. Lodge. He appears 
to me to regard his hypothesis as inevitable and permanent, or at least as 
approximating to the permanent and inevitable. The scaffolding, in fact, is 
made as important as the building. It behoves us, therefore, to examine 
into the security of its foundations. 

We may take the hypothesis as based on two statements : 

I. Kegarding electricity at rest, 'Whenever we perceive that a thing is 



AN EXAMINATION OF PROF. LODGE's ELECTROMAGNETIC HYPOTHESIS 265 

produced in precisely equal and opposite amounts, so that what one body- 
gains another loses, it is convenient and most simple to consider the thing not 
as generated in the one- body and destroyed in the other, but as simply trans- 
ferred' (p. 9). 

II. Kegarding electricity in motion: 'Magnetism is nothing more nor 
less than a whirl of electricity' (p. 171). 

Let us examine some cases which should come under the first statement. 

We may regard the upper and under surfaces of a vessel of liquid as 
respectively positive and negative, and if we measure them by their projections 
on a horizontal plane they are equal and opposite element by element. We 
clearly cannot create a positive without an equal negative. Bring the two 
elements together, and they neutrahse each other, for both cease to exist. 
We have no idea here of something transferred. 

Or, take the case of a rotating cord. Its two ends, as viewed from the 
outside, have equal and opposite properties, as may be seen at once if we think 
of them as brought close beside each other. If the rotations were transferred 
to one and the same body the net result both to themselves and to the body 
would be no motion. Or if the cord were cut away in slices, bit by bit, when 
it is all cut away the two ends have come together, and have, in a sense, 
neutrahsed each other. Here again we think of nothing transferred. 

As a third case take a magnet. We always create equal and opposite 
poles at the same time with mutually neutralising properties, and to bring 
this within Dr. Lodge's statement we have only to consider the magnet as 
made up of two bodies joined together in the middle. Or perhaps we might 
take the case of the creation of N. and S. poles by the breaking of a magnet. 
No one has yet made a displacement- or transfer-hypothesis for magnetism 
except for the purpose of illustration. This, by the way, is rather remarkable, 
for magnetism with its re-entrant tubes of induction is so much more ready 
to lend itself to such a hypothesis. We seem to have finally made up our 
minds that electric is strain-energy, and that magnetic is rotational energy, 
and we do not even consider the alternative of magnetic as strain and electric 
as kinetic energy. No doubt the existence of magnetic rotation of the plane 
of polarisation of light strongly suggests the rotational nature of magnetism, 
but we can hardly claim any explanation of the phenomenon yet put forth 
as complete, and certainly none is exclusive. The other facts would possibly 
be equally well explained if we thought of electric tubes of induction as 
strings of wheels with their spindles along the axis of the tubes, and magnetic 
tubes as fines of flow of ether or of two ethers, with a certain displacement- 
coefiicient, greater in diamagnetics, less in paramagnetics. I do not put this 
forward as worth following out, but merely to illustrate the contention that 
we need not necessarily think of -f and — electrification as due to a longi- 
tudinal transfer of something. It appears to me quite possible then to 



266 AN EXAMINATION OF PROF. LODGE'S ELECTROMAGNETIC HYPOTHESIS 

symbolise the positive and negative charges facing each other across a dielec- 
tric in other ways than by the transfer of something from one towards the 
other. 

Turning now to the second fundamental idea, Ampere's hypothesis, that 
a magnetic molecule is a small closed electric current, this is really closely 
connected with the idea that there is something moving in the direction of 
an electric current and constituting the current. It is in fact dependent on 
our acceptance of the first idea. For consider what an Amperean current is 
phenomenally. There is no resistance, and therefore no heat developed, and 
no fall of potential; there is no junction, and therefore no Peltier effect; 
there is no break, and therefore no chemical effect. The one effect left is 
the magnetic one. If, then, we take away the idea of some substance whirling 
round and round in a channel, all that we have left is a little permanent 
magnet. In other words, we explain the constitution of a magnetic molecule 
by supposing that it is a molecule having a magnetic constitution. 

It is true that if we get a bundle of Hues of magnetic force and tie them 
together with a perfectly conducting cord with its ends joined up we have 
a permanent magnet, for no more and no less Hues can pass through the ring 
of perfectly conducting stuff. But this does not really explain. It does not 
show that the unknown is a case of the known. For a perfect conductor is 
far more difficult to think of than a permanent magnet. 

It is getting time that these so-called 'perfects' were abohshed from 
Physics. We have to deal with matter as it is and not as we should have had 
it if we had been consulted as to how it should be made in order to simplify 
our equations. The perfect gas has nearly gone and I shall be glad to see the 
perfect conductor preparing also to depart. 

If, however, we grant substantiality to the electric current, we are met by 
a difficulty in connection with Ampere's hypothesis which, at any rate, requires 
examination. 

Let us suppose that we have a circuit of self-induction L and resistance 
R, and let N be the number of outside lines passing through the circuit. If 
E is the E.M.F. of the battery kind in the circuit the current- equation is 

li E = and i? = we have an Amperean circuit for which 

01* LC + N = constant. 

In words, the total number of Hues of force threading the circuit is 
constant. 



AN EXAMINATION OF PROF. LODGE's ELECTROMAGNETIC HYPOTHESIS 267 

If, now, N increases, either L or C must diminish. In order that L may- 
diminish the circuit must contract and pinch in, as it were, the Hues of force. 
But meanwhile, since the total is constant, the number of hues passing through 
the same area is increasing, and we should expect the spin, therefore, to increase 
and tend to make the tubes of force widen out. That is, there would be 
resistance to the contraction. The contraction is contrary, too, to our 
ordinary experience of a circuit with a current in it, for such a circuit tends 
to expand. If, on the other hand, the circuit is rigid, L is constant and C 
naust diminish. If, as I gather from the general tenor of Dr. Lodge's work, 
the Amperean circuit is one of the ether cells peculiarly constructed or at least 
of the same order of magnitude, a diminution of C means a diminution of spin 
of this cell. But the total number of lines of force through it is constant, 
which seems to imply that the spin remains the same. This seems to 
force us to suppose that the Amperean circuit is not of the order of the 
ether cells, but larger, so that it may include within its contour a number 
of these cells. Then when N increases, C round the boundary may diminish, 
and still LC + N he constant, for the internal wheels may be spinning at the 
same rate as before, but the external wheels more slowly, the current being 
a function of their difference. It would require further examination to see 
whether the total spinning energy could thus be accounted for. At first sight 
it would appear that that energy is diminished by the lessening of C just when 
we want it to increase. 

If we are to suspend our judgment as to the substantiality of electricity, 
and are not as yet to conclude that a current is a rushing round of a thing 
of some kind, we are bound also to suspend our judgment as to Ampere's 
hypothesis, and if we are in this state of suspense, and, for my own part, I must 
confess to being so, Dr. Lodge's hypothesis and its accompanpng models are 
to be used rather as illustrations — as analogies — than as ultimate solutions. 
For the purpose of illustration they are of the greatest value. Like a bank- 
reserve, ready in case of emergency to cash the symboUc bank-note, they 
are in the background in the mind ready to turn into mechanical reahty the 
ordinary electromagnetic symbols, such as hues of force, which are, I think, 
much more easy to deal with, but which are, doubtless, wanting in reahty. 

Perhaps Dr. Lodge may consider that my criticism is somewhat carping, 
in that I have nothing constructive to put in the place of the hypothesis to 
which I object. But I beheve that the time has hardly come for ultimate 
mechanical construction, and that, at present, progress is more Ukely to be 
made if we are content with an electromagnetic explanation — if we merely 
carry down to the molecules and their interspaces the electric and magnetic 
relations which we find between large masses and round large circuits, and 
leave the ether out of account. I beheve that we may symbolise electric 
and magnetic actions by means of fines of force and their motions in a way 



268 AN EXAMINATION OF PROF. LODGE's ELECTROMAGNETIC HYPOTHESIS 

which allows us to think clearly of the phenomena, and though the ultimate 
nature of the lines of force is unknown, we can only say the same of the ether. 
In the application of these lines of force to the molecules, and their constituent 
atoms, there appears to be hope for some kind of explanation of the chemical 
phenomena of the circuit and of the distinction between electrolytic and 
metalhc conduction. The difficulties in the way are by no means small, as 
everyone knows. I may, at some future time, attempt to set forth some of 
the difficulties as they occur to me, in the hope that a plain statement of them 
may lead some reader to help in their solution. 



18. 

MOLECULAR ELECTRICITY. 

[Electrician, 35, 1895, pp. 644-647, 668-671, 708-712, 741-743.] 

In a Paper in The Electrician of October 13, 1893*, after some criticism 
of the 'wheelwork' hypothesis of electromagnetic actions, I concluded by 
saying that I believed that ' the time has hardly come for ultimate mechanical 
construction, and that, at present, progress is more hkely to be made if we are 
content with an electromagnetic explanation — if we merely carry down to 
the molecules and their interspaces the electric and magnetic relations which 
we find between large masses and round large circuits, and leave the ether 
out of account.' 

I propose to follow out this idea, to see where it leads us, and what 
difficulties we have to face. The idea is in many minds, and we have 
examples of its use in the papers of Prof. J. J. Thomson and in the articles by 
Mr. Chattock in Dr. Lodge's Modern Views of Electricity and the Philosophical 
Magazine. The fullest account yet published is, I think, in Prof. Thomson's 
Recent Researches, Chapter i, but there are some difficulties not there brought 
to the front. I believe there will be some advantage in a new statement, 
starting with the very alphabet of the subject, and following it up by full 
examination of the consequences. In the belief that some way may yet be 
found to remove the difficulties, and that even now it gives us a valuable 
picture of some electrical actions, I venture here on a full description of the 
shape it has taken in my own mind. 

The hypothesis with which we start is that electrical and chemical forces 
are identical; that electrification is a manifestation of unsatisfied chemical 
affinities, and that chemical union is a binding together of oppositely-charged 
atoms or groups of atoms. 

Before descending to the atoms, let us briefly consider what we observe 
on the large scale coming within the range of experiment. The foundation- 
stone of our electrical knowledge is the experimental result that the two 
kinds of charge are always found in equal amounts with opposite or neutrahsing 
mechanical properties, and that they always face each other on the two sides 
of the insulating matter between them. We add to this the idea that the 

* [Collected Papers, Art. 17.] 



270 MOLECULAR ELECTRICITY 

energy of the charges accompanies some kind of strain — alteration, perhaps, 
of atomic or molecular configuration — in the insulating medium. The two 
charges, in fact, always join hands through the dielectric; or, putting the 
same idea in another form, the charges are manifestations on the bounding 
surface of a state of strain within the dielectric, somewhat as hydrostatic 
pressure is a manifestation on the surface of the vessel containing a Hquid 
of a strain to which the liquid is subjected. When this idea of strain was 
enunciated by Faraday and rendered precise by Maxwell we had only the 
variation of inductive capacity to support it. But now the double refraction 
of dielectrics in the field discovered by Kerr, and the wave phenomena dis- 
covered by Hertz, give as near a positive proof of the existence of the electric 
strain as we can ever hope to get. 

In a system at rest the electric strain ends at the surface of a conductor 
and the electric stress or pull on the surface 2tto'^IK per sq. cm. is resisted on 
the other side of the surface by ordinary mechanical stress accompanying 
ordinary mechanical strain in the matter of the conductor. Some day, 
perhaps, we may be able to manufacture a hypothesis identifying mechanical 
and electrical strain ; but at present we are obliged to separate them and 
think of them as different in kind, since mechanical strain in moving off or 
dying away does not give rise to the magnetic or chemical actions which 
characterise the moving off or dying away of electric strain. 

We symbolise the relative value of the strain at different points of the 
field by unit tubes of induction each beginning at + 1 and ending at — 1 of 
electrification. The strain varies inversely as the cross-section of a tube, and 
we regard 1/^, the reciprocal of the specific inductive capacity, as a kind of 
modulus of electric elasticity. Though tubes of induction give us a better 
description of the imagined physical condition of the dielectric, it is easier 
to use lines of induction, one for each tube, when we have to draw figures. 

These ideas of duality of charge, and of strain between them, which we 
owe to Faraday and Maxwell, though now accepted by everyone, have hardly 
yet so saturated our minds that we instinctively use them in every case. We 
still too often find descriptions of elementary phenomena which entirely leave 
them out of account. Like charges are still described as repelling each other, 
as when the gold leaves of an electroscope diverge, without a hint that the 
apparent repulsion is really a pull on each body along the fines of induction 
stretching out to the opposite charges induced on the surrounding conductors. 
The earth is still looked upon as a big conductor which will hold any reasonable 
amount of electricity without showing signs of it, and we speak of discharging 
a body into it instead of saying that the opposite charge is usually on the 
earth or the walls of the room in which we work, and that when a conducting 
bridge is made the two charges can come together so that the inductive strain 
is refieved, and its energy is dissipated. Or to take another example where 



MOLECULAR ELECTRICITY 271 

common language and thought lag behind the more precise ideas given to us 
by science: the earth, as the 'return' circuit of a telegraph wire, is still 
described as a reservoir from which, say, + can be pumped up at the sending 
end and into which it can be emptied out at the receiving end, just as if 
a telegraph wire corresponded to a pipe running above a water reservoir with 
a pump at one end and a spout at the other. Sometimes even we find the 
process described as if the charge going to earth at the receiving end knew 
its way back through the earth to the particular battery from which it started. 
Whereas the earth is really not a return circuit at all, but a parallel out-going 
circuit for the opposite charge, enabUng the two electrifications to travel in 
company and to face each other — one on the wire, the other on the earth — 
whether the earth be the bare ground under an aerial line or the sheath of 
a cable in the bed of the ocean. 

This travelling of opposite charges to meet each other is to be regarded, 
I am convinced, as the essential electrical part of the ordinary current. The 
lines of induction connecting the charges sweep through the air or other 
surrounding insulator, and this motion of the induction, or travelling onwards 
of the electric strain, is accompanied by — indeed, is rendered possible by — 
the magnetic induction which surrounds the conducting wire. The only case 
of current without motion of charge is the gradual cessation of charge in an 
imperfectly insulating condenser. Here the charges simply die away in situ 
— their strain decays and their energy is dissipated ; and as there is no motion 
of induction there is no magnetic field. 

To realise how ordinary current may be described in terms of motion of 
lines of induction let us suppose that, instead of the more usual dynamo or 
battery as source, we have a charged condenser with the — terminal to earth 
and the + terminal insulated, but capable of connection by a key with a wire 
earthed at the farther end. Up to the instant when the key is put down the 
two charges are almost entirely in the condenser, joining hands in the dielectric 
between the plates. The still prevalent mode of describing the current on 
contact at the key is equivalent to supposing that, as soon as the + found 
a door open to it, it freed itself from the embrace of its — , put its hands in 
its pockets, ran along the wire, setting all the neighbouring magnetic machinery 
spinning as it rushed past, and finally plunged into the earth at the farther 
end. The — , no longer kept up or 'bound' by the +, sank into the earth at 
the condenser end, and both were lost in that vast electrical abyss, the globe. 

But let us see what is really implied in the process of discharge if we 
keep to our fundamental principle that + and — always have induction-tubes 
or lines between them. Let us try the various suppositions which seem open 
to us, and let us first imagine as a possibility that the charges are able to start 
on their travels by breaking their induction-tubes somewhere in the dielectric 
between the condenser-plates, as in Fig. 1. At the broken ends we must 



272 



MOLECULAR ELECTRICITY 



have opposite charges (just as in a broken magnet we have opposite poles), 
for a charge is the end on matter of an induction-tube. If each + unit 
setting out along the wire drags the broken half of its tube after it into the 
wire, we shall have on the whole equal + and — travelling in the same direction 
along the wire and no external magnetic effect. Indeed, as soon as the leading 
+ has drawn its piece of tube completely into the wire there does not appear 
to be any reason against the union of the + and — at opposite ends of the 
piece, and then nothing need occur in or around the rest of the wire. But 
as we have magnetic and other efiects in and around the whole length of wire, 
and everything is against the passage of + and — in the same direction, we 
are bound to reject this supposition of breaking tubes. As another supposition 
let us imagine the tubes as remaining in the dielectric of the condenser but 
growing longer where they stand, and as it were pushing their end-charges 
before them, the one through the wire, the other into the earth. But this 



Fig. 1. 

will leave the tubes still in full strength in the dielectric, and when the dis- 
turbance in the wire has died away the condenser will still be fully charged, 
which is absurdly at variance with experience. The only possible supposition 
still left to us is that the tubes of induction spread out sideways from the 
dielectric of the condenser into the surrounding air, connecting the -|- and — 
charges as these move respectively along the wire and the earth. The 
induction— that is, the condition we call electric strain — is propagated by 
the working machinery from point to point through the air, and the motion 
of this machinery is manifested as magnetic induction, symbolised by lines 
of magnetic force which form closed circuits round the wire. We cannot 
explain this mode of transfer, but the mechanical models of Maxwell and 
Lodge and FitzGerald help us to accept it as possible. 

We must now introduce the idea that conductors are dielectrics — of 
a special kind, no doubt — admitting of electric strain, though allowing it 
to decay rapidly and dissipating the energy they receive in the form of heat. 
We may take as a helpful analogue to this behaviour of a conductor the 
behaviour of a liquid under ordinary elastic shear-strain. A liquid can 
receive shear-strain just as much as a solid, but the strain decays, that is, 
rapidly loses its energy, and the lost energy is transformed into heat. But 



MOLECULAR ELECTRICITY 273 

the still undecayed strain remaining at any instant gives a tangential resistance 
to shear, which we recognise as viscous resistance to the motion of one layer 
relative to the next. So in a conductor such as a wire, electric strain may be. 
produced and electric energy may be taken in ; but the strain decays almost 
as fast as it is received and the energy is changed to heat. The e.m.f. along 
the wire is proportional to the strain remaining still undecayed at any instant^ 
and this e.m.f. corresponds to the elastic stress — the viscous resistance — of a 
sheared liquid. 

In the case of the condenser which we are considering, as the induction- 
tubes spread out from it their end-portions are continually moving sideways 
into the wire, and to a less extent into the earth, and there melting away 
and dissipating their energy as heat. New ends are continually being formed 
at the junctions of the decayed and undecayed portions, and these ends travel 
further and further along the circuit until the tubes are entirely propagated 
into the conductors, when the discharge is complete. 

A full account of this mode of regarding current, and the relations involved 
between electric and magnetic induction, with an attempt to show that it 
applies to the voltaic circuit as well as to such a circuit as we have here con- 
sidered, was given in the Philosofhical Transactions for 1885*. Prof. J. J. 
Thomson has modified and elaborated the theory in the Philosophical Magazine 
for March, 1891, and in his Recent Researches in Electricity and Magnetism, 
Chapter i. The full theory shows that if we assume that the magnetomotive 
force round a closed curve is equal to 47r x number of electric tubes cutting 
the curve per second, then the magnetic field is accounted for. If, then, we 
make the electric tubes move so as to account for the right electrical quantities, 
the magnetic properties will follow as a matter of course, and we may therefore 
concentrate our attention on the electric tubes, as we have done in the pre- 
ceding account of one case of current. I have given this case at length, by 
way of introduction, to show that here at least the only way in which we can 
think consistently of current in a wire is in terms of motion of + and of — , 
and of tubes of electric induction connecting them and sweeping through the 
surrounding medium into the wire. We shall see later how such tubes may 
be supposed to be furnished by the recombinations which occur in the voltaic 
circuit, and I have no doubt that in all other cases of current the same kind 
of explanation may be given. 

There is another reason, perhaps, for giving here the foregoing account of 
a condenser- discharge, in that the hypothesis which I am going to describe 
may not inaptly be termed a condenser-hypothesis of electricity. The 
properties which we find in condensers will be carried down to the molecules, 
which we shall suppose to be small condensers with equally and oppositely 
charged atoms forming the two plates. Of course, this will not explain 

* [Collected Papers, Art. 11.] 

p. n. w. is 



274 MOLECULAR ELECTRICITY 

electricity. It is a purely electrical hypothesis, and it shifts all responsibility 
of further explanation on to the shoulders of whatever atomic hypothesis we 
adopt, just as Weber's magnetic hypothesis gives no explanation of magnetism, 
but assumes the molecules to be ready-made magnets, and leaves to them the 
burden of accounting for themselves. 

We are naturally led to make the hypothesis from the consideration of the 
chemical action of current. When a voltameter is included in a circuit a 
perfectly definite quantity of electrolyte is decomposed for a given number 
of induction-tubes moving sideways into it. All parts of the liquid between 
anode and cathode are concerned in what goes on, and there are two opposite 
processions of ions. This is all experimental fact. Kepresenting the action 
atomically we may obtain the net result by the old Grotthus chain method of 
picturing the process, and each atom set free requires the supply of a definite 
constant amount of electric induction, calculable if we know the number of 
atoms per gramme of the substance. 

Let us now consider how electrolysis may occur. We shall imagine 
a somewhat abstract kind of electrolyte, one consisting of molecules, each 
with a pair of atoms, an abstraction, a simplification, which we shall have to 
discard later. To account for the exact chemical equivalence of electrolysis 
in different cases we shall have to suppose each atom to be supplied with the 
same amount of electric induction, to have the same minute fraction of an 
induction-tube starting from it if it is of one kind, ending on it if it is of the 
other kind. To bring the induction up to a thinkable size we must choose 
a new unit — I suppose miUions of billions of times less than the ordinary unit, 
and it will be convenient to picture the atom as supplied with two, four, or 
some such small number of tubes in terms of this new unit. 



Here, then, is our molecule, represented in Fig. 2. A is 
the positive and B the negative atom. The molecule is, in 

fact, a little condenser with fixed charges, and the distance — ' g 

between A and B is usually small compared with their dis- pig. 2. 

tances from neighbouring molecules. But we must suppose 
that in general A and B do not come in contact, perhaps through motion of 
vibration or of rotation relative to their centre of gravity. There is therefore 
persistence or conservation of their tubes. The electrical attractions sym- 
bolised by these tubes are absolutely identical with the chemical attractions 
of the molecules, and the electrical energy they contain with the chemical 
energy of the molecule. It is easy to see, however, that two molecules 
colHding may become connected, and form a more complex group, and may 
even effect an exchange of atoms. 

Thus, in Fig. 3, (a) to (/) represent successive stages in the process of 
exchange. Just as, when two condensers are brought near with their tubes 
of induction running in opposite directions, cross-connections are formed by 



MOLECULAR ELECTRICITY 



275 



the coalescence and neutralisation of parts of the tubes, so here we may 
suppose that in a molecular collision the tubes straying out coalesce, and the 
parts which run in opposite directions destroy each other. In (c) we have 
a more complex molecular group. If the process is continued we may arrive 
at (/), which gives the original molecular configuration but with exchange 
of partners. If we are ever to attempt an electrical hypothesis of elasticity 
and cohesion we shall have to suppose that straying of tubes is always going 
on in solids and liquids, and that complex groups like (c) are always being 
formed, so that each molecule is attached to its surroundings. In gases not 
in an electric field the molecules may be regarded as much more self-contained. 



(rt) 



>' '' /k j\ 



(L) 



X 





,(/) 



Fig. 3. 

When a line or tube of induction moves sideways into an electrolyte com- 
posed of such condenser-molecules it finds the molecules with their axes 
distributed equally in all directions. It will pick out those already facing 
in the direction suitable for it — those requiring the least energy for its purpose, 
and those alone need be drawn. 

In Fig. 4 (a) two lines of induction, XX, YY, are represented as ready to 
move into an electrolyte, in which AB, AB represent the suitable molecules. 

The successive stages are represented by (6), (c), (d), (e), and the final result 
is that the highest B atom is delivered up to the + electrode, and the lowest 
A atom to the — electrode, while all the intermediate atoms in the chain 
change partners, merely forming molecules like the original ones, but with 
their axes reversed. 

If more lines come in they will find new molecules directed as they require, 
and soon also the collisions occurring in the liquid will distribute the axes of 

18—2 



276 



MOLECULAR ELECTRICITY 



the molecules of the first chain into various directions, so that some of these, 
too, will be ready for later lines of induction. 

The coalescence and destruction of the parts of two tubes which run 
alongside each other in opposite directions, upon which the whole process 

•f Etectrode. ..«___ 



1^ /s 



X —Electrode 

Fig. 4 (a). 





Fig. 4(6). 

depends, will naturally tend to take place, as less potential energy will be 
needed for the coalesced tubes than for the two side by side. The energy of 
the final configuration is the same as that of the initial configuration, except 
at the two ends, so that if XX and Y Y bring in enough energy to provide for 
the new configurations at the electrodes the process will continue. If XX 



MOLECULAR ELECTRICITY 



277 



and 77 bring in an excess of energy we may regard the change of partners 
as occurring with more or less of a rush, and the excess is converted into 
internal molecular energy, partly kinetic, partly potential; in fact, the chain 





y 








\ 




\ 






J 




> 


f 


It 




\ 


f 




i 


i 








V 1 


< 


Fig. 4 (c). 




Y ) 


c 


Sr 


B 


Sr 


f 
Q 


Sr 


B 


r^-] 





V 

Fig. 4 {d). 

of recombined molecules is warmer than it was, and we have the Joule C^R 
effect. 

For simplicity we have considered the tubes XX, 77 as stretched 
through free space till they come to the chain of molecules which they 
are to rearrange. But it is easy to see how a tube may be handed on from 



278 



MOLECULAR ELECTRICITY 

y X -f- Electrode 



N' V 



B 



V X 

Fig. 4 (e). 



— Electrode 



Fig. 5 (a). 




X 



Fig. 5 (&). 



MOLECULAR ELECTRICITY 



279 



chain to chain without any free existence. In Fig. 5 (a) a tube XX already 
threads a chain of AB molecules and in (6), (c), (d) are shown the successive 
stages of its transfer to a neighbouring chain. The process is rendered more 
capable of representation by supposing each member of the second set of 
molecules lifted its own depth upwards. The derivation of each stage from 
the preceding is evident. This is doubtless the process which we must 




Fig. 5 (c). 



Fig. 5 id). 

suppose to occur in all cases, and the tubes must be regarded as threading 
such chains in the dielectric, even before they enter the electrolytic cell. 

Let us now consider what must happen at the electrodes. If the ions 
unite chemically with the electrodes, the positive and negative atoms find 
respectively negative and positive atoms ready to combine with them, and 
no difficulty is introduced at this point. But if the ions are set free, as are 
hydrogen and oxygen at platinum electrodes in dilute acid, we are brought 



280 MOLECULAR ELECTRICITY 

face to face with the great, and, it is to be confessed, the yet unsolved difficulty 
in the hypothesis. Take the case of the hydrogen atoms released at the 
cathode, the atoms such as the lowest A in Fig. 4 (e). We may think of them 
as connected at first by tubes of induction going from them to platinum atoms 
on the surface of the cathode. But soon the gas bubbles up, and there is no 
manifestation of the charge all of one kind which we have ascribed to the 
separate hydrogen atoms. We may attempt several explanations. The 
common notion appears to be that the hydrogen gives up its charge to the 
platinum, and then rises up quite deprived of electricity. This might occur 
by the breaking away of the tubes from the hydrogen atoms and the with- 
drawal of the positive ends into the platinum, the positive charge moving 
across the separating space, like a disembodied soul, from its hydrogen 
habitation into the platinum. The platinum having two neutralising souls, 
becomes as merely material as the deserted hydrogen. But this is strongly 
against our experimental knowledge of charge, which is always, so far as we 
know, on matter, whereas here charge crosses over a separating space. We 
might suppose this got over by thinking of the hydrogen and platinum atoms 
as coming absolutely in contact, shortening the connecting tubes to nothing, 
and putting them out of existence. If the tubes are not re-created when 
the hydrogen rebounds the gas rises up without electricity. 

But, according to the current ideas of chemistry, the hydrogen issuing 
from the cell is not atomic, but molecular, and consists of paired atoms at 
least; that is to say, it is a chemical compound, differing from ordinary 
compounds it is true, in that the two members of each molecule are the same 
in kind, but the atoms are held together by chemical attraction, and satisfy 
each other's chemical affinity just as much as if they were different elements. 
If our hypothesis has any truth in it, this means that electric induction exists 
between the atoms and holds them together, a view supported, I think, by 
the phenomena of electric discharge in gases and the electrolysis which appears 
to take place in that discharge. To be consistent, then, we must suppose 
that the hydrogen, when it rises up, consists of pairs of + and — atoms, and 
for us the difficulty is to explain, not what has become of the hydrogen charge, 
but how half the atoms have succeeded in exactly reversing their charges, 
so that they are able to combine with the other half to form neutraHsing pairs. 
And, of course, we must extend our supposition of the constitution of an 
element as being made up of + and — atoms to the platinum also. When 
the lines of induction move into the platinum, the — atoms of the surface 
pairs are freed from their + partners, and are free to form pairs with the 
+ hydrogen atoms. But when half the hydrogen reverses its charge half 
the platinum must do the same in order that it too may effect the neutral 
combination which we find when the electrolysis has ceased. Perhaps the 
best course is to say that if our main hypothesis of identity of chemical 
attraction and electric induction is real, then the reversal of charge, such as 



MOLECULAR ELECTRICITY 



281 



occurs on our supposition at the electrodes, is up to the present an unexplained, 
an ultimate fact. Without attempting explanation, but rather as a crude 
mode of picturing a process which would lead to the result, we might think 
of a + H and a — Pt as coming absolutely in contact, so that the tube-ends 
can shde round on the surface of the atoms and across the bridge of contact 
until + is on the Pt and — on the H. Possibly Fig. 6 will make this clearer. 




Fig. 6. 



In (b) two atoms have come in contact, in (c) the tubes are beginning to 
slide round, in (d) the ends have changed places, and in (e) the atoms have 
drawn apart with reversed charges. Having thus, or otherwise, reversed half 
the pairs, combination with the other half may ensue as in Fig. 7, the result 
being hydrogen molecules and platinum molecules each consisting of + and 
— pairs. 

At first sight it seems as though the reversal occurring in the spark-dis- 
charge of a Ley den jar or a Hertz vibrator would give us the key to the 
reversal pictured in Fig. 6. But further examination takes away this hope. 
The jar-reversal is never complete and exact such as we have to suppose the 
atomic reversal. Indeed, the induction may have any value, -f or — , less 
numerically than the initial value, as it gradually dies down to zero through 
the successive vibrations. We shall see later how some account may be 
given of the reversal in vibrators, an account which quite destroys any analogy 
with atomic reversal. 

Perhaps a better analogy is afforded by two vortex rings A and B when 
they are playing at leap-frog. The first widens out and lets the second go 
through it. Before the passage the liquid is streaming through A towards B. 
After the passage it is streaming from B towards A, so that if we think of 
induction as corresponding to direction of flow we have here a reversal. 



282 



MOLECULAR ELECTRICITY 



Pt 



Pt 




(a) 



(b) 



Pt 



But I suspect that this liquid- analogy is much more appropriate to a 
magnetic than to an electric hypothesis. 

Were it not for the magnetic difficulties involved there would be some 
temptation to attempt a theory in which the sign of charge is merely a state- 
ment of the kind of atom concerned, and that the tubes, Hke gravitation- 
tubes, have nothing but ahgnment and neither + nor — direction. But this 
would, I fear, be getting over one difficulty only by introducing much greater 
difficulties hereafter. 

We shall assume, then, that in some way or 
other reversal can take place in certain cases, 
and we may apply it at once to explain the 
apparently neutral condition of the ions in the 
electrolytic cell. We suppose that half the ionic 
atoms in Fig. 4 reverse or change charges with 
the electrode atoms, as in Fig. 6, and that then 
they go through the process of Fig. 7 with the 
other half, and so form + and — pairs, exhibiting 
no external electrification. 

A similar reversal will enable us to give an 
electrolytic account of metallic conduction. The 
great distinction between electrolytes and metals 
is that in the former there are opposite atomic 
processions, while in the latter there is apparently 
no motion in either direction. Electrolysis with- 
out procession means, as we have just seen with 
the electrodes, where the procession stops, re- 
versal of charge. Suppose, then, that such reversal is possible with every 
metallic molecule. Imagine a copper wire carrying a current to be made up 
of molecules, as represented in Fig. 8 {a), and let tubes of induction XX, YY 
be just movmg into the wire. Before the current is established the molecular 
axes are evenly distributed in all directions, but the tubes entering in select 
those most suitable, as shown in {a), turning the configuration to that shown 
in (6) ; if now reversal takes place we get (c). Let two more tubes come in, 
and we get (d), a reversal of which gives the same arrangement as (a) as far 
as the copper is concerned, but the four entering tubes of induction have 
entirely disappeared. 

We must now attempt to give some account of the action at the source 
of a voltaic current. As we simplified the electrolyte so we shall simplify 
the active liquid of the voltaic cell by imagining that we have merely sulphuric 
acid, that is, we shall neglect the solvent, water. We shall take as the two 
metals zinc and copper. 

We know that the result of putting the copper and zinc in the acid is that 




(c) 



Fig. 7. 



MOLECULAR ELECTRICITY 



283 



the air above the acid tends to become the seat of tubes of induction running 
from copper to zinc. This will be neutralised by the oxidising tendency of 
the air on the zinc— at any rate that is the 'chemical theory' of voltaic 
action. To eliminate this action of the air we shall suppose the zinc to have 



Fig. 8 (a). 



-Cu. 



■Cu^ 



. Cu, 



•Cu, 



.Cu, 



ii 



Ou. 



-Cu. 



.Cu^ 



-Cu. 



-Cu, 



T 



Fig. 8 (6). 



U 



OUc 



-Cug 
.Cu^ 



Cuj 



Cu. 






Cu, 



■ Cu, 



Cu, 



Cu, 



-Cu, 



Fig. 8 (c). 



Fig. 8 id). 



a copper terminal in the air, as it always has in practice. Experiment shows 
that these tubes of induction always run from copper terminal to zinc terminal 
with a fall of potential which may be as much, say, as a volt and a half. To 
represent the establishment of this induction let us imagine the cell initially 
to consist of such molecules as are shown in Fig. 9 (a), where we represent the 



284 



MOLECULAR ELECTRICITY 



acid molecules as consisting of + Hg paired with - SO4, and the copper and 
zinc plates as ± Cu and ± Zn. 

Of course, the molecules in each substance are evenly distributed in all 
directions, but only those are shown in the figure which are suitably directed 
for the action which is going to occur. The first stage is represented in 
Fig. 9 (6), where change of partners has occurred between metal and acid 
both with copper and zinc. The HgZn molecules do not reverse, for it is an 
experimental fact that the hydrogen does not rise up where it is first turned 
out. The reversal may perhaps be prevented by the presence of the ZnS04 



Cu- 



Cu 



j\ /( 



■Cu 



• Cu 



Ou- 



Cn- 



SO^ 



>S Ji 



Zn — 






H 












SO4 






> 


1 d 


k 


, , 


f 


—< 




— < 


r- H 


7n- 


*^ 





SO4 H2 SO4 Hg 

Fig. 9 (a). 



•Cu 



■Cu 



-Cii 



Cu 



Zn 



Z-n 



SO^ 



SO4 Hj SO4 Hj 



Fig. 9 (&). 



SO4 



Cu 



Cu 



molecules, but it is more probably due to the electrical energy put into the 
HgZn. For the ZnS04 contains much less energy than the H2SO4 it replaces ; 
so that the HgZn contains much more than the ZnZn it replaces. The 
electrical energy put in makes the atoms separate too widely, we may imagine, 
to allow of the contact needed for reversal. Probably at first there is no 
reversal in the HgCu molecules at the other plate, and for similar reasons. 

The electrical energy in the HgZn and HgCu molecules will imply that the 
acid is at a higher potential than the metals, and tubes of induction will 
spread out from the Hg atoms. Since the HaZn molecules .contain the most 
energy, we may represent their tubes as spreading rather than those of the 



MOLECULAR ELECTRICITY 



285 



HgCu, and in Fig. 9 (c) we have the tubes shown as going out towards the 
copper and then doubling back again. In (d) the tubes have entered into 
the neighbouring molecules by coalescence with oppositely-directed tubes, 
and we are left with two tubes running from the + to the — terminal with 
a ZnS04 molecule and an HgCu molecule, the CUSO4 molecule having been 
dissociated in the process. The molecules of acid have changed partners, but 
still have the same constitution. 

If the action is not continuous, but merely goes on till the terminals are 
charged, we must suppose that HgZn and HaCu pairs are left against each 




Zn 



SO4 



Cu 



8O4 Hj 8O4 Hg 



Fig. 9 (c). 



Ou 



Cu 



„J_L M M M 



Zn 



80^ 



-Cu 
-Cu 



Cu 



Fig. 9 (d). 

plate with an average fall from H to metal of, say, 1| volts. The pairs against 
the copper plate are no doubt the agents in polarisation. The reverse current 
which we get on replacing the zinc by a fresh copper is to be set down to the 
straying-out of the tubes of induction of these HgCu pairs. If the action is 
made continuous by connecting the terminals with a wire, the hydrogen rises 
up from the copper plate, and we must suppose that half the HgCu pairs have 
reversed and have then changed partners with the other half. Perhaps this 
reversal is rendered possible by the resolving of the CUSO4 molecules, perhaps 
by the outlet provided for the energy of the HgCu pairs in the external circuit. 



286 



MOLECULAB ELECTRICITY 



In a very similar way we may account for the contact- difEerence of potential 
of copper and zinc in air. We know that if the two metals are brought into 
contact a fall of potential occurs from the air near the zinc to the air near the 
copper ; that is, electric induction passes from the neighbourhood of one to 
the neighbourhood of the other. Both metals can be oxidised, but zinc by 
far the more readily. Let us suppose the oxygen molecules in the air to be 
made up of pairs of opposite atoms, and that before contact we have a state of 
affairs, represented by Fig. 10 {a), developing into (6) by the actions at the 
metals. On each metal we have the normal oxides ZnO, CuO, where the 
positive atom comes first, and the unstable molecules OZn and OCu. If the 
tubes of induction of these unstable molecules stray out they have to double 



Zn- 



Zn- 



U V 



Zn- 



Zn- 









qO 




_ 

o o 


""^~ 


Q 









. . 


' 1 


' ' 


O A-« 


/■H ■ 








— < — 








Oo 




o 





Fig. 10(a). 



Zn- 



.1 >i 



r V 



-Cu 

-CU 

-Cu 
-Cu 

-Cu 
-Cu 



Zn 



Zn 



O 

< 

o 

*: 



Cu 



Cu 



o o 

Fig. 10 (&). 

back on themselves, so that as long as there is no contact there are equal 
numbers in the two directions passing through the air, and no fall of potential 
except from the surface-layer of the air to the surface-layer of the metal close 
to it. This fall will be greater at the zinc than at the copper, since there is 
presumably more energy given up at the zinc surface. Probably reversal is 
prevented either by the presence of the normal oxides or by the want of 
outlet for the energy. If, however, it does occur in some of the unstable 
molecules there will be re-pairing and the formation of new metal molecules 
and new oxygen molecules, with the net result that each metal is left slightly 
oxidised. But let us make the two metals touch as at J in Fig. 11 {a). If the 
tubes of induction of the OZn molecule now move out Uke those of the HgZn 
molecule in Fig. 9 (c), the upper returning part can enter into the continuous 
metal bridge and there be dissipated; while the lower outgoing part will 



MOLECULAR ELECTRICITY 



287 



thread the oxygen molecules, as shown in Fig. 11 (b), and decompose the CuO 
molecule already found. We shall as a net result have ZnO on the surface 
of the zinc, a fall of potential from on the zinc surface to on the copper 
surface, and the unstable OCu, which possibly reverses and re-pairs, leaving 
the copper surface unacted on. If it remains then the copper is polarised, and 
if the zinc were suddenly removed and replaced by a new copper it would 
appear that the old copper should show a fall of potential towards the new. 



Zn 



Zo 

Zn 
Zn 

Zn 



Zn Oy 



Co Cu 



Cu 



n >i 



< — — < — — < — — e 

O O GO 

< 1 t < — I I < 1 I ^ 

> 1 I — < 1 I < — I I ► 

o o O O O o 

> — — ^ — — < — — )> 



Cii 

-Cu 
Cu 



Cu 



Fig. 11 (a) 




o o o o 

Fig. 11 (b). 



Zn 



Cu 



Fig. 11 (c). 

But any such fall would probably be disguised by the induction ending in 
the atoms next to the copper. We have probably gone too far in this 
account in supposing that all the induction of the OZn molecules goes out in 
this way. If, for example, we suppose that only half goes out, we get a chain 
extending from Zn to Cu, as in Fig. 11 (c). Probably some such supposition 
must be made in order to explain how the + atoms remain at the zinc 
surface and the — atoms at the copper surface after the break of contact. 
That they do remain is shown by the -i- electrification of the zinc and the 



288 



MOLECULAR ELECTRICITY 



— of the copper when tested by an electrometer. But our account must be 
regarded as a first attempt and not as a complete explanation. 



m 



L i ' M i 





f . 








•^ 


i 




t. 


< ' 


' 


k , . . ^ 




r 



Fig. 12(a). 





\< 




.1 • 



Fig. 12(c) 





■!■' 


- - w 


1 ' y 


't 


— u 


!■ 


, ,, 



Fig. 12(e). 



Fig. 12(6). 



V ^^ >^ 



V V V 



u >r >r 



Fig. 12 (cZ). 



Leaving electrolysis, let us consider how an ordinary insulator may be 
affected by induction, and how we may represent the condition of affairs 
leading up to spark-discharge. We shall thus get some useful ideas which 
will supplement our account of electrolysis and make it somewhat easier to 
understand the true nature of the process. 



MOLECULAR ELECTRICITY 



289 



We shall suppose that we are deaUng with a dielectric consisting of paired 
+ and — atoms, and we shall suppose, further, that these molecules are 
initially self-contained — that is to say, that their tubes do not stray out to 
surrounding molecules. This is, no doubt, a simplification not existing in 
nature; but, as with electrolysis, so here also we must be content to begin 
with an abstract case. When there is no apparent electrification in the system 
the axes of the molecules will be equally distributed in all directions. But 
if electrification is communicated to a pair of conductors bounding the 
dielectric, tubes of induction move into the dielectric, and, selecting the 
suitably arranged molecules, connect these in chains. It will be convenient 
now to suppose at least four tubes of induction to pass from atom to atom 
in a molecule. Let Fig. 12 (a) represent a number of molecules ready for 
a tube of induction to affect them. In (6) it has moved in and formed the 
molecules into a chain stretching right through the dielectric. If we suppose 
another tube to move in, as in (c), we get a condition of instability, for now 
we are just half-way to a change of partners all along the line. A third tube 
will give us (d), a configuration with the same amount of energy as (b), since 
the molecules in the two cases are similar, the axes only being reversed. 
Hence in passing from (c) to (d) energy is given up, another way of saying 
that (c) is unstable. A fourth line entering will change {d) to (e), where the 
change of partners is complete, and where electrolysis has occurred. 




We may usefully follow out the process by the aid of a diagram representing 
the relation between energy put in and induction. Beginning with an ordinary 
condenser, let distances along OX (Fig. 13) represent induction put in per unit 
area cross-section, distances along Y difference of potential between the end- 
plates. 

At first we have V = r^ > 

p. c. w. 19 



290 MOLECULAR ELECTRICITY 

where V is the potential difference, d the thickness, a the surface- density, 
and K the specific inductive capacity. 
But since the induction D = cr. 

If then K is constant, the relation between F and D is represented by a straight 
line, which makes with OX an angle 6 given by 

4:77(Z 

tan c/ = -^ . 

The energy stored per unit cross-section is equal to -^, or equal to the 

area from the origin up to the ordinate V, and bounded by the hne representing 
the relation between F and D. We may indeed regard the diagram as showing 
the relation between induction and energy stored, and this is probably a better 
point of view, since we cannot attach much idea to potential in the later 
parts of the process now to be considered. From this point of view the 
abscissa is the induction through unit area and the ordinate is the energy 
added per unit addition of induction. 

All experiments hitherto made appear to show that K is practically 
constant so long as the medium can continue to store up energy, though 
the Kerr effect shows that K does alter slightly as D increases, apparently 
sometimes increasing and sometimes diminishing, or, perhaps, always in- 
creasing, but sometimes most along the lines of induction, at other times 
most at right angles to them. 

