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Full text of "X rays, an introduction to the study of Röntgen rays"

* 
















FIG. 1. Photograph obtained by C. T. R. Wilson of the path of a beam of 
X rayg in supersaturated air. The beam of rays, some 2 mm. in diameter, 
was sent through the moist air (from left to right in the figure) immediately 
after the expansion which produced the supersaturation (see p. 149). The 
axis of the camera was horizontal, and the magnification of the photograph 
is about 2$ times. 




FIG. 2. Photograph obtained by C. T. B. Wilson showing the passage of X rays 
(from left to right) through a thin copper plate (see p. 152). 



Frontispiece, 



X RAYS 



AN INTRODUCTION TO THE STUDY 
OF RONTGEN RAYS 



BY 

G. W. C. KAYE, B.A., D.Sc. 

HEAD OK THE RADIUM DEPARTMENT AT THE NATIONAL PHYSICAL LABORATORY 

EXAMIXER IX MEDICAL PHYSICS FOR THE UNIVERSITIES OF LONDON AND OLAS'iOW 

MKMBER OF COUNCIL OF THE RoNTOEX SOCIETY 



LONGMANS, GREEN AND CO. 

39 PATERNOSTER ROW, LONDON 

FOURTH AVENUE & SOTH STREET, NEW YORK 

BOMBAY, CALCUTTA, AND MADRAS 

1914 



K 






K 

as 



PREFACE. 

THIS little book does not profess to be a treatise or hand- 
book on X rays. It aims merely at giving an account of 
such of the present-day methods and apparatus as appear 
valuable or novel, and which, in many cases, can only 
be found scattered throughout many journals ; it treats 
critically, and here and there somewhat comprehensively, 
some of the features which have laid claim to the interests 
of the writer from time to time ; it is concerned to some 
extent with the development of theory as well as of experi- 
ment ; and it attempts to convey a notion, however dis- 
connected and ill-proportioned, of the historical trend of 
events from Prof. Rontgen's world-famous discovery in 1895 
down to the end of the year 1913. 

The author trusts that the form of the book will be accept- 
able, not only to the student of physics, but to the man of 
general scientific interests, and particularly to the members of 
the medical profession, most of whom are keenly alive to the 
possibilities of the rays which Rontgen has placed at their 
service. He is aware from experience as teacher and 
examiner of medical students, at Cambridge and London, 
of their need of a book on the subject which is neither 
recondite nor mathematical. 

To two of his colleagues at the National Physical Labora- 
tory, the writer wishes especially to record his grateful 
thanks. Mr. E. A. Owen has revised both manuscript and 
proof, and has co-operated extensively in the treatment of 
Chapter XII., on the Interference and Reflection of X Rays 
by Crystals, a section which the writer believes to be the 
first collected account of this new and fascinating branch 

298534 



viii PREFACE 

of physics. Mr. W. F. Higgins has given freely and gene- 
rously of his time and energies, and rendered invaluable 
aid in all the different stages of the work. He has executed 
with great care a large proportion of the diagrams, and is 
responsible for the preparation of the index and some of the 
more lengthy tables. 

Sir J. J. Thomson, Prof. Bragg, and Mr. C. T. R. Wilson 
have kindly given permission to include original photo- 
graphs. The writer's obligations are also due to the Councils 
of the Royal Society, the Cambridge Philosophical Society, 
and the Rontgen Society, the Editor of the Archives of the 
Rontgen Ray, Messrs. J. and A. Churchill, and Messrs. 
Taylor and Francis, for the loan of original blocks ; and 
to Messrs. F. R. Butt & Co., A. C. Cossor, H. W. Cox & Co., 
C. H. F. Muller, Newton & Wright, The Sanitas Electrical 
Co., Schall & Son, and Siemens Bros. & Co. for various 
trade blocks. 

Finally, the author would wish to thank his wife and 
Mr. J. R. Willis for general criticism, and Mr. A. A. Robb 
of St. John's College, Cambridge, for permission to include 
his verses on Maxwell's famous equations and the birth 
of .an X ray. Mr. Robb's skill in parody is not so well 
known outside Cambridge as his mathematical researches ; 
and the author ventures to hope that the " Revolution of 
the Corpuscle," which first saw light in the Post-Prandial 
Proceedings of the Cavendish Laboratory, will serve to 
" temper the wind " of those critics who can see only the 
numerous shortcomings in the book. 

The writer will be content if his work can be regarded as 
one of the many tokens of esteem with which old students 
of the Cavendish School of Research have delighted to 
honour their distinguished professor, Sir J. J. Thomson. 

G. W. C. K. 

February 1914. 



CONTENTS. 



PAGB 

PREFACE - vii 

TABLE OF NATURE AND PROPERTIES OF VARIOUS KINDS OF 

" RAYS " - xiv 

" THE REVOLUTION OF THE CORPUSCLE " xv 

INTRODUCTION - - xvii 

TABLE OF ABBREVIATIONS OF REFERENCES TO JOURNALS - xx 

CHAPTER I. 

THE PHENOMENA OF A DISCHARGE TUBE. 

THE POSITIVE COLUMN 1 

THE CATHODE DARK-SPACE - 2 

THE FLUORESCENCE OF THE TUBE - 3 

CHAPTER II. 
CATHODE RAYS. 

HISTORICAL - 4 

THE NATURE OF THE CATHODE RAYS - 5 

J. J. THOMSON'S EXPERIMENTS - 7 

CORPUSCLES AND ELECTRONS 8 

WEHNELT CATHODES - 8 

PROPERTIES OF CATHODE RAYS - 10 

ELECTRON THEORY OF MATTER - 18 

POSITIVE RAYS. 