Sooner or later, however, a point of instability is reached — sooner in gases, 
later in liquids and solids ; and discharge occurs along one track and all the 
energy is dissipated. We can see how the curve in Fig. 13 must run in order to 
represent this instabiHty. Making the unit area small enough to represent the 
cross-section of a molecule, the curve expresses the relation between induction 
going right through a chain of molecules from plate to plate and energy put 
in. At first, while only a small part of the induction is continuous, as in 
Fig. 12 (6), we know that the energy stored is proportional to the square of 
the induction, and the curve is, as we have seen, a straight line. But as more 
induction becomes continuous from plate to plate, and as the atoms are 
pulled in both ways, the force resisting separation does not go on increasing 
so rapidly, and the curve falls below a straight line. 

At some point on the way to instability and subsequent change of partners 
the force will reach a maximum, and after that the curve will turn down, 
successive equal additions of induction requiring diminishing additions of 
energy. At last, when the fines of induction from each atom run half one 
way, half the other, as in Fig. 12 (c), the energy put in is a maximum, and the 
curve crosses the X axis, as at A (Fig. 13). The configuration is now unstable. 



MOLECULAR ELECTRICITY 



291 



and will of itself pass through (d) and (e), absorbing two more positive hnes or 
extruding two negative hnes till we arrive at (e), represented by the point 
B in Fig. 13, when the condition is again stable, like that at the beginning. 
The energy put in between and A is given out again between A and B in 
part, no doubt, as the Hght, heat, and so on of the discharge. The curve 
OAB may be termed, perhaps, a molecular characteristic. It should be noted 
that the passage from (c) to (d) (Fig. 12), or past A (Fig. 13), may be effected 






D 



D 




Fig. 14(a). 



Fig. 14(6). 



Fig. 14 (c). 



M V 



^' " A 



V ^r 



Fig. 14 id). 



either by absorbing more positive tubes, or by sending out negative tubes. 
If the passage through the position A occurs with a rush, then we may regard 
the chain as sending out negative tubes by some such process as is illustrated 
in Fig. 14, (a) to (d). Though the instability of a single chain will not be 
reached till its condition is represented by A (Fig. 13), the instability for 
a great number of parallel chains is reached as soon as they are all at or near 
the highest point of the characteristic. In the ascending part of the curve 
all the parallel chains will tend to have the same amount of induction through 

19—2 



292 



MOLECULAR ELECTRICITY 



them, for if any one has more than another, energy will be yielded up by 
a redistribution between them. Thus let one chain have induction OM 
(Fig. 15), another near it induction ON, the energies stored being OMP, ONQ. 
If L bisect MN the second chain may give up induction LN = LM to the 
first, and at the same time there will be a yield of energy equal to the difference 
between the areas PL and QL. Probably the induction is really distributed 
about an average, some of the chains having more, others less, for no doubt 
the condition is kinetic, and when there appears to be equilibrium it is not 
static but ' mobile.' 




Fig. 16. 

If the average condition is represented by a point far up the slope and near 
the crest, and if any one of the chains gets past the crest, as far below it on 
the other side as the average is below on the first slope, then this advanced 
chain will at once receive induction from the others, and continue to move 
down the second slope towards the point of instabihty. Thus, let there be 
n chains in all. Let OH A (Fig. 16) be the characteristic of that with the 
greatest induction, OKA' the sum of the other n — 1 characteristics. If the 
first chain is at P while the others are at p on the same level, by a transfer 
of induction mn = MN from the general body in the first chain, there is 
a yield of energy, since the area of pmnq is greater than the area of PMNQ, 
for the two slips are of the same breadth, but the slope of PQ is steeper than 
that of fq. The transfer will therefore take place, and the chain moves from 
P to Q. At Q, a fortiori, a new transfer will take place, and so on, and the 
chain will move towards the position of instability. This will all probably 



MOLECULAR ELECTRICITY 293 

occur even when the average is far below the crest if the induction is widely 
distributed about that average, for as soon as one chain gets over the crest 
it will probably find others between it and the average ready to hand on their 
induction and energy to it, and send it down the second slope. 

As, then, the general average rises towards the crest, the most advanced 
chains, as soon -as they pass the crest, tend to discharge the rest, and there 
will at a certain point be a rush in sideways on to these advanced tubes. 
They will move down to the condition of instability represented by A on the 
diagram. They can of themselves move past this point, extruding negative 
tubes, till they arrive at the point B, where the change of partners is complete. 

It appears at least probable that this yield of negative tubes gives the 
opposite charging of the medium which takes place in the second quarter- 
period of the oscillation accompanying rapid discharge, now made so familiar 
by the work of Hertz. We may, perhaps, think of the process somewhat 
as follows: If the inrush of positive tubes during the first stage is rapid 
they will concentrate on the chains in the neighbourhood of that which began 
the breakdown, and carry them past the unstable point. Then will begin 
the outrush of negative tubes, and when this is complete the central chains 
will be discharged and in the condition of Fig. 12 (e), while the surrounding 
medium will contain negative induction. There will be now an inrush of 
negative induction into the locus of the first discharge, for not only is this 
free from induction, but also its molecules are suitably arranged to take up 
negative tubes, and during the second half-period of swing from negative 
to positive there will be a second change of partners along the same line. 
And so on with the successive alternations of charge, and there is a tendency, 
evidently, to keep the same line of discharge. Each change will give atoms 
at the two end-plates, which will combine either with each other or with the 
electrodes, or perhaps be taken again into the chains in the following changes. 
In the sudden changes of partners some of the energy goes to atomic vibration, 
and we have evidence of this in the atomic radiation which we call spark. 
There is also energy of translation of the atoms from one partner to another, 
which appears as heat in the molecules. This heat possibly produces the 
sound of the spark through the sudden expansion. We have then dissipation 
of energy as well as the radiation out to space in the Hertzian waves, and the 
two gradually reduce the electrical energy of the system. 

We can see, too, how the amplitude of charge may lessen in the process. 
For if the negative tubes begin to move out at the middle of each oscillation 
before the positive tubes have all moved in, or vice versa, then the first of the 
issuing kind will destroy the last of the incoming kind, and the final charge 
will be diminished by the amount of this overlap. In the extreme case of 
slow discharge, as through a wet thread, the issuing tubes are neutralised as 
they come out by the incoming ones, and the motion is dead-beat. 



294 



MOLECULAR ELECTRICITY 



These negative tubes turned out may also supply the negative tubes 
required in the theory of Prof. J. J. Thomson (Recent Researches, Chapter i). 

Now let us take a conducting dielectric such as water. . As guiding us to 
an account of what goes on we have the facts (1) that, however small the 
E.M.F., conduction and presumably electrolysis take place, and no finite 
difference of potential can be maintained between the electrodes unless we 
continually supply fresh energy, and (2) that a solute such as sulphuric acid, 
which appears to combine with the solvent in some way, enormously increases 
the conductivity. From (1) we gather that some of the molecules must be 
just ready for change of partners, and from (2) we may at least guess that 
molecular groups are formed much more complex than the atomic pairs we 
have hitherto dealt with. We can see how such molecular groups might 
arise by the straying out and coalescence of induction-tubes of neighbouring 
molecules, and the consequent formation of new connections. Thus, if two 



OC 



'^ (fl) 




a {(') 



Fig. 17. 




Fi2. 18. 



pairs come together, as in Fig. 17 (a), we may have them connected into 
a single group, such as (6), and any number of molecules may be brought 
into circuit in the same way, so that we may have a parallel arrangement 
as in Fig. 18. Or if the coalescence occurs by the approach of two or more 
groups, such as Fig. 17 (6), we may have a series arrangement as in Fig. 19, 
the kind which we shall suppose to be effective in conduction. We must 
further suppose that small quantities of acids or salts in solution enormously 
increase the number of complex molecules. Some of these groups would 
appear to be more energetic than the initial pairs. Probably on that account 
they are continually breaking up and re-forming, the energy of translation 
being no doubt converted at each collision and reformation into energy of 
electrical separation. There are no doubt all degrees of connection from those 
of Fig. 17 (h) and Fig. 19 (where the result may be obtained practically by 
adding one closed ring-tube of induction to a series of pair-molecules) to the 
case where the tubes from each atom go half one way and half the other. 



MOLECULAR ELECTRICITY 



296 



But I imagine that at any given instant only a small fraction of the molecules 
are thus connected into groups or circuits. 

If a tube of induction running from above downwards moves sideways into 
such a group as that in Fig. 19 or Fig. 20 (a), it finds the right-hand side made 
ready for it, and we may possibly have in succession Fig. 20 (a), (b) and (c), 
where we suppose that the left-hand side splits up into pairs, while the right- 
hand side remains threaded on the incoming hne. When a liquid contains many 
such groups, some of the chains of molecules are almost ready-made, and the 
chains are very easily completed from plate to plate, since the entering tubes 
have only to furnish a link, as it were, here and there. The ' electric elasticity' 






\t yr yr 

>r V y 



Fig. 19. 



Fig. 20(a). 



>f >r 1, 
>r V ^' 



;. M '< 



>r \f >r 



i^ A M 



>f V >' ^f >k i^ ik 



Fig. 20 (&). 



Fig. 20 (c). 



IjK will be a sort of average of the elasticities or of the difficulties of forming 
chains in all the various kinds of molecules present. To give a numerical 
illustration, let us have two condenser-plates in a liquid in which we suppose 
that in the simply paired molecules, such as those on the left in Fig. 20 (c), 
the value of K is 1-77. Let us suppose also that these occupy 99 per cent, 
of the paths from plate to plate, while the other 1 per cent, is occupied by very- 
much-connected molecules with K equal to 7500. Then if the capacity of the 
condenser in air were 0, it would be with this arrangement of molecular paths 

99 X 1-77C + 7500C 



100 



= 76-76C, 



or the resultant specific inductive capacity is 76-76. 



296 MOLECULAR ELECTRICITY 

But this is only true in the mass. If we have very minute electromagnetic 
waves going through the liquid they will, for 99 per cent, of the molecules, use 
K= 1-77. For a very large fraction of the remaining 1 per cent, they will 
probably also use this K, for they are not concerned with the group as a 
whole, but only with the individual members to which they add or from which 
they subtract small quantities of induction ; and as long as the points repre- 
senting these members of groups are on the straight part of the characteristic 
their K for waves is still 1-77. We may therefore expect the value for small 
waves generally to be very little more than 1-77. 

This appears to indicate a possible explanation of the high inductive 
capacity of such substances as water and alcohol. Accompanying this high 
value there is generally conductivity and no doubt electrolysis. If we 
suppose that some of the groups are already so far towards decomposition 
that half the tubes from each atom run one way and half the other, tubes 
entering the substance will concentrate on such groups, for a further addition 
of induction will yield energy instead of requiring it, and the point of instability 
being passed electrolysis will occur. Perhaps even before the tubes are thus 
evenly divided, tubes entering the substance may prefer to pass through the 
groups rather than through the simply paired molecules, for while more 
energy may be stored in the one half of such a group as that represented in 
Fig. 20, when a new tube enters in it, less may be required in the discarded 
half, and so, on the whole, energy may be given up. But it does not seem 
possible to give any satisfactory account of the process in our ignorance of 
the real constitution of the complex molecules. All we can say is that some 
such process probably occurs, inasmuch as conduction does occur even with 
the smallest external e.m.f. 

We may perhaps suppose that in metallic conduction we have a similar 
process. If the metallic molecular structure is very complicated, with many 
groups having unstable, or nearly unstable, construction, then tubes of 
induction entering a metal will select such groups in preference to the more 
simple stable molecules and electrolyse them. To account for the absence 
of transfer of atoms along the line of current we must, I think, introduce 
the supposition of change of charge as already explained on p. 282, and to 
account for the continuance of conduction we must suppose that there is 
a continuous formation of new groups as the old ones are broken up. Perhaps 
we have here some key to the rise of resistance with rise of temperature. As 
the molecules become more energetic we may expect that the groups will be 
more broken up by the motions of vibration and translation, and that the 
number of unstable groups is diminished and their rate of formation is 
decreased. Hence the entering tubes will find fewer groups ready for them, 
and if the external e.m.f. remains constant, the rate at which the tubes are 
dissipated will be decreased. The difficulty of such explanation consists in 



MOLECULAR ELECTRICITY 297 

understanding its inapplicability to electrolytes. Perhaps, too, we have here 
a hint as to the superior conductivity for heat of metals. If a metal consists 
largely of many-atomed groups entangled together, and continually breaking 
up and re-forming, energy supplied to one molecule will, we may imagine, 
be more readily transferred to its neighbours than if each molecule is self- 
contained and permanent. This will certainly be the case if energy given to 
an atom in a molecule is more rapidly transferred to its fellows in the same 
molecule than to atoms at the same distance in the surrounding molecules. 
To use an illustration which may put the suggestion in a clearer way, news 
will be transmitted through a population dwelling in villages and towns much 
more rapidly than through an agricultural population of the same average 
density scattered over a country in isolated homesteads. 

The theory which I have been trying to set forth may be regarded as 
a theory of the conservation of induction-tubes, and of their beginning and 
ending on atoms. That the atoms always have charges on them, and there- 
fore have tubes proceeding to or from them, appears to be generally held as 
necessary if we accept the electromagnetic theory of light. If the molecules 
give rise to waves of electric induction they must necessarily be electric 
systems, and their parts must almost certainly be bound together by electric 
forces. Whether it is possible for an induction-tube to exist without atoms 
is at present merely a matter of speculation. At present we know of no such 
thing. If such a tube exist, it can only be as a closed ring, like a closed 
ring-tube of magnetic induction round a current, for an unclosed tube would 
have opposite charges at its ends in free space, and charges not on matter 
are so entirely outside experience that we cannot accept their existence. The 
weight of evidence appears to me rather against the view of matter-free 
induction, and though at first we might be inclined to think that the passage 
of light-waves across interstellar space implied such induction, yet even in 
this case we have possibly quite enough matter to supply atomic ends for 
the tubes to attach themselves to. If we accept the electric discharge theory 
of comets' tails we apparently assume the existence of enough matter to 
carry electric induction from a cometic nucleus outwards. I suppose that 
the theory implies that the nucleus is charged in one way, say positively, and 
that it has become separated from the matter bearing the negative, which 
remains far out in space. The sun is itself to be regarded as charged with 
the same sign as the nucleus, while the corresponding solar negative is also 
somewhere in space. When the induction of the comet is added to that of 
the sun the strain is sufficient to break down the feeble insulation of inter- 
planetary space, and a discharge results straight out, or nearly straight out 
'from the nucleus, and this discharge is through the interplanetary matter. 
But though this appears to be the only reasonable account of the discharge- 
theory, after all it is bringing Httle more than a speculation to bolster up 



298 MOLECULAR ELECTRICITY 

another speculation, viz., the existence of matter sufficient to carry waves of 
induction in the interatomic form wherever light- waves travel. 

At first sight this theory of molecular electricity appears to be very different 
from the chemical dissociation-theory now generally held ; but if the dissociated 
atoms of that theory have charges, they have also tubes of induction proceeding 
from the charges. When the tubes are taken into account they must, I believe, 
lead to some such hypothesis as that of which I have attempted an inadequate 
and imperfect explanation. I am only too deeply conscious of the difficulties 
unsurmounted. But in working at the subject I have felt all through that, 
since so much is nearly but not quite explained, there must be hope of progress 
on these or similar Hues if we can only supply some as yet unrecognised idea. 
Perhaps the very imperfections of my account may stimulate some reader 
to take up the subject afresh from some better point of view, and, with new 
ideas, achieve success. 



PAET III. 

WAVE PROPAGATION— KADIATION— PRESSURE 
OF LIGHT— AND RELATED SUBJECTS. 



19. 

NOTE ON AN ELEMENTARY METHOD OF CALCULATING THE 
VELOCITY OF PROPAGATION OF WAVES OF LONGITU- 
DINAL AND TRANSVERSE DISTURBANCES BY THE RATE 
OF TRANSFER OF ENERGY. 

[Birmingham Phil. Soc. Proc. 4, (1885), pp. 55-60.] 
[Bead Nov. 8, 1883.] 

Waves of Longitudinal Disturbance. 

A wave of sound may be considered as energy of a particular type, partly 
potential and partly kinetic, which is being passed on from point to point 
through the medium, so that the energy which is at any moment occupying 
a particular portion of space will have passed in a second later to a distance 
equal to the velocity of sound. Of the two energies the potential is due to 
the strain of the medium, and, when in this strained condition, each part of 
the medium exerts force on the neighbouring parts. But it also has kinetic 
energy, that is, the part considered is in general in motion and there is therefore 
motion of the point of application of the force which it exerts on the contiguous 
parts through the existence of its potential energy ; that is, it does work and 
passes on energy to the contiguous parts. If we consider for instance a series 
of plane waves of sound which move on unchanged, the work done in any 
small time t at any plane perpendicular to the direction in which the sound 
is moving must be equal to the sound-energy contained in the space immedi- 
ately behind the plane through which the sound will travel in the time t. 
This gives us one relation between the various quantities, and we obtain 
another from the consideration that any condition as to velocity and dis- 
placement which is now at a particular point will have travelled on unchanged 



300 VELOCITY OF PROPAGATION OF WAVES OF LONGITUDINAL AND 

in the time t to a distance Ut where TJ is the velocity of sound. From these 
two relations we can at once find the velocity TJ*. 

Let BA be the direction in which the sound is travelling, and let AP be 
the trace of a plane perpendicular to AB. Draw a curve CPQD whose height 
above each point of AB shall represent the displacement of the particle at 
that point (these displacements are actually of course along AB). Thus AP 
represents the displacement at A along AC, and MQ the displacement at M 
along MA. 

Consider a small volume V with unit area on the plane through AP as 
base and height AM = V. The volume of the medium which had height F 
before the disturbance reached A will now be compressed; for the end A 
has moved forward a distance AP, while the end M has moved forward 
a greater distance MQ. The compression is therefore the difference between 
these, viz.; QR = v, say. But if the sound takes a time t to travel over MA, 





p 


Q,__ 








1 — "^'^ n 






^ 


■^ R 


^^^^^-\ 


c 


A 


M 




DB 









after that time the displacement of A will be equal to the present displacement 
of M. Or if u be the actual velocity of the particle at A, 

Qn = v= ut (1) 

Now, considering the energies, we have the kinetic energy of the volume 

V = p -^, where p is the density of the medium. The potential energy equals 

the work done in compressing. If P is the original pressure, and P + f the 

pressure in the compressed state, P -f ^ is the average pressure during the 

compression, and the distance through which this has moved is numerically 
equal to the diminution of volume v. Then the potential energy is 



(p.g. 



* This method of treating the subject of wave- propagation is given by Lord Rayleigh in 
a note in vol. 9, no. 125, of the Proceedings of the London Mathematical Society (republished at 
the end of vol. 2 of his Theory of Sound). This paper is merely an application of his method to 
two particular cases. 



TRANSVERSE DISTURBANCES BY RATE OF TRANSFER OF ENERGY 301 

The work done in t sees, across unit area at A is equal to the pressure 
exerted by the medium to the right on the medium to the left, multiplied 
by the distance which the particle has moved in the time t, or (P + p) ut. 
We may equate this to the sum of the two energies, potential and kinetic, 
contained in 7, for this energy is passed across the plane in the time t. 

Then (P + ^)^,^=(p + g^ + pIJ.^ (2) 

But ut = -y by equation (1) ; 



or ^ = P t^ 



2^2 

Vv 



But V = AM = (distance travelled by the sound in t) = Ut; 

Vv V V^ V ^^„ 

p V 

Vv V 
But — - , or — , that is the ratio of a small increase of pressure to the 

V 

change per unit of volume thereby produced, is the elasticity of the medium. 
We therefore obtain 

jj^ elasticity 
density ' 

or velocity = Velasticity -f- density. 



Waves of Transverse Disturbance. 

The velocity of propagation of plane waves in which the disturbance is 
in the plane of the wave can also be easily found by this method. Since 
the displacements are all perpendicular to the direction of propagation, the 
force acting across a plane perpendicular to this direction will be entirely 
tangential and the rigidity of the medium will alone be brought into play. 

As before, let the waves be travelling in the direction BA and let the 
curve CPQD represent the displacement. The curve may now represent the 
actual displacements since they are perpendicular to AB. 

If, as before, u is the velocity of the particle displaced from A, andt the 
time the wave takes to travel over the small distance MA, then the displace- 
ment AP becomes equal to MQ in t, or QR = ut. 



302 VELOCITY OF PROPAGATION OF WAVES OF LONGITUDII^AL AND 

Now consider the energies in the volume V, with unit area on the plane 
through AP and height AM numerically equal to V. 

If p is the density, the kinetic energy is p —^ . 

The potential energy is equal to the work done in bringing the medium 
from its normal state into its present state of shear — the angle of shear at 
A being QPR, and since MA is very small this angle is measured by 

QR QR ut 

— — or — — = — 

PR V V 

If G be the modulus of rigidity, the tangential force per unit area is 

ut 
G X angle of shear = G ^. 

Now the average value of this force in bringing the shear to its present 
value is half this, and the distance of displacement of the force is QR. Then 
the potential energy in the volume V is 

The work done in t sees, across unit area at A is equal to the tangential force 
at A multiplied by the distance through which the particle at A moves in t ; 

lit u^t^ 

or ^ V ■ ^^^ "" ^ 'v • 

Equating this to the sum of the two energies, potential and kinetic, con- 
tained in V we have 

or Gj-pV. 

But V = AM = (distance through which the wave travels in t) = Ut; 

G V^ 

P t' 



or U ■-= Vmodulus of rigidity h- density. 

It follows at once from the above that the energy in any part of the 
medium is equally divided between the two forms kinetic and potential, for 
the equation 

~r ^ 2T "^ ^ 2 

gives G ^y ^^'2 ' 

or potential energy in F =^ kinetic energy in F. 



TRANSVERSE DISTURBANCES BY RATE OF TRANSFER OF ENERGY 303 

This result also holds for waves of longitudinal disturbance if the initial 
pressure is zero. That is if we may put P = in the equation 



(P + V)v=(^P + l)v + p^^ 



for the potential energy is then ^, and the above equation gives us 



pv Vv^ 
or potential energy in F = kinetic energy in V. 



20. 

RADIATION IN THE SOLAR SYSTEM: ITS EFFECT ON 
TEMPERATURE AND ITS PRESSURE ON SMALL BODIES. • 

[Phil. Trans. A, 202, 1903, pp. 525-552.] 
[Received June 16. Read June 18, 1903.] 

PART I. 

Temperatuee. 

When a surface is a full radiator and absorber* its temperature can be 
determined at once by the fourth-power law if we know the rate at which 
it is radiating energy. If it is radiating what it receives from the sun, then 
a knowledge of the solar constant enables us to find the temperature. We 
can thus make estimates of the highest temperature which a surface can 
reach when it is only receiving heat from the sun. We can also make more 
or less approximate estimates of the temperatures of the planetary surfaces 
by assuming conditions under which the radiation takes place f, and we can 
determine, fairly exactly, the temperatures of very small bodies in inter- 
planetary space. 

These determinations require a knowledge of the constant of radiation 
and of either the solar constant or the effective temperature of the sun, either 
of which, as is well known, can be found from the other by means of the 
radiation-constant. It will be convenient to give here the values of these 
quantities before proceeding to apply them to our special problems. 

* A surface which absorbs, and therefore emits, every kind of radiation is usually described 
as 'black,' a description which is obviously bad when the surface is luminous. It is much better 
described as 'a full absorber' or 'a full radiator.' 

t This was pointed out by W. Wien in his report on 'Les Lois Theoriques du Rayonnement ' 
{Congres International de Physique, vol. 2, p. 30). He remarks that Stefan's law enables us to 
calculate the temperatures of celestial bodies which receive their light from the sun, by equating 
the energy which they radiate to the energy which they receive from the sun, and states that 
for the earth we obtain nearly the mean temperature, using the reflecting power of Mars, while 
the temperature of Neptune should be below - 200° C. 



RADIATION IN THE SOLAR SYSTEM 305 



The Constant of Radiation. 

If R is the energy radiated per second per square centimetre by a full 
radiator at temperature 6° A (where A stands for the absolute scale), the 
fourth-power law states that 

R = ad\ 

where a is the constant of radiation. 

According to Kurlbaum* the constant is 

CT = 5-32 X 10-5 erg/cm. 2 sec. deg.*. 

The Solar Constant. 

The solar constant is usually expressed as a number of calories received 
per minute by a square centimetre held normal to the sun's rays at the distance 
of the earth. The determinations by different observers differ so widely that 
it is not necessary for our present purpose to consider whether the constant 
really exists or whether there are small periodic variations from constancy. 

Angstrom estimated the value as 4 calories per square centimetre per 
minute, and this value is adopted by Crova as very probable I . When 
converted to ergs per second this gives 

Sa = 0-28 X 10' ergs/cm.2 sec, 

where the suffix denotes that it is Angstrom's value. 

LangleyJ assumed that the atmosphere transmits about 59 per cent, of 
the energy from a zenith sun, and from his measurement of the heat reaching 
the earth's surface he estimated the value of the constant at 3 cal./cm.^ min. 
This gives 

Si = 0-21 X 107 ergs/cm.2 sec. 

Rosetti§ assumed a transmission of 78 per cent, from the zenith sun, but 
Wilson and Gray|| consider that 71 per cent, represents Rosetti's numbers 
better than 78 per cent. If in Langley's value we replace 59 per cent, by 
71 per cent., we get 2-5 cal./cm.^ min. This gives 

Sr = 0-175 X 107 ergs/cm.2 sec. 

* Wied. Ann. vol. 65, 1898, p. 748. 

f Congres International de Physique, vol. 3, p. 453. 

J Phil. Mag. vol. 15, 1883, p. 153, and Researches on Solar Heat. 

§ PhU. Mag. vol. 8, 1879, p. 547. 

II Phil. Trans. A, 1894, p. 383. 

p. c. w. 20 



306 RADIATION IN THE SOLAR SYSTEM: 



The Radiation from the Sun's Surface. 

If s is the radius of the sun's surface, R the radiation per square centi- 
metre, then the total rate of emission is 4:7ts^R. This passing through the 
sphere of radius r, at the distance of the earth and with surface 4^r^, gives 

where S is the solar constant. 

Hence R = p= (ifx" lO^' ^ = ^^'''^^^- 

Corresponding to the three values of S just given we have three values of R, 
viz., 

Ra = 1-29 X 10" ; Ri = 0-945 x lO^i ; R^ = 0-805 x lO^i. 

The Effective Temperature of the Sun. 

If we equate the sun's radiation to g6^, where g is the radiation-constant, 
we get 6, the 'effective temperature' of the sun, that is the temperature of 
a full radiator which is emitting energy at the same rate. 

Thus 5-32 X 10-5 ^^4 _ 1.29 x lO^S 

whence 0^ = 7000° A approximately. 

Similarly 6', = 6500° A ; 9, =--- 6200° A. 

Wilson* made a direct comparison of the radiation from the sun with 
that from a full radiator at known temperature. Assuming a zenith trans- 
mission of 71 per cent., he obtained 5773° A as the effective solar temperature. 
If we put 

46,0006' = 5-32 X 10-^ x 5773^, 

we get S^ 0-128 x 10^ 

This is no doubt too low a value. Either then Wilson's zenith transmission 
was less than 71 per cent, or Kurlbaum's constant is too small. 

The low value is probably to be accounted for chiefly by the first supposition. 
Wilson points out that if x is the true value of the transmission, his value of 
the temperature is to be multiplied by (lljx)^. If we take 9^ = 6200°^ as 
the true value, then x will be given by 

This low value is not necessarily inconsistent with the much higher value 
71 per cent, used above in finding Rosetti's solar constant, for no doubt the 
transmission varies widely with time and place, and we have no reason to 

* Roy. Soc. Proc. vol. 69, 1901-2, p. 312. 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 307 



I assume that 1-77 calories per minute, obtained by Langley, would have been 
received from the zenith at the time and in the place where Wilson was making 
his determination. 
The Effective Temperature of Space. 

In determining the steady temperature of any body as conditioned by 
the radiation received from the sun, we have to consider whether it is necessary 
to take into account the radiation from the rest of the sky. If it receives S 
from the sun, p from the rest of the sky, and if its own radiation is R, then in 
the steady state 

R = S-\-p or R-p = S. 

It behaves therefore as if it were receiving S from the sun, but as if it 
were placed in a fully radiating enclosure of such temperature that the 
radiation is p. This temperature is the 'effective temperature of space.' 

The temperature may perhaps be more definitely described as that of 
a small full absorber placed at a distance from any planet and screened from 
the sun. Various well-known attempts have been made to estimate this 
temperature, but the data are very uncertain. The fourth-power law however 
shows that it is not very much above the absolute zero, if we can assume that 
the quality of starlight is not very different from that of sunlight. 

According to Hermite* starlight is one-tenth full moonlight. Full 
moonlight is variously estimated in terms of full sunlight. Langley f takes 
it as 4QoVoo- These two values combined give sunlight as 4 x 10^ starlight. 
But starlight comes from the whole hemisphere, while the sun only occupies 
a small part of it. In comparing temperatures we 
have to use the brightness of sunlight as if the 
whole hemisphere were paved with suns. 

If B is the illumination of a surface at 0, 
Fig. 1, lighted by the sun in the zenith at S, and 
if TTS^ is the area of the sun's diametral plane, 
then B/tts^ is the illumination at due to each 
square centimetre. If the hemisphere were all 
of the same brightness as the sun, the illumina- 
tion at due to the ring of sky between 6 and 6 -\- dd would be 

^27rf2sin6>cos^^^, 

7TS^ 

where r is the distance of the sun. 

Integrating from ^ = to ^ = 7r/2, we have 

Total illumination = Br^js^ = 46,000 B. 

* U Astronomie, vol. 5, p. 406. 

t 'First Memoir on the Temperature of the Surface of the Moon.' National Academy of 
Sciences i Memoirs, vol. 3, 1884. 

20—2 




308 RADIATION IN THE SOLAR SYSTEM I 

The illumination from a hemisphere paved with suns is therefore 
46,000 X 4 X 10^ = 1-84 X lO^^ times that from the stellar sky. 

If we assume that the quality of the radiation is the same in both cases, 
that is, if we assume that the energy is proportional to the light-part of the 
spectrum, we have by the fourth-power law 

^ ^ . „ effective temperature of sun 

Enective temperature oi space = i 

(0-184 X 10i2)t 

effective temperature of sun 
f ^ 655 

As the temperature of the sun probably lies between 6000°^ and 7000^^, 
this gives 

Effective temperature of space = 10° A. 

If, then, a body is raised by the sun to even such a small multiple of 10° 
as, say, 60°, the fourth-power law of radiation implies that it is giving out, 
and therefore receiving from the sun, more than a thousand times as much 
energy as it is receiving from the sky. 

The sky-radiation may therefore be left out of the account when we are 
dealing with approximate estimates and not with exact results, and bodies 
in the solar system may be regarded as being situated in a zero enclosure 
except in so far as they receive radiation from the sun. 



Temperature of a Planet under Certain Assumed Conditions when 
placed at a Distance from the Sun equal to that of the Earth. 

The real earth presents a problem of complexity far too great to deal 
with. I shall therefore consider an ideal earth for which certain conditions 
hold, more or less approximating to reality, and determine the temperature 
of its surface on the assumption that it receives heat from the sun only. 

Let us suppose : 

1. That the planet is rotating about an axis perpendicular to the plane 
of its orbit, which is circular. 

This will give us too high a temperature at the equator, and the absolute 
zero, which is too low, at the poles. The mean, however, over the planet 
will probably be not much affected by the supposition. 

2. That the effect of the atmosphere is to keep the temperature in any 
given latitude the same, day and night. 

This is not a great departure from reality. On the sea, which is more than 
two-thirds of the earth's surface, the daily range is very small, of the order of 
1° or 2° C, while even on the land it is, in extreme cases, not more than 
15° C, which is not a large fraction of the absolute temperature. 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 309 

3. That the surface and the atmosphere over it at any one point have one 
effective temperature as a full radiator. This is no doubt a departure from 
reality. How wide a departure we have no present means of estimating. 

4. That there is no convection of heat from one latitude to another. 
This is a very wide departure from reality. But, as we shall see below, the 

mean temperature of the planet is very little affected by convection, even if we 
assume that it is so extensive as to make the surface of uniform temperature. 

5. That the reflection at each point is -fjj of the radiation received. 

This is probably of the order of the actual reflection from the earth. 
According to Langley* the moon reflects about J of the radiation received. 
The earth certainly reflects less. The temperatures determined hereafter are 
proportional to the 4th root of the coefficient of absorption. Even if this 
coefficient is as low as 0-9 its 4th root is 0-974. Hence if the actual value is 
anywhere between 0-9 and 1, the assumed value of 0-9 will not make an error 
of more than 2J per cent, in the value of the temperature. 

6. That the planet ultimately radiates out all the heat received from the 
sun, no more and no less. 

This again is very near the condition of the real earth, which, on the whole, 
radiates out rather more than it receives — perhaps on the average a calorie 
per square centimetre in three days. 



r cos \ d\ T"" / \r dA 




Fig. 2. 
Making these six suppositions, let us calculate the temperature of various 
parts of this ideal planet. 

Consider a band between latitudes A and A + dX. The area receiving 
heat from the sun at any instant, if projected normally to the stream of 
solar radiation, is (Fig. 2) 

2r cos XrdXcosX= 2r^ cos^ XdX, 

where r is the radius of the planet. 

* 'Third Memoir on the Temperature of the Moon.'. National Academy of Sciemes, Memoirs 
vol. 4, part 2, p. 197. 



310 BADIATION IN THE SOLAR SYSTEM I 

If S is the solar constant, this band is absorbing, with coefficient 0-9, 

0-9^ X 2r^cos^XdX. 

But the band all round the globe is radiating equally, according to the second 
supposition, and the radiating area is 

27rr cos A . rc^A = 27Tr^ cos A^^A. 
Hence the radiation emitted per square centimetre per sec. is 

0'9S . 2r^ cos^ XdX _ 0-98 cos A 
27rr^ cos XdX tt ' 

If the effective temperature in this latitude is 6^, we have 

0-9iS cos A 



5-32 X 10-^ 6>A^ 



77 



or 



/ 0-9 X 10^S \i 

[ 5-3277 ; 



cos* A. 



If we put A = 0, we get the equatorial temperature corresponding to each 
of the different values of S given above, viz. : 

Equatorial 6^ = 350° A approximately. 
e^ - 325° A 
„ e, ^ 312° A 

The temperature in latitude A is 

6^ = equatorial temperature x cos* A. 

Thus, in latitude 45°, it is 0-917 x equatorial temperature. 

The average temperature over the globe is 



J r2 



f 1 

277/-2 cos A 6e cos* XdX, 

'J 

where 6j^ is the equatorial temperature. 



The average temperature, then, is httle more than 1 per cent, above the 
temperature in latitude 45°. 

If we use the three values of 6^ just given, we have 

Average 6^ = 325° A approximately. 
„ 01 = 302° A 
d, = 290° A 

Our fourth supposition was that there is no convection by wind or water 
from one latitude to another. Let us now go to the other extreme and 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 311 

suppose that the convection is so great that the temperature is practically 
uniform all over the globe. We then have a receiving surface virtually irr^, 
and a radiating surface iTrr^. Then we get the radiation emitted per square 
centimetre 

47rr2 ~ 40' 
and if 6 is the temperature required for this, 

5-32x10-5^4=^; 

whence Uniform d^ = 330° A approximately, 

01 = 307° A 
Or = 293° A 

values not more than 5° above those obtained for the average on the 
supposition of no convection. 

Comparing these results with the temperature of the real earth, it is seen 
at once that they are of the same order. 

The average temperature of the earth's surface is usually estimated at 
about 60° F., say 289° A. The temperature of the atmosphere is on the 
whole decidedly lower than that of the surface below it. We should therefore 
conclude that the earth's effective temperature is somewhat below 289° A. 

Again, the earth and the atmosphere, taken as one surface, do not con- 
stitute a full absorber, but are to some extent selective. Hence we should 
expect the earth to be, if anything, of a higher temperature than a full absorber 
and radiator under the same conditions. 

For both these reasons, then, the ideal planet might be expected to have 
a temperature below rather than above 289° A. The lowest estimate obtained 
above is therefore probably nearest to the truth, and it would appear that 
even that is somewhat too high. This tends to show that, if we accept 
Kurlbaum's value of the radiation-constant, we cannot put the solar constant 
so high as 3 or 4, but must accept a value much nearer to that which I have 
called Rosetti's value, viz., 2-5. 

In what follows I shall therefore take Rosetti's value and the resulting 
value of the solar temperature, viz., 6200° A. 

The calculation made above may be turned the other way round, and may 
be used for a 

Determination of the Effective Temperature of the Sun from the Average 
Temperature of the Earth. 
Assuming that the real earth may be replaced by the ideal planet already 
considered, the radiation per square centimetre from the equatorial band is 

. But the radiation per square centimetre from the sun's surface is 



312 RADIATION IN THE SOLAR SYSTEM: 

46,000/S'. If then B^ is the earth's equatorial temperature, and 6^ is the solar 
temperature, 

— ^ : 46,000>S = Se^ : Os^ 

7T 

whence 0^ = ^sl^^- 

The average temperature of the earth is 0-93 of the equatorial temperature. 
If this average is ^^ , then 

e^ = es/21-5. 

If we take the temperature of the real earth as 289° A, and as being equal 
to that of the ideal, 

d^ = 21-5 X 289° = 6200° A approximately. 

Upper Limit to the Temperature of a Fully Radiating Surface exposed normally 
to Solar Radiation at the Distance of the Earth from the Sun. 

The highest temperature which a full radiator can attain is that for which 
its radiation is equal to the energy received. This will only hold when no 
appreciable quantity of heat is conducted inwards from the surface. 

To obtain the upper limit in the case under consideration, we have to 
equate the radiation to the solar constant, which we shall now take as 
Sr = 0-175 X 10^. Then 

5-32 X 10-5^4 = 0-175 X 10^ 
whence 6 = 426° A. 

If the surface reflects some of the radiation and absorbs a fraction x of 
that falling on it, then the effective temperature is 

x^ X 426° A. 

The Limiting Temperature of the Surface of the Moon. 

We may apply this result to find an upper limit to the temperature of the 

moon's surface. This upper limit can only be attained when it is sending 

out radiation as rapidly as it receives it, and is therefore conducting no 

appreciable quantity inwards. 

117 r, n . 1 T 1 ? ^- . /7 •. X i- reflected radiation 1 

We shall take Langley s estimate (loc. cit.) of — ^——-^ :rr~-. — = -— - . 

^ -^ ' emitted radiation 6-7 

This is represented nearly enough by ;z; = | . 

The upper limit of temperature of the surface exposed to a zenith sun is, 
therefore, 

^ = 426 X (I)* = 426 X 0-967 - 412° A. 

This, then, is the upper Hmit to the temperature of the hottest part of an 
airless moon. 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 313 

For a surface with normal at angle A with the line to the sun, 

^x = 412 cos* A. 

If we take this as the law of temperature of the side of the moon exposed 
to the sun, we can find the effective temperature of the full moon as seen 
from the earth, i.e., the uniform temperature of a flat disc of radius equal to 
that of the moon, sending to us the same total radiation. 

If Ndo) is the normal stream of radiation from 1 cm.^ of surface of 
the moon immediately under the sun sent out through a cone of angle doj, 
that sent out in direction A to the normal is iVcosA<?a;. But 1 cm.^ on the 
moon's surface, with normal inclined at A to the sun's rays, only receives 
cos A of the radiation received by the surface immediately under the sun. 
It therefore sends in the direction of the earth, also at A to the normal, onlv 
iVcos^A^o). Hence the total radiation to the earth, obtained by inte- 
grating, is 



/ 



2 N cos^ A . 27rm2 sin XdX 
^^ 



where m is the radius of the moon and r is its distance from the earth. 

Let Nj^ be the normal stream from the equivalent flat disc, then 

Trm^Nn _ 277 m- ^ 

2 
whence Nz) = ^N. 

o 

The effective temperature of the flat disc is therefore Vf that of the 

surface immediately under the sun at the same distance from it. 

Then the effective average = 412 x v^f = 412 x 0-9 = 371° A. The upper 
limit, then, to the average effective temperature of the moon's disc is just 
below that of boiling water. 

This is very considerably above Langley's estimate, that the surface of 
the full moon is a few degrees above the freezing-point. There can be no 
doubt that a very appreciable amount of heat is conducted inwards. The 
observations during eclipses by Langley* and by Boeddicker show that some 
heat is still received from the moon's surface when it has entered the full 
shadow, and that it takes time after the eclipse has passed to establish a 
steady temperature again. It might be possible to make some rough estimate 
of the amount conducted inwards from the Fourier equation, but the problem 
is not an easy one. Perhaps we get the best estimate by comparing the actual 
temperature with that found above. 

* 'Third Memoir,' p. 159. 



314 RADIATION IN THE SOLAR SYSTEM: 

If the actual temperature is taken as about^ | of the upper limit, say 297°^, 
then the radiation outwards is of the order (|)* = 0-41 of that where no con- 
duction exists. Then nearly f of the heat is probably conducted inwards. 

If the moon always turned the same face to the sun instead of to the 
earth, the upper limit would be approached. 



Temperature of a Spherical Absorbing Solid Body of the Order 1 cm. 
in diameter at the Distance of the Earth from the Sun. 

The calculation of the temperature of such a body is interesting for two 
reasons. Firstly, the body will be at nearly the same temperature through- 
out, and secondly, as we shall show in the second part of this paper, the 
mutual repulsion of two such bodies, due to the pressure of their radiation, 
is of the same order as their gravitative attraction. 

If the radius of the body is a, its effective receiving area is 7ra^, and it 
receives 

TTa^S ergs/sec. 

Its radiating surface is iira'^, and therefore its average radiation per square 
centimetre per sec. in the steady state is 

Tra^S/iTTa^ = iS. 

If we take S=2'6 cal./cm.^ min. or 0-04 cal./cm.^sec, and if the conductivity 
is of the order of that of terrestrial rock lying, say, between 0-01 and 0-001, it 
is evident that a difference of temperature of only a few degrees between the 
receiving and the dark surfaces will convey heat sufficient to supply radiation, 
0-01 cal./cm.^ sec, equal to the average. Thus, if the conductivity is 0-001 
and the diameter is 1 cm., a difference of temperature of 10° suffices. 

We may therefore take the temperature of the surface as approximately 
uniform when the steady state is reached. Let the temperature be 6, and 
let the solar temperature be Og. Then we have 

and 

If ^e = 



This will be the temperature of fully absorbing bodies of diameter less 
than 1 cm., so long as they are not too small to absorb the radiation 
falling on them. 



: 0s' 


= 1 : 46,000;S 


e- 


20-7' 


6200° 


A, 


300°. 


4 approximately 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 315 

Variation of Temperature with Distance from the Sun. 

Since the radiation received varies inversely as the square of the distance 
from the sun, that given out varies in the same ratio. The temperature of 
the radiating surface varies therefore as the fourth-root of the inverse square, 
that is inversely as the square-root of the distance. 

This enables us to deduce at once the temperatures of the various surfaces 
and bodies which we have considered, if placed at the distances of different 
planets as well as at the distance of the earth. We have merely to multiply 

i-u ^J. 1,'j.v. J. £ 1 T_ /earth's distance 

the results hitherto found by </ -^ , ... . 

V planet s distance 

The following table contains the values of the temperatures at selected 
distances, all on the absolute scale : 



Table of Temperatures of Surfaces at Different Distances from the Sun. 
All on the Absolute Scale. 



I 


II 


III 


IV 


V 


VI 


VII 


VIII 


IX 








Equatorial 


Average 
tem- 


Upper 
limit of a 


Average 

tem- 
perature 
of 


Tem- 
peiature 


Tem- 
perature 


At the 


Distance, 


Square- 


tempera- 


surface 


four- 


of 


distance of 


Earth's 


root of 


ture 


perature 

of 

ideal 

planet 


reflecting 


fifths 


small 


the planet 


distance = 1 


(distance)-i 


of ideal 


one-eighth 


that of 


absorb- 








planet 


under 
zenith sun 


lent disc 


equiva- 
lent disc 


ing 
sphere 


Mercury 


0-3871 


1-61 


502 


467 


664 


598 


478 


483 


Venus 


0-7233 


1-18 


368 


342 


486 


438 


350 


358 


Earth 


1-0000 


1-00 


312 


290 


412 


371 


297 


300 


Mars 


1-5237 


0-81 


253 


235 


337 


300 


240 


243 


Neptune 


30-0544 


0-18 


56 


52 


74 


67 


53 


54 



We have omitted the larger planets except Neptune, since in all probability 
they radiate heat of their own in considerable proportion. Neptune is inserted 
merely to show how low temperatures would be at his distance if there were 
no supply of internal heat. 

The results given in the table may not be exactly applicable to any of the 
planets, but they at least indicate the order of temperature which probably 
prevails. 

If, for instance, Mars is to be regarded as having an atmosphere with-, 
regulating properties like our own, his equatorial temperature (Column IV) 
is probably far below the temperature of freezing water, and his average 
temperature (Column V) must be not very different from that of freezing 
mercury. If, on the other hand, we suppose that his atmosphere has no 
regulating power, we get the upper limits not very different from those in 



316 RADIATION IN THE SOLAR SYSTEM: 

Columns VI and VII. These are the Hmits for the bright side, and they 
imply nearly absolute zero on the dark side. If we regard Mars as resembling 
our moon, and take the moon's effective average temperature as 297° A, the 
corresponding temperature for Mars is 240° A, and the highest temperature 
is I X 337 = 270°. But the surface of Mars has probably a higher coefficient 
of absorption than the surface of the moon — it certainly has for light — so 
that we may put his effective average temperature on this supposition some 
few degrees above 240° A , and his equatorial temperature some degrees 
higher still. 

It appears exceedingly probable, then, that whether we regard Mars as 
like the earth, or, going to the other extreme, as like the moon, the temperature 
of his surface is everywhere below the freezing-point of water. The only 
escape from this conclusion that I can see is by way of a supposition that an 
appreciable amount of heat is issuing from beneath his surface. 

We cannot draw any definite conclusions as to the temperatures of Mercury 
and Venus till we know whether they have atmospheres and whether they 
rotate on their own axes. If we make both these suppositions and further 
suppose that their conditions approximate to those (given in Columns IV 
and V) of the ideal planet at their distances, then they may well be surrounded 
by hot clouds, as is sometimes supposed, entirely screening their solid bodies 
from us. If, on the other hand, their atmospheres are ineffective as regulators 
and if they always present the same face to the sun, the hottest part of Mercury 
is probably not far from 650° A, and that of Venus not far from 500° A. 

If a comet consist of small solid particles of diameter of the order 1 cm. 
or less, then the temperatures of these particles are given in Column IX. At 
one-quarter of the earth's distance, say 23 million miles from the sun, the 
temperature is 600° A, about the melting-point of lead. At one-twenty-fifth, 
say 3| million miles, it will be about 1500° A, say the melting-point of cast- 
iron. Nearer than this the temperature no doubt increases rapidly, but the 
law of temperature, deduced from the inverse-square law for the radiation 
received, requires amendment, as that law was based on the supposition that 
a hemisphere only is lighted by the sun, and that the whole of his disc is 
visible from every part of that hemisphere. Both of these suppositions 
cease to hold when the distance from the sun is only a small multiple of his 
radius. 

PART 11. 

Radiation-Pressures. 

The pressure of radiation against a surface on which it falls, first deduced 
by Maxwell from the Electromagnetic Theory of Light, is now established 
on an experimental basis by the work of Lebedew, confirmed by that of 
Nichols and Hull. 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 317 

Though this pressure was first deduced as a consequence of the Electro- 
magnetic Theory, Bartoli showed, independently, that a pressure must exist 
without any theory as to the nature of light beyond a supposition which 
may perhaps be put in the form that a surface can move through the ether, 
doing work on the radiation alone and not on the ether in which the radiation 
exists. Professor Larmor* has given a proof of this pressure and has shown 
that it has the value assigned to it by Maxwell, viz., that it is numerically 
equal to the energy- density in the incident wave, whatever may be the nature 
of the waves, so long as their energy-density for given amplitude is inversely 
as the square of the wave-length. We may, in fact, regard a pencil of radiation 
as a stream of momentum, the direction of the momentum being the axis of 
the pencil. If E is the energy-density of the pencil, U its velocity, the 
momentum-density may be regarded as EjU. 

If the stream of "radiation is being emitted by a surface, the surface is 
losing the momentum carried out with the issuing stream, and is so being 
pressed backwards. If the stream is being absorbed by the surface, then it 
is gaining the momentum and is still being pressed backwards, the forces 
being in the line of propagation. 

As the expressions for the radiation-pressure in various cases are probably 
not very well known, it may be convenient to state them here for use in what 
follows. 

Values of Radiation-Pressure in Different Cases. 

If 1 cm.'-^ of a full radiator is emitting energy R per second, and if N dw 
is the energy it is emitting through a cone dco, with axis along the normal, 
then in direction d its projection is cos d, and it is emitting iVcos ddoj through 
a cone do). Putting dcxi = 2tt sin Odd, and integrating over the hemisphere, 
we have 

R= \ iV cos ^ . 277 sin (^(9 = ttN. 
J 

If we draw a hemisphere, radius r, round the source as centre, the energy 
falling on area r^doj is iVcos ddoj per second, and, since the velocity is U cm. 
per second, the energy-density just outside the surface on which it falls is 
iVcos diUr^, and this is the rate at which the momentum is being received, 
that is, it IS the normal pressure. The total force on area r^dcj is iVcos Odw/U. 
This is the momentum sent out per second by the radiating square centimetre 
through the pencil with angle dw, in the direction 6, and is therefore the force 
on the square centimetre due to that pencil. 

Resolving along the normal and in the surface we have 
Normal pressure = N cos^ ddco/U, 
Tangential stress = iV cos ^ sin Odco/U. 

* Brit. Assoc. Report, 1900; Encyc. Brit. vol. 32, Art. 'Radiation.' 



318 RADIATION IN THE SOLAR SYSTEM I 

Putting d(x) = 277 sin ddd and integrating over the hemisphere, we get 

IT 

Total normal pressure = ( [N cos^ d . Iir sin d dOjU) - ^ttNJW = 2i?/3C7. 

J 

Total tangential stress = 0, since the radiation is symmetrical about the 

normal. 

If the surface is receiving radiation, let us suppose that the stream is 

a parallel pencil S ergs per second per square centimetre held normal to the 

stream, and that it is inclined afc an angle 6 to the normal to the receiving 

surface. The momentum received per second is S cos djTJ. This produces 

Normal pressure ^ S cos^ djU , 
Tangential stress =^ S cos d sin djU. 
If the stream is entirely absorbed both these forces exist. 

If the stream is entirely reflected, the reflected pencil exerts an equal 
normal force and an equal and opposite tangential force, and we have only 
normal pressure of amount 2S cos^ d/U. 

If only a fraction /x is reflected, the incident and reflected streams will give 
Normal pressure = (1 + /jl) S cos^ O/U, 
Tangential stress = (1 — /jl) S cos 6 sin d/U. 

To the normal pressure must be added the pressure due to the radiation 
emitted from the surface. 

Radiation- Pressure in Full Sunlight. 
If a full absorber is exposed normally to the solar radiation at the 

distance of the earth the pressure on it is S/U, or - -^ .w.f.-~ = 5-8 x 10~^ 

^ ' 3 X 10^^ 

dyne/cm.^. 



The Radiation-Pressures between Small Bodies. Comparison ivith 
their mutual Gravitation. 

It is well known that the radiation-force on a small body, exposed to solar 
radiation, does not decrease so rapidly as gravitative pull on the body when its 
size decreases. If the body is a sphere of radius a and density p, and with 
a fully absorbing surface, and if it is so small that it is practically at one 
temperature all through, it is receiving a stream of momentum 

• ira^SjU 

directed from the sun. Its own radiation outwards being equal in all 
directions has zero resultant pressure. 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 319 

The gravitative acceleration towards the sun at the distance of the earth 
is about 0-59 cm./sec.^. Then we have 

Radiation-pressure. tto^S 

Gravitation-pull U x ^ira^p x 0-59 ' 

The two will be equal when 

a -- 



S 



Up X 0-59' 
If we put /o=l; /S = 0-175 X 107; U = ^xlO^^; 



we get 



a = 74 X 10-6. 



This is the well-known result that a body of diameter about two wave- 
lengths of red light would be equally attracted and repelled if we could 
assume that a surface so small still continued to absorb. But, of course, when 
we are getting to dimensions comparable with a wave-length that assumption 
can no longer be made. 

It is not, I think, equally well recognised that if the radiating body is 
diminished in size, the radiation-pressure due to it also decreases less rapidly 
than the gravitative pull which it exerts. For the radiation decreases as 
the square of the radius of the emitting body and its gravitative pull as the 
cube. 

We can easily compare the forces due to radiation and gravitation between 
two bodies, if for simplicity we assume that their distance apart is very great 
compared with the radius of either. 





Fig. 3. 



Let A, B, Fig. 3, be two spheres with full radiating surfaces. Let their 
radii be a, b, and let their centres o, o' be d apart. If this distance is great 
compared with a and 6, each may be regarded as receiving a parallel stream 
from the other. 

Let A send out a normal stream N dco per square centimetre through cone 
do), while B sends out N' dco. 

B receives the stream of cross-section nb'^ or the angle of the cone is nb^/d'^, 
and it issues virtually from area na^, for at B, A will appear as a uniformly 
bright flat disc. 



320 RADIATION IN THE SOLAR SYSTEM: 

Then the total force on B is 

where R — ttN. 

The force on A due to B is ira^hm' jUd^, where E = ttN\ 

These are not equal unless R = R' , i.e., unless the two bodies have the 
same temperature, an illustration of the fact that equality of action and 
reaction does not hold between the radiating and receiving bodies alone. 
They no longer constitute the whole of the momentum-system. The ether, 
or whatever we term the light-bearing medium, is material, and takes its 
part in the momentum-relations of the system. 

If the surfaces are partially or totally reflecting, the forces are easily 
obtained. Thus if one is totally reflecting, it can be shown that the force is 
only half as great as when it is fully absorbing. But it will be sufficient to 
confine ourselves to the case of complete absorption, followed by radiation 
of the absorbed heat equally in all directions from all parts of the surface. 
More general assumptions do not alter the order of the forces found. 

If G is the constant of gravitation = 6-67 x 10"^, and if p, p' are the 

densities of A and B, the gravitation-pull, P, is 6^ ^ ar}2^^^ • 

Radiation-push F _ 97Ta^b^R 
Gravitation-puff P WGUn^a^b^pp" 



F 9R 



P WGUTTabpp" 

lia = b; p=p'; R^ 5-32 x 10-^^^ we have 

0-69 X 10-4^2 /p ^ 



or 



If we suppose the two bodies to have the temperature of the sun, say 
6200°^, and its density, say 1-375, then F = P, when a =1930 cm. or 
19-3 metres f. 

Of course two globes of this size would soon cool far below the temperature 
of the sun, even if for an instant they could be raised up to it. 

If we suppose 6 = 300° A — the approximate temperature of small bodies 
at the distance of the earth from the sun — and if we take /o = 1, then F = P 
when a = 6-2 cm. I 

* [The original has 2-18, instead of 0-69, which is a shp due to the omission of ^l^- "^^^ 
density of the sun is also wrongly taken as 0-25 instead of 1-375 (see Art. 65, p. 709). This 
necessitates some corrections in the succeeding part of the paper, which have in all cases been 
marked with a f.' Ed.] 

t [See note above. Ed.] 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 321 

Thus two globes of water — probably nearly full absorbers at 300° A — 
will at that temperature neither attract nor repel each other if their radii 
are about 6 cm.f 

If the density of the spheres is 11, about that so often used for masses 
in the Cavendish experiment, F = P when 

a = 0-564 cm.t 

This does not throw any doubt on the results of Cavendish experiments, 
for it only holds when the radiators are in an enclosure of very low absolute 
temperature. In all Cavendish experiments the greatest care is taken to 
make the attracted body and its enclosure of one uniform temperature. 

The really interesting case is that of two small meteorites, in interplanetary 
space. To judge from the specimens which succeed in penetrating the earth's 
atmosphere they are very dense. Let us suppose them to have density 5-5 — 
that of the earth — and temperature 300° A, that which they will have at 
the earth's distance. Then F = P when 

a = 1-13 cm.f 

If the radii of the bodies are less than the values found for equality of 
F and P in the different cases, the net effect is repulsion. 

The ratio of -F to P is inversely as the square of the radius, so that, as 
the radii are decreased from the values giving F = P, the radiation -repulsion 
soon becomes enormously greater than the gravitation-pull, and the latter 
may be neglected in comparison. Thus for two drops of water at 300° A in 
a zero enclosure, with radii 0-001 cm., the pressure is nearly 40,000,000 times 
the pullf. 

It is not, however, that the radiation-force is great, or even its acceleration. 
The force becomes exceedingly minute, but the gravitation much more minute. 

Thus consider two drops of water at 300° A placed in a zero enclosure at 
a distance d = 10a apart. Our assumption of parallel radiation from one to 
the other is now only a rough approximation, but the result will be of the 
right order. 

The radiation-push is Tra^R/Ud^, and the acceleration is 

3aR/4:Ud^ =" TTv? ^ ~ approximately. 

This only becomes considerable when the drops approach molecular 
dimensions, and long before this they cease to absorb fully the stream of 
momentum falling on them. Still, even molecules are selective absorbers, 
and absorb especially each other's radiations. And we may expect that if 
two gas-molecules collide and set each other radiating much more violently 
than before, they will be practically in an enclosure of much lower temperature 
than their own, and their mutual radiation may result in very rapid repulsion 
— repulsion of the order of the fourth power of the temperature reached. 

t [See note on p. 320. Ed.] 

P. c. w. 21 



322 RADIATION IN THE SOLAR SYSTEM: 

Radiation-Pressure between Small Bodies at Different Distances from the Sun. 
We have seen above, that if two small spheres of density 5-5 are at the 
distance of the earth from the sun, their gravitation will be balanced by their 
radiation-pressure when the radius of each is 1-13 cm.f Now the balancing 
radius is proportional to the square of the temperature, that is, inversely 
proportional to the distance, since the temperature (Part I) is inversely as 
the square-root of the distance. Thus, at the distance of Mercury, the radii 
would be about 3 cm.f ; a million miles from the sun's surface they would 
be about 100 cm.j ; out at Neptune they would be about 0-4 mm.| 

We see then that the mutual action between small bodies of density that 
of the earth, will, at different distances, change sign for different sizes of 
body, ranging from something of the order of 2 metres diameter")* near the sun 
to the order of 1 mm. diameter f at the distance of Neptune. A ring of 
small planets, each of radius 1-13 cm.f, and density 5-5, would move round 
the sun at the distance of the earth without net mutual attraction or repulsion, 
and each might be regarded as moving independently of the rest. It appears 
possible that if Saturn is hot enough, considerations of this kind may apply 
to his rings. 

The repulsion between small colliding bodies, even if not heated by the 
sun, must lead to some delay in their final aggregation. This is obvious 
when there are only two small bodies, and their temperature is very con- 
siderably raised by the collision. But there is also delay if instead of a 
single pair we suppose two swarms to collide. Near the boundary of the 
colliding region, a body will experience radiation-pressure chiefly on one side, 
and will tend to be driven out of the system. Of course, if the swarms are 
so dense that a member near the outside cannot see through the rest, this 
effect will be less. A body in front of another entirely screens its radiation, 
but the gravitation is not screened. Hence, a body near the boundary of 
a densely-packed region of collision may be repelled only by the colliding 
bodies just round it, while it will be attracted by all ; or, to put the same idea 
in another way, a body in a spherical swarm of uniform temperature will 
only be pulled equally in all directions at the centre of the swarm, but it will 
be equally repelled in all directions as soon as it is sufficiently deep to be 
surrounded by its fellows wherever, so to speak, it looks. 

Inequality of Action and Reaction between Two Mutually Radiating Bodies. 

We have seen that two distant spheres push each other with forces 
TTa^b^R/Ud^ and -na^b'^R' jU d^, and that these, though opposite, are not equal 
unless R = R' . 

It would be easy to imagine cases in which the forces were not even 
opposite or in the same directions. At first sight, then, it would appear that 

t [See footnote, p. 320. Ed.] 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 323 

we have two bodies acting upon each other with unequal forces, but of course 
this statement is inexact. The bodies do not act upon each other at all; 
each sends out a stream of momentum into the medium surrounding it. 
Some of this momentum is ultimately intercepted by the other, and in its 
passage the momentum belongs neither to one body nor to the other. If we 
assume that the momentum is conserved, and of course everything in the 
methods of this paper depends on that assumption, the action on one of the 
bodies is equal and opposite to the reaction on the light-bearing medium 
contiguous to it. There is no failure of the law of action and reaction, but 
an extension of our idea of matter to include the medium. There should 
be no difficulty in this extension ; indeed, we have made it long ago in endowing 
the medium with energy-carrying properties. Whether the momentum in the 
medium is in the form of mass m moving with velocity v in the direction of 
propagation is perhaps open to doubt. We may, perhaps, have different 
forms of momentum just as we may have different forms of energy, and 
possibly we ought not to separate the momentum in radiation into the factors 
m and v, but keep it for the present as one quantity M. 

An interesting example of inequality of the radiation-forces on two 
mutually radiating bodies is afforded by two equal spheres, for which, at 
a given temperature, the radiation-push F balances the gravitation-pull P. 
Raise* one in temperature so that the push on the other becomes F\ Lower 
the other so that the push on the first becomes F", but adjust so that 

r + F" = 2F=: 2P, 
then P-F" = F' -P. 

There will then be equal accelerations of the two in the same, not in opposite 
directions, and a chase will begin in the line joining the centres, the hotter 
chasing the colder. If the two temperatures could be maintained, the velocity 
would go on increasing; but the increase would not be indefinitely great, 
inasmuch as a Doppler effect would come into play. Each sphere moving 
forward would crowd up against the radiation it emitted in front, and open 
out from the radiation it emitted backwards. This would increase the front 
and decrease the back pressure, and ultimately the excess of front pressure 
would balance the accelerating force due to mutual radiation. 

Let us examine the effect of motion of a radiating surface on the pressure 
of its radiation against it. 

Application of Doppler^s Principle to the Radiation- Pressure against 

a Moving Surface. 

If a unit area A, Fig. 4, is moving with velocity u in any direction AB, 

making angle with its normal AN, the effect on the energy-density in the 

stream of radiation issuing in any direction AP is two-fold. If the motion 

is such as to shorten AP, the waves and their energy are crowded up into 

21—2 



324 



RADIATION IN THE SOLAR SYSTEM: 



less space, and if such as to lengthen AP, they are opened out. At the same 
time, in the one case A is doing work against the radiation-pressure and in 
the other is having work done on it. We shall assume, as in the thermo- 
dynamic theory of radiation, that this work adds to or subtracts from the 
energy of radiation. Both effects, (1) the crowding, and (2) the work done, 
or the reverse of each, combine to alter the energy and therefore the radiation- 
pressure. We have no data by which we can determine whether the motion 
alters the rate at which the surface is emitting radiation, but it appears worth 
while to trace consequences on the assumption that the radiation goes on as 
if the surface were at rest*, but that it is crowded up into less space or spread 
over more, and that we can superpose on this the energy given out to, or 
taken from, the stream by the work done by, or on, the moving surface by 
the radiation- pressure. This work can evidently be calculated to the first 
order of approximation by supposing the pressure equal to its value when 
the surface is at rest. 

Let us draw from A as centre a sphere of 
radius U, equal to the velocity of radiation. 
The energy which, in a system at rest, would 
be radiated into a cone with A as vertex, 
length U, and solid angle do), in the direction 
AP making an angle x with the direction of 
motion AB, will now be crowded up into a cone 
of length U — u cos x, since u cos x is the velocity 
of A in the direction AP. We shall suppose that 
u/U is very small. Hence the energy-density 
in the cone is increased in the ratio U + u cos x 

ucosx 

Considering now the effect of the work done, the force on A due to the 
stream in dw is N cos ddoj/U, and the work done in one second is 

{N cos ddco/U) X u cos X- 

When A is at rest the energy in this cone is 

N cos ddo). 




U or by the factor 



* Added August 20, 1903. Since the above was written Professor Larmor has pointed out 
to me that the results obtained in the text from this assumption, along with the hypothesis of 
crowding of the radiation and its increase by an amount equivalent to the work of the radiation- 
pressure, can be justified by an argument based on the following considerations. A perfect 
reflector moving with uniform speed in an enclosure, itself also moving at that speed, and so in 
a steady state, must send back as much radiation of every kind as a full radiator in its place. 
Now the electrodynamics of perfect reflection are known ; hence the effect of motion of a full 
radiator on the amount of its radiation can be determined. The result is equivalent to the 
statement that the amplitudes of the excursions of the optical vibrators are the same at the 
same temperature whether the source to which they belong is moving or not. 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 325 

When A is movijig it is increased to 

T,, ^ , N cos ^c?ct> 

iv cos Udoj H ^ u cos ;)^, 

that is N cos 6>^a; (l + ^i^^ . 

Thus the effect of the work done is equal to that of the crowding, and the 
energy-density on the whole is increased in the ratio 

2i^cosx.-. 
"^ U 

The pressure is increased in the ratio of the energy-density*. Then the force 
on A due to the radiation through dco is increased from 

Ncosddco , N cosddco /^ 2w cos v i 
to == 1 + ^ 



U U \ ' u 

If we resolve this along the normal to the surface A and integrate over 
the hemisphere we obtain the total normal pressure. As we only want to 
know the change in pressure P we may neglect the first term which gives 
the pressure on A at rest, and we have 

^ [N cos^ 9 2u cos y , 

If cf) is the angle between the normal planes through B and P we have 
cos X = cos 6 cos e/f + sin ^ sin ijj cos cj). 
Putting dco = sin Odddcf), we get 



P = 



r2 c'i-^2Nu 

I TtT ^^^^ ^ ^^^ ^ (^^^ ^ ^^^ 'A + si^ ^ ^^^^ 'A COS (f)) ddd(f) 



ttNu cos i/r Ru cos j/f 

The change in the tangential stress, T, is evidently in the direction AC, 
that of the component of u in the plane of A. 

We may therefore resolve each element of tangential stress in the direction 
AC. Omitting the first term again, since in this case it disappears on inte- 
gration, the element due to dco in the direction AP will contribute 

N cos 6 sin 6 cos 2u cos x 



U ' U 

and integrating over the hemisphere we have 



dco, 



T = \ -jj^ cos 6 sin^ 6 cos cf) (cos 6 cosip + sin 6 sin ip cos ^) (?^ defy 

ttNu sin j/f i??i sin iff 
^ 2JP '~^ ~~2W~' 



* [See note, p. 330. Ed.] 



326 RADIATION IN THE SOLAR SYSTEM: 



Force on a Sphere moving with Velocity 'u' in a Given Direction. 

If a sphere, radius a, is moving with velocity u, we may from symmetry 
resolve the forces on each element in the direction of motion. The resolutes 
will be P cos ifj and T sin ifj. Evidently it is sufficient to integrate over the 
front hemisphere and then double the result. We have the 

2 



-r. , T T^ ^ r /^^ cos^ w , Ku sm2 e/f\ ^ . , , , 
Ketardmg _borce = -^ I fj^ \ i^jj2 j ^tt"^ sm i/jdifj 



va 



3 U2 

It is noteworthy that one half of this is due to the normal, the other half 
to the tangential stresses. 

If the sphere has density p the acceleration is obtained by dividing by 
iTTCi^p, then 

du/dt = - IRujU^pa. 



Effect on Rotation. 

If the sphere radius a is rotating with angular velocity co, then any element 
of the surface, A from the equator, is moving with hnear velocity aa> cos A in 
its own plane. This does not affect the normal pressure, but it introduces 
a tangential stress opposing the motion 

Rii/2U^ = RacocosXI2U\ 

Taking moments round the axis and integrating over the sphere, we 
obtain a couple 

TT 

ina^p . ^a^ , = ^_^- 27ra^ cos'^ A dX, 

^ "" dt 2U^ j _^ 

2 

whence dco/dt = — qRco/U^pa. 

The rate of diminution of oj is therefore of the same order as that of v. 

To obtain an idea of the magnitude of the retardation of a moving sphere, 
let us suppose that one is moving through a stationary medium. Let its 
radius he a = 1 cm., its density p = 5-5, its temperature 300° A. 

Idu 2 X 5-32 X 10-5 X 300* 



Then 



u dt 9 X 1020 X 5.5 

= 1-75 X 10-16. 



This will begin to affect the velocity by the order of 1 in 10,000 in, say, 
10^2 seconds, or taking the year as 3-15 x 10^ seconds, in about 30,000 years. 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 327 

The effect is inversely as the radius, so that a dust-particle 0-001 cm. 
radius will be equally affected in 30 years. 

The effect is as the fourth power of the temperature, so that with rising 
temperature it becomes rapidly more serious. 

Equation to the Orbit of a Small Spherical Absorbing Particle Moving in 
a Stationary Medium Round the Sun. 

It is evident from the above result, that the effect of motion on radiation- 
pressure may be very considerable in the case of a small absorbing particle 
moving round the sun. 

We shall take the particle as spherical, of radius a and distance r from the 
sun. We shall suppose the radius so small that the particle is of one tempera- 
ture throughout, the temperature due to the solar radiation which it receives, 
but that it is still so large as to be attracted much more than it is repelled by 
the sun. Both attraction and repulsion are inversely as the square of the 
distance, so that we shall have a central force which we may put as producing 
acceleration A/r^, where A is constant. 

We know that at the distance of the earth, putting r = b, A/b^ = 0-59 
cm. /sec. ^ say 0-6 cm./sec.^. Then A = O-Qb^. The force acting against 
the motion produces retardation — 2RulU^pa. 

If S is the solar constant at the distance 6, its value at distance r is 

Sb^rK 

Putting iTram = ira^Sb^/r^, 

R = (S/i) {b^lr% 

then the acceleration in the line of motion is 

_ Sb"^ u__Ts 
2lPpa ' r2 ~ /•2 ^ 

where T = Sb^l2U^pa, and -s- is now written for the velocity u. 

The accelerations along and perpendicular to the radius-vector give the 
equations 

'-"•=-^s <■' 

ls"''>-W^^ ■ ; <^> 

From (2) we get | {r^O) = - ^^^ 

whence r^O = C - TO, (3) 

where C is the constant of integration. 



328 RADIATION IN THE SOLAR SYSTEM: 

If ^ is when t = 0, then C is the initial value of rW. Further, as 6 increases 
rW decreases and is when d = C/T. This gives a limit to the angle described. 

Equation (1) may be written 

r-re^=-i-^ (4) 



Putting u for r~^ we have 

dr X 1 du A ,„ „^, du 



^ = -^ = -^^i^ = -(^-^^)i f-^^-^^^)' 



T{C-Te)n^^g-{C^Teru^^^ from (3). 



Substituting in (4) 



de^ ' (C - TOY' 

This can probably only be integrated by approximation. We can see the 
effect on the motion at the beginning by putting 

d'^u A / 2T 

4- ^^ -= ^ 1 - ^. 



dd^ ' 02 



{■-¥•). 



since TjC is small if we begin at the distance of the earth and with a particle 
having the velocity of the earth. 
An integral of this is 

The complementary function will be periodic and may be omitted. To the 
order of approximation adopted 

Then initially rjr = - (ITjC) 6. 

In applying these results, we may note that T = Sb^l2U^pa is constant 
for all distances, and that b, the earth's distance, is 493 U. Inserting the 
value of the solar constant, 0-175 x 10'^, and taking p ^ 5-5, we get 

T - 3-9 X 1010 . a-i. 

C will depend on the initial conditions. Assuming that the body considered 
is initially moving in a circle, then, at the beginning 

re^^~ or ^= /^= /^, 

since at r = 6 the acceleration to the centre is 0-6. 
Then C =^ r^O = V6^¥r. 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 329 

Substituting these values in r/r we have 

r__ 7-8 X IQio 

This gives only the initial value of - and cannot be taken to hold for a time 

which will make T^d^/C^ appreciable. But by (3) we see that r = if 
6 = C/T, so that CI27tT is a superior limit to the number of devolutions, 
even if we suppose the way clear right up to the centre. 

Putting the numerical values we get 

C/27rT= 46-5A*. 

Suppose, for example, that r = b = 493 x 3 x 10^^ ; a = 1, then 

r/r = - 3-5 X 10-16. 

If we multiply by 3-15 x 10^, the seconds in a year, we obtain 
(r/r) X 3-15 x 10^ = M x IQ-^. 

This implies that a sphere 1 cm. radius and density 5-5, starting with 
the velocity of the earth, and at its distance from the sun, will move inwards 
10,0 00 ^^ ^^^ distance in about 10,000 years. It cannot in all make so many 
as 46-5 X 6^ = 1-79 x 10^ revolutions. 

If we put a = 0-001 cm., since the effects are inversely as a, then its 
distance will decrease by about 1 in 10,000 in 10 years, and it cannot make 
in all so many as 1-79 x 10^ revolutions. 

If instead of starting from the distance of the earth, the particle starts 
from, say, 0-1 the distance, the effect in the radius is 100 times as great and 
the number of revolutions is v'^lO times less. Then with radius 1 cm. the 
distance decreases by foTooo ^^ ^^^ years, and there are not so many as 
80 X 10^ revolutions §, while with radius 0-001 cm. the distance decreases 
by ^Q QQQ in 0-1 year, and there are not so many as 80,000 revolutions §. 

Small particles, therefore, even of the order of 1 cm. radius, would be 
drawn into the sun, even from the distance of the earth, in times not large 
compared with geological times, and dust-particles if large enough to absorb 
solar radiation would be swept in in a time almost comparable with historical 
times. Near the sun the effects are vastly greater. The application to 
meteoric dust in the system is obvious. 

There should be a similar effect with dust and small particles circulating 
round the earth. If, for example, any of the Krakatoa dust was blown out 
so far beyond the appreciable atmosphere, and was given such motion that 
the particles became satellites to the earth, at no long time the dust will 

* [The original has 6 Iria. In what follows, the necessary alterations consequent upon tliis 
correction have been made. Ed.] 

§ [The original has 80,000 and 80 respectively. The correction necessitates a modification in 
the views expressed in the succeeding paragraphs. Ed.] 



330 - RADIATION IN THE SOLAR SYSTEM: 

return. A ring of dust-particles moving round a planet and receiving heat 
either from the sun or from the planet will tend to draw in to the planet. 

[Note added October 31. Since the foregoing paper was printed I have 
re-examined the theory of the pressure on a fully radiating surface when in 
motion, and have come to the conclusion that the change in pressure due to 
the motion is only half as great as that obtained on p. 325. In that investi- 
gation the pressure was assumed to be equal to the energy-density, whether 
the surface was at rest or in motion, whereas it appears, if the following mode 
of treatment is correct, that the pressure on a radiating surface moving 

u 
forward is only 1 — yy of the energy-density of the radiation emitted. 

Let us suppose that a surface A, a full radiator, is moving with velocity 
u towards a full absorber J5, which, with the surroundings, we will suppose 
at 0° ^. Consider for simplicity a parallel pencil issuing normal from A with 
velocity U towards B. Let the energy-density in the stream from AhQ E 
when A is at rest, and E' when it is moving. Let the pressure onAhaf^E 
when it is at rest, and f' when it is moving. When moving, A is emitting 
a stream of momentum f' per second and this momentum ultimately falls 
on B. Let A start radiating and moving at the same instant; let it move 
a distance d towards B, and then let it stop radiating and moving. It emits 
momentum f' per second for a time dju and therefore emits total momentum 
f'dju. Since B is at rest, the pressure on it, the momentum which it receives 
per second, is W . But since A is following up the stream sent out, B does 
not receive through a period as long as dju, but for a time less by djU. If we 
assume that the total momentum received by B is equal to the total sent out 
by A, we have 

f'dju = E' (dju - djU), 
or rp' =.E' (I- ujU). 

To find E' in terms of E we must make some assumption as to the effect 
of the motion on the radiation emitted. In the paper I have assumed that 
the emitting surface converts the same amount of its internal energy per 
second into radiant energy as when it is at rest, but that ^p'u of the energy 
of motion of the radiating mass is also converted into radiant energy. Since 
the radiation emitted in one second is contained in length TJ — ^/, we have 

E' {U - u) =^EU + p'u =EU-\-Fy (^— ) ^, 

whence E' = E - ^' ,, = ^ (1 + 2WC/). 

The same result is obtained if we assume that the amplitude of the 
emitted waves is the same whether the surface is moving or not, and that 



ITS EFFECT ON TEMPERATURE AND ITS PRESSURE ON SMALL BODIES 331 

the energy-density is inversely as the square of the wave-length for given 
amplitude. 

We have, therefore, if the above application of the equality of action and 
reaction is justified, 

In a similar way we can find the effect of motion of an absorber on the 
pressure against it due to the incident radiation. 

Let a stream of energy-density E be incident on a fully absorbing surface 
moving towards the source with velocity u. Let the surface be at 0° ^, so 
as to obtain the effect of the incident radiation only. When the surface is 
at rest, we may regard the stream as bringing up momentum E per second, 
or as containing momentum of density E/U brought up with velocity U to it. 
If the surface is moving towards the source, it takes up in one second the 

E 

momentum in length U + u, or receives yj{U + u), and the pressure on it is 



,'^.(i.a = ,(i.-^). 



It is easy to show that when a perfect reflector is moving, the pressure 

2u' 



upon it is altered from ^^ to ^ ( 1 + jj 

In the paper, the case of a full radiator in an enclosure at zero has alone 

1 -IT 11 •<• . ^ u ^ u cos Y , 
been considered, so that the correctmg factor is 1 + yy or 1 H ^,- when 

the motion is at an angle x to the line of radiation. Hence the forces obtained 

2u 
in the paper when the factor was 1 + 77 are all double those obtained with 

the factor now given. The process of drawing in small particles to the sun 
is correspondingly lengthened out. 

It is, perhaps, worth noting that the motion of a body round the sun 
produces a small aberration-effect. If the body is a sphere, the sunlight 
does not fall on the hemisphere directly under the sun, but on one turned 
round through an angle u/U. The pressure of the radiation, though still 
straight from the sun, does not act through the centre but through a point 

^jj^ X (radius of sphere) in front of the centre. Thus, in the case of the 

earth, it will tend to stop the rotation. But the effect is so minute that 
if present conditions as to distance and radiation were maintained, it 
would take something of the order of 10^^ years to stop the whole of the 
rotation. J. H. P.] 



21. 

NOTE ON THE TANGENTIAL STRESS DUE TO LIGHT 
INCIDENT OBLIQUELY ON AN ABSORBING SURFACE. 

[PM. Mag. 9, 1905, pp. 169-171.] 

[Read at Section A, British Association, Cambridge, August, 1904.] 

The existence of pressure on a surface due to tlie incidence of a normal 
beam of light, first deduced as a consequence of the electromagnetic theory 
by Maxwell, has been fully confirmed by the experiments of Lebedew, and 
quite independently by the exact work of Nichols and Hull. These experi- 
ments show that the pressure exists and that it is equal to the energy per c.c. 
or to the energy-density in the incident beam. 

In so far as it produces this pressure we may regard the beam as a stream 
of momentum, the direction of the momentum being along the line of 
propagation, and the amount of momentum passing per second through unit 
area cross-section of the beam being equal to the density of the energy in it. 
Let E denote this energy-density. If the beam is inclined at 9 to the normal 
to a surface on which it falls, the momentum-stream on to unit area of the 
surface is E cos 9 per second, and this is the force which the beam will exert 
in its own direction. If the beam is entirely absorbed, the result is a pressure 
E cos^ 9 along the normal and a tangential stress in the plane of incidence 
E sin 9 cos 9 = IE sin 29. If ^ of the incident beam is reflected, the normal 

pressure is {I -\- ^x) E cos^ 9 , and the tangential stress is - -^sin2^*. 

When there is absorption the tangential stress has a maximum value at 45° 
if IX is constant. When there is no absorption the tangential stress disappears. 

The tangential stress is much more easily detected than the normal 
pressure. For the action of the gas surrounding the surface is normal to it 
and is with difficulty disentangled from the normal light-pressure. But the 
gas-action is at right angles to the tangential stress, and it is merely necessary 
to arrange a surface free to move in its own plane to eliminate the action of 
the normal forces and to reveal the tangential stress. 

* These expressions are given in 'Radiation in the Solar System,' Phil. Trans. A, 202, p. 539. 
[Collected Papers, Art. 20.] 



NOTE ON THE TANGENTIAL STRESS DUE TO LIGHT, ETC. 333 

With the assistance of my colleague Dr. Guy Barlow, to whom I am much 
indebted for help in the work, I have made the following experiment to show 
the existence of the stress. 

Two circular glass discs, each 2-75 sq. cm. area, were fixed at the ends of 
a horizontal light glass rod 5*3 cm. long, the discs being perpendicular to the 
rod and fixed to it at their highest points. One of the discs was lampblacked, 
and the other silvered. The rod was placed in a light wire cradle and 
suspended by a fine quartz fibre about 25 cm. long in a brass case with glazed 
sides. On the cradle was a mirror by which deflections could be observed 
with a telescope on a millimetre-scale 1-8 metres distant. The moment of 
inertia of the system was 2*35 gm. cm.^ and the time of vibration was 
146 seconds. A deflection of 1 scale-division therefore corresponded to a 
tangential force on a disc of about one two-millionth of a dyne — more exactly 
0-483 X 10-6 (jyne. 

The air was pumped from the case till the pressure was less than 1 cm. of 
mercury. At this pressure the irregularity of the disturbances due to the 
residual gas is very greatly reduced. A parallel beam of light from a Nernst 
lamp was then directed so as to be incident obliquely on the lampblacked 
disc. From the arrangement of the discs it is obvious that a uniformly 
distributed normal force would have no moment tending to twist the system, 
while a tangential force would have a moment and would twist it. In all 
cases the disc moved away from the source of light. The deflection was 
a maximum when the incidence was not very far from 45°, and fell off on each 
side of the maximum value. 

As there are various sources of error not yet removed, we have not made 
a complete series of measurements but have only made sure that the effect 
is of the order to be expected from the theory, by finding the deflection for 
an angle of 45°. 

The beam from the Nernst lamp when incident at 45° turned the rod 
through 16-5 scale-divisions. Assuming total absorption, the tangential 
force should be ^E sin 2d x area of disc == ^E x 2-75. 

Equating to the value of the force given by the deflection, viz., 

0-483 X 10-6 X 16-5, 

we have E = 5-8 x 10-^ erg/cm.^. 

The same beam was then directed on to a small lampblacked silver disc 
of known heat-capacity, through a glass plate of thickness equal to that of 
the side of the case. The initial rise of temperature per second was measured 
by a ther mo junction of constantan wire soldered to the disc. The energy- 
density of the stream was thus found to be ^ = 6-5 x 10"^ erg/cm. 3. 



334 NOTE ON THE TANGENTIAL STRESS DUE TO LIGHT, ETC. 

The agreement of the two values is quite as close as could be expected 
in so rough a determination*. 

When the beam was directed on to the silver disc at the other end of the 
torsion-rod, the deflection was much less, as was to be expected. 

We have also made some qualitative experiments with a blackened glass 
cyhnder — a ring cut from a test-tube — suspended by a quartz fibre with its 
axis vertical. When a beam fell on this in any direction not along a diameter, 
there was always a twist in the direction corresponding to the tangential 
stress. 

* [A redetermination of the various constants and a revision of the calculations gave a still 
closer agreement. The observed torque was 21 x 10"^ cm. dynes, while the torque calculated from 
the energy was 22 x 10"^ cm. dynes. This correction is given in The Pressure of Light, p. 55. 
(Romance of Science Series, S.P.C.K. 1910.) Ed.] 



Wl 



[Note by G. Barlow, July 1916. 

Particular care was taken to make the torsion-system very symmetrical 
th respect to the axis of suspension. The success of the experiment 
depended, also, on using a uniform parallel beam of light of cross-section 
slightly greater than the projected area of the disc. During the small oscilla- 
tions of the system the disc, therefore, remained uniformly illuminated. 
It was found that when a circular patch of light was focussed centrally on the 
disc the deflections were irregular. On one occasion we used a beam of 
sunlight reflected from a heliostat, but owing to the very variable absorption 
by the town atmosphere the observed pressure showed great fluctuations. 

The experiment with the cylinder has since been repeated, and was 
shown to some members of the British Association at the Birmingham meeting 
of 1913. The cylinder was of aluminium and was turned very accurately 
by a watchmaker. The ends were closed, but near the axis two small air- 
holes were drilled. The surface was blackened with a deposit of asphaltum. 
The observed torques due to the light-pressure were always of the order of 
magnitude expected, and variation of the gas-pressure over a considerable 
range did not greatly afi^ect the results. This method is suitable for lecture 
demonstration but it does not appear satisfactory for exact measurements.] 



22. 

RADIATION-PKESSURE *. 
[Phil. Mag. 9, 1905, pp. 393-406.] 

[Presidential Address, delivered at the Annual General Meeting of the 
Physical Society, February 10, 1905.] 

A hundred years ago, when the corpuscular theory held almost universal 
sway, it would have been much more easy to account for and explain the 
pressure of light than it is to-day, when we are all certain that light is a form 
of wave-motion. Indeed, on the corpuscular theory it was so natural to 
expect a pressure that numerous attempts were madef in the eighteenth 
century to detect it. But the early experimenters had a greatly exaggerated 
idea of the force they looked for. Even on their own theory it would only 
have double the value which we now know it to possess, and their methods of 
experiment were utterly inadequate to show so small a quantity. But had 
these eighteenth-century philosophers been able to command the more refined 
methods of to-day, and been able to carry out the great experiments of 
Lebedew and of Nichols and Hull, and had they further known of the emission 
of corpuscles revealed to us by the cathode-stream and by radioactive bodies, 
there can be little doubt that Young and Fresnel would have had much 
greater difficulty in dethroning the corpuscular theory and setting up the 
wave-theory in its place. 

The existence of pressure due to waves, though held by Euler and used 
by him 160 years ago to explain the formation of comets' tails by repulsion, 
seems to have dropped out of sight, till Maxwell, in 1872, predicted its existence 
as a consequence of his Electromagnetic Theory of Light. It is remarkable 
that it should have been brought to the front through the investigation of 
such a special type, such an abstruse case, of wave-motion, and that it was 
not seen that it must follow as a consequence of any wave-motion, whatever 
the type of wave we suppose to constitute Light. I believe that the first 
suggestion that it is a general property of waves is due to Mr. S. Tolver 
Preston, who in 1876 { pointed out the analogy of the energy-carrying power 

* [This address is included here because it contains an account of some original work not 
elsewhere described. Ed.] 

t Some account of these methods is given by Nichols and Hull in 'The Pressure due to 
Radiation,' Proc. Am. Ac. vol. .38, no. 20, p. 559. See also Priestley, On Vision, p. 385. 

t Engineering, 1876, vol. 21, p. 83. 



336 KADIATION-PRESSURE 

of a beam of light with the mechanical carriage by belting, and calculated 
the pressure on the surface of the Sun by the issuing radiation, obtaining 
a value equal to the energy-density in the issuing stream, without assumption 
as to the nature of the waves. But though the analogy is valuable, I confess 
that Mr. Preston's reasoning does not appear to me conclusive, and I think 
it still remains an analogy. There is, I suspect, some general theorem yet 
to be discovered, which shall relate directly the energy and the momentum 
issuing from a radiating source. It seems possible that in all cases of energy- 
transfer, momentum in the direction of transfer is also passed on, and there- 
fore there is a back pressure on the source. Such pressure certainly exists 
in material transfer, as in the corpuscular theory. It exists too, as we now 
know, in all wave-transfer. From the investigation below (p. 338) it appears 
to exist when energy is transferred along a revolving twisted shaft. In heat- 
conduction in gases, the kinetic theory requires a carriage of momentum from 
hotter to colder parts ; so that there is some ground for supposing the pressure 
to exist in all cases. 

Though we have not yet a general and direct dynamical theorem accounting 
for radiation-pressure. Professor Larmor* has given us a simple and most 
excellent indirect mode of proving the existence of the pressure, which applies 
to all waves in which the average energy-density for a given amplitude is 
inversely as the square of the wave-length. Let us suppose that a train of 
waves is incident normally on a perfectly reflecting surface. Then, whether 
the reflecting surface is at rest, or is moving to or from the source, the perfect 
reflection requires that the disturbance at its surface shall be annulled by the 
superposition of the direct and reflected trains. The two trains must there- 
fore have equal amplitudes. Suppose now that the reflector is moving 
forwards towards the source. By Doppler's principle, the waves of the 
reflected train are shortened, and so contain more energy than those of the 
incident train. This extra energy can only be accounted for by supposing 
that there is a pressure against the reflector, that work has to be done in 
pushing it forward. When the velocity of the reflector is small, the pressure 

is easily found to be equal to E ll + jj), where ^ is the energy-density 

just outside the reflector in the incident train, U is the wave- velocity, and 

a the velocity of the reflector. If u = 0, the pressure is E ; but it is altered 

2u 
by the fraction -^ when the reflector is moving, and the alteration changes 

sign with ii. A similar train of reasoning gives us a pressure on the source, 
increased when the source is moving forward, decreased when it is receding. 