NATURE OF THE POSITIVE RAYS - 19 

J. J. THOMSON'S EXPERIMENTS 20 



x CONTENTS 

CHAPTER III. 
X RAYS. 

PAGE 

THE DISCOVERY OF X RAYS 24 

METHOD or PRODUCTION OF X RAYS - 25 

CHAPTER IV. 
AN X-RAY BULB. 

EARLY X-RAY TUBES - 29 

THE ANODE - 33 

THE CATHODE - 34 

THE ANTICATHODE 35 

SUITABLE METALS FOR THE ANTICATHODE - 38 

METHODS OF COOLING THE ANTICATHODE - 41 

CHAPTER V. 
HIGH-POTENTIAL GENERATORS. 

INFLUENCE MACHINES - 47 

INDUCTION COILS - 49 

CORE - 50 

PRIMARY WINDING 51 

CONDENSER - 51 

SECONDARY WINDING - 53 

FEATURES OF COIL DESIGN - 55 

STEP -UP TRANSFORMERS 59 

BREAKS AND INTERRUPTERS 62 

THE HAMMER BREAK - 62 

THE WEHNELT INTERRUPTER 62 

MERCURY BREAKS - 64 

RECTIFIERS AND VALVE -TUBES - 66 

CHAPTER VI. 

THE "HARDNESS" OF AN X-RAY BULB. 

THE FACTORS CONTROLLING THE HARDNESS OF AN X-RAY 

BULB 68 



CONTENTS xi 

PAGE 

THE PROGRESSIVE HARDENING OF, AN X-RAY BULB WITH USE 71 
DEVICES FOR SOFTENING AN X-RAY BULB ... 74 



CHAPTER VII. 

THE BLACKENING OF AN X-RAY BULB. 

CATHODIC DISINTEGRATION OR " SPUTTERING " 76 

THERMAL VOLATILISATION OF THE ANTIC ATHODE 81 

VIOLET COLORATION OF THE BULB - 83 

CHAPTER VIII. 

THE MEASUREMENT OF X RAYS. 

THE INTERNATIONAL AND BRITISH RADIUM STANDARDS 84 

METHODS OF MEASURING INTENSITY - 85 

CURRENT THROUGH THE TUBE 86 

THERMAL METHODS OF MEASURING INTENSITY - 87 

IONISATION METHODS - 87 

PHOTOGRAPHIC METHODS 91 

FLUORESCENCE METHODS 93 

METHODS USED IN MEDICINE : THE PASTILLE, ETC. - 93 

METHODS OF MEASURING HARDNESS - 94 

WAVE-LENGTH 95 

POTENTIAL APPLIED TO X-RAY TUBE - 96 

ABSORPTION METHODS - 99 

BENOIST'S PENETROMETER, ETC. - 104 

CHAPTER IX. 
SECONDARY RAYS. 

SCATTERED X RAYS - 108 

DISTRIBUTION - no 

POLARISATION - no 

CHARACTERISTIC OR "MONOCHROMATIC" X RAYS - 112 

K AND L SERIES OF RADIATIONS - 113 
CHARACTERISTIC RADIATIONS FROM RADIOACTIVE ELEMENTS 118 



xii CONTENTS 

PAGE 

CHARACTERISTIC LIGHT RAYS 119 

DIRECT GENERATION OF CHARACTERISTIC RAYS - - 121 

CONNECTION WITH CATHODE RAY VELOCITY - 124 

SELECTIVE ABSORPTION OF CHARACTERISTIC RAYS - 129 

CORPUSCULAR RAYS - 139 

DISTRIBUTION - 139 

VELOCITY - 140 

ABSORPTION - - - - - - - - ,- 141 



CHAPTER X. 
FURTHER PROPERTIES OF THE X RAYS. 

lONISATION PRODUCED BY X RAYS - 145 

C. T. R. WILSON'S FOG CONDENSATION EXPERIMENTS - 149 

VELOCITY OF X RAYS - 152 

CHAPTER XI. 

PRACTICAL APPLICATIONS OF X RAYS. 

RADIOGRAPHY - 154 

INSTANTANEOUS RADIOGRAPHY - - 159 

PHYSIOLOGICAL AND CURATIVE ACTIONS OF X RAYS - - 161 

X-RAY " BURNS " - 161 

PROTECTIVE DEVICES - - 161 

GLASSES ESPECIALLY TRANSPARENT TO X RAYS - - 165 

THERAPEUTIC USE OF HOMOGENEOUS X RAYS 166 

CHAPTER XII. 

INTERFERENCE AND REFLECTION OF X RAYS. 

LAUE'S THEORY OF DIFFRACTION BY CRYSTALS - - 169 

THE EXPERIMENTS OF FRIEDRICH AND KNIPPING - 171 

BRAGG'S THEORY --------- 179 

EXPERIMENTS ON RADIUM y RAYS - 185 

BRAGG'S EXPERIMENTS ON CRYSTAL -REFLECTION - - 186 



CONTENTS xiii 

PAGE 

CRYSTAL-STRUCTURE - 191 

W A VE -LENGTHS OF X RAYS AND ATOMIC DISTANCES IN 

CRYSTALS - 196 



CHAPTER XIII. 
THE NATURE OF THE X RAYS. 

PLANCK'S QUANTUM THEORY 205 

BRAGG'S NEUTRAL-PAIR THEORY - - 205 

IDENTITY OF X RAYS AND ULTRA-VIOLET LIGHT RAYS 206 

STOKES' SPHERICAL PULSE THEORY - - 208 

THE LOCALISED PULSE THEORY OF J. J. THOMSON - - 213 

THE OUTSTANDING PROBLEM OF RADIATION - 215 

APPENDIX I. 