It is essential, I think, to Larmor's proof that we should be able to move 
the reflecting surface forward without disturbing the medium except by 

* Encyc. Brit. vol. 32, ' Radiation,' p. 121. 



RADIATION-PRESSURE 337 

reflecting the waves. In the case of light-waves it is easy to imagine such 
a reflector. We have to think of it as being, as it were, a semipermeable 
membrane, freely permeable to ether, but straining back and preventing 
the passage of the waves. In the case of sound-waves, or of transverse waves 
in an elastic solid, it is not so easy to picture a possible reflector. But for 
sound-waves I venture to suggest a reflector which shall freeze the air just in 
front of it, and so remove it, the frozen surface advancing with constant 
velocity u. Or perhaps we may imagine an absorbing surface which shall 
remove the air quietly by solution or chemical combination. In the case 
of an elastic solid, we may perhaps think of the solid as melted by the ad- 
vancing reflector, the products of melting being passed through pores in the 
surface and coming out to solidify at the back. 

Though Larmor's proof is quite convincing, it is, I think, more satisfying 
if we can realise the way in which the pressure is produced in the different 
types of wave-motion. 

In the case of electromagnetic waves. Maxwell's original mode of treatment 
is the simplest, though it is not, I believe, entirely satisfactory. According to 
his theory, tubes of electric and of magnetic force alike, produce a tension 
lengthways and an equal pressure sideways, equal respectively to the electric 
and magnetic energy-densities in the tubes. We regard a train of waves as 
a system of electric and magnetic tubes transverse to the direction of propaga- 
tion, each kind pressmg out sideways^that is, in the direction of propagation. 
They press against the source from which they issue, against each other as 
they travel, and against any surface upon which they fall. Or we may take 
Professor J. J. Thomson's point of view*. ' Let us suppose that the reflecting 
surface is metallic; then, when the light falls on the surface, the variation 
of the magnetic force induces currents in the metal, and these currents pro- 
duce opposite effects to the incident light, so that the inductive force is 
screened off from the interior of the metal plate : thus the currents in the plate, 
and therefore the intensity of the light, rapidly diminish as we recede from the 
surface of the plate. The currents in the plate are accompanied by magnetic 
force at right angles to them ; the corresponding mechanical force is at right 
angles both to the current and the magnetic force, and therefore parallel to 
the direction of propagation of the light.' In fact, we have in the surface of 
the reflector a thin* current-sheet in a transverse magnetic field, and the 
ordinary electrodynamic force on the conductor accounts for the pressure. 

In sound-waves there is at a reflecting surface a node— a point of no 
motion, but of varying pressure. If the variation of pressure from the 
undisturbed value were exactly proportional to the displacement of a parallel 
layer near the surface, and if the displacement were exactly harmonic, then 
the average pressure would be equal to the normal undisturbed value. But 

* Maxwell's Electricity and Magnetism, 3rd edition, vol. 2, p. 441, footnote. 
P. c.w. 22 



RADIATION-PHESSURE 

consider a layer of air quite close to the surface. If it moves up a distance y 
towards the surface, the pressure is increased. If it moves an equal distance 
y away from the surface, the pressure is decreased, but by a slightly smaller 
quantity. To illustrate this, take an extreme case, and for simplicity suppose 
that Boyle's law holds. If the layer advances half-way towards the reflecting 
surface, the pressure is doubled. If it moves an equal distance outwards from 
its original position, the pressure falls, but only by one-third of its original 
value ; and if we could suppose the layer to be moving harmonically, it is 
obvious that the mean of the increased and diminished pressures would be 
largely in excess of the normal value. Though we are not entitled to assume 
the existence of harmonic vibrations when we take into account the second 
order of small quantities, yet this illustration gives the right idea. The 
excess of pressure in the compression-half is greater than its defect during 
the extension-half, and the net result is an average excess of pressure — a 
quantity itself of the second order — on the reflecting surface. This excess in 
the compression-half of a wave-train is connected with the extra speed which 
exists in that half, and makes the crests of intense sound-waves gain on the 
troughs. 

Lord Rayleigh*, using Boyle's Law, has shown that the average excess on 
a surface reflecting sound-waves should be equal to the average density of 
the energy just outside ; and I think the same result can be obtained by his 
method if we use the adiabatic law. But the subject is full of pitfalls, and 
I am by no means sure that the result is to be obtained so easily as it appears 
to be. It is perhaps worth while to note one of these pitfalls, of which I have 
been a victim. It is quite easy to obtain the pressure against a reflecting 
surface by supposing that the motion just outside it is harmonic. But the 
result comes out to (y + 1) x energy-density, where y is the ratio of the specific 
heats. Lord Rayleigh kindly pulled me out of the pit into which I fell, pointing 
out that when we take into account second-order quantities the ordinary 
sound-equation does not hold. In fact we cannot take the disturbance as 
harmonic, and the simple mode of treatment is illusory. 

The pressure in transverse waves in an elastic solid is, I think, to be 
accounted for by the fact that when a square, ABCD, is sheared into the 
position aBCd (Fig. 1) through an angle e, the axes of the shear, aC and Bd, 

6 

no longer make 45° with the planes of shear AD, BC. Since ACa = -, the 

6 . . 

pressure-line aC is inclined at 45° — - to the direction of propagation, and 

the tension-line at 45° + ^ to that line. The result is a small pressure 

perpendicular to the planes of shear, that is, in the direction of propagation ; 
.and this small pressure is just equal to the energy-density of the waves. 
* Phil. Mag. vol. 3, 1902, p. 338, 'On the Pressure of Vibrations.' 



RADIATION-PRESSURE 



339 



For let PQR (Fig. 2) be a small triangular wedge of the solid, PQ being 
a plane of shear perpendicular to the direction of propagation. Let this 
wedge have unit thickness perpendicular to the plane of the figure. Let 
FR be along a pressure-line and QR along a tension-line, and let pressure 
and tension each be P. Resolve the forces on PR and QR perpendicular 
to PQ. Then we have a force from right to left, 

P . QR cos PQR -P .PR cos QPR 

= P.PQ |cos2 ^45° - - cos^ ^45° + ^\i = P . PQ .e. 




oz/T-ccr/OAA^ 



Of^ f/fOPAOATWA/ 




Fig. 1. 

Thus, to prevent motion in the direction of propagation there must be 
a pressure on PQ equal to Pe = ne^, where n is the rigidity- modulus. But 

the strain-energy per unit volume is -^ , and the kinetic energy is equal to 

it. The total energy-density is therefore ne^, and the pressure is equal to this. 
The pressure of elastic-solid waves appears to be beyond experimental 
verification at present. But that of sound-waves has been demonstrated 
most successfully by Altberg*, working in Lebedew's laboratory at Moscow. 

A small wooden cylinder, 21 mm. diameter, was suspended at one end of 
a torsion-arm, with its axis horizontal and transverse to the arm. One end 
of the cylinder occupied a circular hole in the middle of a board, there being 
just sufficient clearance to allow it to move, and the plane end was flush 
with the outer surface of the board. When very intense sound-waves 10 cm. 
in length, from a source 50 cm. distant, impinged on the board, the cyHnder 
was pushed back, the pressure sometimes rising to as much as 0-24 dyne/cm. 2. 
The intensity of the sound was measured independently by the vibrations 
of a telephone-plate, in a manner devised by M. Wien, and through a large 
range it was found that the pressure on the cylinder was proportional to the 
intensity indicated by the telephone- manometer. 

* Ann. der Physik, vol. 11, 1903, p. 405. 

22—2 



340 RADIATION-PRESSURE 

Just lately Professor Wood* has devised a strikingly simple experiment 
to illustrate sound-pressure. The sound-waves from strong induction-sparks 
are focussed by a concave mirror on a set of vanes like those of a radiometer, 
and when the focus is on the vanes as they face the waves the mill spins 
round. 

Theory and experiment, then, justify the conclusion that when a source 
is pouring out waves, it is pouring out with them forward- momentum as well 
as energy, the momentum being manifested in the reaction, the back-pressure 
against the source, and in the forward pressure when the waves reach an 
opposing surface. The wave-train may be regarded as a stream of momentum 
travelling through space. This view is most clearly brought home, perhaps, 
by considering a parallel train of waves which issues normally from a source 
for one second, travels for any length of time through space, and then falls 
normally on an absorbing surface for one second. During this last second, 
momentum is given up to the absorbing surface. During the first second, 
the same amount was given out by the source. If it is conserved in the 
meanwhile, we must regard it as travelling with the train. 

Since the pressure is the momentum given out or received per second, and 
the pressure is equal to the energy-density in the train, the momentum-density 
is equal to the energy-density H- wave-velocity. 

This idea of momentum in a wave-train enables us to see at once what is 
the nature of the action of a beam of light on a surface where it is reflected, 
absorbed, or refracted, without any further appeal to the theory of the wave- 
motion of which we suppose the light to consist f. 

It is convenient to consider the energy per linear centimetre in the beam, 
and the total pressure-force, equal to this linear energy-density, so as to avoid 
any necessity for taking into account the cross-section of the beam. 

Thus, in total reflection, let a beam AB (Fig. 3) be reflected along BC, and 
let AB = BC represent the momentum in each in length V equal to the 
velocity of light. 

Produce AB to D, making BD = AB. 

Then DC represents the change in the momentum per second due to the 
reflection — the force on the beam, if such language is permissible ; and CD is 
the reaction, the total light-force on the surface. 

If there is total absorption, let AB (Fig. 4) represent the momentum of 
the incident beam. Eesolve AB into AE parallel and EB normal to the 
surface. Then, since the momentum AB disappears as light-momentum, 
there must be a normal force EB on the surface and a tangential force AE 

* Phys. Zeitschrift, 1 Jan. 1905, p. 22. 

•j- A discussion, on the electromagnetic theory, of the forces exerted by light is given by 
Goldhammer, A7in. der Phys. vol. 4, 1901, p. 483. 



RADIATION-PRESSUKE 



341 



parallel to the surface. I have lately* described an experiment which shows 
the existence of the tangential force AE. 

If there is total refraction, let AB (Fig. 5) be refracted along BC with 
velocity V. HE is the energy in unit length of AB, and if E' is the energy 
in unit length of BC, the equahty of energy in the two beams is expressed by- 

VE = V'E'. 




Fig. 3. 




Fig. 4. 




But if M is the stream of momentum passing per second along AB, and 
if M' is that along BC, then 

M=E and M' = E'. 



Whence 
and 



VM = F'M' 



M' = y, M = fxM. 



Let AB = M, and BC along the refracted beam = M' = fjuM = fiAB. 
Draw CD parallel to BA, meeting the normal BN in D. Then 



CD = CB sin r/sin 



CB 



= AB=M. 



Phil. Mag. Jan. 1905, p. 169. [Collected Papers, Art. 21.] 



342 



RADIATION-PRESSURE 



Hence, by the refraction, momentum DC has been changed to momentum 
BC, or momentum BD has been imparted to the light. There is therefore 
a reaction BB on the surface. The force DB may be regarded as a pull-out 
or a pressure from within, and it is along the normal*. 

If the refraction is from a denser to a rarer medium, CB will now represent 
the incident stream and BA or CD the refracted stream. BD is the stream 
added to CB to change it to CD, and DB is the force on the surface, again 
a force outwards along the normal. 

In any real refraction with ordinary light, there will be reflection as well 
as refraction. The reflection always produces a normal pressure, and the 
refraction a normal pull. But with unpolarised light, a calculation shows 
that the refraction-pull, for glass at any rate, is always greater than the 
reflection-push, even at grazing incidence. 

The following table has been calculated from Fresnel's formula for 
unpolarised light by Dr. Barlow : 

P = total pull on surface. 

M = momentum per second in incident beam. 

R = reflection-coefflcient for angle i. 

/x = 1-5. 



i 


R 


PjM 





•0400 


•4000 


20 


•0402 


•4240 


40 


•0458 


•4925 


50 


•0572 


•5310 


60 


•0893 


•5720 


65 


•1205 


•5771 Maximum 


70 


•1710 


•5683 


75 


•2531 


•5329 


80 


•3878 


•4521 


89 


•9044 


•0738 


90 -f/^ 




2^,^de 


90 


i"ooo6 


•0000 



If a ray of light passes obliquely through a parallel plate, there is a normal 
pull outwards at incidence and a normal pull outwards at emergence : and 
if the refraction were total, this would result in a couple. But since some of 
the light returns into the first medium, it is easy to see that the net result is 
a normal repulsion and a couple. 

An experiment which I have lately made in conjunction with Dr. Barlow 
will serve as an illustration of the idea of a beam of light regarded as a stream 

* It has been pointed out by J. J, Thomson, Electricity and Matter, p. 67, 'that even when 
the incidence of the light is oblique, the momentum communicated to the substance is normal 
to the refracting surface ' The change of momentum of a beam of light is, it may be noted, 
the same on the wave and on the corpuscular theory. 



RADIATION-PRESSURE 343 

of momentum. A rectangular block of glass, 3 cm. x 1 cm. x 1 cm., was 
suspended by a quartz fibre so that the long axis of the block was horizontal. 
It hung in a case with glass windows, which was exhausted to about 15 mm. 
of mercury. A horizontal beam of light, from either a Nernst lamp or an 
arc, was directed on to one end of the block so that it entered centrally as AB 
in Fig. 6, and at an angle of incidence about 55°. After two internal reflections 
it emerged centrally as EF from the other end. Thus a stream of momentum 
AB was shifted parallel to itself into the line EF, or a counter-clockwise 
couple acted on the beam. The reaction was a clockwise couple on the block. 
Using mirror, telescope, and a millimetre-scale about 184 cm. distant, with 
the strongest light a very small deflection in the right direction could just 
be detected. But the quartz fibre was rather coarse, indeed needlessly 
strong; and as the time of vibration was only 39 seconds, the deflection was 
very minute. To render the effect more evident we used intermittent passage 
of the beam, sending it in during the half-period of vibration while B was 




F 

Fig. 6. Plan. 

moving from A, and shutting it off while B was moving towards A. The 
swings then always increased. When the beam was sent in during the 
approaching half and shut off during the receding half, the swings always 
decreased, and always rather more rapidly than they increased during the 
first half. For in the first case the natural damping acted against the light 
couple, and in the second with it. In one experiment the average increase 
was -55 scale-division and the average decrease -61 per period, and was fairly 
regular in each case. The mean was -58. The steady deflection is half this, 
or 0-29 division, giving a couple 11 x 10"^ cm. dyne. We made a measure- 
ment of the energy in the beam by means of the rate of rise of a blackened 
silver disc; but it was necessarily very inexact, as we had no means of 
securing constancy in the arc used in this experiment. This energy- measure- 
ment gave as the value of the couple 6 x 10"^, and the agreement is sufficient 
to show that the order of the result is right. 

An analysis of this experiment shows that the couple was really due to 
the pressures at the two internal reflections ; for, as we have seen, the forces 
at incidence at B and emergence at E are normal and produce no twist. 



344 RADIATION-PRESSURE 

Another experiment which we have made is, I think, more interesting, 
in that it brings into prominence the pull outwards or push from within 
occurring on refraction. Two glass prisms, each with refracting angle 34°, 
another angle being a right angle, and with refracting edge 1-6 cm. long, were 
arranged as in Fig. 7 (which shows the plan) at the ends of a thin brass torsion- 
arm suspended at its middle point from a quartz fibre in the same case as 
that used in the last experiment. The two inner faces were 3 cm. apart, 
and their width was 1-85 cm. A mirror gave the reflection of a millimetre- 
scale 171-4 cm. distant. The moment of inertia of the system was 48 gm. cm.^, 
and the time of vibration was 317 seconds. The air-pressure was reduced as 
before. When a beam of light from a Nernst lamp was sent through the 
system, as shown in the figure, it was shifted parallel to itself through a 
distance about 1-64 cm. The torsion-arm moved round clockwise by an 
easily measurable amount. In one experiment the deflection was 3-3 scale- 
divisions, indicating a couple 1-84 x 10~^ cm. dyne. The same beam directed 





Fig. 7. Plan. 

on to the blackened silver disc gave the linear energy-density as 9-8 x 10"^, 
which should have given a couple 1-6 x 10~^. Though the agreement is 
perhaps accidentally close, yet, as we could use a Nernst lamp, the measure- 
ments were much more trustworthy than in the last experiment*. 

The interesting point here is that the effect could only be produced by 
a force outwards at B and E. Whatever forces exist at C and D would be 
normal to the surfaces and would give no twist. 

A very short experience in attempting to measure these light-forces is 
sufficient to make one realise their extreme minuteness — a minuteness which 
appears to put them beyond consideration in terrestrial affairs, though I have 
tried to showf that they may just come into comparison with radiometer- 
action on very small dust-particles. 

In the Solar system, however, where they have freer play and vast times 
to work in, their effects may mount up into importance. Yet not on the 
larger bodies ; for on the earth, assumed to be absorbing, the whole force of 

* [See The Pressure of Light, p. 61 (S.P.C.K. 1910), where these values are slightly corrected, 
and results are given for a similar experiment with smaller prisms. Ed.] 
t Nature, Dec. 29, 1904, p. 200. ^Collected Papers, Art. 65.] 



RADIATION-PRESSURE 345 

the light of the sun is only about a 50 million-millionth of his gravitation- 
pull. But since the ratio of radiation-pressure to gravitation-pull increases 
in the same proportion as the radius diminishes if the density is constant, 
the pressure will balance the pull on a spherical absorbing particle of the 
density of the earth if its radius is a 50 billionth that of the earth — a little 
over a hundred-thousandth of a centimetre, say, if its diameter is a hundred- 
thousandth of an inch. 

We may illustrate the possible effects of radiation-pressure without 
proceeding to such fineness as this. Let us imagine a particle of the density 
of the earth, and a thousandth of an inch in diameter, going round the sun 
at the earth's distance. There are two effects due to the sun's radiation. 
In the first place, the radiation-push is j^^y of the gravitation-pull ; and the 
result is the same as if the sun's mass were only 99/100 of the value which it 
has for larger bodies like the earth. Hence the year for such a particle would 
be longer by ^^-^, or about 367 instead of 365 J days. In the second place, 
the radiation absorbed from the sun and given out again on all sides is crushed 
up in front as the particle moves forward and is opened out behind. There 
is thus a slightly greater pressure due to its own radiation on the advancing 
hemisphere than on the receding one, and this appears as a small resisting 
force in the direction of motion. Through this the particle tends to move 
in a decreasing orbit spiralling in towards the sun, and at first at the rate of 
about 800 miles per annum. 

Further, if there be any variation in the sun's rate of emitting energy, 
there will be a corresponding variation in the increase of the year and the 
decrease of the solar distance, and the particle, if we could only observe it^ 
would form a perfect actinometer. 

Though, unfortunately, we cannot observe the motion of independent 
small particles circling round the sun at the distance of the earth, there is 
good reason to suppose that some comets at least are mere clouds of dust. 
If we are right in this supposition, they should show some of these effects. 
Encke's comet at once suggests itself as of this class ; for, as everyone knows, 
it shortens its journey of 3 J years round the sun on every successive return, 
and on the average by about 2i hours each revolution. Mr. H. C. Plummer* 
has lately been investigating this comet's motion; and he finds that if it 
were composed of dust- particles, each of the earth's density and about J^j mm. 
or rather less than a thousandth of an inch in diameter, the resisting force 
due to radiation-pressure would account for its accelerating return. But the 
sun's effective mass would be reduced by about 1/80; and on certain sup- 
positions he finds that the assumed mean distance as calculated from Kepler's 
law, without reference to radiation, is greater than the true mean distance 

* Monthly Notices B.A.S. Jan. 1905, 'On the Possible Effects of Radiation on the Motion 
of Comets, with special reference to Encke's Comet.' 



346 RADIATION-PRESSURE 

by something of the order of 1 in 400, and he thinks such a large error is hardly 
possible. So that radiation-pressure has not yet succeeded in fully explaining 
the eccentricities of this comet. But comets are vague creatures. As 
Mr. Plummer suggests, we hardly know that we are looking at the same matter 
in the comet at its successive returns ; and I still have some hope that the 
want of success is due to the uncertainty of the data. 

There is one more effect of this radiation-pressure which is worthy of 
note : its sorting action on dust-particles. If the particles in a dust-cloud 
circling round the sun are of different sizes or densities, the radiation-accelera- 
tions on them will differ. The larger particles will be less affected than the 
smaller, will travel faster round a given orbit, and will draw more slowly in 
towards the sun. Thus a comet of particles of mixed sizes will gradually 
be degraded from a compact cloud into a diffused trail lengthening and 
broadening, the finer dust on the inner and the coarser on the outer edge. 

Let us imagine, as an illustration of this sorting action, that a planet, 
while still radiating much energy on its own account, while still in fact a small 
sun, has somehow captured and attached to itself as satellite a cometary cloud 
of dust. Then, if the cloud consists of particles of different sizes, while all 
will tend to draw in to the primary, the larger particles will draw in more 
slowly. But if the larger particles are of different sizes among themselves, 
they will have different periods of revolution, and will gradually form a ring 
all round the planet on the outside. Meanwhile the finer particles will drift 
in, and again difference in size will correspond to difference in period and 
they too will spread all round, forming an inner fringe to the ring. If there 
are several grades of dust with gaps in the scale of size, the different grades 
will form different rings in course of time. Is it possible that here we have 
the origin of the rings of Saturn ? 

The Radiation Theory is only just starting on its journey. Its feet are 
not yet clogged by any certain data, and all directions are yet open to it. 
Any suggestion for its future course appears to be permissible, and it is only 
by trial that we shall find what ways are barred. At least we may be sure 
that it deals with real effects and that it must be taken into account. 

[Compare Rayleigh, 'On the Momentum and Pressure of Gaseous 
Vibrations,' Phil. Mag. vol. 10, 1905, p. 364. Ed.] 



23. 

ON PKOF. LOWELL'S METHOD FOR EVALUATING THE SURFACE- 
TEMPERATURES OF THE PLANETS ; WITH AN ATTEMPT 
TO REPRESENT THE EFFECT OF DAY AND NIGHT ON THE 
TEMPERATURE OF THE EARTH*. 

[Phil. Mag. 14, 1907, pp. 749-760.] 

Prof. Lowell's paper in the July number of the Philosophical Magazine 
marks an important advance in the evaluation of planetary temperatures, 
inasmuch as he takes into account the effect of planetary atmospheres in 
a much more detailed way than any previous writer. But he pays hardly 
any attention to the 'blanketing effect,' or, as I prefer to call it, the 'green- 
house-effect' of the atmosphere. He assumes in fact that the fourth power 
of the temperature is proportional to the fraction of solar radiation reaching 
the surface, and he neglects both the surface-radiation reflected down again 
and the radiation downwards of the energy absorbed by the atmosphere. 

This is brought out clearly in the footnote on p. 172, where he uses a 
formula of Arrhenius, to which I am unable to refer, but wliich I think he 
must misinterpret in making it give his result. The inadequacy of his method 
is well shown by its application to the cloud-covered half of the earth's surface. 
He finds that this half only receives 0-2 of the radiation which the clear sky 
half receives. The surface-temperature under cloud should therefore be only 
V 0-2 = 0-67 of that under clear sky. If the latter is 300° A. the former is 
only about 200° A. Common observation contradicts this flatly, for the 
difference is at most but a few degrees. 

On another point common observation appears, at any rate at first sight, 
to contradict Professor Lowell. He assumes that the loss in the radiation of 
the visible spectrum in its passage through the atmosphere is practically all 
due to reflection, and he puts it down as about 0-7 of the whole in clear 
sky. If this were true the reflection from the sky opposite to the sun would 
I think be vastly greater than it is. White cardboard reflects diffusely 
about 0-7 of sunlight. But when a piece of white cardboard is exposed 
normally to the sun's rays it is several times brighter than the cloudless sky. 

* In Phil. Trans. A, vol. 202, p. 525 f I attempted an evaluation, in which the atmosphere 
was taken into account as keeping the temperature at a given point practically the same day 
and night. I did not then know that Christiansen {Beibldtter zu den Ann. der Physik und Chemie, 
vol, 10, 1886, p. 532) had nearly twenty years earlier applied the fourth-power law to calculate 
planetary temperatures. His work deserves recognition as the first in which this law was applied. 

t [Collected Papers, Art. 20.] 



348 

The * greenhouse- effect' of the atmosphere may perhaps be understood 
more easily if we first consider the case of a greenhouse with horizontal roof 
of extent so large compared with its height above the ground that the effect 
of the edges may be neglected. Let us suppose that it is exposed to a vertical 
sun, and that the ground under the glass is ' black ' or a full absorber. We 
shall neglect the conduction and convection by the air in the greenhouse. 

Let S be the stream of solar radiation incident per sq. cm. per sec. on the 
glass. Of this let rS be reflected, aS be absorbed, and tS be transmitted by 
the glass. Then r + a+ t = 1. Let the ground send out radiation R per sq. 
cm. per sec. and of this let r^R be reflected, a^R be absorbed, and t^R be 
transmitted by the glass. Here also rj + a^ + ^^ = L It is to be noted that 
since the edges are far distant R is incident on each sq. cm. of glass. The 
glass, then, absorbs aS + a^R, and as it is thin it may be taken as having 
the same temperature on each side, so that it sends down to the ground 
J {aS + a-^R), the other half going upwards into space. Equating receipt 
and expenditure of radiation by the ground, 

R=.tS+riR+ i {aS + a^^R), 

whence on putting 7\= 1 — n^ — t^ we obtain 

a 

i2 = -S. 

The values of t and a depend upon the glass. By way of illustration 
let us take t = 0-6, a = 0-3. For radiation from a surface under 100° C. 
Melloni found that even thin glass is quite opaque. We have then t-^ = 0, 
and if we neglect reflection, probably small, Oi == L 

Then R = ^S=l'bS. 

If the glass were removed we should have 

R^S. 

The temperature of the ground is therefore \/'i-D = 1-1 times as high 
under the glass as it is in the open. If, for instance, it is 27° C. or 300° A. 
in the open, it is 330° A. or 57° C. under the glass. 

If the glass reflects some of the radiation R then a^ is less and the ground 
temperature is still higher. 

If the ground, instead of being black, reflects a fraction p of the incident 
sunlight, or has total albedo p, the formula must be modified. If we take 
into account merely the first reflection from the ground and assume that the 
glass has absorption a for it, then we easily find 



SURFACE-TEMPERATURES OF THE PLANETS 349 

t + 



n^sJl^ 



If we take p = 0-1 the numerator is 0-78 instead of 0-75, and if we assume 
the fourth-power law for the low-temperature radiation emitted by the 
surface, the temperature is about 1 per cent, higher*. But the ground will 
probably reflect a much smaller fraction of the whole spectrum, and the 
correction for total albedo becomes inconsiderable. 

If we replace the sun by cloud the radiation is, on the average, of much 
lower temperature, and t and a are much nearer to t^ and a^. The value of 
R/S is then much nearer to 1, and the covered ground has a temperature 
much ]ess raised above that of the open ground. This agrees of course with 
common experience. 

A planetary atmosphere no doubt acts in some such way as the green- 
house glass. Let us, for the sake of comparison with Prof. Lowell's results, 
assume, as he has done, that we have a steady state, with the incident radiation 
normal to the surface. I do not see how to estimate the distribution of the 
radiation from the air between the upward stream into space and the down- 
ward stream to the surface. Since the lower layers of air are warmer than the 
upper probably more than half comes down, and the truth probably lies 
between the assumptions that the atmospheric radiation is J {aS + a^R) as 
it is with the greenhouse, and that it is aS + a^R when all the radiation would 

be downwards. Let us suppose that - (aS + a^R) comes downward. 

The albedos of the surfaces of both the Earth and Mars average, according 
to Lowell, 0-1 for visible radiation. They must be much less for the whole 
spectrum. Where all the data are uncertain the effect of small albedo may 
be neglected, and indeed in our ignorance of the dependence of temperature on 
radiation in the case of a partially reflecting surface, it is safer to neglect it. If 
dg is the actual surface-temperature under a vertical sun, and 6 is the tempera- 
ture which the surface would have without atmosphere, it is easily found that 

t-\- a/n 
^1 ^- {n— l)a-^/n 

Earth. If we use Lowell's figures for the Earth under a clear sky, 

t = 0-42, a = 0-5 X 0-65 = 0-325, 

t^ = 0-5, since of the invisible radiation half is transmitted, 

a^ = 0-5, very httle is reflected. 

* [It would appear that in the preceding equation for R the term ( 9 ~ P ) ^ should he(^-l]pt. 
This would give the numerator as 0-70, and the temperature about 2 per cent, lower. Ed.] 



350 PROF. Lowell's method for evaluating the 

We shall suppose in succession that 

(a) half of the radiation is downwards or that n = 2, 

(b) two-thirds ^ = f , 

(c) all n=l. 

We then find 

{a) 1 = 0-94; (6) J=0-99; (c) f=M2. 

For the case of a cloud-covered earth the data are very uncertain. Lowell 
takes t = 0-2 of 0-42 = 0-084, assuming that the atmosphere has already 
reflected and absorbed 0-58 before the cloud is reached, surely an overestimate, 
since the cloud-surface is in the higher air. Let us guess that ^ = 0-L The 
absorption without cloud is according to Lowell about 0-3. With cloud 
much is reflected back without reaching the lower and more absorbing regions. 
Let us guess that a = 0-2. Of the radiation from the surface we may suppose 
perhaps that 0-2 passes through, that 0-7 is reflected, and that 0-1 is absorbed. 
Of the 0-2 passing we may suppose that 0-1 is absorbed and 0-1 goes into 
space. Then ^^^ = 0-1 and a^ = 0-2. 

With these values we get for the different values of n 

. («) J=l; (b) J =1-08; (c) 1=1-31. 

These guesses, then, make the temperature under a cloudy sky at least 
as great as under a clear sky. But this is certainly not true in common 
experience, where, however, we may have clouds accompanied by cold winds 
and no approach to the steady state here assumed. The results merely serve 
to show that with certain absorptions and transmissions clouds might actually 
raise the surface-temperature, and that for the present it is better to neglect 
them. 

Mars. If we apply Lowell's data for Mars we have 

t = 0-64, and a = 0-40 x 0-65 = 0-26, 
ti = 0-6, and a^ = 0-4, since R is dark radiation. 

With these values we get for the different values of n 

(a) 1 = 0-99; (b) |=]-02; (e) |=M0. 

Comparison of the Earth and Mars. Let us take the temperature of the 
Earth as 17° C. or 290° A. If it were removed to the distance of Mars 
its temperature would be inversely as the square-root of the distance, which 
is 1-524 that of the Earth, or 290/1-235 = 235° A. 



StlRF ACE-TEMPERATURES OF THE PLANETS 351 

With the different values of n the temperature of Mars should be 

(a) 235 X If =247° A. or -26°C., 

(b) 235 X -^2. _ 242° A. or -31°C., 

(c) 235 X j|§ = 231° A. or - 42° C. 

Of course the data are very uncertain and the formula used is only an 
approximation. But with these data it is hard to see how the temperature 
of Mars can be raised to anything like the value obtained by Professor Lowell. 
Perhaps the data are quite wrong. It is conceivable that Mars has a quite 
peculiar atmosphere practically opaque to radiations from the cold surface. 
Those who believe that there is good evidence for the existence of intelligent 
beings on that planet, should find no difficulty in supposing that they have 
been sufficiently intelligent to cover the planet with a glass roof or its equiva- 
lent. Then we might easily have ^+- = 0-77 and ^^+^^ = 0-5, and then 

the temperature might be raised to 281° A. or 8° C. Indeed, if the glass were 
of such kind as to transmit solar radiation, and if it were quite opaque to 
dark radiation while still reflecting a considerable proportion, the temperature 
might easily be raised far above this. 



An Attempt to represent the Effect of Day and Night 
on the Temperature of the Earth. 

The 'greenhouse' formula, which has been used in the foregoing dis- 
cussion, would hold only if all the conditions were steady. But in reality 
the alternations of day and night prevent a steady state, and we can only 
hope that the neglect of these alternations does not greatly affect the ratios 
of the temperatures found for different planets or for different elevations on 
the same planet. 

I shall now attempt to represent the effect of the diurnal variation in the 
supply of solar heat to the Earth, or rather to an abstract Earth. For even 
if we could represent the actual conditions we should obtain differential 
equations so complicated that they would be useless for practical purposes. 

To simplify matters, let us suppose that we are dealing with the equatorial 
region of the earth at the equinox, that the air is still, that the surface is solid 
and black, and that the sky is clear. 

The temperature of the air except near the surface can change but little 
during 24 hours. For over each square centimetre at sea-level we have 
1000 gms. of air with specific heat 0-2375, and therefore with heat capacity 
237-5. Consider a band of the atmosphere 1 cm. wide round the equator. 
A stream of solar radiation of length equal to the diameter 2r of the earth 



352 PROF. Lowell's method for evaluating the 

enters a band of air of length equal to half the circumference. If the solar 
constant is 3 the average energy entering a sq. cm. column is 

2rS 2S 6 , . . 
= — = - cal./mm. 

Then in 12 hours 1375 cal. enter on the average, and if this heat were all 
absorbed and retained it would raise the temperature on the average about 
1375/237-5 = 5°-8 C. 

As the absorption is only partial and as radiation takes place from the 
air, the rise cannot really average nearly as much as this. 

Again, consider the radiation during the twelve hours of night. If the 
air ^vere a black body and of temperature 300° A., and these are absurdly 
exaggerated estimates of its radiating power and of its average temperature, 
it would only radiate about 1-2 cal./min. per sq. cm. column from its two 
surfaces, or 864 calories in the twelve hours, and neglecting the radiation from 
the ground the temperature would only fall about 864/237-5 or 3°-6 C. 
Obviously, then, the air as a whole cannot undergo much variation in tempera- 
ture as day alternates with night. It is indeed a flywheel storing the energy 
of many diurnal revolutions. We may, then, in a rough estimate consider 
that its temperature, and therefore its radiation, remains constant during the 
24 hours. 

If the total radiation from a sq. cm. column per second is A, there will 
be a stream D downwards and U upwards where Z) + U = A. We can find 
an expression for A by equating it to the average absorption. Considering 
an equatorial band 1 cm. wide, the average energy entering it per sq. cm. in 

the 24 hours is - . Let the average amount absorbed be - . The value of 

77 77 

a at sea-level varies for clear sky from perhaps 0-3 with the zenith sun to very 
nearly 1 with the setting sun. Let the average radiation from the surface 
during the 24 hours be /?, of which a^ R is absorbed by the atmosphere. Then 
neglecting conduction through the air, the constant-temperature assumption 
gives us 

A=^ V a^R. 

77 

If a fraction is radiated downwards 

n _ 

j^ dS a^R 

rnr n 

The actual surface-temperature depends not only on radiation but also 
on conduction both by ground and air. But we shall neglect this conduction 
and shall suppose that the surface has reached an equilibrium between receipt 
and expenditure of radiation. This is a condition to which the surface tends 
at or soon after noon by day, and before dawn at night. We shall suppose 



SURFACE-TEMPERATURES OF THE PLANETS 353 

that the low-temperature radiation from the surface is either transmitted 
or absorbed, so that, using the previous notation, 

^j + ^1 = 1 and r^ = 0. 

If R^ is the equilibrium surface-radiation reached, we suppose about noon, 

yiTT n 

If Rn is the equilibrium surface-radiation in the later part of the night, we 
have to omit tS, and _ 

fiTT n 

To proceed further, we must express R in terms of S. We can only do 
this by some assumption. Probably it is not very far from the truth to 
assume that R = i (Ra + Rn), and we shall take this value. It gives us 

t a 

n 
and substituting in the values of day and night radiations we get 











t 


a 


s 




a 


+ <Li 


2 + 


mr 


= t + 










rnr n 


1- 


"^ 










n 








t 


d 




i-> 






ai2'^ 






Rn 


a 


+ 


mr 




s 


rnr 


^1- 


~' 










n 





Though these formulae are only obtained by making large assumptions, 
and by neglecting important considerations, they nevertheless show the 
tendency of the day and night effect, and it is worth while to apply them to 
the Earth, taking the best data at our command. 

At the surface let us take t = 0-4:2 and aj = 0-5 as before. For d we have 
no trustworthy observations, and I doubt whether a calculation from Langley's 
observations is of any more value than an estimate. Since a varies from 
perhaps about 0-3 to 1, let us take a = 0-628 or 27r/10, a value simpHf5H[ng 
arithmetic. 

At the level of Camp Whitney 3550 metres above sea-level, with barometer 
about 500 mm., and therefore with about | of the atmosphere below it, we 
may take ^ = 0-6 and a^ = 0-4. For a we must take a value much smaller 
than that at sea-level. Since the most absorbing third of the atmosphere is 
below, I do not think it is far wrong to take d as having half the value at the 

p. c. w. 23 



354 PROF. Lowell's method for evaluating the 

lower level, and I therefore put a = 0-314. But I have also examined the 
consequences of putting it equal to 0-419, i.e. f of its value at the lower level, 
and the results are given below to show how much the figures are affected by 
the variation in the value taken. 

We have no data for n. I have therefore calculated the values of R^ and 
Rn in terms of S for successive values of n equal to 1, |, |, f , 2 ; corresponding 
to D equal to A, ^A, ^A, %A, and \A respectively. 

In the following tables the values of RajS and RJS are given, and also 
the mean RjS = J (R^ + Rn) S- Then follow the ratios of the day and night 
temperatures, 6^ and 6^, to the temperature ^ of a black surface radiating S, 
and then the mean value djd. The last column gives the range 6^ — 9^ on 
the supposition that = 300° A. 

The third table is only given to show that the change in the value of a 
does not greatly affect the results. The value of a of Table II is much more 
reasonable if that of Table I is near the truth. We need, therefore, only 
compare the results given in the first two tables. 

If we take the same values of n in each table, the value of 7^ is less at the 
higher level than at the lower in every case except that in which n has the 
extreme and probably inadmissible value of 2. The value of 6 is less at the 
higher level in every case. But it appears most probable that 1/n or DjA 
is greater at the lower level than at the higher. For consider a thin layer 
of air at sea-level. It is radiating equally up and down, but of the half 
going upwards a considerable fraction will be intercepted by the superin- 
cumbent and strongly absorbing layers. Now consider a thin layer close 
to the surface at the higher level. It, too, radiates half up and half down. 
But of the half going upwards a less fraction will be intercepted since the 
superincumbent layers are now less absorbing. Thus DjA will be greater 
at the lower than at the higher level*. We should, therefore, compare the 
results for any value of DjA in Table I with the results in Table II for a some- 
what lower value. 

We may exclude the extreme cases of 9^ = 2 and n= 1, as the true value 
is certainly between these, and confine our examination to intermediate values. 

Suppose, for example, that DjA = 4/5 at the lower level, while it is 3/4 
at the upper level. Then djd = 0-88 from Table I at the lower level, while 
O/e = 0-83 from Table II at the upper level. Or if D/A = 3/4 at the lower 
level, while it is 2/3 at the upper level, O/O = 0-86 below, while ejO - 0-81 
above. Or in each case the mean temperature is higher at sea-level by about 
5 in 87 or by about 17° in 300°. 

* Another consideration leading to the same conclusion is that the atmosphere acts like 
a plate with its lower surface much warmer than its upper. When we only have the part above 
an elevated region the difiference of temperature between the surfaces is much less than for the 
whole air, and the radiations up and down are more nearly equal. 



SURFACE-TEMPERATURES OF THE PLANETS 



356 



Table I. 

At sea-level, t = 0-42, a-^ = 0-5, a = 0-6^8. 



















Range 


n 


DIA 


^d/S 


iiJS 


R/S 


eje 


^nl^ 


ejd 


about 
300° A. 


1 


1 


103 


0-61 


0-83 


1-01 


0-88 


0-95 


41° 


5/4 


4/5 


0-83 


0-41 


0-62 


0-95 


0-80 


0-88 


51° 


4/3 


3/4 


0-79 


0-37 


0-58 


0-94 


0-78 


0-86 


56° 


3/2 


2/3 


0-72 


0-30 


0-51 


0-92 


0-74 


0-83 


65° 


2 


1/2 


0-62 


0-20 


0-41 


0-89 


0-67 


0-78 


85° 



Table II. 

At 3550 m. above sea-level. Barometer 500 mm. 
t = 0-6, ai = 04, a = 0-314. 



















Range 


n 


D/A 


Rd/S 


njs 


R/S 


Sdie 


Kid 


dfd 


about 
300° A. 


1 


I 


0-97 


0-37 


0-67 


0-99 


0-78 


0-89 


71° 


5/4 


4:15 


0-86 


0-26 


0-56 


0-96 


0-71 


0-84 


89° 


4/3 


3/4 


0-84 


0-24 


0-54 


0-96 


0-70 


0-83 


94° 


3/2 


2/3 


0-80 


0-20 


0-50 


0-95 


0-67 


0-81 


100° 


2 


1/2 


0-74 


0-14 


0-44 


0-93 


0-61 


0-77 


125° 



Table III. 

At 3550 m. above sea-level and with. ^ = 0-6, ai = 0-4, 
but with a = 0419 = 2/3 of 0-628. 



















Range 


n 


D/A 


RdlS 


Rjs 


R/S 


dale 


eje 


did 


about 
300° A. 


1 


1 


102 


0-42 


0-72 


101 


0-81 


0-91 


66° 


5/4 


4/5 


0-90 


0-30 


0-60 


0-97 


0-74 


0-86 


80° 


4/3 


3/4 


0-87 


0-27 


0-57 


0-97 


0-72 


0-85 


88° 


3/2 


2/3 


0-83 


0-23 


0-53 


0-95 


0-69 


0-82 


95° 


2 


1/2 


0-76 


0-16 


0-46 


0-93 


0-63 


0-75 


120° 



23—2 



356 METHOD FOR EVALUATING SURFACE-TEMPERATUBES OF THE PLANETS 

It is to be observed that the lower mean temperature at a higher level 
must hold good if the higher level is so much higher that there is practically 
no atmosphere above. For then t = 1 and a^ = 0, so that R^^ S and i?„ = 0. 
Therefore djd = 1 and 6 JO = and 0/6 = 1/2. 

The lower mean temperature of elevated parts of the earth's surface is 
a well-established fact. Perhaps if it were only observed in the case of 
mountain peaks it might be ascribed to the cold air blowing against them. 
The fall of temperature in free air as we go upwards tends towards that 
given by convective equilibrium, though recent observations show that it 
is not so great as that given by the adiabatic law. Thus for a rise of 3500 
metres the adiabatic law would give a fall of about 32° C. if the sea-level 
temperature were 300° A. ; whereas the observations of Teisserenc de Bort 
at Trappes show a mean annual fall of about 16° C. for this rise (Encyc. Brit. 
vol. 30, Meteorology, p. 695). A continual blast of air thus cooled might of 
course reduce the temperature on the mountain peaks, even if radiation did 
not tend to any such reduction. But we can hardly account in this way for 
the equally well-established lower temperature of elevated continental plateaus. 
According to Abbe {loc. cit. p. 694) 0°-5 C. must be subtracted from sea-level 
temperature for every 100 metres general elevation of the land-surface or 
about 18° for an elevation of 3500 metres, and this fall may be ascribed to 
radiation in some such way as that here set forth. 

If the atmosphere of Mars is comparable with our own atmosphere at 
high levels, and if the effect is of the same general character in the two cases, 
it appears probable that the surface-temperature of Mars is actually lower 
by many degrees than that which the surface of the Earth would have at 
the same distance from the Sun. 



24. 

THE MOMENTUM OF A BEAM OF LIGHT. 

[Atti del IV Congresso internazionale dei Matematici 
(Rome), 3, 1909, pp. 169-174.] 

[The substance of this paper is contained in the Address to the French 
Physical Society, March 1910. Collected Papers, Art. 70. Ed.] 



25. 

ON PKESSURE PERPENDICULAR TO THE SHEAR-PLANES IN 
FINITE PURE SHEARS, AND ON THE LENGTHENING OF 
LOADED WIRES WHEN TWISTED. 

[Roy. Soc, Proc. A, 82, 1909, pp. 546-559.] 

[Read June 24, 1909.] 