AN INTERVIEW WITH PROF. RONTGEN by Sir James 

Mackenzie Davidson - 217 

APPENDIX II. 

COOLIDGE'S X-RAY BULB WITH HOT CATHODE - 219 

APPENDIX III. 

THE PRODUCTION OF HIGH VACUA - 222 

APPENDIX IV. 

ELECTRICAL INSULATORS - 226 

APPENDIX V. 

TABLE OF ATOMIC WEIGHTS 229 

TABLE OF DENSITIES - 231 

TABLE OF VALUES of e~ xd - 232 

TABLE OF CATHODE-RAY VELOCITY AND POTENTIAL - - 233 

TABLE OF CHARACTERISTIC y RAYS - - 234 

TABLE OF WAVE LENGTHS OF LINES IN X-RAY SPECTRA - 235 

INDEX - .... 236 



NATURE AND PROPERTIES OF VARIOUS KINDS OF "RAYS." 

f = charge carried by hydrogen ion in electrolysis, 
e -unit of electricity^ =4-7 x 10~ 10 electrostatic units (E.S.U.), 
[ =l-57x 10~ 20 electromagnetic units (E.M.U.) 


NATURE. 


RAYS. 


RATIO OF CHARGE TO 
MASS, (elm). 


VELOCITY. 
cms. /sec. 


RANGE, ETC. 


E.M.u.grm.-i 


E.s.u.grm.- 1 


ELECTRI- 
CALLY ; 
NEUTRAL. 


Hertzian waves. 
Infra-red rays. 

Visible light rays. 

Ultra-violet light. 
Entladung- 
strahlen. 


106 to 0-4 cms. \ 
0-013 to 7-7 xlO-5 
7- 7x10-5 to (Wave 
1 3-6x10-5 Hength 
3-6 x 10-5 to 10-5 
About 10-6 ? J 


3 x low. 


From a fe> 
mm. of air t 
infinity. 


X rays. 


Wave length 


about 10-8 


3 x 1010. 


From a fei 
cms. to ove 
100 metres i 
air at N.T.P. 


Y rays of Ra, U, 
Ac. Th, etc. 


Wave length 


about 10-9? 


3 x 101. 


Reduced to 
per cent, by 
mile of air a 

N.T.P. 


Ordinary atoms 
and molecules. 


Hj atom 
w= 1-64 x 10-^4 grm. 
Diam. = 2-2x10-8 cm. 


H 2 : 18-4x104) at 
O 2 : 4-6xl04/0C. 


Mean free pat 
of H 2 at C 
1-8 x 10- 5 cm 


CARRIERS 
OF 
NEGATIVE 

ELECTRI- \ 
CITY. 


Electrons. 
Corpuscles. 
Cathode rays. 
Lenard rays. 
Negative ion at 
low pressures. 


(For small 
1-77 x 107 

rw = 1/183 
LDiam. =4 


velocities.) 
5-31 x 1Q17 j 

3 Ho atom. ] 
x 10- 13 cm J 


Photoelectrons 
107 to 108. 


Very small. 


Wehnelt cathode rays 
108 to 109. 


Very small. 


Cathode rays 
109 to lOio. 


Range in air 
few mms. 


/3 rays of Ra, U, 
Th, Ac, K, etc. 


ft rays of Ra 
lOio to 2-99 x IQio. 


Stopped by 
about 1 cm. c 
lead. 


8 rays 
(slow /3 rays). 


As low as 3-2 x 108. 


Too slow t 
ionise. 


Negative ion. 


May have several charges 
(though usually one) and 
up to 30 molecules. 


In air; 1-8 for unit 
electric field. 





Negatively 
charged atoms 
and molecules in 
discharge tubes. 


104 
(for H 2 ). 


3 x 1014 

(for Ho). 


Up to 108. 





CARRIERS 
OF 
POSITIVE 

ELECTRI- \ 
CITY. 


a rays of Ra, U, 
Th, Ac, etc. 
(helium atoms 
with charge 2e). 


1 4-8 x 10 : * 
Hm=6-56 
J x 10-24] 


1-4 x 1014. 


Initial value 1-6x109 
to 2-2 x 109 (depend- 
ing on source). 


3 to 8 cms. i 
air at N.T.P. 


Recoil atoms. 


47 (RaB). 


1-4 x 10 12 . 


5xlQ4(RaC). 


1/10 mm. in ai 
at N.T.P. 


Positively 
charged atoms 
and molecules 
(Kanalstrahlen) 
in discharge 
tubes. 


10^ 
(for Ho). 


3 x 1014 
(for Ho). 


Up to 10 s . 





Positive ion. 


May have several charges T _ _ ir . -,.- fnr .. nli . 
(though usually one) and ln 'SLUrfc fldd umt 
up to 30 molecules. 






THE REVOLUTION OF THE CORPUSCLE. 1 



A CORPUSCLE once did oscillate so quickly to and fro, 

He always raised disturbances wherever he did go. 

He struggled hard for freedom against a powerful foe 

An atom who would not let him go. 

The aether trembled at his agitations 

In a manner so familiar that I only need to say, 

In accordance with Clerk Maxwell's six equations 

It tickled people's optics far away. 

You can feel the way it's done, 
You may trace them as they run 
dy by dy less d/3 by dz is equal K . dX/dt 



While the curl of (X, Y, Z) is the minus d/dt of the vector (a, b, c). 



Some professional agitators only holler till they're hoarse, 

But this plucky little corpuscle pursued another course, 

And finally resorted to electromotive force, 

Resorted to electromotive force. 

The medium quaked in dread anticipation, 

It feared that its equations might be somewhat too abstruse, 

And not admit of finite integration 

In case the little corpuscle got loose. 

For there was a lot of gas 

Through which he had to pass, 

And in case he was too rash, 

There was sure to be a smash, 

Resulting in a flash. 
Then dy by dy less d/3 by dz would equal K . dX/dt 



While the curl of (X, Y, Z) would be minus d/dt of the vector (a, 6, c). 



The corpuscle radiated until he had conceived 

A plan by which his freedom might be easily achieved, 

1 Air : " The Interfering Parrot " (Geisha). 



xvi REVOLUTION OF THE CORPUSCLE 

I'll not go into details, for I might not be believed, 

Indeed I'm sure I should not be believed. 

However, there was one decisive action, 

The atom and the corpuscle each made a single charge, 

But the atom could not hold him in subjection 

Though something like a thousand times as large. 

The corpuscle won the day 

And in freedom went away 

And became a cathode ray. 

But his life was rather gay, 

And he went at such a rate, 

That he ran against a plate ; 

When the aether saw his fate 

Its pulse did palpitate. 
And dy by dy less df3 by dz was equal K . dX/dt. 



While the curl of (X, Y, Z) was the minus d/dt of the vector (a, b, c). 

A. A. R. 



INTRODUCTION. 

IN the early nineties, it was not infrequently maintained 
that the science of physics had put its house in complete 
order, and that any future advances could only be along 
the lines of precision measurement. Such pessimism has 
been utterly confounded by a sequence of discoveries since 
1895, unparalleled in their fundamental nature and promise. 

Even many not specially concerned have had their atten- 
tion directed to the recent attempts at solving the riddle 
which has excited interest and taxed ingenuity since the 
beginning of civilisation the problem of the ultimate struc- 
ture of matter. The chemist and physicist have long built 
upon a theory of atoms and molecules ; though information 
as to the existence and behaviour of individual atoms was 
only based on speculation, however justifiable. 

But within the last decade we have not only isolated 
the atom, but we have learnt a great deal about its internal 
structure. Radioactivity has, for example, introduced us 
to an electrically charged atom of helium (the a ray) with 
characteristics such that it can, in spite of its extreme small- 
ness, 1 make individual appeal to our senses. The speed of 
a rays is so abnormally high, 2 that if, for instance, they 
are allowed to strike a fluorescent screen, as in the Spin- 
thariscope of Sir Wm. Crookes, each atom possesses enough 
energy to record its arrival by a visible flash of light. This 
provided what was probably the first instance of the regis- 
tering of a single individual atom. Rutherford and Geiger 
similarly turned to account the electric charge, and have 

1 Mass about 7 x 10~ 24 gramme ; diameter about 2 x 10~ 8 cm. 
-About 12,000 miles or 2x 10 9 cm. per sec. 



xviii INTRODUCTION 

actually recorded the arrival of single atoms by means of 
a delicate electrometer. 

More recently, C. T. R. Wilson, by the aid of his most 
beautiful and ingenious experiments on fog condensation, 
has succeeded in rendering visible and photographing the 
paths, not only of single charged atoms, but of electrons 
and X rays as well. 

The emission of such charged atoms from the radioactive 
elements proceeds entirely regardless of let or hindrance on 
our part : we have, however, the ability and means to 
create similar abnormal carriers for ourselves they are, 
for instance, to be found in abundance in a rarefied gas 
through which an electric discharge is passed. Under these 
conditions, the gas molecules, which are ordinarily electri- 
cally inert, assume, in many cases, electrical charges and, 
in addition, may have their usual velocities 1 increased a 
thousand-fold, so that they acquire properties which single 
them out from their fellows. 

These facts are sufficiently attractive in themselves, but 
even those few among us who, severely practical, stil] look 
askance at ' ionics,' cannot but agree that the close study 
which has been given to the phenomena of a discharge 
tube has already been more than repaid by the further 
discovery of electrons by J. J. Thomson and of the X rays 
by Rontgen. 

The discovery of electrons provided us with the present 
accepted theory of the constitution of matter ; it paved 
the way for a ready recognition of the properties of the 
radioactive elements, then on the point of discovery ; and 
it led to a new school of physics which accepted as a creed 
the transmutation of the elements, an idea entirely repug- 
nant to the orthodox chemist who had been taught to 
regard the elements as fundamental and immutable the 
foundation stones of a science which had been well and 
truly laid. 

The amazing properties of the X rays excited universal 
astonishment at the time of their discovery. An X-ray 
outfit is now indispensable to the surgeon and physician ; 

1 About 20 miles per minute in the case of air molecules. 



INTRODUCTION xix 

and the debt which the world of humanity owes to the 
X rays is a heavy and increasing one. 

Through the efforts of a devoted band of workers, with 
an outlook on life not immediately utilitarian, the Rontgen 
rays have thrown a searchlight on many phases of atomic 
physics not susceptible to other methods of attack. 

And, last of all, and quite recently, X rays have come 
to the aid of the crystallographer and triumphantly dis- 
played in the hands of Laue. Friedrich and Knipping, Bragg 
and others, the regular grouping of the atoms in a crystal. 
The experiments, which have opened up an immense field 
of enquiry, have at the same time given the long-deferred 
answer to the problem of the nature of the unknown or 
; <X" rays. 

The Geissler discharge tube * the former beautiful play- 
thing of the scientist has proved the pioneer of some of the 
most wonderful discoveries and speculations that physical 
science of this or any generation has known. Truly, as 
Maxwell predicted in the early severities, the vacuum tube 
has shed light upon the whole domain of electrical science, 
and even upon the constitution of matter itself. 