In the Philosophical Magazine, vol. 9, 1905, p. 397*, I gave an analysis of 
the stresses in a pure shear which appeared to show that if e is the angle of 
shear and if n is the rigidity, then a pressure ne^ exists perpendicular to the 
planes of shear. That analysis is, I believe, faulty in that the diagonals 
of the rhombus into which a square is sheared are not the lines of greatest 
elongation and contraction, and are not at right angles after the shear, when 
second-order quantities are taken into account, i.e., quantities of the order 
of 6^ ; I think the following analysis is more correct, and though it does not 
give a definite result, it leaves the existence of a longitudinal pressure an 
open question. The question appears to be answered in the affirmative by 
some experiments, described in the second part of the paper, in which loaded 
wires when twisted were found to lengthen by a small amount proportional 
to the square of the twist. 

I. Stresses in a Pure Shear. 

Let a square ABCD (Fig. 1) of side a be sheared into EFCD by motion 
through AE = d, the volume being constant. The angle of shear is ADE = e, 
and tan e = dja exactly ; neglecting e^, we may put e = dja. 

To find which line is stretched most by the shear, consider the Kne r 
drawn from B to P and making an angle 6 with DC before stretching. 

Let it stretch to p, making an angle 6' with DC ; we have r = a /sin 6 and 
p = a/sin 6' ; 

also p2 ^ ^2 _|_ 2rd cos 6 -{- d^; 

thus /)2/r2 = 1 + 2dlr . cos ^ -h d^/r^ 

= 14- 2dla . sin ^ cos ^ -1- d^/a^ . sin^ 6. 

* [Collected Papers, Art. 22, p. 338.] 



ON PRESSURE PERPENDICULAR, TO THE SHEAR-PLANES 



359 



Differentiating p^jr^ with respect to 6, it is a maximum when 
Mja . cos 2d + d^la^ . sin 26 = 0, 
or tan 29 = — 2a/d = — 2 cot e. 

Put 6 = 45° + 8, then tan 28 = J tan e, 

or 8 = Je to the second order, so that r makes an angle J e with the diagonal 
DB of the square through D, and on the upper side. 

If the same shear is now made in the opposite direction p contracts to r, 
and the same directions of p before, and r after shear, give the maximum 
contraction. It is almost obvious that p makes an angle Je with DB on the 
lower side, but it may be verified by putting 

r^ = p^- 2pd cos d' + d^, 
and finding the maximum value of r^/p^ after putting p = a/ sin 6' on the 
right. 





J) C A J2a. ^ 

Fig. 1. 

Hence the lines of maximum elongation and contraction are at Je with 
the diagonals of the square, and are at right angles before and after the strain, 
to the order of e^. It is noteworthy that as the shear increases the fibres 
which undergo maximum elongation and contraction change. 

To find r and p put 

r = a/sin 6 = a/sin (45° + \e), 

then r - y/2a (1 - Jc + A^') ; 

and changing the sign of e we get 

p = V2a (1 + ie + i-^e^). 
It is easily seen that the elongation and contraction are respectively 
e=(p- r)lr = ^c (1 + Jc) ; c = {r - p)/p = Jc (1 - Je). 
We shall now consider the stresses. We shall assume that a pressure P 
is put on in the direction of maximum contraction and a tension Q in the 
direction of maximum elongation, these being, as we have seen, at right 
angles ; and we shall consider the equilibrium of the wedge ABC (Fig. 2) when 



360 ON PRESSURE PERPENDICULAR TO THE SHEAR-PLANES 

sheared, assuming that P and Q are the only forces on AC and BC, Let 
AB = 2a ; AC = p\ BC = r; these having the values just found. 

Kesolve in a direction perpendicular to the base, and let R be the pressure 
against the base, then 

R.2a = Pp cos (45° - Je) - Qr cos (45° + Je) 
= Pa cot (45° -i€)-Qa cot (45° + Je) 

= Pa (1 + Je + le2) - Qa (1 - Jc + Je^) 

= (P - g) a + (P + Q) . ia€ + (P - g) . lac^, 

where P and Q can only be taken as equal to the first order. Proceeding to 
the second order, we must put 

P ^ 7l€ + fe^ ; 
then Q = ne - pe^, 

where p is a constant to the second order. 

Thus P-Q = 2pe^ and P + Q = 2ne, 

and P = (Jn + p) e^, 

the third term being negligible. If we resolve parallel to the base, it is easily 
found that the tangential stress is 

T = 1 (P + Q) = nc. 

If the shear is produced by a tangential stress T, then it requires the 
system P, Q, and R to maintain equilibrium with it. 

It is possible that a stress exists perpendicular to the plane of the figure 
in Fig. 1. It can only be assumed that the changes of dimension in that 
direction neutralise each other to the first order when equal pushes and 
pulls are put on in the plane of the figure ; when the dimensions perpendicular 
to the figure are constrained to remain the same to the second order — and 
this is our supposition — it may require a tension or pressure to effect this. 

Let us suppose that a pressure S = qe^ is introduced, a tension if q is 
negative. To make P = we should require to have f = — ^w, also P would 
then be less than Q. If pressure perpendicular to AC is exerted alone, and 
then tension perpendicular to BC is exerted alone, it appears probable that 
for very large equal compressions and extensions P is greater than Q. If we 
suppose that when they are simultaneous the tendency is in the same direction, 
then R should have a positive value, or the longitudinal pressure perpendicular 
to AB should exist. 

Let us examine the consequences of the supposition that both R and S 
exist. Let a thin tube of length I and of radius a be fixed at one end, and let 



IN FINITE PURE SHEARS, ETC. 361 

the other end be twisted through an angle 6 so that the angle of shear is 
€ = ad/l. Let an end-pressure R = (^n + p) e^ be put on, and also a side- 
pressure S = qe^ so as to maintain constant dimensions. 

The side-pressure S may be replaced by a uniform pressure S over the 
whole surface, and a tension S over the ends. We have then an end-pressure 
R — S and a pressure S all over. 

Now suppose that these forces are removed. Through the removal of 
R — S we shall have a lengthening dl^ given by 

(in + p- q) £2 = Ydljh 
and a contraction S^ of the diameter given by 

Sj2a = adljl, 
where Y is Young's modulus and o is Poisson's ratio. 

Through the removal of the pressure S we shall have a lengthening dlz 
given by qe^ = ^Kdljl, where K is the bulk-modulus, and an expansion §2 
of the diameter given by qe^ = ZKh.J2a. 

The end-lengthening is therefore 

dl = dl^ + dl^ = {{in + p-q)IY+ q/SK} k^ ; 
or putting 1/3Z = 3/7 - 1/n, 

dl = {(Jn + p)IY+ (2/F - l/n) q} k^ = sU^ = sa^d% 
where s is put for (Jn + p)/7 + {2/Y — l/n) 5'. 

The diameter decreases by 

S = Sj _ §2 = {{in +p-q)alY- q/SK} 2a€^ 
= {{in + p)cjIY- [(3 + c7)/r - l/n] q} 2a^6^ll\ 

It would not be easy to test this result with a thin tube. But if we suppose 
that a wire extends by the amount equal to the average extension of the tubes 
into which it may be resolved, we get 



dl=—J ^"""^dr = isa^e% 



I now proceed to describe some experiments which show that such an 
extension exists. 



II. The Lengthening of Loaded Wires when twisted. 

Experiments were made on several wires hung vertically from a fixed 
support, and loaded in order that kinks or remnants of the spiral due to the 
coihng to which they had been subjected might be taken out. This was 
considered to be effected when the stretch for a given addition of load was 



362 



ON PRESSURE PERPENDICULAR TO THE SHEAR-PLANES 



sensibly the same whether the wire was twisted or not. An account of the 
twisting of a steel wire before this stage was reached will be given later. 
Fig. 3 represents the arrangement more or less diagrammatically. The upper 
end of the wire to be twisted was fixed to a 
stout bracket B near the ceiUng. The wire 
was always about 231 cm. long ; its lower end 
was clamped in jaws in the upper end of a 
turned steel rod rr, 51 cm. long, which passed 
through a hole in the table T on which was 
the observing microscope M, and a parallel- 
plate micrometer m. One division of this 
micrometer was equal to 0-00974 mm. At 
the lower end of the rod was a horizontal 
iron cross-piece cc, 19 cm. long and 1-6 cm. 
square. From the lower end was suspended 
a carrier for the weights, or for the two stouter 
wires the weight itself, connected to the rod 
by a flat steel strip twisted in its middle, so 
that the upper and lower halves were in two 
vertical planes at right angles. Below the 
weights was a set of vanes immersed in a 
shallow bath of oil to damp vibrations. This 
bath rested on a circular turntable tt, on 
which were two uprights ii, ii at opposite ends 
of a diameter, with horizontal screws at their 
upper ends which could be brought to bear 
against the ends of the cross-piece as shown 
in the plan, Fig. 4. The screws ended in small 
steel balls and the sides of the cross-piece were 
polished. On rotating the turntable the screws 
came against the cross-piece and turned it 
round ; and so the wire was twisted by a couple 
with vertical axis. The axis of the turntable 
was made vertical by means of the levelling- 
screws I, I. To adjust this axis in the axis of 
the wire prolonged, the turntable could be 
moved over the base-plate by means of the 
horizontal screws s, of which only one is 
represented in Fig. 3. All are shown in Fig. 4. 

A horizontal microscope, not represented in the figure, was attached to one 
of the uprights and focussed on the edge of the rod rr. The adjustment by 
the screws s was continued until the microscope always saw the edge of the 
rod in the middle of the field, however the turntable might be turned. 




Fig. 3. Elevation of Arrange- 
ment for Twisting the Wires. 



IN FINITE PUEE SHEARS, ETC. 



363 




To give a definite point of view in the microscope M, in the earlier experi- 
ments starch-grains were put on the wire about 1 cm. from the lower end. 
These were illuminated, and a suitable one was selected. 

In the later experiments a needle, about 1 cm. long, was fixed, point 
upwards, on the upper end of the rod close alongside the wire, and the needle- 
point was viewed. This was better than the starch-grains. 

In the earlier work the temperature of the room was fairly steady, and 
the changes in length due to temperature- variations were too slow to give 
trouble. But in some gusty weather occurring later there were such rapid 
and considerable variations in the temperature of the room that it was 
necessary to enclose the wire in a wooden tube. After this was done tempera- 
ture gave no further trouble, whatever the weather. 

In order to observe the effect of a twist the 
turntable was levelled and adjusted axially when 
the wire and cross-piece were free. The turntable 
was rotated till the screws on the uprights just 
touched the cross-piece. Then chalk-marks were 
made on the turntable and on the plate below, one 
just over the other. The microscope was adjusted 
exactly to sight the upper or lower edge of a starch- 
grain on its horizontal cross- wire, and the micrometer 
was read. Then the turntable was rotated so many 

whole turns, and the micrometer-plate was moved till the edge of the grain 
was again on the cross-wire and the micrometer was read again. 

Except in the case of a wire stretched only by the weight of the rod and 
cross-piece, in some experiments described later, there was always a lengthening 
on twisting, of the same order whether the twist was clockwise or counter- 
clockwise. The lengthening was nearly proportional to the square of the 
twist put on. It was necessary to limit the twist to a few turns to avoid 
permanent set, and when such a small twist had been given and the wire 
was untwisted it returned sensibly to its original length. 

The lowering was entirely due to twisting and not to any giving of the 
support, for when a microscope was sighted on a point on the wire close to 
the upper end, no change in level could be detected, when the wire was 
twisted through 5. turns at its lower end. This was further verified by an 
experiment on a steel wire from the same piece as No. 3 below, which 
showed that the extension half-way down the wire was, within the limits of 
experimental error, half that at the lower end. A microscope and micro- 
meter were fixed on a table half-way up the wire, and a needle-point was 
fixed here as well as at the lower end. At each twist and untwist both 
micrometers were read. I give the observations in this experiment in full, 
as they will show the sort of accuracy attained. 



Fig. 4. Plan of the Cross- 
piece and Turntable. 



364 ON PRESSURE PERPENDICULAR TO THE SHEAR-PIANES 

The lower end was twisted from a starting twist of J turn to 4 J turns. 
Micrometer-Readings at Lower End. 



J turn 


4J turns 


Lowering 


22-3 


18-6 


3-7 


22-5 


19-0 


3-5 


23-0 


19-2 


3-8 


22-6 


19-4 


3-2 


22-9 


19-6 


3-3 


22-6 


19-5 


31 


23-0 


19-5 


3-5 


23-0 


18-9 


41 


22-7 


19-2 


3-5 


22-9 


19-6 


3-3 



Mean lowering, 3-50 divisions. 

One division of micrometer = 0-00974 mm. 

The lowering is 0-0341 mm. 

Micrometer-Readings Half-way up the Wire. 



^ turn 


4J turns 


Lowering 


30-4 


28-3 


2-1 


30-5 


28-4 


2-1 


30-2 


28- 1 


2-1 


30-5 


28-5 


20 


30-5 


28-0 


2-5 


30-4 


28-7 


1-7 


310 


28-4 


2-6 


30-6 


28-5 


21 


31-8* 


29-9 


1-9 


31-6* 


29-lt 


2-5 



Mean lowering, 2-16 divisions. 

One division of micrometer = 0-00751 mm. 

The lowering is 0-0162 mm. 

If the lowering at the end is accurate, that half-way up should be 
0-0171 mm. The observed lowering is as nearly equal to this as could be 
expected. 

With the first wire, determinations of extension due to an addition of 
520 grammes were made both in the untwisted and twisted conditions, as 
it was only when these became sensibly equal that the lowering on twist 
became equal for different loads. The extra load could be put on or taken 
ofi by lowering or raising a lever, not represented in Fig. 3. It is unnecessary 



* Another point on the needle sighted, 

■f [This number has been corrected from 29-9 in accordance with the original MS. 
other small shps in the results which follow have been similarly corrected. Ed.] 



A few 



IN FINITE PURE SHEARS, ETC. 365 

to describe the details of this arrangement. The experiments with the other 
wires were made with such loads that it was not considered necessary to 
observe the stretch due to addition of load. 

Results. 

la. Steel piano- wire, diameter 0-720 mm. (mean of 10 measurements 
at different points), length to observing point in this and all cases 230 cm. 
Permanent set, after putting on eight turns twist and then untwisting, only 
a very few degrees. Total load, 7081 grammes. 

The twist is termed clockwise when the turntable as viewed from above is 
moved clockwise. 

Clockwise twist, 0-4 turns ; lowering 0-0181 mm., mean of 10 observations. 
„ 0-8 „ „ 0-0732 

The ratio of these is 4-04 : 1. 
The extension due to an addition of 520 grammes was : 

No twist on the wire 0-143 mm., mean of 10 observations. 
4 turns „ 0-141 „ „ „ 

8 ,, ,, u-14o „ ,, ,, 

lb. Same wire. 

Total load, 9081 grammes. 

Clockwise twist, 0-4 turns; lowering 0-0180 mm., mean of 20 observations. 
„ 0-8 „ „ 0-0749 

The ratio of these is 4-15 : 1. 
The extension due to an addition of 520 grammes was : 

No twist on the wire 0-142 mm., mean of 10 observations. 
8 turns „ 0-144 „ „ „ 

Taking the mean lowering for the two loads of 7081 and 9081 granmies 
for eight turns twist, viz., 0-074 mm., and taking it as proportional to the 
square of the twist, the lowering for one turn is 0-00116 mm., and 

s = 2ldllaW^ = 1-043. 

The moduli of elasticity of this wire were found to be 

n = 0-769 X 1012, y _ 2-013 x 10^2, 

whence n/Y = 0-382. The value of n, found for loads of 1081 grammes and 
9081 grammes respectively, was identical. 

2. The same wire was raised to a red heat, by an electric current, with 
the load of 9081 grammes on it. It lengthened about 3 cm., and this length 
was cut off. The surface oxidised, and when the oxide was rubbed off the 
diameter was 0-696 mm. (mean of 10 measurements). 



366 ON PRESSURE PERPENDICULAR TO THE SHEAR-PLANES 

The permanent set after twisting and untwisting was greater, and so only- 
three turns were given. 

Total load, 9081 grammes. 
Clockwise twist, 0-3 turns ; lowering 0-0129 mm., mean of five observations. 
The extension due to an addition of 520 grammes was : 

No twist on the wire 0-155 mm., mean of 10 observations. 
3 turns „ 0-154 „ „ „ 

The lowering for one turn according to the square-law is : 0-00143 mm., 
whence 5 = 1-376 mm. 

The five values of the lowering were : 1-5, 1-2, 1-2, 1-2, 1-5 divisions, mean 
1-32 divisions. With such small lowering no accuracy could be expected, 
and it would be difficult to verify the square-law. 

The moduli of elasticity for the softened wire were : 

n = 0-809 X 1012 and Y = 2-06 x IO12, 
whence n/ 7 = 0-393. 

3. Steel piano- wire, diameter 0-970 mm. (mean of 10 measurements). 

A needle-point fixed at the side of the wire was viewed in the microscope. 
After twisting and untwisting, a slight permanent set threw the point out of 
focus if the start was from no twist. A quarter- turn was therefore put on 
initially, and the twisting was from this, and the untwisting was back to it. 

Total load, 19,504 grammes. 

Clockwise twist, J-2J turns; lowering 0-0088 mm., mean of 10 observations. 

55 55 4~^4 '5 " 0-0343 ,, ,, ,, 

Counter-clockwise twist, ^-2^ turns; lowering 0-0090 mm., mean of 10 obs. 

1-41 0-0844 

Mean lowering, J-2J turns, 0-0089 mm. 
1-41 turns, 0-0344 „ 

By the square-law the lowerings for 4|, 2J, and J should be as 289 : 81 : 1, 
and the difference should be as 288 : 80 = 18 : 5. 

The observed differences are as 19-4 : 5. 

The lowering for one turn deduced from the difference between J and 4J is 
0-00191 mm., whence s = 0-946. 

Comparing the lowerings for one turn of this wire with the hard wire 
No. 1, if the lowering is proportional to the square of the diameter, we ought 
to have for No. 1 a lowering of 0-00191 x (72/97)2 = 0-00105 mm. The 
observed lowering was 0-00116 mm., which is as near the calculated value as 
could be expected. 



EST FINITE PURE SHEARS, ETC. 367 

4. The same wire was then raised to a red heat by an electric current with 
the load on. After being rubbed down its diameter was 0-947 mm. (mean of 
10 measurements). Same load as in experiment 3. 

Clockwise twist, J-3J turns, lowering 0-0207 mm., mean of 10 observations. 

The deduced lowering for one turn is 0-00197 mm. 

The value of s is 1-025 mm. 

Comparing the lowerings for one turn of this wire with the softened wire 
No. 2, the square-law for the diameter should give for No. 2 a lowering 
0-00197 X (696/947)2 = 0-00106 mm. The observed lowering was 0-00143 mm., 
a considerable divergence. 

6. Copper wire, diameter 0-655 mm. (mean of 10 measurements). Load, 
7081 grammes. 

Clockwise twist, J-2J turns; lowering 0-0066 mm., mean of 10 observations. 
Counter-clockwise twist, ^-2^ turns ; lowering 0-0083 mm., mean of 10 obs. 

It was not safe to give a greater twist owing to the largeness of the 
permanent set. With 2^ turns the set was still small. 

The larger value of the lowering for the counter-clockwise twist is almost 
certainly real, and not merely error of observation. Some other observations 
showed an even greater excess, though they were very irregular owing to 
temperature- variations, and are not worth recording. The extension due to 
an addition of 520 grammes was : 

No twist on the wire 0-268 mm., mean of 20 observations. 
3 turns „ 0-269 ,, „ ,, 

Taking the mean for clockwise and counter-clockwise twist, the lowering 
for one turn is 0-00149 mm., and s = 1-62 mm. 

6. Brass wire, diameter 0-928 mm. (mean of 10 measurements). Load 
19,504 grammes. 

Clockwise twist, i-2J turns; lowering 0-0169 mm., mean of 10 observations. 
1-41 0-0540 

Counter-clockwise twist, j-2J turns; lowering 0-0135 mm., mean of 10 obs. 
„ i-41 „ „ 0-0479 

The difference between clockwise and counter-clockwise twisting is too 
large for errors of observation. 

For the square-law the lowerings for J-4J and for J-2 J turns should be in the 
ratio 18 : 5. They are in the ratios 16 : 5 for clockwise, and 17-7 : 5 for counter- 
clockwise twisting. The lowering for one turn clockwise, as deduced from 
i-4J turns, is 0-00300 mm., and for one turn counter-clockwise is 0-00265 mm. 

The mean value of s = 1-537 mm. 



368 ON PRESSURE PERPENDICULAR TO THE SHEAR-PLANES 

Experiments with Smaller Loads. 

When the piano- wire diameter 0-72 mm. was loaded only with the rod and 
cross-bar weighing 1081 grammes, there was a rise on twisting. 

Clockwise twist, 0-4 turns, rise 0-041 mm. 

0-8 „ 0-139 „ 

Counter-clockwise twist, 0-4 „ 0-023 „ 

„ 0-8 „ 0-108 „ 

The extension for an addition of 520 grammes was : 



No twist on wire 0-137 


mm.. 


mean 


of 6 observations. 


4 turns clockwise 0-170 


jj 


55 


6 


55 


8 „ „ 0-237 


?j 


55 


3 


55 


4 turns counter-clockwise 0-156 


5J 


55 


6 


55 


8 „ .. ,. 0-238 


55 


55 


3 


55 



If by means of the observed extensions we calculate the positions of the 
point viewed, when the load of 1081 grammes is taken off, we find that the 
total rise for clockwise twist would be: for four turns 0-110 mm., and for 
eight turns 0-347 mm. 

The rise appears to be due to coiling up of the wire on twisting, through 
some remnant of the spiral condition in which it existed before suspension. 
This is confirmed by the very large increase in extension, due to addition of 
load as the twist on the wire is increased. It may be a coincidence that the 
rise on twisting and the increase of stretch are both nearly proportional to 
the square of the number of turns. 

Experiments were then made with greater loads to find how the lowering 
and extension changed. Only clockwise twist was observed. 

Load 3081 grammes, the rise changed to lowering. 

Twist, 0-4 turns; lowering 0-0131 mm., mean of 20 observations. 
55 0-8 „ „ 0-0498 

The extension due to an addition of 520 grammes was : 

No twist on the wire 0-144 mm., mean of 10 observations. 
4 turns „ 0-143 „ „ „ 

^ 55 55 U-l-iy ,, ,, ,, 

Showing a still slight excess of extension in the most twisted condition. 

Load 5081 grammes. 

Twist, 0-4 turns; lowering 0-0164 mm., mean of 20 observations. 
„ 0-8 „ „ 0-0660 



m FINITE PURE SHEARS, ETC. 369 

The extension due to an addition of 520 grammes was : 

No twist on the wire 0-141 mm., mean of 10 observations. 
4 turns „ 0-142 „ „ 15 

8 „ „ 0-144 „ „ 15 

The results for loads of 7081 and 9081 grammes are already recorded 
under 1 a and 1 h. There is obviously a tendency for the lowering to increase 
with load until the extensions under different twists become more nearly 
equal with equal added load. 

When the same wire was softened and loaded with 3081 grammes the 
lowering for three turns was 0-0093 mm. (mean of 10 observations). 

The extension due to an addition of 520 grammes was : 

No twist on the wire 0-149 mm., mean of 10 observations. 
3 turns „ 0-147 

With load 9081 grammes the same wire gave the results recorded under 2, 
which show a greater lowering for an equal twist but the same extension with 
added load. " 

The copper wire diameter 0-655 mm. (No. 5 above) with load 4081 grammes 
gave: 

Clockwise twist, 0-3 turns, 0-00965 mm., mean of 10 observations. 
Counter-clockwise twist, 0-3 turns, 0-0156 mm., mean of 10 observations. 

Taking the mean of these, the lowering for one turn is 0-0014 mm. 

The extension due to an addition of 520 grammes was : 

No twist on the wire 0-268 mm., mean of 5 observations; 
3 turns „ 0-270 

extensions agreeing very nearly with those recorded above for a load of 
7081 grammes on the same wire. 

Remarks on the Results of Measurements. 

The lowering was never so much as 0-1 mm. and was usually much less. 
The accuracy attained could hardly be expected to be great. The measure- 
ments, however, appear to show that when a wire is sufficiently loaded to be 
straightened, it is lengthened by twisting by an amount proportional to the 
square of the twist and, with a given number of turns, inversely as the length. 

It might be thought possible that the effect observed was due to rise of 
temperature, either through adiabatic strain or through dissipation of strain- 
energy as heat. But the observations give no support to this explanation. 
When the wire was extended by twisting, it remained extended, and when 
untwisted it returned. Temperature-effects would be a maximum the instant 
after twisting, and would then gradually subside. It may be noted that 

p. c.w. 24 



370 ON PRESSURE PERPENDICULAR TO THE SHEAR-PLANES 

the adiabatic change of temperature is proportional to a^B^jl, but it is a 
cooling, and its amount is such as to shorten the wire, in the case of steel, by 
something of the order of 1/100 of the observed extension. If we suppose 
that some definite fraction of the strain-energy put in is dissipated, again the 
change, now a warming, is proportional to a^O^jl. The whole strain-energy, 
in the case of steel, would only raise the temperature by an amount accounting 
for something of the order of 1/10 the observed extension, and, in fact, only 
an exceedingly minute fraction of the strain-energy is dissipated. 

A comparison of the wires (1) and (3) appears to show that the lengthening 
for a given twist is proportional to the square of the radius. 

If we put the lengthening 

s for steel is in the neighbourhood of 1. For copper and brass s is in the 
neighbourhood of 1-5. The lowering for the copper and brass wires tested for 
twists in opposite directions is not the same. 

With a hard steel wire with small load the end of the wire rises on twisting, 
probably through coiling. 

The value of s = (\n + p)/Y — (21 Y — Ijn) q appears to be measurable, 
but its value gives us no clue to the values of f and q. 

If we could assume q = 0, then for steel we should have f about 2n, but 
I see no justification for the assumption. 

If we could measure the decrease in diameter, we should obtain the value 
of (\n + f — q) (jjY — {(?> + g)IY — Ijn) q, and knowing n, Y and a we should 
be able to find p and q. But a thin wire is quite unsuitable for this 
measurement. The decrease is probably of the order of 2a/? x lengthening. 
With the wires I have used this is of the order 1/1000 x lengthening, and an 
accuracy of measurement of 10"^ mm. would be required at least. With a 
shaft of considerable diameter it might be possible to measure the quantity, 
though the experimental difficulties are obviously very great*. 

The Effect of the Lengthening of a Wire on its Torsional Vibration. 

If a wire is loaded with mass M having moment of inertia /, when M is 
set vibrating torsionally it falls and rises as it swings, its distance below the 
highest point being given by 

X = isaW/l. 

The kinetic energy is T = ^16^ + iMx\ 

The last term is easily found to be negligible. 
The potential energy is 

F = Inira^e'^ll - Mgx = Inira'^d^ll - iMgsaW% 

* [For the experimental carrying out of these measurements see Collected Papers, Art. 30. Ed. ] 



IN FINITE PURE SHEARS, ETC. 371 

The equation of motion is 

• 10 + (JwTraV^ - Mgsa^/l) d = 0. 

Whence T^ - ^^^^^ 

~n7Ta^{l-2Mgsln7Ta^)' 

and T is greater than it would be if s were 0, by the factor 

1 + Mgslmra^. 

If Y is Young's modulus and if e is the elongation of the wire due to the 
load Mg, 

Mg/ira^ = Ye, 

so that the factor may be conveniently written as 1 + seY/n. 

If the vibrations are used to determine the modulus of rigidity n, then the 
value of n will be greater than that deduced by neglect of s, and by the factor 

1 + 2seY/n, 

To give an idea of the effect on the determination of the modulus of 
rigidity, let us suppose that a quite straight steel wire, diameter 0-7 mm., has 
a load of 2000 grammes. For steel Y/n is about 2-6. For the given diameter 
e is about 2 x 10"*. We have found that s is about 1. The correcting 
factor is then about 1-001, or the true rigidity exceeds the value calculated 
in the ordinary way by about 1 in 1000. If the wire is not sufficiently loaded 
to be straight the value of s is less. If very lightly loaded the sign of s may 
be changed and the true rigidity may be less than the value as ordinarily 
calculated. The correction is hardly needful in practice, as the modulus of 
rigidity is probably not measurable to three figures. 

Distortional Waves. 

In purely distortional waves in a medium of great extent it is evident 
that the pressure S perpendicular to the axes of shear, if it exists, will not 
produce any motion. To keep the waves purely distortional, i.e. with 
motion perpendicular to the direction of propagation only, a force must 
be applied from outside dRjdx per cubic centimetre in the direction of 
propagation. If this force is not applied then longitudinal motion must 
result, obviously of the second order, unless Jn + p = 0. This is probably 
the condition for an incompressible medium. If \n + p is not zero it 
appears possible that dispersion may exist. If the longitudinal motion is 
neglected the pressure in the direction of propagation is (|n + f) e^ and all 
that we can say, at present, is that it is probably of the order of ne^. 



24—2 



2t). 

THE WAVE-MOTION OF A EEVOLVING SHAFT, AND A SUG- 
GESTION AS TO THE ANGULAR MOMENTUM IN A BEAM 
OF CmCULAELY POLARISED LIGHT. 

[Roy. Soc. Proc. A, 82, 1909, pp. 560-567.] 

[Read June 24, 1909.] 

When a shaft of circular section is revolving uniformly, and is transmitting 
power uniformly, a row of particles originally in a line parallel to the axis 
will lie in a spiral of constant pitch, and the position of the shaft at any 
instant may be described by the position of this spiral. 

Let us suppose that the power is transmitted from left to right, and that 
as viewed from the left the revolution is clockwise. Then the spiral is a 
left-handed screw. Let it be on the surface, and there make an angle e with 
the axis. Let the radius of the shaft be a, and let one turn of the spiral have 
length A along the axis. We may term A the wave-length of the spiral. 
We have tan e = IrrajX. If the orientation of the section at the origin at 
time t is given by ^ = 27rNt, where N is the number of revolutions per second, 
the orientation of the section at x is given by 

e = 'lirNt - - tan e = ^ (iVA^ - x), (1) 

a A 

which means movement of orientation from left to right with velocity iVA. 

The equation of motion for twist-waves on a shaft of circular section is 

^^^U^^ (2) 

where JJ^^ = modulus of rigidity/density = njp. 

Though (1) satisfies (2), it can hardly be termed a solution, for d^6/dt^, and 
dW/dx^ in (2) are both zero. But we may adapt a solution of (2) to fit (1) if 
we assume certain conditions in (1). 

The periodic value 

e = esm^{Uj-x) 

satisfies (2), and is a wave-motion with velocity Un and wave-length I. Make 



WAVE-MOTION OF A REVOLVING SHAFT 373 

t SO great that for any time or for any distance under observation Unt/l and 
x/l are so small that the angle may be put for the sine. Then 

e=9^{Uj-x) (3) 

This is uniform rotation. It means that we only deal with the part of 
the wave near a node, and that we make the wave-length I so great that 
for a long distance the 'displacement-curve' obtained by plotting 6 against 
t coincides with the tangent at the node. We must distinguish, of course, 
between the wave-length I of the periodic motion and the wave-length A of 
the spiral. 

We can only make (1) coincide with (3) by putting 
0/?=l/A and NX= C/„. 

Then it follows that for a given value of N, the impressed speed of uniform 
rotation, there is only one value of A or one value of € for which the motion 
may be regarded as part of a natural wave-system, transmitted by the elastic 
forces of the material with velocity = \/(nlp). There is therefore only one 
'natural' rate of transmission of energy. 

The value of e is given by 

tan e = 27Ta/X = 27TaNINX = 27raiV/t/'„ = 27TaN^{pln). 

Suppose, for instance, that a steel shaft with radius a = 2 cm., density 
/) = 7-8, and rigidity n = 10^^ ig making iV = 10 revs, per sec. We may put 
tan € = €, since it is very small. The shaft is twisted through 2?? in length 
A or through 27r/A per centimetre, and the torque across a section is 



smce 



A- 



U^ _ 1 In 



The energy transmitted per second is 

27rNG ^ 27T^a^NWM- 

Putting 1 H.p. = 746 x 10^ ergs per second, this gives about 38 h.p. 

But a shaft revolving with given speed A^ can transmit any power, subject 
to the limitation that the strain is not too great for the material. When the 
power is not that 'naturally' transmitted, we must regard the waves as 
'forced.' The velocity of transmission is no longer j[7„, and forces will have 
to be applied from outside in addition to the internal elastic forces to give 
the new velocity. 

Let H be the couple applied per unit length from outside. Then the 
equation of motion becomes 

^ _ p 2 ^ ^ 2^ 



dt^ ' "" dx^ TraV 



374 WAVE-MOTION OF A REVOLVING SHAFT, AND A SUGGESTION AS TO 

where ^Tra* is the moment of inertia of the cross-section. Assuming that the 
condition travels on with velocity TJ unchanged in form, 

or H has only to be applied where dW/dx^ has value, that is where the twist 
is changing. 

The following adaptation of Rankine's tube-method of obtaining wave- 
velocities* gives these results in a more direct manner. Suppose that the 
shaft is indefinitely extended both ways. Any twist- disturbance may be 
propagated unchanged in form with any velocity we choose to assign, if we 
apply from outside the distribution of torque which, added to the torque due 
to strain, will make the change in twist required by the given wave-motion 
travelling at the assigned speed. 

Let the velocity of propagation be U from left to right, and let the dis- 
placement at any section be 6, positive if clockwise when seen from the left. 
The twist per unit length is 

dd_ I d_d^ I 
dx" Udt~' U' 

The torque across a section from left to right in clockwise direction is 

1 ^ dd mra'^ X 

Let the shaft be moved from right to left with velocity U ; then the dis- 
turbance is fixed in space, and if we imagine two fixed planes drawn 
perpendicular to the axis, one, ^, at a point where the disturbance is 6 and 
the other, B, outside the wave-system, where there is no disturbance, the con- 
dition between A and B remains constant, except that the matter undergoing 
that condition is changing. Hence the total angular momentum between A 
and B is constant. But no angular momentum enters at B, since the shaft 
is there untwisted and has merely linear motion. At A, then, there must be 
on the whole no transfer of angular momentum from right to left. Now, 
angular momentum is transferred in three ways : 

1. By the carriage by rotating matter. The angular momentum per unit 
length is ^p-na'^O, and since length V per second passes out at A, it carries 
out IpTra^dU. 

2. By the torque exerted by matter on the right of A on matter on the 
left of A. This takes out - nTra^d/^U. 

3. By the stream of angular momentum by which we may represent the 
forces applied from outside to make the velocity U instead of Un . 

* Phil. Trans. 1870, p 277. 



ANGULAR MOMENTUM IN A BEAM OF CIRCULARLY POLARISED LIGHT 375 

If H is the couple applied per unit length, we may regard it as due to the 
flow of angular momentum L along the shaft from left to right, such that 
H = — dL/dx. There is then angular momentum L flowing out per second 
from right to left. Since the total flow due to (1), (2), and (3) is zero, 

ifma*dU - n7ra^e/2U - L = 0, 
and X = '?(.^-^) = '^(^-"^„^)=-^|(^-.A 

li H = 0, either U^ = C7„2 when the velocity has its 'natural value,' or 
d^d/dx^ = 0, and the shaft is revolving with uniform twist in the part con- 
sidered. 

Now put on to the system a velocity U from left to right. The motion 
of the shaft parallel to its axis is reduced to zero, and the disturbance and 
the system H will travel on from left to right with velocity U. A 'forced' 
velocity does not imply transfer of physical conditions by the material with 
that velocity. We can only regard the conditions as reproduced at successive 
points by the aid of external forces. We may illustrate this point by con- 
sidering the incidence of a wave against a surface. If the angle of incidence 
is i and the velocity of the wave is F, the line of contact moves over the 
surface with velocity v = 7/sin i, which may have any value from F to 
infinity. The velocity v is not that of transmission by the material of the 
surface, but merely the velocity of a condition impressed on the surface from 
outside. 

Probably in all cases of transmission with forced velocity, and certainly 
in the case here considered, the velocity depends upon the wave-length, and 
there is dispersion. 

With a shaft revolving N times per second U = NX, and it is interesting 
to note that the group- velocity, U — XdU/dX, is zero. It is not at once evident 
what the group-velocity signifies in the case of uniform rotation. In ordinary 
cases it is the velocity of travel of the 'beat' pattern, formed by two trains 
of slightly different frequencies. The complete 'beat' pattern is contained 
between two successive points of agreement of phase of the two trains. In 
our case of superposition of two strain-spirals with constant speed of rotation, 
points of agreement of phase are pomts oi intersection of the two spirals. 
At such points the phases are the same, or one has gained on the other by 
27T. Evidently as the shaft revolves these points remain in the same cross- 
section, and the group- velocity is zero. 

With deep-water waves the group- velocity is half the wave- velocity, and 
the energy-flow is half that required for the onward march of the waves*. 

* 0. Reynolds, Nature, August 23, 1877 ; Lord Rayleigh, Theory of Sound, vol. 1, p. 477. 



376 WAVE-MOTION OF A REVOLVING SHAFT, AND A SUGGESTION AS TO 

The energy-flow thus suffices for the onward march of the group, and the 
case suggests a simple relation between energy-flow and group- velocity. 

But the simpHcity is special to unforced trains of waves. Obviously, 
it does not hold when there are auxiliary working forces adding or subtracting 
energy along the waves. For the revolving shaft the simple relation would 
give us no energy-flow, whereas the strain existing in the shaft implies trans- 
mission of energy at a rate given as follows. 

The twist per unit length is dd/dx, and therefore the torque across a 
section is — ^rnra^ dd/dx, or ^nTra^d/U, since dO/dx = — 6/U. The rate of 
working or of energy-flow across the section is ^mra^d^lU. 

The relation of this to the strain and kinetic energy in the shaft is easily 
found. The strain- energy per unit length being J (couple x twist per unit 
length) is ^rnra^ (dO/dx)'^, which is ^mra'^b^lU^. The kinetic energy per unit 
length is Ipna^O^, or, putting p = n/Un^, is l^iTra^d^/UJ. 

In the case of natural velocity, for which no working forces along the 
shaft are needed, when U = Un = s/in/p), the kinetic energy is equal to the 
strain- energy at every point and the energy transmitted across a section per 
second is that contained in length [/"„ . 

But if the velocity is forced this is no longer true*, and it is easily shown 

2U 
that the energy transferred is that in length fj^Tfj—2> which is less than 

U a U > Un, and is greater than U ii U < 17^. 

It appears possible that always the energy is transmitted along the shaft 
at the speed Un • If the forced velocity U > 11^, we may, perhaps, regard the 
system in a special sense as a natural system with a uniform rotation super- 
posed on it. 

Let us suppose that the whole of the strain- energy in length U^ is trans- 
ferred per second while only the fraction ijl of the kinetic energy is transferred, 
the fraction 1 — /x being stationary. 

The energy transferred : strain-energy in U„ : kinetic energy in 11^ 

= IIU:VJW^:UJ2U„\ 

Put U = fU.n , and our supposition gives 

pu: 2im,jiu„ " /" /* p-^ V p) ■ 

If the forced velocity U < 11^, we may regard the system as a natural one, 
with a uniform stationary strain superposed on it. 

* In the Sellmeier model illustrating the dispersion of light, the particles may be regarded 
as outside the material transmitting the waves and as applying forces to the material which 
make the velocity forced. The simple relation between energy-flow and group- velocity probably 
does not hold for this model. 



ANGULAR MOMENTUM IN A BEAM OF CIRCULARLY POLARISED LIGHT 377 

We now suppose that the whole of the kinetic energy is transferred, but 
only a fraction v of the strain-energy, and we obtain 

It is perhaps worthy of note that a uniform longitudinal flow of fluid may 
be conceived as a case of wave-motion in a manner similar to that of the 
uniform rotation of a shaft. 



A Suggestion as to the Angular Momentum in a Beam of Circularly 

Polarised Light. 

A uniformly revolving shaft serves as a mechanical model of a beam of 
circularly polarised light. The expression for the orientation 6 of any section 
of the shaft distant x from the origin, d = 27t\-'^ (Ut — x), serves also as 
an expression for the orientation of the disturbance, whatever its nature, 
constituting circularly polarised light. 

For simpUcity, take a shaft consisting of a thin cylindrical tube. Let the 
radius be a, the cross-section of the material s, the rigidity n, and the density 
p. Let the tube make N revolutions per second, and let it have such twist 
on it that the velocity of transmission of the spiral indicating the twist is the 
natural velocity 11^ = \^{n/p). 

Repeating for this special case what we have found above, the strain- 
energy per unit length is ^neh, or, since e = adO/dx = — aO/Un, the strain- 
energy is Inahe^jTIr? = Ipahd^. 

But the kinetic energy per unit length is also ^pahd^, so that the total 
energy in length Z7„ is pahO^Un. The rate of working across a section is 

nesaO = na^sO^/Ur, = pa^sd^U^, 

or the energy transferred across a section is the energy contained in length ?/„ . 

If we put E for the energy in unit volume and G for the torque per unit 
area, we have 

GsO=EsUn, 

whence G = EUJO = ENXjIirN = EXj^tt. 

The analogy between circularly polarised light and the mechanical model 
suggests that a similar relation between torque and energy may hold in 
a beam of such light incident normally on an absorbing surface. If so, a 
beam of wave-length A containing energy E per unit volume will give up 
angular momentum ^A/27r per second per unit area. But in the case of 
light- waves E = P, where P is the pressure exerted. We may therefore 
put the angular momentum delivered to unit area per second as 

PA/27r. 



378 WAVE-MOTION OF A REVOLVING SHAFT, AND A SUGGESTION AS TO 

In the Philosophical Magazine, 1905, vol. 9, p. 397*, I attempted to show 
that the analogy between distortional waves and light-waves is still closer, 
in that distortional waves also exert a pressure equal to the energy per unit 
volume. But as I have shown in a paper on ' Pressure Perpendicular to the 
Shear-Planes in Finite Pure Shears, etc.f,' the attempt was faulty, and a 
more correct treatment of the subject only shows that there is probably a 
pressure. We cannot say more as to its magnitude than that if it exists it 
is of the order of the energy per unit volume. 

When a beam is travelling through a material medium we may, perhaps, 
account for the angular momentum in it by the following considerations. On 
the electromagnetic theory the disturbance at any given point in a circularly 
polarised beam is a constant electric strain or displacement / uniformly 
revolving with angular velocity 6. In time dt it changes its direction by dd. 

This may be effected by the addition of a tangential strain /c?^; or the 
rotation is produced by the addition of tangential strain fO per second, or by 
a current fO along the circle described by the end of /. We may imagine 
that this is due to electrons drawn out from their position of equilibrium so 
as to give/, and then whirled round in a circle so as to give a circular con- 
vection-current/^. Such a circular current of electrons should possess angular 
momentum. 

Let us digress for a moment to consider an ordinary conduction-circuit 
as illustrating the possession of angular momentum on this theory. Let the 
circuit have radius a and cross-section s, and let there be N negative electrons 
per unit volume, each with charge e and mass m, and let these be moving 
round the circuit with velocity v. If i is the total current, i = Nsve. The 
angular momentum will be 

Ns27ra . mva = ^iraHmje = 2Aim/e = 2Mm/e. 

where A is the area of the circuit and M is the magnetic moment. This is 
of the order of 2M/10'. 

It is easily seen that this result will hold for any circuit, whatever its 
form, if A is the projection of the circuit on a plane perpendicular to the 
axis round which the moment is taken, and if M = Ai. If we suppose that 
a current of negative electrons flows round the circuit in this way and that 
the reaction while their momentum is being established is on the material 
of the conductor, then at make of current there should be an impulse on 
the conductor of moment 2M/W. If the circuit could be suspended so that 
it lay in a horizontal plane and was able to turn about a vertical axis in 
a space free from any magnetic field, we might be able to detect such impulse 
if it exists. But it is practically impossible to get a space free from magnetic 
intensity. If the field is H, the couple on the circuit due to it is proportional 
to HM. It would require exceedingly careful construction and adjustment 

* [Collected Papers, Art. 22, p. 338.] j [Collected Papers, Art. 25.] 



ANGULAR MOMENTUM IN A BEAM OF CIRCULARLY POLARISED LIGHT 379 



•Pi 
■Pi 



[of the circuit to ensure that about the vertical axis the component of the 
jouple due to the j&eld was so small that its effect should not mask the effect 
Fof the impulsive couple. The electrostatic forces, too, might have to be 
[considered as serious disturbers. 

Returning to a beam of circularly polarised light, supposed to contain 
electrons revolving in circular orbits in fixed periodic times, the relations 
between energy and angular momentum are exactly the same as those in 
a revolving shaft or tube, and the angular momentum transmitted per second 
per square centimetre is EXJ^tt = PXj2tt, where P is the pressure of the 
light per square centimetre on an absorbing surface. 