Our present intent, that of the study of the X rays, is 
approached most naturally by way of a scrutiny of the 
general phenomena of a discharge tube and of electrons in 
particular. To these branches of the subject we accord- 
ingly first direct ourselves. 

1 Known at different times and in different countries as the Pliicker, 
Hittorf, or Crookes tube. 



ABBREVIATIONS OF REFERENCES TO JOURNALS. 

A.d.P. Annalen der Physik. 

A.Rt.R. Archives of the Rontgen Ray. 

C.R. Comptes Rendus. 

D.P.G.V. Verhandlungen der Deutschen Physikalischen Gesell 

schaft. 

J.d.P. Journal de Physique. 

J.Rt.S. Journal of the Rontgen Society. 

N. Nature. 

P.C.P.S. Proceedings of the Cambridge Philosophical Society. 

P.M. Philosophical Magazine. 

P.P.S. Proceedings of the Physical Society. 

P.R. Physical Review. 

P.R.S. Proceedings of the Royal Society (Series A). 

P.R.S.E. Proceedings of the Royal Society of Edinburgh. 

P.T. Philosophical Transactions. 

P.Z. Physikalische Zeitschrift. 



CHAPTER I. 
THE PHENOMENA OF A DISCHARGE TUBE. 

WHEN a current of electricity from an induction coil or 
influence machine is sent between two metal electrodes 
fused into the ends of a glass tube (say 12 inches long) 
from which the air is gradually withdrawn by a pump, the 
tube presents a continuous succession of striking appear- 
ances. 

At high pressures, air is a very bad conductor of electricity ; 
and a large force is necessary to produce a visible discharge 
while the pressure remains in the region of atmospheric. 
But a reduction of pressure facilitates the passage of the 
spark, which after a time loses its noisy character and is 
replaced by a collection of sinuous and irregular pink 
streamers which later broaden and fill almost the whole of 
the tube with a pink diffuse glow known as the positive 
column. Simultaneously the alternative spark-gap of the 
coil diminishes to a small fraction of an inch evidence 
that the rarefied air is now conducting well. 

Meanwhile the cathode the electrode by which the cur- 
rent leaves the tube l assumes at its tip a luminous tuft 
the negative glow^ violet in colour, which later grows until 
it completely envelops the cathode. Between these two 
luminous glows comes a darker ill-defined region called the 
Faraday dark-space. These general appearances correspond 
to a pressure of some 8 to 10 millimetres of mercury. 

As the pressure is further reduced, the alternative spark- 
gap begins to lengthen, the anode becomes tipped with a 

1 The electrode by which the current enters the tube is the anode. 

A 



X R,AYS 



vivid speck of glow, and the positive column proceeds, if 
the current density is suitable, to break up into thin fluctu- 
ating pink discs or striae, which subsequently thicken and 
diminish in number, intensity and extent. The Faraday 
dark-space enlarges, and in the meantime (at about 1 mm. 
pressure), the violet negative glow increases in brightness 
and volume, 1 and the glass walls of the tube are seen to 
fluoresce with an olive-green light which, as J. J. Thomson 
(P.C.P.S. 1910) has shown, is due to the action of extremely 
active ultra-violet light from the negative glow. 2 



N eg ^rive or 
C&fhode Glow. 



Srria.Nons in 
PosiNve Column. 



Anode 
Glow. 

/ 




Crookes 

D<^rk Spxce. Dark Spa.ce. 

FIG. 3. Discharge tube at low pressure, showing cathode dark-space and 
positive striations. 



As the exhaustion proceeds, this fluorescence disappears, 
the negative glow detaches itself like a shell from the cathode, 
while a new violet film forms and spreads over the surface of 
the cathode. Thus the negative glow now consists of two 
parts : they are separated from each other by a narrow dark 
region called the Crookes or cathode dark-space, which has a 
sharply defined outline running parallel to that of the 
cathode. (See Fig. 3.) 

With a reduction of pressure, the dark space increases in 
width, and pushes the outer negative glow before it. The 
dark space is often used as a rough indication of the pres- 
sure, though its width depends also on both the current 

1 The length of the glow on the cathode depends also on the current : 
the two are indeed roughly proportional (Hehl, 1903). 

2 This is especially marked in hydrogen. The fluorescence is bluish- 
green with lead glass, 



THE PHENOMENA OF A DISCHARGE TUBE 3 

density and the metal of the 'cathode, and is not really a 
reliable guide to the degree of exhaustion. 1 

With higher rarefactions (say -V mm.) both positive and 
negative glows become less bright and definite in outline, 
and finally lose almost all traces of luminosity. Meanwhile 
the cathode dark space has grown at the expense of all 
else, until finally it becomes so large that its boundaries 
touch the glass walls of the tube. It is at this stage that 
the tube begins to shine, first in the region of the cathode, 
and then (as the dark space extends) over its whole surface, 
with the brilliant apple-green fluorescence 2 well known 
to those who are accustomed to X-ray tubes. 

All this time the length of the alternative spark-gap has 
been steadily increasing. If the exhaustion is pressed still 
further, the fluorescence diminishes, and the resistance of 
the tube increases, until finally it becomes impossible for the 
discharge to pass at all. The pressure at which this comes 
about depends partly on the induction-coil and partly 
on the size of the discharge tube. A large tube always 
runs more easily than a small one at the same pressure. 3 

The above are the more salient features to be observed 
in a discharge tube. At atmospheric pressure the discharge 
consists mainly of positive column ; at low pressures it is 
the phenomena of the cathode dark-space which are most 
conspicuous and have attracted widespread attention. 