The value of this in any practical case is very small. In 
light-pressure experiments, P is detected by the couple on a 
small disc, of area A say, at an arm h and suspended by a 
fibre. What we observe is the moment APh. If the same 
disc is suspended by a vertical fibre attached at its centre and 
the same beam circularly polarised in both cases is incident 
normally upon it. according to the value suggested the torque 
is APXI27T. 

The ratio of the two is A/27r6. Now b is usually of the 
order of 1 cm. Put A = 6 x 10~^ cm., or, say, 27r/10-^, and 
the ratio becomes 10~^. . 

It is by no means easy to measure the torque APb accurately, 
and it appears almost hopeless to detect one of a hundred- 
thousandth of the amount. The effect of the smaller torque 
might be multiplied to some extent, as shown in accompanying 
diagram. 

Let a series of quarter wave plates, Pi, ^2' Vs^ ..., be suspended by a fibre 
above a Nicol prism N, through which a beam of light is transmitted upwards, 
and intermediate between these let a series of quarter wave plates, g'l, g'2^ ?3> •••> 
be fixed, each with a central hole for the free passage of the fibre. The beam 
emerges from N plane polarised. If N is placed so that the beam after passing 
through 2?i is circularly polarised, it has gained angular momentum, and there- 
fore tends to twist p^ round. The next plate q^^ is to be arranged so that the 
beam emerges from it plane polarised and in the original plane. It then 
passes through ^2 > which is similar to pi , and again it is circularly polarised 
and so exercises another torque. The process is repeated with q^ and f^, and 
so on till the beam is exhausted. By revolving iV through a right angle round 
the beam, the effect is reversed. But, even with such multiplications, my 
present experience of light-forces does not give me much hope that the effect 
could be detected, if it has the value suggested by the mechanical model. 



27. 

PKELIMINARY NOTE ON THE PRESSURE OF RADIATION AGAINST 
THE SOURCE: THE RECOIL FROM LIGHT. 

By J. H. PoYNTiNG and Guy Barlow, D.Sc. 

[British Association Report, 1909.] 
[This is merely a preliminary account of the following paper Art. 28.] 



• 28. 

THE PEESSUKE OF LIGHT AGAINST THE SOURCE: 
THE RECOIL FROM LIGHT. 

BAKERIAN LECTURE. 

By J. H. PoYNTiNG and Guy Barlow, D.Sc. 

[Roy. Soc. Proc. A, 83, 1910, pp. 534-546.] 

[Read March 17, 1910.] 

All experiments on the pressure of radiation have hitherto been made on 
the force exerted by light or radiation on a receiving surface. The experiment 
now to be described shows the pressure of radiation against the source from 
which it starts, and from analogy with a gun we may term this the recoil 
from hght. It does not appear practicable to show this effect by using a 
source in which heat is developed intrinsically. But if radiation falls on an 
absorbing body it heats the body and the heat so developed issues again as 
radiation, and it is possible to detect the efEect of this issuing radiation. 

Theory. 

We may see the nature of the action to be looked for by considering an 
ideal case in which we allow a beam of light with energy P per cubic 
centimetre to faU normally in a perfect vacuum in turn on each of four discs, 
the front and back surfaces of these discs being respectively as in Fig. 1, 




BB BS SS SB 

' BACK 

Fig. 1. 

where B represents a fully absorbing or * black' surface, and S a fully reflecting 
or non-radiating surface. 

When the radiation falls on an absorbing face, as in the case of either of 
the discs (1) and (2), the temperature of the disc rises till a steady state is 
reached in which emission equals absorption. We may suppose that the 
discs are so thin that the two faces are sensibly at the same temperature. If 
we did not take into account the pressure due to issuing radiation, or if we 



382 THE PRESSURE OF LIGHT AGAINST THE SOURCE: 

only considered the initial effects before heating took place, the pressures on 
the first two discs would be P in each case, due to the incident beam alone, 
and on the last two would be 2P, due to the sum of the incident and reflected 
beams. We should have, therefore, pressures respectively 

(1) (2) (3) (4) 

P P 2P 2P 

But when a steady state is reached, the discs (1) and (2) must be giving 
out as much radiant energy as they receive. The first disc gives out equal 
amounts on the two sides, producing equal and opposite pressures. All the 
radiation from the second disc is given out at the front side and is equal in 
energy to that of the incident beam. Assuming this emitted radiation is 
distributed according to the cosine-law, the pressure resulting from it is 
easily shown to be |P, so that the total pressure on this disc is fP. 

Since there is no absorption by discs (3) and (4), we still have the pressures 
2P ; hence we have now 

(1) (2) (3) (4) 

P fP 2P 2P 
In a real case these results are modified in two ways : 

(i) By the possession of some small reflecting power by surface B, and of 
some small absorbing and radiating power by surface S. 

(ii) By an inequality of temperature between front and back surfaces 
conditioned by the energy which is carried through from front to back to be 
radiated thence. The vacuum is not perfect, and there is radiometer-action 
due to the residual gas, which, owing to the inequality of temperature, is not 
the same on the two sides. This is probably the only way in which gas- 
action is sensible, for the effects due to ordinary convection and conduction 
in the residual gas are negligible. The temperature-diflerence, though 
sufficient to produce a differential radiometer-action, is so small that in 
estimating the radiation from the two sides of a disc we may take them as 
being at the same temperature. 

In the experiment to be described the diffusion is so sHght that we dis- 
regard it. Considering, then, only the reflection and absorption, let r be the 
coefficient of reflection of the surface B for the incident radiation, p that of S, 
a the coefficient of emission of B for the emitted radiation, a that of S. 

It is then easy to show that the total radiation-pressures on the four discs 
are respectively 

(1) (2) (3) (4) 

The emitted radiation is not of the same quality as the incident radiation ; 
and strictly we are not justified in assuming that the emissive powers for the 



THE RECOIL FROM LIGHT 



383 



two surfaces for the one quality of radiation are in the same ratio as the 
absorbing powers for the other. But we shall for simpUcity suppose that 
the ratios are the same, a supposition which enables us to proceed, and which 
is probably not very far from the truth. We have, therefore, 

a=l— r, a=l— /). 

On this assumption Table I has been constructed, giving the four pressures 
for different values of r and p. The pressure-ratios in the last two columns 
are of interest in connection with the experimental results, and will be referred 
to later. 

Table I. Pressures on Discs, calculated for Different Values of the 
Reflection- Coefficients . 



Reflecting 
power of B 


Reflecting 
power of 8 


Pressures taking P = 


1 


Pressure-ratios 














r 


P 


BB 


BS 


88 


8B 


B8 
BB 


BS 


\{88 + 8B) 





1-00 


1-00 


1-67 


200 


2-00 


1-67 


1-67 





0-95 


1-00 


1-60 


1-95 


1-92 


1-60 


1-66 





0-90 


1-00 


1-54 


1-90 


1-85 


1-54 


1-65 


0-05 


1-00 


105 


1-68 


200 


200 


1-60 


1-68 


005 


0-95 


105 


1-62 


1-95 


1-92 


1-54 


1-67 


005 


0-90 


1-05 


1-56 


1-90 


1-85 


1-49 


1-66 


010 


100 


110 


1-70 


2-00 


200 


1-55 


1-70 


010 


0-95 


MO 


1-64 


1-95 


1-92 


1-49 


1-70 


010 


0-90 


MO 


1-58 


1-90 


1-85 


1-44 


1-69 



The modification of the values in the above table by radiometer- action 
will be greater for the BB disc than for the others. In that disc energy 
proportional to JP has to be carried through the disc, and therefore the 
temperature-difference is the greatest. It is only possible to guess at the 
relative magnitudes of the radiometer-actions for the different discs. We 
have therefore sought to make the vacuum so high that the action nearly 
disappeared. 

The Experiment. 

In the final form of the experiment each disc consisted of a pair of 
circular cover-glasses, 1*2 cm. in diameter and about 0-1 mm. thick, between 
which was squeezed a layer of asphaltum also about 0*1 mm. thick, the 
temperature being first raised sufficiently to render the asphaltum molten. 
It is difficult to make the discs of uniform thickness and free from bubbles 
of gas ; but a great number were made and the four best were selected for 
use. Such a compound disc appears to be perfectly opaque, and its surface 
is the blackest black and the least diffusing that we have yet been able to 
obtain. 



384 



THE PRESSURE OF LIGHT AGAINST THE SOURCE 



The reflecting surface was made by depositing silver on the outside of the 
compound disc by means of the discharge from a silver cathode in an 
exhausted receiver. A similar deposit on clear glass just allowed an arc 
light to be seen through it. 

Four holes the size of the discs were cut in a stout plate of mica ABCD, 
the centres of the holes being at the corners of a 2 cm. square (Fig. 2). 
The discs were then fixed in these holes by a minute amount of celluloid 
varnish. The suspension-rod E and the mirror-holder F were attached to 



H 



:i 



<}. 





the mica plate, at the middle points of its top and bottom edges respectively, 
by very small copper clips without any cement. A platinised mirror was 
cut in half, and the two portions M^, M^ were mounted back to back in 
a suitable clip of copper foil at the extremity of the rod F, the plane of the 
mirrors being perpendicular to the mica plate. This system was suspended 
by a quartz fibre G, 9 cm. long, in the centre of a glass flask of 16 cm. 
diameter. The upper end of the quartz fibre was fixed to a brass collar H 
held by friction in the neck of the flask. Both ends of the fibre were silvered 
and coppered, so that they could be soldered to E and to the support. After 



THE RECOIL FROM LIGHT 



385 



suspension the mouth of the flask was sealed off, a lateral tube in the neck 
being still available for connection with the exhausting apparatus. 

To carry out the exhaustion of the flask to a very high degree, the general 
arrangement shown in Fig. 3 was adopted, and the successive stages in the 
process were as follows : 

(1) Prehminary exhaustion by aid of a Gaede pump. The experimental 
flask A was kept hot by gas-burners below it, and the charcoal bulbs Cj and 
Cg were strongly heated electrically by enclosing them in asbestos tubes con- 
taining coils of platinum wire. From time to time the whole apparatus was 
washed out with dry oxygen generated from the manganese dioxide in the 
bulb E. A small discharge tube G indicated the state of the vacuum, and 
examination of the spectrum gave useful information as to the gas given off 
from the bulbs C^ and C2. At the end of three days these bulbs appeared to 




Fig. 3. 



have ceased to give off any gas, and the Gaede pump would bring the vacuum 
down to a hard X-ray stage in about 10 minutes after admitting oxygen. 
The temperatures of C^ and C2 were then somewhat reduced, and the apparatus 
was sealed off at Si. B and F in the figure represent phosphorus pentoxide 
drying tubes, and H is a manometer. 

(2) The charcoal bulb Cj was put in liquid air, and the temperature of the 
second bulb Cg and of the flask A was then slowly reduced to the room 
temperature. After 30 hours the apparatus was finally sealed off at 82- 

During both these stages the U-tube D was kept always immersed in liquid 
air. This arrangement formed a perfect trap for mercury vapour, which 
otherwise diffused from the pump into the flask and attacked the silver 
mirrors. A roll of silver foil was placed in one limb of the U-tube, with the 
object of making the trap more effective, but this precaution was probably 
unnecessary. 



p. c. w. 



386 THE PRESSURE OF LIGHT AGAINST THE SOURCE: 

(3) In the final stage of the exhaustion, the charcoal bulb Cg was 
surrounded by liquid air, which was boiled off continuously at the reduced 
pressure of about 2 cm, of mercury for several hours before and during the 
whole of the measurements. Experience showed that the highest vacuum 
was obtained probably two hours after the application of fresh liquid air, and 
that it was necessary to renew the liquid air after every four or five hours. 

The source of light S (Fig. 4) was an Ediswan 50- volt ' Focus lamp,' which 
was fed from accumulators. By means of an adjustable resistance in series 
with the lamp, the voltage was maintained exactly at 60 volts, the lamp then 
taking a current of 5-37 amperes. The light was so steady for hours at a 

B 
o 





s 


Li. 




La. 


; ,' 




G : 


::©----- 


-:::::::: 


.■:::;:;i:: 


--'€ 


o 






so 

I 1 :. 


IOC 


A 

Cms- 



Fig. 4. 

time that adjustment of the resistance was seldom necessary. A photo- 
graphic lens Li, of 15 cm. focal length, provided with an iris diaphragm, was 
arranged to throw an image of the lamp filament on an achromatic lens Zg 
of 19 cm. focal length. This seeond lens then formed a uniformly illuminated 
image of the iris diaphragm on the disc to be worked with. By adjustment 
of the diaphragm this image was made rather smaller than the disc, so that 
when the beam was centred on the disc an unilluminated margin about | mm. 
wide was left all round it. The lamp and lenses were fixed to a board which 
could be moved parallel to itself vertically and horizontally between guides, 
so that the beam could be easily directed on to the four discs in succession. 
The centring of the image on the disc was made by eye. 

The flask was mounted on an iron turntable with the quartz fibre 
accurately in the axis of rotation. By rotation of the flask through 180° 
it was possible to experiment on the reverse sides of the four discs. We 



9 THE RECOIL FROM LIGHT 387 

shall refer to the observations raade in the two positions as 'direct' and 
'reverse.' Thus the 'direct' BS disc becomes the 'reverse' SB disc. The 
flask was shielded from electrification and from extraneous radiation by 
enclosure in a cylindrical case of tinned iron, blackened inside and provided 
with windows to admit the beam of light and to allow the deflections to be 
observed. 

For reading the deflections the image of an electric lamp B (Fig. 4) on a 
millimetre scale C, at a distance of 113 cm., was used. The definition of the 
image was sufficiently good to allow deflections to be read accurately to 
0-2 mm., although the optical irregularities of the glass flask rendered a 
telescope useless. 

In finding the centre of swing from the deflections, a curious periodic 
motion was observed in its position, the centre moving to and fro with simple 
harmonic motion. The complete period of the torsional vibrations of the 
suspended system was 74-6 seconds with no appreciable damping; and the 
period of the motion of the centre of swing was found to be about seven 
minutes, that is nearly 11 torsional half-periods. The periodic motion was 
ultimately traced to pendulum-motions of the system set up by external 
disturbances, for the amplitude of the displaced centre of swing was found 
to increase with such pendulum-motion. The pendulum-period of the system 
was slightly longer for motion in the plane of the mica plate than for motion 
in a perpendicular plane. Hence a pendulum-motion once set up changed 
periodically from motion in a straight line to motion in an ellipse, and the 
complete cycle of these changes was actually gone through in seven minutes. 
This meant a periodic change in the angular momentum of the system about 
the axis of suspension, and to neutralise this the mica plate tended to turn 
with equal and opposite angular momentum about the vertical axis, and so to 
give a twist to the fibre. Accordingly, we should expect the maximum twist 
to take place when the vibrations are lineat, and this was observed to be the 
case. To eliminate this effect, 12 consecutive turning points were always 
taken (i.e. observations over a period of seven minutes), and the mean centre 
of swing calculated from these. 

When the beam was allowed to fall on a disc, some initial effects were 
observed, of which we shall give an account below, and shall then suggest a 
tentative explanation. 

The Results. 

The observed deflections for the four discs are given in Table II. 

These results are divided into two series, A and B. In Series A the beam 
of Hght was kept on the disc under experiment until the deflection seemed 
nearly constant, the time of exposure being generally about 20 or 30 minutes 
before the deflection was read. Attention was chiefly given to the BB and 

25—2 



388 



THE PRESSURE OF LIGHT AGAINST THE SOURCE *. 



BS discs ; the deflections for the SS and SB discs are not so reUable, as it 
became afterwards evident that these discs require even longer time to recover 
from the initial disturbances (see below). 

Table II. 



i 


BB 


BS 


SS 


SB 


Series A 

Light on for about 20 or 
30 mins. ; zero observed be- 
tween each exposure to light 


D 


14-91 
15-56 
15-03 


20-45 
23-29 
20-77 


32-32 


27-22 


Mean D 


15-17 


21-50 


32-32 


27-22 


R 


17-15 
17-01 


22-89 
23-94 


27-36 


26-19 


Mean R 


17-08 


23-42 


27-36 


26-19 


Mean D and i? 


16-13 


22-46 


29-84 


26-71 


1 

Series B 

Light on for 1 hour; zero 
observed only at beginning 
and end of observations on 
several discs 


D 


(13-78)* 
15-20 
14-71 


2107 
20-44 


30-71 

28-51 


29-15 
27-95 


Mean I) 


14-96 


20-76 


29-61 


28-55 


R 


1711 


23-37 


27-72 


27-42 


Mean D and R 


16-04 


22-07 


28-67 


27-99 


Final Values 

BB and BS taken as mean for 
Series A and B 
! SS and SB from Series B alone 




16-1 


22-3 


28-7 


28-0 



Deflections in scale-divisions for the four discs. Observations on the 'direct' 
and 'reverse' sides are denoted by D and R respectively. 

* Rejected in taking the mean as, owing to a breakdown of the pump for exhausting the 
liquid air, the vacuum had doubtless deteriorated. 



In the Series B, made a month later, the beam was kept on each disc for 
one hour before reading the deflection, with the object of obtaining a still 
closer approach to a steady state. One hour was also allowed before taking 
the zero after cutting off the light. The much greater time now required 
made it impracticable to observe the zero more often than twice while making 
observations on all four discs. Experience showed that, provided the vacuum 
was well maintained, the zero seldom changed more than one or two tenth- 



THE RECOIL FROM LIGHT 



389 



divisions during five hours. There appeared, therefore, no objection to this 
course. 

Examination of the table shows that in most cases rather different values 
are given by the * direct' and 'reverse' observations. This is particularly- 
evident in the case of the BB disc, where the values, roughly 15 and 17 
divisions respectively, differ by about 13 per cent. The explanation of 
this appears to be that the radiometer-action was still sensible and acted 
differently in the ' direct ' and ' reverse ' positions of the disc. The difference 
was probably due to inequality in the thickness of the two cover-glasses 
enclosing the asphaltum layer. 

Some observations were made without using liquid air, i.e. with the charcoal 
bulb at room-temperature. The deflections were then always very unsteady, 
and the zero showed erratic behaviour. A few deflections obtained under 
these conditions are given in Table III. They merely serve to indicate the 
general effects of the gas-action. On the BB disc the want of symmetry 
referred to above is now greatly exaggerated, and points to the gas-action 
being a suction on the 'direct' side and a pressure on the 'reverse.' It is 
also to be noted that the action is a suction on the BS disc. Any tendency 
towards this action in the high-vacuum experiments would, therefore, tend 
to mask the recoil-pressure sought for. 

Table III. 





BB 


BS 


SS 


SB 


Direct 


5 
37 


- 19 

- 77 


no obs. 
45 


93 

58 


Reverse 



Deflections for the four discs with charcoal bulb at room-temperature. (The 
minus sign indicates suction.) 

On account of the uncertain values of the various corrections required, and 
on account of the existence of outstanding disturbances, it was felt that mere 
multiplication of observations would not lead to more exact results. As the 
final values, in scale-divisions, for the pressures on the four discs given by the 
experiment we therefore take 

BB BS ss SB 

16-1 22-3 28-7 28-0 

The values for BB and BS are the means given by both Series A and B, 
the values for SS and SB are from Series B alone, since for these discs the 
steady state was not attained properly in the observations of Series A. 

Hence we have 



BS 
BB 



= 1-39, 



BS 



J (SS + SB) 



= 1-58. 



390 THE PRESSURE OF LIGHT AGAINST THE SOURCE: 

We select for comparison with these ratios those calculated for r = 0-05, 
p = 0-95 in the last columns of Table I, i.e. the values 

m~^'^^' UssTsB)- ''"''• 

The latter ratio agrees better with the experiment than the former. This 
is what we might expect, since the latter ratio does not involve the pressure on 
the BB disc, and that is the pressure which is most affected by the radiometer- 
action. As to the actual reflection-coefficients for the surfaces of the discs, 
we can do but httle more than make a guess. For the black surface we may 
take the glass surface as reflecting 4 per cent, of the incident Ught, and 
allowing another 1 per cent, for the asphaltum — probably a reasonable 
estimate — we have, finally, r =- 0-05. In the case of the silver surface the 
reflection was tested by means of a thermopile, and it was concluded that at 
least 96 per cent, of the beam used was reflected. 

The Energy of the Beam. 

A determination of the energy of the beam used was made and afforded a 
means of calculating the absolute values of the pressures to be expected. 

As in the experiments of Nichols and Hull, and in other experiments on 
light-pressure which we have made, the energy was measured by allowing the 
beam to fall on a blackened disc of pure silver (2 cm. diameter and 0-28 cm. 
thick), and by observing the initial rate of rise of temperature by means of 
a constantan-silver thermo-electric junction soldered to the disc. A Rubens 
panzer-galvanometer was used and was adjusted to have a period of about 
two seconds and was then made dead-beat. The transit of each centimetre- 
division of the scale across the field of view of the observing telescope was 
recorded on the drum of an electrically driven chronograph. Just before and 
just after each series of transits, a set of 5-second intervals were also recorded 
in order to give the peripheral speed of the drum. The number of micro- 
volts per scale-division was then determined, and the thermo-electric power 
of the couple being known from a separate experiment, it was possible, by 
means of a graphical representation of the chronograph-record, to calculate 
the rate of rise of temperature, and hence the energy of the beam was 
calculated to be 33 x 10~^ erg per centimetre length. This would be the 
force in dynes on a fully absorbing surface. The moment of inertia of the 
suspended system was 0-770 gramme-cm. 2, and its period was 74-6 seconds. 
The arm was 1-00 cm. Hence the beam should give a deflection of 13-6 
divisions of the scale used when falling on a disc fully absorbing on both 
sides. Assuming that the BB disc reflects 5 per cent., the deflection should 
be 14-3 divisions. This is in close agreement with half the value obtained 
with SS and SB, and the excess over 14-3 of the observed value 16-1 obtained 



THE RECOIL FROM LIGHT 391 

with BB is probably to be ascribed to residual radiometer-action. The 
smallness of the excess shows that the radiometer-action was reduced to 
a very small amount. 

Initial Effects. 

We have already referred to the necessity for exposing the discs in general 
to the beam for a long time before taking the observations of the deflections. 
On the BB disc, however, there was no marked initial effect, and after a few 
periods the deflection was nearly constant. When the beam was first allowed 
to fall on one of the other discs an initial disturbing effect was evident, and 
in some cases even half an hour seemed insufficient to produce a steady state. 

On the BS disc there were sometimes indications of a pressure for the first 
few seconds, but a strong suction always set in, and this suction, after reaching 
a maximum, rapidly subsided, giving place finally to a pressure increasing to 
the Umiting value corresponding to the steady state. 

The SS disc showed want of symmetry. On the direct side there was 
suction followed by pressure, as in the last case, but on the reverse side there 
was at first excessive pressure, reaching a maximum and then slowly falling 
to the value given in the steady state. 

The SB disc showed an initial excess pressure very similar to that on the 
reverse side of the SS disc. 

For both the SS and SB discs the duration of these initial effects was so 
drawn out that an appreciably steady state was not attained in much less 
than an hour. In Fig. 5 the effects on all four discs are represented by curves 
in which the time is taken as abscissa and the deflection as ordinate. The 
residual effects which follow the cutting off of the Hght are also indicated. 

As an explanation we suggest that these effects are due to the heat of the 
beam driving off occluded gas from the silver films, the expulsion of the gas 
causing a back pressure on the film. In the case of the SS disc it was known 
that the silver film on one side was decidedly thicker than that on the other, 
so that there is no difficulty in accounting for the want of symmetry observed 
in that case. 

This explanation is supported by two observations : first, that with a 
stronger beam the steady state is sooner reached ; secondly, that in the case 
of the BS disc it was noticed that if after the estabhshment of the steady 
state one cut off the light for a minute or two, and then put it on again, an 
initial suction took place but was much less than originally. 

Assuming that the effects observed were really due to the expulsion of 
occluded gas as suggested above, it is possible to form an estimate of the total 
mass of gas given off by calculating from the curves (Fig. 5) the total impulse 
given to the disc. Thus, if we assume the gas to be oxygen and suppose, 



392 



THE PEESSURE OF LIGHT AGAINST THE SOURCE: 



further, that the molecules leave the film normally with the ordinary molecular 
velocity, we find that for the BS disc the total mass given off was about 
1*7 X 10-' gramme. Taking the volume of the experimental flask as 2 litres. 



BB 



fi 




a 



SB 



mr/7S 



Fig. 5. Initial Effects. The ordinate represents the deflection, plotted against the time as 
abscissa. The direction of the deflection for pressure is shown by the arrow. For the 
SS disc curves D and R correspond to the ' direct ' and ' reverse ' sides respectively. 
The light is put on at a and cut off at ^. 



this quantity of gas would give a partial pressure of about ^^ dyne/cm. 2. 
So that unless the gas is rapidly absorbed by the charcoal it would appear 
that the vacuum might be sensibly affected. Unfortunately we had no 
means of forming even a rough idea of the actual pressure of the residual 
gas in the apparatus. 



THE RECOIL PROM LIGHT 393 

The Temperature of the Discs. 

In the steady state all the energy of the beam absorbed by a disc must be 
! radiated from the faces. By assuming the fourth-power law of radiation, 
we may therefore estimate roughly the rise in temperature of the disc. The 
results are 55° C. and 90° C. for the rise in temperature of the BB and BS 
discs respectively. 

For the BB disc it was also estimated that the temperature-difference 
between the two faces was probably less than J^° C. The temperature- 
differences for the other discs should be still less. It should be noticed that 
the glass is black for the issuing radiation, hence the asphaltum layer alone 
counts in the case of the BB disc. 

Early Experiments with Platinum Discs. 

In some early experiments we used two discs, BB and BS, oi platinum foil 
J mm. thick, the black surfaces being formed by depositing platinum black. 
Results were obtained somewhat like what we expected as to the ratios, but 
the deflections depended very greatly on the state of the vacuum, and under 
the best conditions were about 50 to 100 per cent, greater than the values 
calculated from the energy of the beam. This disagreement was doubtless 
chiefly due to radiometer-action. The black surface, being flocculent, is 
obviously badly conducting, the temperature-slope is therefore increased, and 
we have in consequence a big differential radiometer-action on the faces of 
the discs. Moreover, the polished platinum is a poor reflector, so that such 
discs quite fail to approach the ideal conditions. These considerations led to 
the use of the asphaltum discs. 



29 



ON SMALL LONGITUDINAL MATERIAL WAVES ACCOMPANYING 

LIGHT-WAVES. 

[Roy. Soc. Proc. A, 85, 1911, pp. 4'74-476.] 

All experiments on the pressure of light agree in showing that there is a 
flow of momentum along the beam. This flow is manifested as a force on 
matter wherever there is a change of medium. When the light is absorbed, 
the momentum is absorbed by matter. When the beam is shifted parallel to 
itself there is a torque on the matter effecting the shift. The momentum 
would therefore appear to be carried by the matter and not merely by the 
ether. Though there is an obvious difficulty in accepting this view when the 
density of the matter is so small as it is in interplanetary space, it appears to 
be worth while to follow out the consequences of the supposition that the 
force equivalent to the rate of flow of momentum across a plane perpendicular 
to a beam of light acts upon the matter bounded by the plane. This rate 
of flow per square centimetre is equal to the energy-density or energy per 
cubic centimetre in the beam. Of course, in experiments, only the average 
of the rate of flow during many seconds and the average energy per cubic 
centimetre in a length of beam of millions of miles is actually measured. 
But on the electromagnetic theory of light, which suggested the experiments 
and which gives the right value for the pressure, this pressure is equal to the 
energy-density at every point of a single wave. 

Let us suppose that we have a train of plane polarised electromagnetic 
waves of sine-form, the magnetic intensity being given by 

H = H^ sin ^ (x — vt), 
A 

where H^ is the amplitude of H. The magnetic energy per cubic centimetre 
at any point is hH^/Stt, and as the electric energy is equal at each point to 
the magnetic energy, the total energy is jjlH^I^^tt. 



The energy per unit volume is ~ — dx = ijuH-^j^tt. 



SMALL LONGITUDINAL MATERIAL WAVES ACCOMPANYING LIGHT- WAVES 395 



The pressure p across a transverse surface is 

AfIT 



p = fiH^I4^ = ^ sin2 ^(x- vt) 



&7T 



1 — COS -y (iC — Vt) 



The force on an element of length dx is 



^ dx = 
dx 



'-^ J- sm jr [x — vt) dx 

fJiH^^ .477. .. 

= — ^-^ sm -Y (x — vt) dx. 

If ^ is the linear longitudinal displacement of the element there will be 
a force due to elastic change of volume 

d^$. 

where q is the elastic constant for compression or extension. 

If p is the density of the material, the equation of motion is 

d^ M^l^,.47^ 
^' dx^ 



d^i 

Pd^ 



2A smy (a;-i;0. 



477 



A sin -Y (x — vt — e). 



Assume 
Then, substituting, 

( p^ . -^ v^ - qA -^ j sm -j-(x-vt-e)= ^^^ sm y (^ - ^'0- 

Putting X =^ and ^ = 0, we see that e = 0. Putting q = pu^, where u 
is the velocity of free elastic waves of the q type, and assuming that the 
longitudinal waves are forced waves, keeping exact time with the waves of 
light, we have 

Xp^H ,^ 

32tt^P (v^ - u^) ' 

As u/v is negligible for all ordinary matter, 

. XuHi' .477. ,, 

. uHi^ 477, . 

The potential energy in these waves is neghgible in comparison with the 
kinetic. We have then 



Energy per unit volume = J p^hlx 

•' 



25677^^^ ' 



396 SMALL LONGITUDINAL MATERIAL WAVES ACCOMPANYING LIGHT- WAVES 

As the electromagnetic energy per unit volume is ^xH-^j^, 

Energy in longitudinal waves _ ^E^ _ 1 ij^H^ jpv^ 
Electromagnetic energy S27Tpv^ 8 Stt / 2 ' 

which is one-eighth of the electromagnetic energy divided by the energy 
which the matter would have if it were moving with the velocity of light 
in that matter. 

This shows how infinitesimal is the fraction of the energy of the beam 
which is located in these waves of compression of the material. 

The fraction is proportional to the intensity of the beam. 

As an example, take a beam of the intensity of full sunlight just outside 
the earth's atmosphere, in which the energy-flow is about 1-4 x 10^ ergs/sec. 
The energy-density ixH^^j^rr is therefore 1-4 x 10^ ^ v. Put v = 2> x lO^^jn, 
where n is the refractive index. The fraction is 

1 1'4 y 10^ ^3 

At the surface of the sun it would be about 40,000 times as much, say, 
5 X 10-22^7^. 

It is interesting to note that if a beam of light is incident on any reflecting 
or absorbing surface and if the pressure of light is periodic with the waves it 
must give rise to ordinary elastic waves in the material of frequency double 
that of the light-waves. 



30. 

ON THE CHANGES IN THE DIMENSIONS OF A STEEL WIKE WHEN 
TWISTED, AND ON THE PRESSUKE OF DISTORTIONAL WAVES 
IN STEEL. 

[Roy. Soc. Proc. A, 86, 1912, pp. 534-561.] 

[Read March 21, 1912.] 

In the Proceedings of the Royal Society* there is an account of some 
experiments which I made to show that wires when twisted lengthen by an 
amount proportional to the square of the angle of twist, a result expected 
from an analysis of the strains in a finite pure shear. In those experiments 
it was necessary to put considerable loads on the wires. 

I have now succeeded in measuring the change in the diameter of a wire 
when twisted, as well as the longitudinal extension, and have found that the 
change, a contraction, is also proportional to the square of the angle of twist. 
It has been now found that the change is sensibly the same for large loads 
and for the smallest load which could be used, when the wire was sufficiently 
straightened before being twisted, so that apparently the only function of the 
load is to straighten the wire. 

To measure change in the diameter the wire was fastened at the bottom 
of a long narrow tube, the 'wire- tube,' filled with water. It passed out from 
the top of the wire- tube through a water-tight leather washer. A capillary 
glass tube rose vertically from an orifice in the side of the tube, into which 
it was cemented, and the change of the water level in the capillary when 
the wire was twisted indicated the change in the volume of the wire within 
the wire-tube. 

Description of the Apparatus. 

The apparatus used for the measurement of the effects is shown in Fig. 1, 
where, for convenience of representation, various parts are put into the plane 
of the figure, though actually they were in different planes. 

An iron bracket B projected from the wall of the laboratory, and a tripod 
rested on it, on three levelHng-screws. The tripod carried a conical bearing 

* Series A, 1909, vol. 82, p. 546. [Collected Papers^ Art. 25.] 



398 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 



for the twisting head-piece T. When the 
axis of this was exactly vertical, the tri- 
pod was fixed to the bracket by clamping- 
screws s, s. The twisting-head was pro- 
vided with a circular plate P, with marks 
at 90° intervals, which could be set against 
a fixed index i. In practice only whole 
turns were given, so that only one mark 
was used, except in one experiment de- 
scribed later. 

At the lower end of T, there was a 
chuck into which the upper end of the 
wire was inserted, and a tightening-screw 
made a firm grip. The wire in all cases 
was very nearly 160-5 cm. long. At its 
lower end it was gripped by a similar 
chuck attached to a steel cross-piece C, 
about 29 cm. long, seen endwise in the 
figure. 

Polished steel plates were screwed on 
to the vertical sides of this cross-piece 
near its ends. Four horizontal screws, 
working in brackets projecting from the 
wall, and with small steel balls at their 
ends, were screwed up so as just not to 
touch the steel plates when there was no 
twist on the wire. But when a twist was 
put on, the cross-piece moved up against 
two of the screws, and was thus fixed in 
position. Below the cross-piece there was 
a rod to which was attached another rod 
carrying a platform p, and on this weights 
could be placed. Each weight was in two 
semicircular halves. Below the platform 
was a lead weight S, which I call the 
sinker, with a volume of 1020 c.c. This 
hung in a can, and near the can was a 
water cistern, not shown, connected to it 
by a rubber tube. When the cistern was 
pulled up water flowed into the can so as 
just to cover the sinker and lessen the 
load by 1020 grm. When the cistern was 




Fig. 1. 



AND ON THE PRESSURE OF DISTORTIONAL WAVES IN STEEL 399 

let down the water flowed back into it and the load increased to its full value. 
This was used to determine Young's modulus and Poisson's ratio. 

A table t was fixed to the wall independently of the bracket B, to carry 
the observing microscope, and the observer sat on a platform built up about 
1-5 metres from the floor. The brass wire- tube had an internal diameter 
about 2 mm. The top of the tube was fixed in a horizontal brass plate, in 
which was a hole about 0-25 mm. wider than the wire. On this plate rested 
a well-vaseHned leather washer about 1-5 mm. thick, drilled so that it was 
fairly tight round the wire. On the washer was another brass plate, with 
a hole in it about 0-35 mm. wider than the wire. Four screws passed freely 
through holes in this upper plate, and were screwed into the lower plate. 
Springs^ between the heads of the screws and the upper surface of the upper 
plate gave sufficient pressure on the washer. It was found necessary to have 
the holes in the plates somewhat larger than the wire, in order to adjust the 
wire and the wire- tube both vertical. An arm, not shown in Fig. 1, projecting 
from the lower part of the apparatus, with a sliding weight on it in the plane 
of the upper side-tubes, sufficed to make this vertical adjustment. At first 
I tried india-rubber washers. They were quite good when first put in, but 
they deteriorated rather rapidly, and, when they began to perish, they let 
a small quantity of water out of the tube when the wire moved. The leather 
washer only required renewal once, when it began to let water escape, and 
then, on examination, it appeared to be due to action on the wire, which 
was perceptibly rough on the surface where it emerged from the tube. A short 
length was cut off the lower end of the wire, and an equal length was let 
down through the upper chuck, so that once more a smooth part of the wire 
passed through the washer. There was no further difficulty, and no evidence 
again of any escape of water. 

At the upper end of the wire-tube there were two side-tubes. A glass 
capillary tube was cemented into one, and bent as shown on the right in the 
figure, the vertical branch being about 10 cm. long. When the tube was filled 
with water, the level in the capillary was adjusted at the level of the micro- 
scope about 7 cm. above the level of the washer, as this was about the rise of 
water in the capillary due to surface-tension, and there would therefore be no 
hydrostatic pressure on the water at the level of the washer. The tendency 
to leak would thereby be lessened, but the precaution was probably needless. 
Into the tube on the left a plunger passed through a leather stuffing-box. 
The plunger had a diameter of 0-2060 cm., and it was driven to or fro by 
a micrometer-screw of J mm. pitch. On the head of this screw was a 10 cm. 
plate, with 500 divisions on its circumference. This plunger was used 
ordinarily to adjust the level of the water in the capillary. But it was also 
used to calibrate the capillary. For this caHbration, the usual observing 
microscope was replaced by a microscope-cathetometer, and the change of 



400 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 

water-level in the capillary was measured for one turn in or one turn out of 
the screw. Turning always in or always out, it was hardly possible to hit 
exactly on a whole turn, but a correction could be made, of course, for the 
fraction of a division in the micrometer head in excess or defect of a whole 
turn. The mean of 10 measurements when the micrometer was driven 
inwards 0-5 mm. gave a rise of 11-135 mm., with a range of 0-155 mm. between 
greatest and least. The mean of 10 measurements when the micrometer 
was drawn out 0-5 mm. gave a fall of 11-190 mm., with a range of 0-045 mm. 
The value was taken as 1 1 - 1 6 mm. This gives the cross-section of the capillary 
as 0-001493 sq. cm., and its diameter as 0-0436 cm. 

At the lower end the wire-tube was soldered on to a screw-cap which could 
be screwed over the chuck gripping the lower end of the wire. 

Below the chuck was a side-tube used to fill the wire-tube with water. 
For this purpose the side-tube was connected with a flask in which water 
Avas boiled. The steam passed up through the crevices in the chuck and out 
at the plunger-tube, from which the plunger was removed. A funnel con- 
taining water was connected on to the plunger-tube, and when the water 
in this was boiling freely, through the passage of the steam, the flask was 
allowed to cool and water was sucked back into the wire-tube. When it 
was full the flask was detached and a cap was screwed on to the lower tube. 
The plunger was replaced and the capillary, which had been closed meanwhile, 
was opened. By driving in the plunger the water was raised up to the level 
of the washer and to any desired point in the capillary. 

When the apparatus was not being used the open end of the capillary was 
under water in a beaker, the plunger being driven in so that the capillary 
was entirely filled with water. The apparatus thus remained full of water 
whatever change of temperature might occur. When required for work the 
beaker was withdrawn and the plunger was screwed out till the meniscus 
was in the field of view of the microscope. 

The wire-tube was surrounded by an outer tube about 2-5 cm. diameter, 
filled with water. This merely served as a means of reducing the effect of 
outside or inside temperature-changes. A wooden casing covered with tin foil 
surrounded the whole from the floor up to the table to lengthen out still 
further any effects due to temperature-change. 

To observe the changes of level due to twisting, a microscope with a 
1-inch objective and provided with a parallel-plate micrometer was used. 
The micrometer-scale was calibrated by means of a millimetre divided to 
tenths on a standard invar bar. Twelve determinations of 0-4 mm, gave 
107*2 micrometer-divisions equal to 1 mm., the determinations falling within 
about 1 per cent, range. Then 1 micrometer-division = 0-00933 mm. Since 
the cross-section of the capillary is 0-001493 sq. cm., one division of the 



AND ON THE PRESSURE OF DISTORTION AL WAVES IN STEEL 401 

micrometer signifies a change of volume of the water in the wire-tube of 
1-393 X 10-6 c.c. 

When it was desired to read the height of the water in the tube the 
micrometer- plate was moved till the cross- wire in the microscope just 
touched the image of the lowest point of the capillary meniscus. The field 
was well illuminated by a small lamp behind the capillary, but the image 
was not always very distinct, and settings of the micrometer could not be 
trusted to, I think, two or three tenths of a division in some cases, though 
usually they were more exact. Close to the tube and between it and the 
micrometer-plate a small vertical plate of glass was fixed to the tube at 45° 
to the line of sight, and this reflected the point of a needle which was also 
fixed to the tube, so that its image was in the same plane as, and close to, 
the image of the meniscus. This enabled the observer to note the position 
of either the meniscus or the needle-point without moving the microscope. 

When the wire lengthened the wire-tube was let down by an equal amount, 
and the needle-point fell. Let us call this fall NP. 

At the same time the wire contracted laterally, and the meniscus fell in 
the tube, and the fall relative to the tube gave the change in volume. The 
fall observed was that relative to the tube plus that of the tube or NP. 

Hence, if the fall observed in the microscope is T the fall relative to the 
tube is T - NP. 

The Wires and their Preparation. 

Two piano-steel wires were used in the experiments here described. No. I 
with a mean diameter 0-0986 cm., the diameters in two planes at right angles 
being measured with a micrometer every decimetre of its length. The 
measurements ranged from 0-0980 to 0-0989 at different points. No. II had 
a mean diameter of 0-1210 cm., measured in the same way, with a range at 
different points from 0-1207 to 0-1212. 

It was found necessary to straighten these wires, for, unstraightened, they 
showed the effect with light loads noticed in the previous paper *, an apparent 
shortening on twisting, due, I think, to coiling. To straighten them they 
were loaded and an electric current was passed through them. No. I was 
loaded with 50 kgrm., and received a current of 10 amperes. No. II was 
loaded with 60 kgrm., and received a current of 16 amperes. In each case 
the wire drew out slightly and then stopped, acquiring a blue temper without 
rising to a red heat. 

Of course the wires became circularly magnetised, but the magnetisation 
can hardly have contributed to the results here to be described, as these 
results are of the same character and order as results obtained with heavily 

* [Collected Papers, Art. 26.] 
p. c. w. 26 



402 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 

loaded unannealed wires in a number of preliminary experiments made 
before the experiments took their final shape. 

A few experiments were made on a hard-drawn copper wire, mean diameter 
0-1219 cm. (with a range from 0-1216 to 0-1224 cm.). 

The Method of Measuring the Lowering on Twisting. 

In making any determination of NP or T the following plan was adopted. 
Suppose, for instance, that the value of T was to be found for four turns 
of the wire clockwise as seen from above. The position of the meniscus 
was read for no twist at a given minute, then my assistant put on four turns 
clockwise — denoted by C^ — then he gave a signal just before, and again 
exactly at the next half minute, and I set the cross- wire on the meniscus at 
the half minute. The micrometer was read, and the twist was taken off. 
At the next half minute the micrometer was set as before. Again C^ was 
put on, and so on, usually for 32 observations. The first two or three 
readings were not taken into account, as initially there was usually some 
irregularity, due probably to settling down in the bearing. The readings 
were combined in threes in the usual way to give T = \ {a -\- c) — h to 
eliminate as far as possible any march of the zero reading. With the 
meniscus there was almost always a march, due chiefly to temperature- 
change, for, of course, the arrangement was a very sensitive thermometer. 
The mean result of the 32 observations was equivalent to 15 or 16 inde- 
pendent determinations. To determine NP the same course was followed, 
except that the time was not noted. For, though there was often a march 
in the zero, it was very much smaller, and the observations were made at 
suflQ-ciently nearly equal intervals of time without noting exact times. This 
small march was doubtless partly due to temperature-change, but also partly 
due, I believe, to further settling down of the cone of the twisting-head into 
its bearing through slow squeezing out of the oil. In reconstruction I should 
try the effect of replacing the conical bearing by a ball-bearing. 

Before the reading was made it was found to be absolutely necessary to 
move the head-piece some ten or fifteen times to and fro through a small 
angle — perhaps diminishing from 20° — on each side of the final position. 
If this was not done the wire did not sink down or rise up to its final position, 
probably owing to some small friction in the leather washer. After the 
alternating motion of the head-piece had been given ten or fifteen times no 
further alternation made any difference in the reading. It was only after 
finding the necessity of this that I obtained consistent readings. 

The maximum and minimum values of NP in a set of 30 determinations 
usually differed by about 0-4 division, only once rising to 0-8 division. 

The maximum and minimum values of T in a set of 30 differed more, as 
might be expected; the difference averaging 1-7 divisions, and once rising to 



AND ON THE PRESSURE OF DISTORTIONAL WAVES IN STEEL 



403 



nearly 5 divisions. This, however, was in the case of the least load, when the 
weight was probably insufficient to keep the wire-tube quite vertical. 