1 See Aston, P.R.S. 1912. 

2 This is quite different in appearance from, and much more brilliant 
than, the olive-green fluorescence at higher pressures (see above). The 
yellow-green colour holds only for soda-glass tubes. Lead and lithium 
glasses yield a bluish fluorescence. 

3 It should be remarked that induction coils do not give pure unidirec- 
tional currents, but, in some cases, show marked " reverse " currents as 
well, more especially with easy discharge paths. So that as the pressure 
is lowered both electrodes may appear to act as cathode until the fluores- 
cence stage is reached. In some cases it is quite difficult at the higher 
pressures to say which electrode will ultimately prove to be the cathode. 



CHAPTER II. 

CATHODE RAYS. 

Historical. 

The study of the green fluorescence on the glass was com- 
menced by Pliicker as long ago as 1859, and carried on 
vigorously by Hittorf (1869) and Goldstein (1876) in Ger- 
many, by Crookes (1879) in England, and by Puluj in 
Austria. It was soon ascertained that the fluorescence was 
produced by something coming from the region of the 
cathode ; for a suitably interposed obstacle cast a sharp 
shadow on the walls of the tube. This " something " was 
given the name cathode rays (Kathodenstrahlen) by Gold- 
stein, who believed that the fluorescence was produced by 
waves in the ether for whose propagation the gas in the 
tube was not necessary. The expression has been retained, 
and rightly, for although the word " ray " has come to be 
associated with a wave-motion in the ether, the connection 
is quite fortuitous ; indeed Newton used the term in his 
corpuscular theory of light. 

The general properties of the cathode rays soon revealed 
themselves. Pliicker had found that their path was bent 
by a magnet. Goldstein, by suitably contrived shadow 
experiments, confirmed Hittorf 's observation that the rays 
travel in straight lines, and showed further that they 
start at right angles to the surface of the cathode. Crookes. 
by the use of cathodes shaped like a concave mirror, 
demonstrated that the rays concentrate near the centre of 
curvature, and there display in a marked degree heating 
properties and an ability to excite phosphorescence in 




CATHODE RAYS 5 

many substances. They tend also to push away any object 
against which they strike, and cause, for example, paddle- 
wheels of mica to rotate, although we now know that such 
experiments depend for their success, not on the momentum 
of the rays, but on heating effects such as prevail in a radio- 
meter. 1 Most important of all, cathode rays, when they 
strike matter, generate X rays, as Rontgen snowed in 1895. 

The Nature of the Cathode Bays. 

For nearly thirty years the English and German schools 
of physics disagreed as to the nature of the cathode rays. 
German physicists, follow- 
ing Goldstein's lead, held 
that the rays were similar 
to light a wave motion 
in the ether with which 
matter had nothing to do. 
This view received much ' Anode 

,, ., . FIG. 4. A Lenard tube for showing the pass- 

Support from the experi- age of cathode rays through a thin aluminium 

ments of Hertz (1892), and 

his friend and colleague Lenard (A.d.P. 1894), who showed 
that solid bodies were not absolutely opaque to the rays 
which could pass, for example, through gold and aluminium 
foil. Lenard's historic vacuum-tube was provided with a 
small " window " of aluminium foil, 0*000265 cm. thick. 
(See Fig. 4.) When the cathode rays were shot against 
this window, he found that they, passed through without 
puncturing it, and were able to excite phosphorescence, 
etc., a few millimetres away in air. 2 There was no diffi- 
culty in accounting for the transparency of thin solids on 
the ether-wave theory, though it was apparent that the 
relation to ordinary optical transparency was slight ; for 
instance, gold leaf is more transparent than clear mica to 

1 A paddle-wheel made of a good thermal conductor such as aluminium 
does not show the effect. In a radiometer the vanes are propelled by the 
recoil of the gas molecules from the warmer face of each vane. 

2 The Lenard rays travel farther in attenuated air. We know now that 
part, at any rate, of the phosphorescence which these " Lenard rays " 
produced was due to X rays generated by the aluminium. X rays were 
not discovered until the following year. 



6 X RAYS 

cathode rays. On the other hand, the normal emission of 
the rays from the cathode could scarcely have been antici- 
pated on any wave theory : one would have inferred a 
general emission in all directions. 

But the magnetic deflectibility of the cathode rays, and 
its unsatisfactory explanation on a wave theory, were re- 
garded as crucial by English physicists, who were unanimous 
in the view that the rays consisted of particles of matter 
charged with negative electricity and projected with im- 
mense speeds from the cathode. Varley in 1870 seems to 
have been the first to suggest, though somewhat vaguely, 
the presence of such particles in the electric discharge ; and 
Sir Wm. Crookes, in a series of papers in the Philosophical 
Transactions (1879-1885), definitely adopted this standpoint 
to explain and coordinate the many striking results of his 
experiments. 

In a flight of intuition, Crookes suggested that the matter 
constituting the cathode rays was neither gas nor liquid 
nor solid as ordinarily known, but in a fourth state tran- 
scending the gaseous condition an extremely happy sur- 
mise, as events proved. It is interesting to recall that 
Faraday as long ago as 1816 speculated on the possibility 
of the existence of such a fourth state of matter. 

Crookes supposed the cathode particles to have the 
dimensions of molecules, but this view became hard to re- 
concile with their penetrating power for metals found (as 
mentioned above) by Hertz and Lenard some years later. 
To meet the difficulty, the suggestion was put forward that 
the metal, under the bombardment of the cathode rays, 
acted as a pseudo-cathode, and itself emitted cathode rays. 
It was further pointed out that instances of the penetration 
of metals by molecules were not unknown ; to wit, the 
passage of hydrogen through hot palladium" and platinum, 
the squeezing of water under hydraulic pressure through 
gold and other metals, and the gradual penetration of lead 
by gold when discs of the two are in close contact. 1 How- 
ever, these explanations were not very satisfying, nor did 

1 The experiments of Roberts-Austen on the interdiffusion of metals were 
being conducted about this time (see P.T. 1896). 