Subsidiary Experiments, 

The three following subsidiary experiments were made in order to justify 
the methods of measuring the changes of dimensions on twisting : 

1. To show that the lengthening on twisting is not due to a change in 
Young's modulus, Y. 

This is satisfactorily proved by the experiments on twisting described 
below. For, suppose that we have a stress P applied to the end of the wire 
by a load stretching length I by dl when the wire is not twisted, we have 
dl = Pl/Y. Now let the wire thus loaded be twisted through, say, four turns 
and let the lowering through twisting be 8. If this is due to a change in 
Young's modulus to F, dl + S = Pl/Y'. Whence S = PI (F-i - F-i), and 
S should be proportional to P, whereas it is found to be very nearly the same 
for loads varying from 5 to 50 kgrm. (approximately). 

It appeared worth while, however, to test the question directly, by finding 
the extension of wire I for very different loads when 1-02 kgrm. was added, 
first with the wire untwisted, then with the wire twisted through four turns 
clockwise. The following results, in micrometer- divisions, were obtained, 
each the mean of a number of measurements : 

Table I. 



Load 


No twist 
Lowering for 1-02 kgrm. 


C4 twist 
Lowering for 1-02 kgrm. 


18-5 
38-5 

48-5 


10-55 
10-68 
10-50 


1 
10-52 
10-80 
10-39* 


Means 


10-58 


10-57 



Thus a twist of four turns produced no measurable change in Young's 
modulus. 

2. To show that the rise and fall of the liquid meniscus were due to, and 
measured, the change in volume of the wire in the wire-tube. 

The most satisfactory way of showing this appeared to consist in using the 
apparatus to measure Poisson's ratio a. The load was altered by 1-02 kgrm. 
by alternately immersing the sinker in water and letting the water run out. 

* [This corrected value is that given in the original manuscript. Ed.] 

26—2 



404 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 

The rise and fall of the needle-point gave the end-extension, and Young's 
modulus Y could then be calculated ; while the rise and fall of the meniscus 
gave the volume-change, and the side-contraction and Poisson's ratio a could 
be calculated. The rigidity-modulus n could be obtained from n = ^ Y/(l + a). 

When the load was increased there was some yield of the supporting 
bracket. To determine its amount, a needle-point was fixed on the bracket 
close to the upper chuck and sighted by the microscope. A series of loads 
up to 50 kgrm. showed that the lowering per kilogramme was 0-06 division. 
An addition of 1-02 kgrm., therefore, lowered the bracket by 0-061 division, 
and this had to be subtracted from the NP reading when used to find Y. In 
the lowering of the meniscus T — NP, it obviously did not come into con- 
sideration. The observed change of volume given by T — NP had to be 
corrected by a factor about 160-5/156, since only 156 cm. of wire were within 
the wire-tube and 4-5 cm. were outside. The actual length outside varied 
from 4-2 to 4-6, and the factor was varied accordingly. 

No doubt better values of Y and a might have been obtained with a larger 
change of load, but, to test the apparatus, it was important to observe lower- 
ings of the same order as those observed in the twisting. In the following 
Table II the values of T — NP and NP, due to an addition of 1-02 kgrm., are 
given in micrometer-divisions corrected as above described. Each value is 
the mean of 30. The range between maximum and minimum in a set averaged 
1-7 divisions for T and 0-57 division for NP : 

Table II. Elastic Moduli and Poisson's Ratio. 



Load in kgrm. 


NP 


T-NP a 


10-12 y 


10-12 n 




Steel Wire I, Diameter 0-0986 cm. 




18-5 

28-5 
38-5 

48-5 

Mean values 


10-65 
10-60 
10-74 
10-44 


29-86 
29-57 
29-37 
29-05 


0-272 
0-271 
0-265 
0-270 


2-11 
2-12 
2-09 
2-16 


0-830 
0-835 
0-828 
0-849 


— 


0-270 


2-12 


0-835 




Same Wire with C^ Twist on it 




48-5 


10-27 1 28-04 1 0-265 j 2-18 
Steel Wire II, Diameter 0-1210 cm. 


0-861 


48-5 


7-05 1 31-22 1 0-287 | 2-12 [ 
Hard-drawn Copper Wire, Diameter 0-1219 cm 


0-825 


18-5 


11-20 61-52 0-331 1-31 


0-493 



AND ON THE PEESSURE OF DISTORTIONAL WAVES IN STEEL 405 

The values found for a steel wire after annealing, given in the paper 
already referred to*, were Y = 2-06 x lO^^, n = 0-809 x 10^2 (by vibration), 
whence or = 0-273. 

The values for the steel wires are sufficiently near to each other and to the 
values previously found to show that the tube-readings gave, at any rate, very 
nearly the true changes in volume. 

3. To show that the changes were very nearly isothermal. 
The change in temperature of a solid sheared adiabatically through e is 
de=- Xnd€y2JC^p, 

where An is the decrease in rigidity per degree rise, 6 is the absolute tem- 
perature, Cp is the specific heat, and p is the density. 

Let us suppose that a steel wire 156 cm. long and 0-05 cm. radius — nearly 
wire I — is twisted through one turn. It is sufficient to investigate the effect 
for one turn, for both the adiabatic temperature-change and the twisting- 
effects are proportional to the square of the shear, and therefore in a ratio 
independent of the shear. Then e = 27rr/156, where r is the distance of an 
element from the axis. The mean change in temperature of such a wire is 

[^ 27Trdedr _ Tr'^a^Xnd 

where a = 0-05. 

For steel we may put n = IO12, A = 2 x lO"*, p = 7-8, C^ = 0-112. Taking 
6 as 300° A. we find the heat in calories developed by the twist to be about 
— 18 X 10-* calories. Or on untwisting + 18 x 10"* calories. 

If this heat were confined to the steel it would alter its temperature by 
about 1/600° C. and its linear dimensions by about 1-8 x 10"^ in 1. The 
twisting through one turn, as will be seen below, alters the radius by about 
3-19 X 10-*^ in 1 and the length by about 1-72 x 10"^. The effects of an 
adiabatic change of temperature would, therefore, be appreciable compared 
with the effects of twisting, especially on the radius. But the wire shares its 
heat, positive or negative, with the water in the wire-tube, and here it may 
produce a serious effect, if it gets no farther than the wire-tube, owing to the 
considerable coefficient of expansion of the water. Let us suppose that the 
heat or cold is shared with the water in the wire-tube so rapidly that both are 
at one temperature. The water, having approximately three times the volume 
of the steel, has about 7/9 the heat-capacity of steel plus water. So that the 
water would receive about 14 x 10"* calories. If a mass of water at a tempera- 
ture at which its cubical expansion is a receives H calories its change of volume 
is Ha, whatever the total volume. In our case the temperature was usually 
about 12° C, at which a is about 10"*. Then the volume-change would be 
about 14 X 10~^ c.c. and since one micrometer-division is about 14 x 10~' c.c. 

* Loc. cit. p. 554. [Collected Paper s^ p. 366.] 



406 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 

along the capillary, one turn, through thermal effect alone, would produce a 
fall on twisting and a rise on untwisting of about 0-1 division. The actual 
change observed on twisting through one turn was about 0-56 division. If 
then the heat or cold only slowly spread from the wire, or again if it were 
rapidly shared with the water in the wire-tube but only slowly spread thence, 
the measurements would be seriously affected. But it is obvious that there 
must, in reality, be a rapid adjustment of temperature between the wire-tube 
and the outer water-jacket, and it was important to find out how rapidly 
the adjustment progressed. Fortunately the wire was insulated from the 
tube where it passed through the washer, so that it was easy to pass an electric 
current through it by connecting the terminals of a battery, one to the bracket, 
the other to the wire-tube. Heating-currents of the order of 1 to 2 amperes 
were thus passed along the wire. The current was put on for 2 seconds, 
the meniscus rushing up meanwhile fairly uniformly, and the point to which 
it rose was read on the micrometer. Then, 15 seconds after the cut off, 
the position of the meniscus was read again and the mean of a number of 
determinations showed that after 15 seconds only 0-032 of the original rise 
remained. The original rise varied from 7 to 18 divisions with different 
currents. If the twisting were made instantaneously and the reading of the 
fall in the tube were made 15 seconds later, about 0-032 x 0-1 -^0-56 = 0-006 
of the fall would be due to the cooling on twisting. But this is a very con- 
siderable over-estimate. The twisting was usually begun 25 seconds before 
reading and ended more than 15 seconds before. The effect of temperature 
change may, I think, be estimated at less than 1/300 of the whole. It was 
impossible to assign even an approximate value to it and as it proved to be 
so small it was neglected. The effect would have been reduced altogether 
beyond consideration if the readings had been taken at intervals of one minute, 
but this would have introduced errors, probably much worse, through 
irregularities in the march of temperature. 

In the experiments on Poisson's ratio the adiabatic change of temperature 
on adding a load which stretches length I by dl is 

dd = - aYddl/JC^pl, 

where a 7 is the change in Young's modulus per degree. The value of a for 
steel is about 1/4000. This gives the heat for a stretch of 10 divisions as 
about — 9 X 10-^ calories. With uniform temperature of steel and water in 
the wire- tube the water would have about 7 x 10~^ calories, and its effect 
would be about 0-5 division. After 15 seconds it would be about 0-016 division. 
As the change of level observed was about 30 divisions the effect is negligible. 
The direct effect in lengthening or shortening the wire is easily shown to be 
very much smaller. 



AND ON THE PRESSURE OF DISTORTIONAL WAVES IN STEEL 407 

Measurement of the Changes of Dimensions on Twisting, 
The method of making the measurements has been described abeady. In 
the case of wire I, NP, or the lengthening of the wire, was observed for various 
loads for four turns and for two turns clockwise twist, denoted by C4 and Cg, 
and for four turns and for two turns counter-clockwise twist, denoted by CC^ 
and CC 2i each value being the mean of 30 determinations — once or twice of 
40 — made as described above. As permanent set came in with five turns, 
four turns was the maximum twist employed. The mean values of T were 
also determined for the same four twists and T — NP was corrected for the 
length of wire outside the wire-tube. For one load on wire I the lowerings 
for Og and COg were also observed. 

In all cases the lowering w could be represented very nearly by the parabola 

L (n -\- cY = w -\- h, 

where n is the number of turns put on and L, c, and b are constants, not, of 
course, the same for NP and T — NP. The constant c represents the fraction 
of a turn, always on the counter-clockwise side of the point of no twist, about 
which the lowering is symmetrical. Putting n = — c, b = — w is a small 
shortening, or for a counter-clockwise twist c the wire has a minimum length. 
The existence of c and b is due to want of homogeneity in the wire. They 
may be explained by supposing that the wire in the apparently neutral 
condition consists of a core and a sheath twisted against each other, as will 
be shown in the theory given later. Owing to want of exact centering the 
image * wobbled' somewhat in the field during twisting, and only returned 
to the same vertical line after a whole number of turns, so that it was futile 
to attempt to measure b. But there was fairly conclusive evidence that it 
had a real existence. 

According to the theory given, L is the all-important quantity. The 
internal strain only shifts the vertex of the parabola without altering its size. 

To find the constants of a parabola which should fairly represent the 
results, it was assumed that the curve went through the point w = 0,n = 0, 
so that Lc^ = b. 

The equation then becomes 

L (n^ -\- 2nc) = w. 

Let w be the lowering for C„, and w' that for CCn, then 

L (n2 - 2nc) = w', 
whence 

L== {w + w')/2n^ and c = {w - w')linL = ^n (w - w')l(w + w'). 

The errors are given by 

hL = h{w-\- w')l2n% and Sc =^ S (w - w')linL, 
assuming that L is without error in c. 



408 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 

If then we find the value of L, say L^, from the lowerings at C^ and OC4, 
and the value of L, say L^ , from those at G^ and CG^ , the value of the former 
should have four times the weight of the latter and we may take the best 
value of X as 1^ (4X4 + L^. 

The value of c determined from C4 and CC4 should have twice the weight 
of that determined from C^ and CC2, and we may take the best value as 

J (2C4 + C2). 

In the following tables the results are set out. In Table III the lengthening 
of the wire I is given in micrometer-divisions, and below each lengthening the 
difference, calculated — observed, is put in italics, the calculated values being 
those given by the parabolas of which the constants are given in Table IV. 
Similar tables are given for wire II, and for the hard-drawn copper wire, but 
for a single load only, sufficient to secure good centering. After the experience 
with various loads with wire I, it appeared unnecessary to vary the load in the 
other cases. With wire II it was not thought advisable to go beyond three 
turns, and with the copper wire beyond one turn owing to permanent set, 
which began to be very considerable beyond those limits. The mode of 
calculating the best parabola was modified accordingly. 



Table III. Lowering 


NP for Steel Wire I, Dimneter 0-0986 cm. 


Load 


c. 


C2 


CC.^ \ CC, 

i 


kgrm. 












48-5 


5-095 


1-452 





0-768 


4-233 


1 


+ 0-06 


- 0-03 


1 + 0-13 


- 0-13 


38-5 


5-353 


1-422 





0-818 


4-152 




- 0-05 


+ 0-06 





+ 0-06 


- 0-05 


28-5 


5-265 


1-500 





0-858 


4-259 




- 0-01 


+ 0-03 





+ 0-04 


- O'lO 


18-5 


5-382 


1-680 





0-905 


4091 




+ O'U 


- 0-13 





- 0-04 


+ 0-06 


4-7 


5-407 


1-742 





0-797 


3-865 


1 

1 


+ 0-13 


- 0-15 





- 0-03 


+ 0-04 



For load 28-5, C3 was 3-048, and CC3 was 2-187, and these were taken into 
account in calculating the parabola, the difference in each case being + 0-05. 

Table IV. Constants of Parabolas for Table III. 



Load 
48-5 


0-289 


c 
0-226 


b 


0-015 


38-5 


0-294 


0-256 


0-019 


28-5 


0-295 


0-237 


0-017 


18-5 


0-302 


0-281 


0-024 


4-7 


0-295 


0-345 


0-035 



AND ON THE PRESSURE OF DISTORTIONAL WAVES IN STEEL 



409 



There was probably some permanent set given in the last two owing to 
accidental over-twisting of the wire. 

The mean value of L is 0-295, and within errors of observation it is inde- 
pendent of the load. It is hardly likely that this is strictly true. 

Table V. Lowering of the Meniscus T — NP for Wire /, corrected for 
length Outside the Tube. 



Load 


c. 


c^ 





CO, 


CC^ 


48-5 


11-69 


3-94 





0-07 


6-65 




■\-0-33 


- 0-17 





+ 0-63 


- 0-78 


38-5 


11-63 


3-94 





0-84 


6-36 




+ 0-22 


- 0-28 





+ 0-06 


- 0-22 


28-5 


11-82 


4-00 





1-13 


6-38 




- 0-25 


- 0-2H 





- 0-23 


+ 0-04 


18-5 


10-99 


2-84 





1-00 


6-17 




+ 0-36 


+ 0-37 





- 0-02 


- 0-02 


4-7 


1017 


2-54 





0-99 


8-24 




- 0-14 


+ 0-30 





+ 0-57 


- 0-72 



For load 28-5, Cg was 7-33 and CCg was 2-86. These were used in calcu- 
lating the parabola, and the differences were respectively — 0-01 and + 0-23. 

Table VI. Constants of Parabolas for Table V. 



Load 


L 


c 


b 


48-5 


0-559 


0-69 


0-26 


38-5 


0-569 


0-61 


0-21 


28-5 


0-578 


0-61 


0-21 


18-5 


0=525 


0-54 


0-15 


4-7 


0-548 


0-29 


0-05 



As the errors of observation with the last two loads were about double 
those for the earlier loads, they are only given half the weight in finding L. 
The value of L is taken as 0-561. 



Table VII. Lowering NP for Steel Wire II, Diameter 0-1210 cm. 
Two Independent Sets. 





Load 


Cs 


c. 





ca. 


cc. 


I 


48-5 


4-922 


2-368 





1-740 


4000 






+ 0-05 


- 0-06 





- 0-04 


+ 0-05 


II 


48-5 


4-983 


2-243 





1-708 


4049 






- 0-06 


+ 0-05 





- 0-01 


0-00 

1 



410 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 

Table VIII. Constants of Parabolas for Table YII. 





L 


c 


6 


I 

II 


0-501 
0-499 


0154 
0147 


0012 
0011 



The mean value of L is 0-500. 



Table IX. Lowering of the Meniscus T — NP for Wire II, corrected 
for length Outside the Tube. 





Load 


c^ 


G, 





CO., 


cc^ ■ 


I 


48-5 


15-16 


7-73 





3-90 


9-22 






+ 0-20 


- 0-25 





- 0-31 


+ 0-33 


II 


48-5 


15-48 


7-61 





3-71 


9-55 






+ 0-04 


- 0-06 





- 0-07 


+ O'lO 



Table X. Constants of Parabolas for Table IX. 





L 


c 


b 


I 
II 

1 


1-385 
1-398 


0-35 
0-35 


0-17 
017 



The mean value of L is 1-392. 

Copper Wire. Diameter, 0-1219 cm. 

I was only able to use C^ and CC^ owing to permanent set. 

The values of NP were 1-043 and 0-415, and the parabola going through 
these points and the origin is 0-73 {n + 0-22)^ = iv + 0-03. 

The values of T — NP, corrected for 4-2 cm. outside the tube, were 8-007 
and 1-433, and the parabola is 4-72 {n + 0-35)^ ^w+ 0-58. 

The work given in the former paper appears to justify the assumption of 
the parabolic law for copper. 

The End- Elongation, Side-Contraction, and Volume-Increase. 

Steel Wire I. Diameter, 0-0986 cm. 

If w is the end-lowering for one turn from the position of minimum length 
assumed to be L divisions, 

w = L X length of one micrometer-division 
= 0-295 X 933 x 10-^ = 2-75 x lO"* cm. 



AND ON THE PRESSURE OF DISTORTIONAIj WAVES IN STEEL 411 

The length is I = 160-5. Then 

w/l = 1-71 X 10-8. 
If u is the decrease in the radius a for one turn, 

27raul = L X volume of one micrometer-division of capillary, 
u = (0-561 X 1-393 X 10-«) -r (2tt x 0-0493 x 160-5) 
= 1-57 X 10-8 cm. 
The radius is 0-0493 cm. Then 

u/a = 3-19 X 10-7. 
The ratio side-contraction/end-elongation, namely, 

u/a ^ w/l = 0-187. 
If dv is the volume-increase in total volume v, 

dv/v = (irahv — '2malu)/iTaH 

= w/l - 2u/a = 1-07 X 10-6. 

All the quantities w/l, u/a, dv/v are proportional to the square of the twist 
from the point of minimum length. The ratio u/a -^ w/l is the same for all 
twists. 

Steel Wire II. Diameter, 0-1210 cm. 
Using the values given in the tables, we have for one turn 

w = 4-66 X 10-4 cm., u/a = 5-24 x 10-', 

w/l = 2-90 X 10-6, u/a -^ w/l = 0-181, 

u = 3-17 X 10-8 cm., dv/v = 1-85 x lO-^. 

Copper Wire. Diameter, 0-1219 cm. 

The corresponding quantities given by the single set of observations are 
not of such weight as those for wires I and II, but I add them here : 

w = 6-81 X 10-4 cm. u/a = 1-75 x 10-^, 

w/l = 4-25 X 10-6, .j^ja ^ w/l = 0-41, 

u = 10-7 X 10-8 cm., dv/v = 0-75 x 10-6. 

On comparing the results for wires I and II we see that side-contraction 
~ end-elongation is very nearly the same for both. The theory given below 
makes both w/l and u/a proportional to the square of the radius for wires of 
the same material undergoing the same twist. But as far as these two wires 
are concerned they are very nearly proportional to (radius)^'^. I do not 
think the discrepancy is to be ascribed to experimental error. Perhaps the 
theory is inadequate, but I think that it is more probable that slight differences 
in the material, not greatly affecting the ordinary elastic moduli, may produce 



412 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 

very considerable changes in what we may term the secondary moduli, which, 
in the theory below, are denoted by f and q. 

I should like to have taken observations on several more steel wires with 
a wider range of diameters, but I am not able to continue the work at present. 

Experimental Verification of a Reciprocal Relation. 

In the Philosophical Magazine for November, 1911, vol. 22, p. 740, 
Dr. R. A. Houston has expressed the reciprocal relation between the stretching 
and twisting of a wire (confined within limits of reversibility) in the form 

dd\ fdw\ 



(S) =(|) . (1) 

\aj^ / Q const. \aix/ p const. 



where F is the end pull and w the increase in length, G the torque and 6 
the twist on the wire (I use letters for length and torque differing from 
Dr. Houston's). 

As the apparatus only needed small modification it appeared to be worth 
while to see how nearly this relation was verified, and wire II was used for 
the purpose. Incidentally, the value of the rigidity was obtained by the 
method of statical torque. 

When the observations needed are worked out it is found that they are 
identical, as of course was to be expected, with those needed to verify the 
relation 

m ^C^l) , (2) 

\du J ,^, const. \^^^/0 const. 

which is the more direct expression of the Conservation of Energy in these 
phenomena. 

Taking equation (1) we require to know on the left the extra twist dd 
which must be put upon the wire to keep G the same when a load dF is 
added. For this purpose the wire was initially loaded with 18-5 kgrm., and 
the head was turned through a right angle. The bar at the bottom was 
also turned through a right angle from its usual position. On the cross-bar 
a mirror was fixed reflecting into a telescope a millimetre-scale 156-5 cm. 
away. The ends of the cross-bar were rounded into arcs of a circle with 
centre in the axis of the wire and radius 14-70 cm. Horizontal threads 
passed ofi these arcs to two very light horizontal spiral springs which stretched 
very uniformly in proportion to the pull up to 40 or 50 grm. These springs 
were attached to the bases of two travelling microscopes, of which the 
horizontal scales merely were used to measure any change in stretch. Initially, 
the wire was without twist, and the position of the cross-bar on the scale was 
read. It would have been at least very difficult to determine directly the 
total stretch of the springs required to keep the cross-bar in position when the 
head was twisted, so the following plan was adopted : 



AND ON THE PRESSURE OF DISTORTIONAL WAVES IN STEEL 413 

A half -turn CC was put on by the head-piece, and the springs were stretched 
so as to bring the cross-bar to its original position. Then a further turn and 
a half CC was put on, and the additional stretch of each spring needed to keep 
the cross-bar in position was read. This additional stretch multiplied by 
4/3 gave the total stretch of the springs, and thus the pull exerted at each 
end of the cross-arm to maintain two turns twist on the wire. The full 
stretches thus computed were 12-340 cm. on the left and 12-384 cm. on the 
right, corresponding to pulls, according to previous calibration, of 49*24 grm. 
and 46-87 grm., mean 48-06 grm. The torque was therefore 
G = 48-06 X 981 x 29-4 = 1-38 x 10^ dyne-cm. 

From this the rigidity is 

n = 0-838 X 1012. 

The tube-method gave 0-825 x 10^^^ ^^d the nearness of the two values 
appears to show that the springs could be trusted fairly well. 

A load dF = 30 kgrm. was then added, and the torque for two turns was 
thereby diminished. The springs therefore contracted, and it was observed 
that they pulled the cross-bar round through 15-85 mm. on the scale — the 
mean of five different observations ranging from 15-45 to 16-55, or through 
an angle 0-00507 radian. 

Denoting this angle by SO, and the radius of the cross-bar arm by k, and 
the decrease of torque by SG, 

f=^>:^ (3) 

where s is the whole mean stretch of the springs for two turns. 

But we require the twist dO, which must be put on the wire from its 
initial two turns, and in the opposite direction to Sd, to restore the torque 
to G. This is given by 

G + dd 



G-hG d-W 



here = i^r. 



whence, on substituting for SG/G from (3), we get 

dd^[^--l)W, and jp=[~-l)j^ W 

Taking the right hand of equation (1), we require to know the lowering dw 
for a change dG in the torque under constant load. We get the lowering 
from the equation 

L{n-cY = w-\-h, 

giving dw = 2 L {n — c) dn = L [n - c) —. 

Also dir = —K-. 

dw L(n-c)e , . 

Ti^^^ m^^-^rG-' *^^ 



414 ON CHANGES IN DIMENSIONS OV A STEEL WIRE WHEN TWISTED, 

Equating (4) and (5), we ought to find 

ttG fkd \ SO 

Substituting the known values on the right, viz., G = 1-38 x 10^, n = 2, 
c = 0-15, h = 14-7, e = 4^,s= 12-37, 86 = 0-00507, dF = 30 x 981000, we get 

L = 4-48 X 10-*. 
The observed value of L is given as w on p. 411, viz., 

L = 4-66 X 10-4, 

showing as close an agreement as could be expected, considering the errors of 
observation. 

Taking the second reciprocal relation (2), to find [-jn] we must twist 

\aC7/^ const. 

through dd and observe dw, and then calculate what load dF must be removed 
to restore the original length. 

We have dw -= L [n — c) — , and dF = 7ra^Ydw/l. 

dF L(n-c)a^Y 

To find If-) , we put on a load W and observe W. If hG is the 

\dwJ0const. 
diminution in torque at this point and dG the diminution in torque with the 
original twist 6, 

G-hG _d-W 
G-dG" 6 ' 

Substituting for SG/G from (3), this gives 

the change in torque for addition W when 6 is constant. 
The value of dw for this load is 

dw=lWl7ra^Y, 
dG\ fhO ^\iTa^YG 



and ^ ^^ ^ 

I const 



Vc^Weconst. V5 / IWd 

dF for ^ 

\ s J 



Equating (7) and (8) and putting dF for W, we get 

ttG fkd ^\ SI 
. OdF 
the same equation as before. 

If we could use a wire without any internal strain when untwisted, c would 
be zero, and we could calculate L, the lowering for one turn, from observations 
on the torque and load alone. 



AND ON THE PRESSURE OF DISTORTION AL WAVES IN STEEL 415 




Fig. 2. 



A Theory of the Changes of Dimension on Twisting: The Stresses in 

a Finite Pure Shear. 

In the paper already referred to* I showed that in a finite pure shear 
€ such as is represented in Fig. 2, in which a cube of section ABCD is sheared 
into a figure of section ABKL through an angle CBK = €, the thicknesses 
perpendicular to AB and to the plane of the figure remaining constant, the 
lines of maximum elongation and contraction are, to the order of c^, at right 
angles before the shear, making an angle e/4 with the diagonals of the square, as 
AE and BG. After the shear they are 
again at right angles to the order of 
€^, and make an angle c/4 with the 
diagonals on the other side as AF and 
BH. Since we have elongation in one 
direction AF, and contraction in a 
direction BH at right angles, the shear 
may be maintained by a pressure P 
along BH and a tension Q along AF 
as far as forces in the plane of the 
figure are concerned. 

If we go to the first order of € only, 

If we go to the second order we must put 

P = ne + pe^, 

where ^ is a constant to that order. 

If we reverse e, P becomes equal to — Q, so that we have 

— Q = — ne -\- fe^ or Q = ne — pe^ 

We can only assume that there is no pressure or tension perpendicular to 
the plane of the figure, if we neglect e^. Going to the second order, we have 
to allow the possibility of a pressure of that order, which we may put as 

S = qe% 
where if q is negative the force is a tension. 

Considering the equilibrium of the wedge ABC, Fig. 3, with AC in the 
direction of greatest elongation and BC in that of greatest contraction, 
I showed that the tangential stress along AB is, to the second order, 

T = n€, 
and that a pressure is required perpendicular to AB given by 

R=(in + p) e\ 
* Loc. cit. p. 546. ICollected Papers, p. 368.] 



416 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 



The analysis stopped here and was incomplete, as no account was taken of 
the stresses on the plane CD, Fig. 3, perpendicular to AB. It requires to be 
supplemented as follows : 

Considering the equilibrium of the wedge CDB, let us suppose that on 
CD there is a tangential stress T' along CD, and a pressure R' perpendicular 
to it. 

C 
P 




R = (^n+p)e2 T=ii£ 



Ii--(-trL+p)&2 




*-T=Tl& 



R=(-Jn+p)£^ 



T=ne -t 

E=(-|n+p)&^ 
Fig. 4. 

Resolving all the forces on CDB in a direction parallel to DB, 

R' . CB sin (45 + Je) - T . C5 cos (45 -^le) + Q.CB sin (45 + Je) = 0, 

whence R' = T cot (45 + Je) - Q ; 

or, since T = ne and cot (45 + Je) = (1 — Je), neglecting e^, as it is multi- 
plied by e, 

R' = n€(l- ic) -n€ + pe^ = {- ^n + f) eK 



AND ON THE PRESSURE OF DISTORTION AL WAVES IN STEEL 417 

Kesolving in a direction parallel -to CD, 

T . CB sin (45 + ^e) - R . CB cos (45 -\- ie) - Q . CB cos (45 + Jc) = 0, 
whence T = (R + Q) cot (45 + Jc) = {^n + p) c^ + w€ - j>€^} (1 - Jc) 
= n€ to the second order. 
On a unit cube of the material in the sheared condition then, we have, a& 
in Fig. 4, 

Tangential stresses along AB and CD each • ne. 

Tangential stresses along ^Z) and 50 each n€. 

Pressures perpendicular to ^5 and CD each (\n -\- f) €^. 

Pressures perpendicular to AD and BC each (— \n + p) e^. 

And pressures perpendicular to the plane of the 

figure each ^e^ or, in more convenient form... (q — f) e^ -{- peK 

The Strains in a Finite Shear-Stress consisting of Tangential Stress T, T' only. 

If an element is subjected to the system of stresses just investigated, when 
we put on to it a system of tensions equal and opposite to the second order 
pressures we have just found, we leave only the tangential stresses T = T' = ne. 
The strains due to these tensions must be superposed on the shear €, and we 
shall then have the strains due to the tangential stresses only. 

We have then to examine the strains due to tensions 
iin + p) e2 on AB and CD (Fig. 4). 
{- in + p) e2 on AD and BC. 
{q ~ p) €^+ pe^ perpendicular to the plajie of the figure. 

Through the tension pe^ on every face we get an extension in all directions 
pe^/SK, where K is the bulk-modulus. 

The tensions ^ne^ on AB and CD and the pressures Jwe^ on AD and BC 
constitute a shear-stress giving an elongation parallel to BC of ^ne^/n = Je^, 
and a contraction parallel to AB also Je^. 

The tensions {q — p) e^ perpendicular to the plane of the figure give an 

elongation perpendicular to that plane y (q — p) e^, and contractions at right 

angles, viz., along AB and AD, y {q — p) e% where Y is Young's modulus and 

a is Poisson's ratio. 

Collecting the results, we have secondary strains accompanying the shear 

€ as follows : 

1 pe^ o 

An elongation parallel to BC = 7 ^^ + o^ — y (9' — /^) ^^ • 

i>e2 1 
,, perpendicular to the plane = f^- + y (5' — V) ^^• 



p. c. w. 



27 



418 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 

As in the experiments, described abovp, Y and a were determined directly, 
it will be convenient to replace K from the equation ^ = — y — , and the 
secondary strains become 

/I 1-(T C7\, f 1 1-G (^ \ 2 /2(7 I \ . 



Equations Representing the Changes in the Dimensions of a Wire 
Subject to a Torque. 

I am indebted to Sir Joseph Larmor for his kindness in indicating how the 
following equations should be formed and solved. Let us assume that we 
put on to a wire of length I and radius a a pure shear-stress proportional to 
the distance r from the axis, and twisting the length I through angle 6. Then 
in addition to the shear € = rd/l, this stress would produce in an element un- 
constrained by neighbouring material what we may term 'free strains' with 
the values just found, which we may write as ar^ radial, ^r^ transverse to 
the radius, and yr^ longitudinal ; where 



/ 2g 1 \ 

-=[-Y^^Y^) 

^=(-4+ F^^-T^j 

/I 1 - cr o \ 

r=(+4 + ^^P-y^^j 



(1) 



If u is the actual radial displacement, and if w is the actual longitudinal 
displacement, the strains in addition to the shear e are, in cylindrical 
co-ordinates, 

dujdr, u/r, and dwjdz. 

The differences between these actual strains and the 'free strains,' viz., 

du 2 ^ w o 2 ^^ 2 /o\ 

^^~dr~ ' f = r~^ ' ^^cfo~^ ' *^ 

imply 'secondary stresses' in the wire due to adjustment of strain in neigh- 
bouring elements. Let these be denoted hj R, 0, W. 

To find R, 0, and W, we treat e, /, g as if they were strains in an inde- 
pendent system. Putting A = e -\-f + g, the equations are 

R=XA + 2fjLe, = AA + 2/x/, W = XA + 2fig, (3) 

where X = K — %n = ,- — -, — ^^^ ^^-^ and jx = n 



(1 + 0-) (1 - 2(7) ^ 2(1+ C7) 




AND ON THE PRESSURE OF DISTORTION AIj WAVES IN STEEL 419 

The forces R, 0, and W must form a system in equilibrium, there being 
no external forces to balance. Considering the equilibrium of the element 
ABCD, Fig. 5, 

d{RrSd) = eSddr, whence r^ + R=e. ...(4) 

We obtain another equation by assuming that the wire is 
so gripped at each end that sections perpendicular to the axis \ ^dr 
remain perpendicular to the axis after twisting. Indeed, we ^ 1^ 
have already assumed this in omitting equations for shear- 
stress in (3). Hence w is independent of r and dw/dz is con- \^d/^ 
stant over a section for a given wire with a given twist. Let 
us put 

dw/dz = h. 

' Fig. 6. 

Further, the load is constant, so that 

r Wrdr - (5) 

•0 

Substituting in (4) from (3) we obtain 

^,du_u_ 2A (g + i8 + y) + 6/xa - 2ju^ ^ 
^ dr^^ dr r~ X + 2fji ^ ^^^ 

By putting u/r = v, we easily find the solution 

u = Ar^-\-Br-i- Gr-\ (7) 

where ^ ^ 2A (a 4- ^ + y) 4- 6/.a - 2^^ 

8 (A + 2/x) 

and B and C are arbitrary constants to be determined by the boundary- 
conditions. 

If the wire is unstrained in all parts before twisting, the solution applies 
with the same constants for all parts. 

In order that u = when r = 0, we must have = 0, so that 

u = Ar^ + Br (8) 

When T = a,R = 0, 

Substituting from (8) in the value of R in (3), and putting J? = when 
r = a, we get 

2 (A + /x) 5 + AA = {A (a + )8 + y) + 2/xa - (4A + 6/x) A} a\ ...(9) 

From equation (5) we obtain another relation between B and h, when we 
substitute for u from (8) in W from (3) and integrate from r = to r = a, viz., 

A5 + (iA + /^) /^ = ttA (a + i8 + y) + J/xy - A^} a\ (10) 

and from (9) and (10) we can find B and h, 

27—2 



420 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 

Since ^ is a linear function of a, ^, and y, and each of these is proportional 
to 6^, h and B are proportional to 6^. Substituting for B in (8), u is also 
proportional to 6^. The theory, then, gives the paraboHc law for the twisting 
of a wire initially unstrained both for lengthening and for side-contraction. 
It also gives the lengthening and side-contraction w/l and u/a for different 
wires of the same material as proportional to a^. 

So far the theory does not, of course, give any account of the fact that the 
wires examined are always unsymmetrical, that the effects always date from 
a point c, on the counter-clockwise side in the wires examined, c being different 
for w and u. This want of symmetry imphes initial internal strain, probably, 
in reahty, very comphcated. Let us examine a simple case in which there is 
a core, radius a, twisted initially against a sheath, outer radius b, and let the 
opposing twists be respectively 6^ and 6^. When we put a twist 6 from 
outside on to the core as a whole the core is twisted through 6 + d^., and the 
sheath through 6 — Og- For the core and sheath respectively we have 

Uc = Ar^ + Br, 

u, = A'r^ + B'r + C'r-\ 

where ^ is a linear function of a, ^, y, and therefore proportional to {6 + 9^)^ 
and A' is the same function of a, ^' , y, say, and therefore proportional to 

To find the constants we have 

Uc = Ug when r = a, 



R. = „ r = L 



and 1 W^rdr^l Wgrdr=--0. 

These give us four equations to find B, B', C , h of the form (it appears 
needless to give the detailed work) 

B' = p2 (0 + e,Y +Q,{e- e,Y, h = p,(e + e,Y+ q, (b -e,f; 

and, substituting for A' , B', C in u^, and putting r = 6, we get 

Both h and Wj, are of the form 

Dd^ + Ee + F, 

where D does not contain 6^ or 6^. As the parabolas depend only on D, 
E and F merely giving the position of the vertex, Oc and Og only affect that 
position. 



AND ON THE PRESSURE OF DISTORTIONAL WAVES IN STEEL 421 

To find that position we may put dhjdd = for the one, dujdJd = for the 
other, and since h and u are different functions of dc and dg the vertices will 
be at different points for the two quantities h and n. 

Taking this simple case as a guide we shall assume that internal strain 
only affects the position and not the size of the parabola representing the 
change of linear dimensions on twisting. 

Hence if we could obtain a wire without internal strain we should have 

Ln^ = w, 

where L has the value found in the experiments on the actual, initially 
strained wire, and we may regard the values uja and wjl for one turn as 
the values for a wire initially without internal strain. 

The Values of p and q in the Secondary Stresses. 

We are now able to find the values of p and q. For the known values of 
Y and a enable us to find a, ^ and y in equations (1) in terms of p and q 
for a known twist, which we shall take as one turn, or as 2?? in length 
I = 160-5 cm. We also know A and /x, since 

A = (7 7/(1 + 0-) (1 - 2(7) and ^i = n = 7/2 (1 + a). 

Substituting for A, fx, a, p, and y we can determine A in terms of p and q. 
Then from equations (9) and (10) we can find B and h in terms of p and q. 
Equating Aa^ + B to the observed value of u/a (which is negative), and h to 
the observed value of w/l, we have two linear equations in p and q. 

The arithmetic is straightforward, though very lengthy, and may be 
omitted. I have used a slide rule in the calculations. 

Using the values of the elastic constants Y and a from Table II, and the 
values of u/a and w/l, on p. 411, I find for wire I 

p = 1-67 X 1012, q= _ 0-70 x lO^^, 

so that the force perpendicular to the plane of the figure in Fig. 4 is a tension 
and not a pressure. 

The Pressure in the Direction of Propagation in Distortional Waves and 
the Longitudinal Waves Produced by the Pressure. 

If we had a train of waves purely distortional, that is, a train in which 
the strain could be represented by a pure shear e, there would be a pressure 
in the direction of propagation (Jw + p) e^. But as e varies from point to 
point in the train, the pressure due to the shear -strain varies, and there must 
be longitudinal disturbance, longitudinal waves, accompanying the distortional 
waves. The longitudinal strain implies that the material yields under the 



422 ON CHANGES IN DIMENSIONS OF A STEEL WIRE WHEN TWISTED, 

pressure, and the pressure will, in general, have a different value from that 
in a pure shear. 

Let us represent the distortional train by 

€ = 7; sm ^ (a:; — vt), 

where v^ = n/p and tj is the ampHtude of the shear. 

If f is the longitudinal displacement at the point where the shear is €, 
d^/dx is the elongation of the element about the point. 

Now if we shear a cube, and remove the pressure (Jw + p) e^, the cube 
elongates in that direction, and if the dimensions in the two directions at 
right angles are maintained the same, the removal of the pressure produces 
elongation 

y-^ {^n + f) e^, where v = X + 2fjL = K + ^n. 

This we may term the 'free elongation' in the direction of propagation 
on the supposition that there is no change of length at right angles to it. 

The pressure due to the shear falls from its full value {^n + p) e^ to 
while the elongation increases from to its full value v~^ (Jn + f) e^. When 
the elongation is d^jdx the pressure remaining is 

= (\n + f) rf sin^ X ^^ ~ ^^^ "" ^^^1^^- 
The equation of motion for the longitudinal waves is 

^ d^ ^~dx^~ '^^ + ?^) "?' X ^^^ T ^^ ~ "^'^^ "^ vd^i/dx^ 
an equation similar in form to that for the longitudinal waves which I have 
attempted to show must accompany light- waves *. 

4:77 

If we put f = ^ sin -y {x — vt — a), 

and substitute in the above equation, we find on putting a; = 0, i = 0, that 
a = 0, and 

_ (jn + v) rj^X 
Stt {pv^ - v) ' 

or if v' is the velocity of free longitudinal waves, since pv'^ = v and v'^ > v^y 

If we substitute for d^/dx in P, we get 

P = J (^n + p)7]^ h — cos -Y {^ — vtn — vA ^ cos -y (x — vt), 

* Roy. Soc. Proc, A, vol. 85, p. 474. [Collected Papers, Art. 29.] 



AND ON THE PRESSURE OF DISTORTIONAL WAVES IN STEEL 423 

We may regard tkis as made up by a steady pressure J (Jw + p) r\^ and 
a purely periodic pressure, of which the average is zero. 

If B is the energy per cubic centimetre at any point in the distortional 
waves, it is half kinetic energy, half strain-energy. The latter is Jw€^, so 
that the total is wc^ or 

E = ntf sin^ -y (ic — vii) = \n'rf y- — cos -^ (a; — v^) k 

Then the average value is 

E = In-qK 

If we denote the average pressure by P, 

p^jro + y^ 
n 

If we use the values of n and p found for wire I, we find 

P = 2-50^. 

If we put the energy per cubic centimetre in the longitudinal waves as 



Average energy in longitudinal waves 1 v'^ + v^ {^n + f) 



2 

2 . -n' 



Average energy in distortional wav^s 8/o [v"^ — v^)'' 

so that the ratio is proportional to tf and therefore in any actual waves it is 
very small. 

The pressures at right angles to the line of propagation will not produce 
any disturbance in a wave-front where t] is constant. Round the edges of the 
wave-front, however, where t] is diminishing as we go outwards, they may 
have effects, and it appears likely that they may give rise to disturbances 
propagated sideways. 

I have much pleasure in recording my hearty thanks to Mr. 6. 0. Harrison, 
mechanic in the laboratory workshop, for his great help in planning the 
apparatus used in the experiments described in this paper, for his skill in 
constructing it, and for his assistance in making the observations. 






31. 

THE CHANGES IN THE LENGTH AND VOLUME OF 
AN INDIA-RUBBEK COED WHEN TWISTED. 

[The India-Rvhber Jomnal, October 4, 1913, p. 6.] 

In some investigations on the way in which pressure might be produced 
by transverse waves in a solid, analogous to the minute pressure produced 
by light-waves, the author was led to expect that a wire with a constant 
load on it would lengthen, when twisted, by an amount proportional to the 
square of the twist, and he gave at the Winnipeg meeting of the British 
Association an account of experiments which fully verified the expectation. 
According to the theor}- used, there should also be accompanying the 
lengthening a diminution in the radijus, and a description has been published 
in the Proceedings of the Royal Society* of experiments which show that the 
diminution exists and follows the same law. The changes are very minute, of 
the order of a millionth in the length and in the diameter when a steel wire 
160 cm. long and 1 mm. diameter is twisted through one turn. The volume is 
also slightly increased. If instead of allowing the length to increase it had 
been kept constant by reducing the load, there would with steel have been a 
slight outstanding increase in the volume. 

The author thought it might be interesting to look for similar effects in 
india-rubber. To investigate the lengthening, he used a rubber cord 118 cm. 
long and 1-2 cm. diameter, of which the upper end was attached to a vertical 
axis which could be rotated in a bearing. The lower end was attached to 
a horizontal cross-piece between four stops, which allowed the cross-piece, 
and therefore the end of the cord, to rise or fall, but prevented rotation. 
To the cross-piece there was attached the ordinary wheel-barometer device 
for magnifying up and down motion. There was a lengthening on twisting, 
somewhat irregular, not proportional to the square of the twist, but increasing 
rather less rapidly. Two turns of twist gave an average lengthening of 
0-088 cm., or about 750 in a million, vastly greater than the lengthening 
of the steel wire with a similar twist. But, as with steel wire, the lengthening 
is rather more than proportional to the square of the diameter. A rubber 

* [Collected Papers, Art. 30.] 



CHANGES IN AN INDIA-RFBBER CORD WHEN TWISTED 425 

cord 1 mm. in diameter, and with the length and twist of the steel, would 
probably have increased in length by an amount of the same order as that 
observed with steel. 