CATHODE RAYS 7 

they prove to be necessary, for. about this time new evidence 
began to accumulate in favour of the charged particle theory. 
Sir J. J. Thomson came to the conclusion that the velocity 
of the cathode rays was appreciably less than that of light ; 
and the French physicist Perrin in 1895, by catching the 
rays in a Faraday cylinder, demonstrated that they carried 
a charge of negative electricity with them. But though 
Perrin's experiment carried conviction to the majority, it 
was not regarded as conclusive by the extreme ether- wave 
supporters, who looked on the electrification as merely 




To Electroscope 



FIQ. 5. J. J. Thomson's modification of Perrin's apparatus for proving that 
cathode rays are negatively charged. 

revealing the presence of electrified particles which were 
an accidental accompaniment of, and were not essentially 
connected with, the cathode waves. 

But J. J. Thomson in 1897, by suitably modifying Perrin's 
apparatus, showed that when a magnet was used to deflect 
the cathode rays the negative electrification followed exactly 
the same course as the rays which produced the fluorescence 
on the glass (Fig. 5). He further succeeded in deflecting 
the cathode rays by an electric field : Hertz had tried this 
experiment fourteen years previously, and had failed 
because the pressure of his gas was too high, the gas con- 
ducting sufficiently well to mask the effect of the electric 
field. 

After these results it could hardly be doubted that the 
cathode rays were negatively charged particles, and the 
objections to this view, on the score of their penetrating 



8 X RAYS 

ability for solids, were finally set at rest by J. J. Thomson 
in 1897-1898. In a series of experiments, which for ingenuity 
and insight have rarely been equalled, Professor Thomson, 
by deflecting the cathode rays in magnetic and electric fields 
of known strength, was able to infer the size of the charged 
particles ; and later, to deduce their mass, velocity and 
electric charge. (See p. 14.) He showed that the cathode 
rays were neither atoms nor molecules, but something far 
smaller : the mass of each of the particles proved to be 
about T ^V?rth part of the smallest mass hitherto recognised 
by chemists that of the atom of hydrogen. Their nature 
depends neither on the nature of the cathode nor on that 
of the residual gas in the discharge tube. The charge is 
invariable, and agrees with that carried by the hydrogen 
ion in liquid electrolysis : the velocity depends upon the 
electric force which is applied to the tube ; the speeds are 
found to be an appreciable fraction of the speed of light, 1 
in some cases as much as one- third. 

Thus the cathode rays proved to be neither ethereal waves 
nor ordinary material particles, but bodies of sub-atomic 
size moving with prodigious velocities, a state of things so 
nearly realising Newton's long-abandoned conception in his 
corpuscular theory of light that J. J. Thomson called the 
small particles which constitute the cathode rays, corpuscles. 
Johnstone Storey had previously suggested the name electron 
for the electrolytic unit, or atom of electricity, and the 
suitability of the expression for the cathode rays was at once 
recognised : both terms have since come into common use. 

It is perhaps difficult to realise the disproportion 2 between 
tne size of an atom and the size of an electron : the two 
have been aptly compared to a fly in a cathedral, or a speck 
of dust in a room ! 

Wehnelt Cathode. 

If the cathode is constructed of a strip of platinum which 
can be raised to a bright red heat, 3 and on which is mounted 

1 The velocity of light is 186,300 miles or 3 X 10 10 cms. per sec. 

2 1 : 100,000 in linear dimensions ; 1 : 10 15 by volume. 

3 By means of an independent current. 



CATHODE RAYS 9 

a speck of lime 1 (see Fig. 6), all the ordinary phenomena 
of a discharge tube can be reproduced by means of quite 
small potentials 100 volts or less between the cathode and 
anode. The hot lime emits torrents of corpuscles and the 
pencil of cathode rays, owing to their low speed (see p. 12), 
produces vivid luminosity in the rarefied gas. The velocity 
of the rays is proportional to the square root of the potential 
applied : the velocities are of the order of T Vth of those in 



Elecrrified 
PlaJe 




Pl&hnum 

Strip *s^ r+ Speck 
\ J of Lime 



Side View 

of 
Cathode 



FIG. 6. A discharge tube with a Welmelt cathode, displaying the repulsion 
of the cathode rays by a negatively charged plate. 

an ordinary discharge tube ; e.g. with 50 volts, the velocity 
= 4- 2 x 10 8 cms. per sec. The magnetic and electrostatic 
deflection of these rays can be strikingly demonstrated 
owing to their relatively small velocity. * 

It is possible with the hot-lime cathodes to send very 
large currents through the discharge tube currents of ^ 
ampere and more are readily attained. With high cathode 
temperatures and voltages of 200 or 300 on the tube it 
requires precautions to prevent the discharge growing into 
"an arc 2 with the consequent destruction of the cathode. 

1 Or any of the alkaline earths (a speck of sealing-wax, ignited in situ, 
is very convenient). See Wehnelt (P.M. July 1905). 

2 A water resistance in the potential circuit serves to prevent this. 



10 X RAYS 

Transmission and Absorption of Cathode Rays. 

Lenard (Wied. Ann. 1895) showed that for fast- moving 
cathode rays, the extent of the absorption in different sub- 
stances is roughly proportional to the density. The pene- 
trating power of a cathode ray varies very greatly with its 
speed. The highest speed rays, which move at the rate of 
about 10 10 cms. per sec., can only penetrate 2 or 3 mms. of 
air at ordinary temperature and pressure. The fastest /3 
rays from radium are cathode rays which have no more 
than about three times this speed, and yet their range in 
air is nearly 100 times as great. The range is, of course, 
increased by lowering the pressure of the gas, as the mole- 
cules are not so closely packed, and the cathode rays suffer 
fewer encounters with the atoms. 