To find whether there was a change in diameter, a cord of the same length 
and diameter as that used for the lengthening was enclosed in a vertical glass 
tube with brass ends, the lower end of the cord being attached to the lower 
brass end, and the upper end to a vertical axis coming into the tube through 
as close fitting a bearing in the upper brass end as could be made. This axis 
could be rotated, and so any twist could be put on the cord while it remained 
of constant length. The tube was filled with water, and as it was provided 
at one side with a capillary tube which issued through a hole near the top 
and rose above the upper end, any change in the volume of the rubber on 
twisting would have been indicated by a rise or fall of the water surface in 
the capillary, and this was viewed by a measuring microscope. When two 
turns were put on the rubber, small changes of volume were observed, now 
one way, now the other, probably due to errors of experiment. But the 
changes were very small, and the mean change so minute that it appears 
safe to say that the real change in volume was not so much as one in two 
millions. It was therefore, if it existed at all, of an order not greater than for 
the steel wire above described. If the cord had been only 1 mm. in diameter 
like the steel, and had been of the same length, and had been subjected to 
the same twist, the change in volume would have been vastly less than in 
the case of steel. 

[This appears to be the only published notice of an account of this work which was given at 
the meeting of the British Association at Birmingham in 1913. Ed.] 



APPENDIX BY SIE J. LAKMOR ON THE MOMENTUM OF 

RADIATION. 

[The following extract from a lecture 'On the Dynamics of Radiation' by Sir Joseph 
Larmor, read before the Fifth International Congress of Mathematicians at Cambridge 
in x4.ugust 1912, is inserted here, after consultation with the Author (whose permission 
was requested), in further elucidation and illustration, chiefly from the side of the electric 
theory, of Poynting's experiments resting on the momentum of radiation. Ed.] 

General theory of pressure exerted hy waves. 

If a perfectly reflecting structure has the property of being able to advance 
through an elastic medium, the seat of free undulations, without producing 
disturbance of structure in that medium, then it follows from the principle 
of energy alone that these waves must exert forces against such a reflector, 
constituting a pressure equal in intensity at each point to the energy of the 
waves per unit volume. Of. p. 432, infra. The only hypothesis, required in 
order to justify this general result, is that the velocity of the undulations in 
the medium must be independent of their wave-length; viz., the medium is 
to be non-dispersive, as is the free aether of space. 

This proposition, being derived solely from consideration of conservation 
of the energy, must hold good whatever be the character of the mechanism 
of propagation that is concerned in the waves. But the elucidation of the 
nature of the pressure of the waves, of its mode of operation, is of course 
concerned with the constitution of the medium. The way to enlarge ideas 
on such matters is by study of special cases : and the simplest cases will be 
the most instructive. 

Let us consider then transverse undulations travelling on a cord of linear 
density pQ , which is stretched to tension Tq . Waves of all lengths will travel 
with the same velocity, namely c = (Tq/pq)^, so that the condition of absence 
of dispersion is satisfied. A solitary wave of limited length, in its transmission 
along the cord, deflects each straight portion of it in succession into a curved 
arc. This process implies increase in length, and therefore increased tension, 
at first locally. But we adhere for the present to the simplest case, where 
the cord is inextensible or rather the elastic modulus of extension is indefinitely 
great. The very beginnings of a local disturbance of tension will then be 
equalised along the cord with speed practically infinite ; and we may therefore 
take it that at each instant the tension stands adjusted to be the same (Tq) 
all along it. The pressure or pull of the undulations at any point is concerned 
onlv with the component of this tension in the direction of the cord ; this is 

where 77 is the transverse displacement of the part of the cord at distance x 



ON THE MOMENTUM OF RADIATION 427 

measured along it; thus, up to the second order of approximation, the pull 
of the cord is 



T 



-*^o(S 



The tension of the cord therefore gives rise statically to an undulation 
pressure 

The first of these three equivalent expressions can be interpreted as the 
potential energy per unit length arising from the gathering up of the extra 
length in the curved arc of the cord, against the operation of the tension Tq ; 
the last of them represents the kinetic energy per unit length of the undulations. 
Thus there is a pressure in the wave, arising from this statical cause, which is 
at each point equal to half its total energy per unit length. 

There is the other half of the total pressure still to be accounted for. That 
part has a very different origin. As the tension is instantaneously adjusted to 
the same value all along, because the cord is taken to be inextensible, there 
must be extra mass gathered up into the curved segment which travels along 
it as the undulation. The mass in this arc is 

or to the second order is approximately 

In the element Sx there is extra mass of amount 

which is carried along with the velocity G of the undulatory propagation. 
This implies momentum associated with the undulation, and of amount at 

each point equal to \pqgI^\ per unit length. Another portion of the un- 
dulation pressure is here revealed, equal to the rate at which the momentum 
is transmitted past a given point of the cord; this part is represented by 

IPqG"^ i-^A or \pq (-^ j , and so is equal to the component previously 
determined* 

In our case of undulations travelling on a stretched cord, the pressure 
exerted by the waves arises therefore as to one half from transmitted intrinsic 
stress and as to the other half from transmitted momentum. 

The kinetic energy of the cord can be considered either to be energy 

belonging to the transverse vibration, viz., \\p {-^\ ds, or to be the energy 



4?8 APPENDIX BY SIR J. LARMOR 

of the convected excess of mass moving with the velocity of propagation G*, 
viz., I 2/° (t^ ) <^^^^ ; for these quantities are equal by virtue of the condition 

of steady propagation -jl = <^ ^ • 

On the other hand the momentum that propagates the waves is transverse, 
of amount p ~ per unit length ; it is the rate of change of this momentum 
that appears in the equation of propagation 

dtV'dt) dx\ dx)' 
But the longitudinal momentum with which we have been here specially 
concerned is J/jf-—] c per unit length, which is |-p.p-7^. Its ratio to 

the transverse momentum is very small, being \j^', it is a second-order 

phenomenon and is not essential to the propagation of the waves. It is in 
fact a special feature, and there are types of wave-motion in which it does 
not occur. The criterion for its presence is that the medium must be such 
that the reflector on which the pressure is exerted can advance through it, 
sweeping the radiation along in front of it, but not disturbing the structure; 
possibly intrinsic strain, typified by the tension of the cord, may be an essential 
feature in the structure of such a medium. 

If we derive the dynamical equation of propagation along the cord from 
the Principle of Action 8 | (T - 110 dt = 0, where 



[i.(5/..a„d>F=|ir„(g) 



2 

dx. 



the existence of the pressure of the undulations escapes our analysis. A corre- 
sponding remark applies to the deduction of the equations of the electro- 
dynamic field from the Principle of Action f. In that mode of analysis the 
forces constituting the pressure of radiation are not in evidence throughout 
the medium ; they are revealed only at the place where the field of the waves 
affects the electrons belonging to the reflector. Problems connected with 
the Faraday-Maxwell stress lie deeper; they involve the structure of the 
medium to a degree which the propagation of disturbance by radiation does 
not by itself give us means to determine. 

We therefore proceed to look into that problem more closely. We now 
postulate Maxwell's statical stress system; also Maxwell's magnetic stress 
system, which is, presumably, to be taken as of the nature of a kinetic 
reaction. But when we assert the existence of these stresses, there remain 
over uncompensated terms in the mechanical forcive on the electrons which 

* [This specification is fictitious; indeed a factor J has been dropped in its expression just 
following. There is however actual energy of longitudinal motion; as it belongs to the whole 
mass of the cord, which moves together, it is very small in amount, its ratio to the energy of 
transverse vibration being \ {d-rjldc^^. J. L.] 

t Cf. Larmor, Trans. Camb. Phil. Soc. vol. 18 (1900), p. 318; or Aether and Matter, Chapter vi. 



ON THE MOMENTUM OF RADIATION 429 

may be interpreted as due to a distribution of momentum in the medium*. 
The pressure of a train of radiation is, on this hypothetical synthesis of stress 
and momentum, due entirely (p. 431) to the advancing momentum that is 
absorbed by the surface pressed, for here also the momentum travels with 
the waves. This is in contrast with the case of the cord analysed above, in 
which only half of the pressure is due to that momentum. 

The pressure of radiation against a material body, of amount given by 
the law specified by Maxwell for free space, is demonstrably included in the 
Maxwellian scheme of electrodynamics, when that scheme is expanded so as 
to recognise the electrons with their fields of force as the link of communica- 
tion between aether and matter. But the illustration of the stretched cord 
may be held to indicate that it is not yet secure to travel further along with 
Maxwell, and accept as realities the Faraday-Maxwell stress in the electric 
field, and the momentum which necessarily accompanies it; it shows that 
other dynamical possibilities of explanation are not yet excluded. And, 
viewing the subject from the other side, we recognise how important have 
been the experimental verifications of the law of pressure of radiation which 
we owe to Lebedew, too early lost to science, to Nichols and Hull, and to 
Poynting and Barlows The law of radiation-pressure in free space is not 
a necessary one for all types of wave-motion ; on the other hand if it had not 
been verified in fact, the theory of electrons could not have stood without 
modification. 

The pressure of radiation, according to Maxwell's law, enters fundamentallv 
in the Bartoli-Boltzmann deduction of the fourth-power law of connection 
between total radiation in an enclosure and temperature. Thus in this 
domain also, when we pass beyond the generalities of thermodynamics, we 
may expect to find that the kAvs of distribution of natural radiant energy 
depend on structure which is deeper seated than anything expressed in the 
Maxwellian equations of propagation. The other definitely secure relation 
in this field, the displacement-theorem of Wien, involves nothing additional 
as regards structure, except the principle that operations of compression of 
a field of natural radiation in free space are reversible. The most pressing 
present problem of mathematical physics is to ascertain whether we can 
evade this further investigation into aethereal structure, for purposes of 
determination of average distribution of radiant energy, by help of the 
Boltzmann-Planck expansion of thermodynamic principles, which proceeds 
by comparison of the probabilities of the various distributions of energy 
that are formally conceivable among the parts of the material system which 
is its receptacle. 

Momentum intrinsically associated with Radiation. 

We will now follow up, after Poynting f, the hypothesis thus implied in 
modern statements of the Maxwellian formula for electric stress, namely that 
the pressure of radiation arises wholly from momentum carried along by the 
waves. Consider an isolated beam of definite length emitted obliquely from 
a definite area of surface A and absorbed completely by another area B. The 

* For the extension to the most general case of material media cf. Phil. Trans, vol. 190 (1897).. 
p. 253. 

t Cf. Phil. Trans, vol. 202, A (1903). [Collected Papers, Art. 20.] 



430 APPENDIX BY SIR J. LARMOR 

automatic arrangements that are necessary to ensure this operation are easily 
specified, and need not detain us. In fact by drawing aside an impervious 
screen from A we can fill a chamber AA' with radiation ; and then closing 
A and opening ^', it can emerge and travel along to B, where it can be 
absorbed without other disturbance, by aid of a pair of screens B and B' in 
like manner. Let the emitting surface ^ be travelling in any direction while 
the absorber B is at rest. What is emitted by A is wholly gained by B, for 
the surrounding aether is quiescent both before and after the operation. 
Also, the system is not subject to external influences; therefore its total 
momentum must be conserved, what is lost by A being transferred ultimately 
to B, but by the special hypothesis now under consideration, existing mean- 
time as momentum in the beam of radiation as it travels across. If v be the 
component of the velocity of A in the direction of the beam, the duration 
of emission of the beam from A is (1 — v/o)-^ times the duration of its 
absorption by the fixed absorber B. Hence the intensity of pressure of 
a beam of issuing radiation on the moving radiator must be affected by 
a factor (1 — vJG) multiplying its density of energy; for pressure multiplied 
by time is the momentum which is transferred unchanged by the beam to 
the absorber for which v is null. We can verify readily that the pressure of 
a beam against a moving absorber involves the same factor (1 — vjc). If the 



aA^ 



/ 



B' 



emitter were advancing with the velocity of light this factor would make the 
pressure vanish, because the emitter would keep permanently in touch with 
the beam : if the absorber were receding with the velocity of light there would 
be no pressure on it, because it would just keep ahead of the beam. 

There seems to be no manner other than these two, by altered intrinsic 
stress or by convected momentum, in which a beam of limited length can 
exert pressure while it remains in contact with the obstacle and no longer. 
In the illustration of the stretched cord the intrinsic stress is transmitted and 
adjusted by tensional waves which travel with velocity assumed to be prac- 
tically infinite. If we look closer into the mode of this adjustment of tension, 
it proves to be by the transmission of longitudinal momentum ; though in 
order that the pressure may keep in step, the momentum must travel with 
a much greater velocity, proper to tensional waves. In fact longitudinal 
stress cannot be altered except by fulfilling itself through the transfer of 
momentum, and it is merely a question of what speeds of transference come 
into operation. 

In the general problem of aethereal propagation, the analogy of the cord 
suggests that we must be careful to avoid undue restriction of ideas, so as, 
for example, not to exclude the operation, in a way similar to this adjustriient 
of tension by longitudinal propagation, of the immense but unknown speed 
of propagation of gravitation. We shall find presently that the phenomena 
of absorption lead to another complication. 




ON THE MOMENTUM OF RADIATION 431 

So long, however, as we hold to the theory of Maxwellian electric stress 
with associated momentum, there can be no doubt 
as to the validity of Poynting's modification of the 
pressure formula for a moving reflector, from which 
he has derived such interesting consequences in 
cosmical astronomy. To confirm this, we have only 
to contemplate a beam of radiation of finite length I 
advancing upon an obstacle A in which it is 
absorbed. The rear of it moves on with velocity c; 
hence if the body A is in motion with velocity 
whose component along the beam is v, the beam will be absorbed or passed 
on, at any rate removed, in a time l/io — v). But by electron theory the 
beam possesses a distribution of at any rate quasi-moTonentum. identical with 
the distribution of its energy, and this has disappeared or has passed on in 
this time. There must therefore be a thrust on the obstructing body, directed 
along the beam and equal to e (1 — n/o), where € is the energy of the beam 
per unit length which is also the distribution of the quasi-moraentum. along 
the free beam. 

The back pressure on a radiating body travelling through free space, 
which is exerted by a given stream of radiation, is by this formula smaller 
on its front than on its rear; so that if its radiation were unaffected by its 
motion, the body would be subject to acceleration at the expense of its internal 
thermal energy. This of course could not be the actual case. 

The modifying feature is that the intensity of radiation, which corresponds 
to a given temperature, is greater in front than in rear. The temperature 
determines the amplitude and velocity of the ionic motions in the radiator, 
which are the same whether it be at rest or in uniform motion: thus it 
determines the amplitude of the oscillation in the waves of aethereal radiation 
that are excited by them and travel out from them. Of this oscillation the 
intensity of the magnetic field represents the velocity. If the radiator is 
advancing with velocity v in a direction inclined at an angle d to an emitted 

ray, the wave-length in free aether is shortened in the ratio 1 cos 9 ; thus 

the period of the radiation is shortened in the same ratio ; thus the velocity 
of vibration, which represents the magnetic field, is altered in the inverse 
ratio, and the energy per unit volume in the square of that ratio, viz., that 

energy is now € (l — - cos ^) ; and the back pressure it exerts involves 
a further factor 1 cos 6 owing to the convection ; so that that pressure 

is e(l — -cos^) , where € is the energy per unit volume of the natural 

radiation emitted from the body when at rest. The pressural reaction on 
the source is in fact E'/o, where E' is the actual energy emitted in the ray 
per unit time. 



432 ''-'■ APPENDIX BY SIR J. LARMOR 



Limitation of the analogy of a stretched cord. 

In the case of the inextensible stretched cord, the extra length due to the 
curved arc in the undulation is proportional to the energy of the motion. 
The loss of energy by absorption would imply slackening of the tension ; and 
the propositions as to pressure of the waves, including Poynting's modification 
for a moving source, would not hold good unless there were some device at 
the fixed ends of the cord for restoring the tension. The hypothesis of 
convected momentum would imply something of the same kind in electron 
structure. 

It is therefore worth while to verify directly that the modified formula 
for pressure against a moving total reflector holds 

good in the case of the cord, when there is no >v 

absorption so that the reflection is total. This 
analysis will also contain the proof of the generalisa- 
tion of the formula for radiant pressure that was 
enunciated on p. 426 sw^m*. 

Let the wave-train advancing to the reflector and the reflected wave-train 
be represented respectively by 

7^1 = Ai cos Ml {x -f ct), 

7]2 = A^ cos mo {x — ct). 

At the reflector, where x = vt, we must have 

jvidt^ ji^.,dt; 
this involves two conditions, 

— = — - and m^ (c + u) = m.. (c — v). 

Now the energies per unit length in these two simple wave-trains are 

ipAj^ and ipA^^; 
thus the gain of energy per unit time due to the reflection is 
8E=(c- v) Jp^2' - {c + ^) \p^i 



= iM.^{,c-.)(^^-J 



1 J 2 O ^ + ^ 



(c + v) 



C — V 



* See Larmor, Brit. Assoc. Report, 1900. [The statement that follows here is too brief, unless 
reference is made back to the original, especially as a minus sign has fallen out on the right of the 
third formula below. The reflector consists of a disc with a small hole in it through which the 
cord passes ; this disc can move along the cord sweeping the waves in front of it while the cord 
and its tension remain continuous through the hole — the condition of reflection being thus 
T7j + 7/2=0 when x = vt. In like manner a material perfect reflector sweeps the radiation in 
front of it, but its molecular constitution is to be such that it allows the aether and its structure 
to penetrate across it unchanged. For a fuller statement, see Encyclopaedia Britannica, ed. 9 or 
10, article 'Radiation.' J. L.] 



ON THE MOMENTUM OF RADIATION 433 

This change of energy must arise as the work of a pressure P exerted by 
the moving reflector, namely it is Pv ; hence 

c + V 



P = lpA,K2 



C — V 



The total energy per unit length, incident and reflected, existing in front of 
the reflector is 

E^ + E^ = yA,^ + yA^^ 

C^ 4- 7j2 



Hence finally P - {E^ + E 



(c - vf 

2^ n2 ' ,.2^ 



becoming equal to the total density of energy E-^ -\- E2, in accordance with 
Maxwell's law, when v is small. 

If we assume Poynting's modified formula for the pressure of a wave-train 
against a travelling obstacle, the value ought to be 

and the truth of this is readily verified. 

It may be remarked that, if the relation connecting strain with stress 
contained quadratic terms, pressural forces such as we are examining would 
arise in a simple wave-train*. But such a medium would be dispersive, so 
that a simple train of waves would not travel without change, in contrast 
to what we know of transmission by the aether of space. 

The question is then suggested how far a cognate momentum can be 
regarded as arising from change of aethereal inertia produced by travelling 
electric strain. It will be represented by inertia attached to moving tubes 
of electric force. The conclusion is reached that such a scheme can be con- 
sistently constructed for any steady electric system convected with uniform 
speed ; also that it holds for any field of pure radiation, that is any field in 
which the electric and magnetic forces are everywhere at right angles : but 
that in other cases it is not possible. On the other hand any changing electro- 
dynamic field whatever is constituted by the superposition of pure radiations 
from all the electrons belonging to its source. 

[A discussion follows of the frictional resistance to the motion through 
space of a radiating body, whose mere existence, as is pointed out, had been 
predicted by Balfour Stewart as early as 1871. Estimates are made for 
bodies of various forms, including one for the sphere which verifies Poynting's 
formula in Phil. Trans. 1893 (p. 3.30 supra). The important applications 
to cosmical astronomy which Poynting has there developed do not seem to 
have yet received the attention they deserve. 

Then it is recalled that if we assume the real existence of the Maxwell 
stress in the aether, suitably modified for modern ideas, as the source of all 

* Cf. Poynting, Boy. Soc. Proc. vol. 86, A (1912), pp. 53^561, where the pressure exerted by 
torsional waves in an elastic medium, such as steel, is exhaustively investigated on both the 
experimental and the mathematical side. [Collected Papers, Art. 30.] 

p. c w. -8 



434 APPENDIX BY SIR J. LABMOR 

mechanical interactions between electric systems, and we retain the ascer- 
tained mechanical electrodynamic forces as part of its effect, then another 
phenomenon is required to make up the complete result, and this can be 
represented as a distribution of momentum in the aether of density equal to 
the vector-product of aethereal displacement and magnetic induction. In 
the case of trains of waves, the latter agrees with Poynting's momentum 
of radiation. As regards the resultant momentum and forces for any 
self-contained svstem, th^ Maxwell stress is eliminated, and no hypothesis 
as to its reality is involved. 

But for such a complete system, free from external disturbance, we 
require to compare this outstanding force, visualised as rate of change of 
some kind of latent momentum, when the system is convected with uniform 
velocity v, with what it would be for the same system at rest in the aether ; 
for although the system remains the same the convection modifies the electro- 
dynamic field around each electron which it contains, and thus may modify 
the effective electromagnetic mass of that electron as well as the distribution 
of latent momentum. When this comparison is made by aid of the classical 
correlation first employed by H. A. Lorentz, it turns out* that the 
forcive acting on the convected system exceeds that acting on the same 
system when stationary in the aether, by the effect of convection of latent 

momentum specified exactly as before, together with a force equal to v _. I 2 ) ' 

where E is the energy in the system. On the principle that force is expressed 

as -7- {mv) we can infer that an increase hE of the electrodynamic energy of 

a system increases tlie effective mass of the system by SEjc^. This additional 
result, as well as the momentum result, is necessitated beyond cavil by the 
ascertained laws of electrodynamics, which however are themselves established 
only when (v/c)^ is negligible! : extension of its validity beyond that limit 
requires new postulates of 'relativity.' In astronomical applications such 
as Poynting's, the effect of any change of mass due to cooling is totally 
insignificant compared with the results which he derives from the latent 
inomentum. J. L.j 

* [Larmor : 'On the Dynamics of Radiation,' Fifth International Congress of Mathe- 
maticians, Aug. 1912, vol. 1.] 

t [Cf. Larmor. Aether and Matter. 1900.] 



PART IV. 

LIGHT. 

32. 



ON A SIMPLE FORM OF SACCHARIMETER. 

[Phil. May. 10, 1880, pp. 18-21.] 

The general principle of the modification of the saccharimeter which 
I shall describe in this paper is well known, and has already been applied 
in the construction of several standard instruments, such as Jellett's and 
Laurent's. This principle consists in altering the pencil of rays proceeding 
from the polariser in such a way that, instead of the whole pencil having the 
same plane of polarisation, the planes of the two halves are slightly inclined 
to each other. The analyser is therefore not able to darken the whole field 
of view at once. In one position of the analyser the one half of the field is 
quite dark; in another position, slightly different, the other half is dark; 
while when the analyser is halfway between these two positions, the two 
halves of the field are equally illuminated. This will be seen from the 
accompanying figure. 

Let CA be the trace of the plane of polarisation of the right half of the 
pencil, and CB that of the other half. Let CD bisect 
ACB. Then, if CE represent the plane of polarisation 
of the light which alone the analyser will allow to 
pass, when the analyser is turned so that CE is 
perpendicular to CA the right-hand side of the field 
is dark. When CE is perpendicular to CB the right- 
hand is partially illuminated (as CA has a component 
along CE), while the left-hand is dark. Halfway 
between these positions, when CE is perpendicular 
to (7D, both sides appear equally illuminated. The 

analyser being turned round till this equality of illumination is obtained, its 
position is noted on the attached circle. When an active substance is now 

28—2 




436 ON A SIMPLE FORM OF SACCHARIMETER 

inserted in the path of the rays, the planes CA, CB are both rotated through 
the same angle, and the analyser has to be rotated through this angle to 
give the equal illumination once more. The circle again being read, the 
difference of readings gives the rotation due to the interposed substance. 

In Jellett's saccharimeter the inclination of the planes of polarisation of 
the two halves of the field is obtained by interposing a prism of Iceland spar. 
This is formed by cutting a rhomb nearly parallel to its optic axis, reversing 
one of the pieces, and then cementing the two together again with the plane 
of separation bisecting the pencil of rays. 

In Laurent's instrument, for which homogeneous light is used, half the 
pencil is passed through a plate of quartz cut with its axis in the surface 
and parallel to its edge, the thickness being such that the extraordinary ray is 
retarded half a wave-length behind the ordinary. On emergence the direc- 
tions of vibration in the two parts of the pencil, one of which has traversed 
the quartz, are equally inclined to the edge of the crystal. The inclination 
of the two to each other can be very easily altered by simply turning the 
polariser. 

The following arrangement is in place of the Iceland spar in Jellett's 
instrument, and of the quartz plate in Laurent's. It seems to be somewhat 
simpler, and gives fairly good results. 

A circular plate of quartz cut perpendicular to the axis is divided along 
a diameter, and one half slightly reduced in thickness. The two halves are 
then reunited and interposed in the path of the pencil and at right angles 
to its direction. Since one half of the pencil passes through a slightly greater 
thickness of quartz, its plane of polarisation is slightly more rotated than that 
of the other half ; and the pencil therefore emerges with the planes of polar- 
isation of its two halves slightly inclined to each other. It is of course 
always necessary to use homogeneous light to avoid dispersion. 

Mr. Glazebrook has very kindly given me the following numbers, which 
are taken at random from a large number of sets of readings he has obtained 
for the electromagnetic rotation of certain solutions of NaCl in water; the 
difference of thickness of the two plates being -1 mm., and the inclination 
of the planes of polarisation being therefore about 2° for the sodium light 
used. The circle to which the analyser was attached reads to 3' ; but the 
vernier-divisions can easily be further subdivided by eye. 

In order to vary the inclination of the two planes of polarisation 
to each other, one of the halves of the quartz plate might be arranged like 
a Babinet's compensator, so that the difference of the two might be varied 
at will. The chief objection to the method seems to be that the quartz 
plate has to be adjusted very exactly perpendicular to the axis of the pencil. 



% 
ON A SIMPLE FORM OF SACCHARIMETER 437 

Circle-readings 

I. Current direct 23° 45' 

23 46 
23 45 
23 45 

Current reversed 



II. Current direct .. 



Current reversed 



III. Current direct ... 



Current reversed ... 21 45 

21 47 
21 48 



21 


36 


21 


34 


21 


39 


23 


15 


23 


16 


23 


18 


22 


19 


22 


19 


22 


20 


23 


30 


23 


30 


23 


28 



A still simpler arrangement, which has as yet only been tried in a somewhat 
rough form, consists in a cell containing some active liquid, say sugar solution. 
This cell is interposed in the path of the pencil ; and in it is inserted a piece 
of plate-glass several millimetres thick, arranged so that one half the pencil 
passes through it. This half therefore passes through a less thickness of the 
active substance than the other half, and is less rotated. The two then 
emerge as before, having their planes of polarisation slightly inclined to each 
other. This inclination, and consequently the sensitiveness of the instrument 
can be varied either by varying the strength of the active solution, or the 
thickness of the plate of glass inserted in the cell. 

This arrangement, as far as it has been tested, gives as good results as the 
previous one, while it is much more easily constructed and adjusted. 



33. 

ON THE LAW OF THE PROPAGATION OF LIGHT*. 
By J. H. PoYNTiNG and E. F. J. Love, B.A. 

[Birmingham Phil Soc. Proc. 5 (1887), pp. 354-363.] 

[Read March 31, 1887.] 

The general law for the propagation of light, applying both to transparent 
and absorbing media, is that the intensity of illumination at a distance d 
from a given source is proportional to e'^^/d^. For transparent media c = 0. 

We shall describe in this paper a new experimental method of showing 
that this is the law of propagation. While the method gives, we believe, 
the first exact experimental verification of the law for absorbing media, a 
combination of the result with those of ordinary photometric observations 
shows that for transparent media c = 0, and so for these our method proves 
the inverse-square law. 

Since various proofs of the inverse-square law already exist it may perhaps 
be necessary to justify an addition to the list by pointing out the weak points 
in its predecessors. 

In what may be called the a ^priori proof a cone is drawn with the source 
as vertex, and cross-sections of the cone are taken at different distances. 
Since the areas of these sections vary as the squares of their distances from 
the vertex, the amount of light falling on unit area of each is stated to vary 
inversely as the square of the distance. This assumes (1) that there is some- 
thing, constant in amount, travelling out with a constant velocity; and 
(2) that the illumination is proportional to the amount of this incident per 
second on unit area. In fact, it assumes the conservation of light-energy, 
and it identifies intensity of illumination with the amount of light-energjy 
received. 

It appears to be sometimes supposed that the law is proved by the con- 
sistency of the results obtained in photometry. Thus let I^ , I^ be the illumina- 
tions on a screen at unit distance from two sources ; assuming the law, when 

the illuminations are equal at distances d-^ and (igrespectivelyjWe have -t\=^ -i\' 

* The substance of this paper was communicated to Section A at the 1886 meeting of the 
British Association. The experiments with sodium light have been carried out since then. 



ON THE LAW OF THE PROPAGATION OF LIGHT 439 

This equation, holding for one pair of distances, will hold for any pair in which 
the ratio d^ : dz is constant. Carstaedt {Pogg. Ann. 150, p. 551), using the 
Bunsen photometer, has shown experimentally that the ratio is constant 
within the limits of experimental error. But with any power of the distance 

^the ratio should be constant. For the equation j^ = t-^ will be satisfied by 

1 
any pair of values oi d^: d^ii -r = (y-) . 

The simplest direct proof is that which shows that 1 candle at a distance 
1 produces an illumination equal to that of 4 candles at distance 2, of 9 
candles at distance 3, and so on. But this method is wanting in exactness, 
and must be considered rather as a lecture-room illustration than as an 
accurate proof. 

Crookes (Phil. Trans., 1876, p. 325) has given another proof depending 
on the radiometer-effect. He shows by subsidiary experiments that this 
effect is proportional to intensity of illumination, and then he determines 
by the effect the intensity of the light received from a standard candle at 
varying distances. The proof is interesting, but does not appear to be very 
.exact. 

Another proof is based on the observation that a uniform illuminating 
surface looked at through a narrow blackened tube appears equally bright 
at all distances so long as the illuminating surface entirely fills up the aperture. 
The area illuminated on the retina is constant, while the area illuminating 
it varies as the square of the distance. Hence the illuminating power per 
unit area of the surface varies inversely as the square of the distance. This 
is probably more accurate than the previous methods, but it requires the 
observer to be assured that there is no gradual change in the illumination 
as the distance of the source changes. The eye, however, is not very sensitive 
to gradual changes of illumination, seeking always to counteract them by 
altering the aperture of the pupil. But the eye is very sensitive to difference 
of illumination of two surfaces presented to it at the same time. 

The proof which we now give has therefore been devised to depend on 
equality of illumination of two surfaces seen together. It may be regarded 
as a development of the last proof, but instead of employing a single illumin- 
ating surface two illuminating surfaces at different distances are viewed 
through a narrow blackened tube, each surface occupying half the field of 
view. The illuminating powers of the two surfaces are adjusted till for a 
given distance of the tube they appear equally bright. They then appear 
equally bright for any other distance of the tube. 

We shall first, assuming the truth of this statement, deduce from it the 
law of propagation, and then give an account of the experimental verification. 



440 ON THE LAW OF THE PROPAGATION OF LIGHT 



I. Theoretical. 

Let two uniform illuminating surfaces be arranged so that when viewed 
through a narrow tube each occupies half the field, and let them be placed 
at distances from the eye-end of the tube d^ and d^ respectively. Then the 
surfaces sending light to a point at the eye-end of the tube may be put equal 
to Mj^, hd^, respectively. 

Let 7i, I2 be the illuminations produced per unit area of the illuminating 
surfaces on a screen held at unit distance from an element of the illuminating 
surfaces. 

f ix) 
Let -~ be the law of propagation, so that if I be the intensity of 

X 

If ix) 
illumination of a screen at unit distance from a source — — is that at 

x^ 

distance x. 

If the two surfaces appear equally bright we have 

j^a.fj^^jj,^.m (1) 

or hf(d,) = IJ(d,) (2) 

But this equation is still true by experiment if both d-^^ and (^2 b^ increased 
by any the same quantity £, 

.-. IJ(d, + e) = IJ(d, + €) (3) 

Put t?i + € = X and d^ — di = y, 

••• hf{^)-hf(x + y) (4) 

For given values of /j, I^ and y this is true for all values of x. Hence 
differentiating (4) we have 

Iif{^)-l2fix + y) (5) 

Dividing (5) by (4) 

f(x) ' f{x-\--y) ^^^ 

^^' 7M = ^^^'^ (') 

.-. xW = x(^ + ^) (8) 

Now this is true for all values of y, and can therefore only be satisfied by 
X (x) = constant = — c, (9) 

or / (x) = Ae-"". 



ON THE LAW OF THE PROPAGATION OP LIGHT 441 

But at distance 1, /(I) = 1, 

.-. 1 = Ae-^, 
)r A^e". 

Hence the intensity of illumination at distance x from the source is 

/g-c(a5-l) 

x^ 

Now ordinary photometric measures in transparent media such as air 
show that if two sources give equal illuminations at distances d^ and d^, the 
equality of illumination is maintained for all distances so long as d^^ : d^ is 
constant. 

The equality of illumination requires that 

/^g-c(di-l) /2^-c(d2-l) 



d^^ d 



2 

Putting d^ = kd-^, we obtain 

d^^ ^ ^Wd^" 

. p-cdx(l-Tc) ^ ^2 

which can only be true for all values of d-^^ when c = 0. 

Hence for transparent media we have the ordinary inverse-square law. 

When c has a value differing from zero e'^ is the 'coefficient of absorption.' 

Of course c varies in general for different colours, so that we get equality 
of tint only when we are using monochromatic light. 

In this case equation (2) becomes 

QY — =: g—cid^—di) 

^2 

1 






If, then, we arrange the apparatus in such a way that Zj and /g can be 
determined e-^ can be found from the above equation. We have not attempted 
to adapt the apparatus for this purpose, but content ourselves with pointing 
it out as a possible method of finding the coefficient of absorption. 



442 ON THE LAW OF THE PROPAGATION OF LIGHT 



II. Experimental. 

The object of the experiments was to prove the statement already made— 
that *if two luminous surfaces at different distances are viewed through a 
narrow blackened tube, each surface occupying half the field of view, and 
the illuminating powers of the two surfaces are adjusted so that for a given 
distance of the tube they appear equally bright, they will then appear equally 
bright for any other distance of the tube.' 

The statement requires to be demonstrated for transparent and absorbing 
media. It was accordingly resolved to make observations in air with both 
white and monochromatic light ; but as liquids exerting a perceptible general 
absorption of white light are not to our knowledge obtainable, the experiments 
with an absorbing medium were carried out with monochromatic light only. 



E O 



p 




o 


'p^xrJ 





., ._ 


1 2 ' 1 1 'k Sl 


: 


H' 

O 


P^ 



Fig. 1. 

The apparatus employed {vide Fig. 1) consisted of a trough, ABCD, 
12" X 4" X 4" internally ; the bottom, sides, and one end are of paraffined 
wood blackened, the other end being closed by a piece of plane parallel 
glass, PP. A hole was pierced in the wooden end through which passed 
a brass tube, HL, f" in diameter, arranged to slide in a stuffing-box, M, 
fixed to the end of the trough. The end, L, of the tube was closed by a 
glass plate. The bright surfaces were obtained as follows : A large plate of 
opal glass was carefully examined, and a part which appeared homogeneous 
was cut out and divided into two. Each of the pieces was then placed in 
the trough (at 0, 0) so as to fill up half the field of view of the brass tube, 
but at different distances from it, care being taken that the adjacent edges 
should be those along which the plate was cut. The plates were illuminated 
by lights, E, E, at some distance from the trough. In front of the lights were 
placed convex lenses, G, G, somewhat nearer than their own focal length, so 
as to produce the effect of a brighter beam coming from a greater distance. 
On the side of each lens next the trough was placed a blackened screen, F, V, 
with a rectangular slit cut in it, so as to produce a well-defined beam. One 
lamp and lens was fixed, the other placed on a wooden tray moving parallel 
to itself in guides, so that its distance from the trough could be varied at 
pleasure, changes in its position being read off on a millimetre-scale, NN, 
attached to the tray, with the aid of a fiducial mark affixed to the experiment 
table. To prevent light from either lamp reaching the opal on the other 
side, a blackened strip of zinc, Z, was fixed in the middle line. 



ON THE LAW OF THE PROPAGATION OF LIGHT 



443 



To get rid of internal reflection in the apparatus, the tube and trough were 
"well blackened. The tube was provided in addition with four stops, H, Z, 
K, L, placed respectively at the two ends, at the middle, and at one-fourth 
of the length from the eye-end. The diameters of the stops were H |", I ^%", 
K ^}", L f^". To prevent stray light from reaching the eyes, a card-board 
screen about 12" in diameter was hung on the tube. The experiments were 
carried on in the dark chamber of the Physics Laboratory at Mason College. 

As sources of white light two galvanometer-lamps, which burn with a very 
steady hght, were employed. With monochromatic light the difficulties were 
greater ; for the ordinary form of sodium flame was quite unsuitable, owing 
to the different quantities of light emitted from different parts of the surface, 
and the altogether uncontrollable fluctuations in brightness. The following 
arrangement, represented in Fig. 2, was, however, found successful : 

A stream of coal-gas, whose pressure was rendered pretty uniform by 
passing through a 'Stott' governor, was sent into a large glass flask, F, by 
a tube, QQ, which just passed through the cork. Some granulated zinc was 




Fig. 2. 

placed at the bottom of the flask, together with a small quantity of con- 
centrated salt-solution, with a large excess of solid salt; hydrochloric acid 
was added by the thistle funnel, S. The hydrogen slowly evolved from the 
zinc and acid kept up a constant spirting of salt-solution into the gas ; and 
as the lower end of the exit tube, RR, was near the surface of the liquid, 
the gas came up charged with a quantity of finely divided salt-solution*. 
This mixture was led to a large Bunsen burner, E, and a light of uniform 
intensity in all parts obtained, which was freed from flickering by surrounding 
the flame with an Argand lamp chimney. One of these arrangements was 
used for each opal. As absorbing medium, a dilute solution of cobalt car- 
bonate in ammonium carbonate, which energetically absorbs yellow hght, 
was employed. 

* This arrangement is similar to that employed in Bunsen' s well-known apparatus for 
exhibiting the sodium absorption. 



444 ON THE LAW OF THE PROPAGATION OF LIGHT 

The experiments were conducted as follows : The point at which the 
intensities of the lights were equal having been ascertained, the tube was 
withdrawn, or pushed in, so as to alter its distance from the opals, and in 
every case the two halves of the field remained equally bright. The distance 
through which it was necessary to move the tray, NN, carrying one of the 
lamps and its lens, in order to produce a sensible difference in the intensity 
of the two halves of the field, was then noted. The distance of a source of 
light which would have produced a pencil of the same dimensions and angle 
as that actually employed was then found by measuring the width of the 
bright image of the rectangular slit formed on a screen interposed for the 
purpose at a known distance. In all the sets of experiments, the relative 
intensities of the lights, their distances, and the distance apart of the opals, 
were varied. 

The accuracy obtained was rather remarkable. It appeared that a 
difference of about 1 part in 500 in the intensities of the two halves of the 
field was perceptible for white light; and of about 1 in 270 in the case of 
sodium light. This could, however, only be obtained by giving the eyes 
a period of rest in darkness before commencing the experiments ; accordingly 
the shifting of the tray and registering of its position had to be done by an 
assistant. 

We append the details of one complete experiment as an example : 
Light employed : Sodium Flame. Medium : Air. 

Distance from image to slit 22-5 cm. 

Width of image 4-4 cm. 

„ „ slit 1-8 cm. 

Effective distance of source from slit 15-6 cm. 

The tube was at first drawn out. 

Distance of slit from opal 40-30 cm. Left side darker. 

40-23 cm. Equal. 
,, ,, ,, 40-18 cm. Right side darker. 

The tube was then pushed in, and with the last distance, 40-18, the right 
side was still the darker. When the distance from the slit to the opal was 
40-23 the equality was restored. 

The eye was therefore sensitive to a change in the distance of the moveable 
source of -05 to -07 cm. — say -07 cm. 

Since the effective distance of source from sht is 15-6 cm., and of slit from 
opal 40-23 cm., the total distance from source to opal is 55-83 cm. 

Smallest perceptible change of distance = j^-g; 

whence „ ,, ,, brightness = o^. 



ON THE LAW OF THE PROPAGATION OF LIGHT 

The results of the experiments are collected in the following table 



445 



Kind of light 


Effective dis- 
tance 


Smallest per- 
ceptible change 
of distance 


Change 

of 
intensity 


White in air ... < 

Sodium in air ... i 

Sodium in absorb- / 
ing medium ( 


112-80 cm. 

103-25 „ 
55-83 „ 
55-60 „ 
54-30 „ 
61-96 „ 


0-10 cm. 
010 „ 
0-07 „ 
0-10 „ 
10 „ 
0-05 „ 


1 in 564 
1 „ 516 
1 „ 399 

1 „ 278 
1 „ 272 
1 „ 620 



Since in no case was there any difference between the positions for equality 
when the tube was drawn out and pushed in, we may say that, within the 
hmits of error above mentioned, the law of propagation holds good for both 
transparent and absorbing media. 



NOTE IN CORRECTION TO ABOVE PAPER. 

[Birmingham Phil. Soc. Proc. 6 (1888), p. 168.] 

In a paper read by the authors on March 31st, 1887, an account was given 
of a method of experimentally verifying the law of propagation of light both 
for transparent and absorbing media. In the paper it was stated that the 
method used seemed to lead to much more than the usual accuracy in the 
estimation of small differences of intensity of illumination. At the suggestion 
of Lord Rayleigh we have re-examined the method, and have come to the 
conclusion that the experiments were affected by some source of error — which 
we cannot now detect — leading us to under-estimate the differences noted, as 
we now find that we only attain the ordinary accuracy, detecting a difference 
not far from 1 % . This does not, of course, affect the method as a proof of 
the law, but only gives a wider range for the possible errors of experiment. 
We much regret, however, that the pubhshed figures should have given to 
the experiments an appearance of accuracy to which they have no real claim. 



34. 

HAZE. 

[Nature, 39, 1889, pp. 323-324.] 

I have for some time given in my lectures an explanation of the common 
summer haze which appears to me to be very probable. I do not know 
whether it is new, but it has not been referred to in the discussion raised by 
Prof. Tyndall's letter on Alpine haze*. Some time since I mentioned it to 
Prof. Lodge, and at his suggestion I send it to you, though its extension to 
other kinds of haze is somewhat speculative. 

It is that haze is often due to local convection-currents in the air, which 
render it optically heterogeneous. The light received from any object is, 
therefore, more or less irregularly refracted, and, through the motion of the 
currents, its path is continually varying. The outline of the object, instead 
of appearing fixed, has a tremulous motion, and so becomes ill-defined. At 
the same time, reflection occurs where there is refraction at the surfaces of 
separation of heterogeneous portions. Much of the light which, in a homo- 
geneous medium, would come straight from the object, is thus lost for direct 
vision, and the contrast between neighbouring objects is lessened. The 
reflected light is diffused as a general glare. The combination of the quivering 
of outline, and the loss of direct light, with the superposition of the reflected 
light as a diffused glare, gives the appearance we call haze. 

This explanation appears to me to accord well with the obvious facts of 
summer haze — the haze which is seen in the middle of a hot, cloudless, summer 
day. The lower layers of air, being heated by contact with the earth, rise 
in temperature till equilibrium is no longer possible, and convection begins, 
streams of the heated air rising, and streams of colder air falling to take its 
place. The variation of temperature and density gives optical heterogeneity. 
The existence of these streams is sometimes shown by the quivering of distant 
objects, looked at through the air close to the ground, but a telescope will 
often show the quivering of outline at higher levels, and when quite invisible 
to the naked eye. Accompanying this refraction, reflection must occur. We 
have a direct proof of its occurrence in the fact that the glare is greatest 
under the sun, where reflection occurs at angles approaching grazing incidence, 

* [Nature, vol. 39, 1888, p. 7.] 



I 



HAZE 447 

for which it is a maximum ; while it is least opposite the sun, where reflection 
occurs at angles approaching normal incidence, for which it is a minimum. 
The opening lines of The Excursion perfectly describe the resulting appear 
ance: 

"'Twas summer, and the sun had mounted high; 
Southward the landscape indistinctly glared 
Through a pale steam; but all the northern downs, 
In clearest air ascending, showed far ofif 
A surface dappled o'er with shadows flung 
From brooding clouds." 

During the night the lower strata become colder than the upper ones, and 
the atmosphere passes into a state of stable equilibrium. We should there- 
fore expect that, if the foregoing explanation is true, there would be complete 
absence of haze, and it is well known that the air is peculiarly clear in early 
morning, when we get above the fog-level. 

According to this account of heat-haze, it stands in sharp contrast to fog, 
of which it is so often supposed to be a relative in reduced circumstances. 
While the one requires convection, the other usually occurs when the air is 
in stable equilibrium, the lowest strata being the coldest. In the fog, for 
example, which so frequently heralds or accompanies the break-up of a frost, 
the lower strata are still cold, while above the wind has changed, and the air 
comes up warm and vapour-laden. The vapour diffuses downwards into the 
lower, cold stra