During its journey, the cathode ray loses velocity both 
by ionising and by deflection. So long as its speed remains 
high, it pursues a fairly even course ; as it slows down, it 
becomes more and more liable to deflection by the encoun- 
tered atoms, until finally it loses so much energy that it 
becomes undistinguishable as a cathode ray. Thus a fine 
pencil of cathode rays gradually becomes fuzzy and scattered. 

Some of the cathode rays are actually swung completely 
round by the surface atoms, and so may be " reflected " 
with velocities up to the original velocity. The more 
obliquely incident the rays, the greater the number " re- 
flected." In regard to the transmitted rays, Whiddington 
(P.E.S. 1912) has recently shown experimentally the truth 
of a relation deduced theoretically by Sir J. J. Thomson 
some years ago. He finds that the maximum velocity ( V d ) 
with which a cathode ray may leave a material of thickness 
d is given by F 4 F * - 7 

K " Y d ~ ia/ ' 

where F is the initial velocity of the cathode ray and a 
is a constant (2 x 10 40 for air ; 732 x 10 40 for Al ; 2540 x 10 40 
for gold ; all in cm.- sec. units). This fourth-power scatter- 
ing law holds also for Ra ft rays, except the very fastest. 
Whiddington was unable to trace any simple connection 
between the value of a and the atomic weight or density 
of the material. 



CATHODE RAYS 11 

Cathode rays lose their speed very quickly in passing 
through solids, and thin metal leaf has to be used in experi- 
mental work. 1 The maximum thickness of aluminium or 
glass which transmits high-speed cathode rays to any 
appreciable extent is about 0-0015 cm. (Cf. p. 45.) 

Heating Effects produced by Cathode Bays. 

The bulk of the energy of the cathode rays is dissipated 
as heat when the rays strike an obstacle. A simple cal- 
culation shows that if in a tube of moderate vacuum 
the current carried by the cathode rays is a milliampere 
(10~ 3 amp.), the energy given up by the rays per minute 
is of the order of 100 calories. 2 Now a milliampere is only a 
moderate current ; as will be seen later, currents up to 50 
or 60 milliamperes and even more (with momentary 
discharges) obtain in practice. No target can withstand such 
currents for any length of time if the rays are concentrated 
by using a concave cathode. Platinum may be fused, 
diamonds converted into coke ; even tantalum and tung- 
sten 3 with melting points in the neighbourhood of 3000 C. 
can be rendered molten. Owing also to the low pressure, 
most metals can be vaporised with ease. 

The heating effects reach a maximum at a certain pres- 
sure, which is not very low, and are not so marked in very 
high vacua. 

lonisation produced by Cathode Bays. 

A cathode ray has the property of ionising a gas, i.e. of 
rendering it electrically conducting. The ionising power is 

1 Al, Cu, Ag, Sn, Pt, and Au can be got in the form of thin leaf. (See 
Kaye and Laby's Physical Constants t p. 35.) 

2 The energy E = ty . . v 2 , where i is the current, v is the velocity of 

C 

the rays, m is the mass of and e the charge on each ray. If E is expressed 
in calories per min., i in milliamperes, and v in cms. per sec., then 
E=4c . 10~ 18 iv 2 . This assumes that all the cathode ray energy is turned 
into heat. 

3 Von Wartenberg (1907) determined the melting point of tungsten by 
means of a cathode-ray vacuum furnace. He used a concave Wehnelt 
cathode (p. 8) for the purpose. See also Tiede (1913) for an account of 
a cathode-ray furnace with a water-cooled cathode. 



12 X RAYS 

especially conspicuous with the slower rays the ionisation 
per centimetre of path was in fact found by Glassori (P.M. 
1911) to vary approximately as l/(velocity) 2 . The faster 
rays have the greater energy, it is true, but they do not 
begin to ionise to any great extent until their velocity has 
dropped. 1 A cathode ray (like the a ray) ionises most 
towards the end of its path, until finally it loses so much 
energy that it can no longer ionise, and ceases to be dis- 
tinguishable as a cathode ray. 

The strong ionisation is responsible for the luminosity 
which cathode rays produce in the residual gas of a dis- 
charge tube a luminosity which at higher pressures is 
displayed as the outer negative glow bounding the cathode 
dark-space (see p. 2), and at lower pressures lights up 
the path of the rays themselves (see p. 34). The luminosity 
of the track reaches a maximum at a certain pressure ; as 
the pressure is further reduced, the rays gradually become 
faster and less luminous, but simultaneously their power of 
exciting fluorescence in the glass walls of the tube increases, 
until finally the rays become quite invisible and are mani- 
fested solely by the fluorescence on the glass. 

Fluorescence produced by Cathode Rays. 

An ordinary X-ray tube affords abundant evidence of the 
fluorescing properties of glass subjected to cathode rays. 
Crookes found in 1879 that glass which had suffered pro- 
longed bombardment by the rays fatigued and lost a good 
deal of its fluorescing ability. Most of the fatigue is only 
temporary, but a portion is very permanent. Crookes 
found, for instance, that complete recovery was not brought 
about even by fusion of the glass ; and Campbell- Swinton 
(P.E.8. 1908) refers to a tube in which the fatigue persisted 
for more than ten years. Swinton showed that the fatigue 
is purely a surface effect, and is removed by grinding 
away the surface of the glass. He found that the thickness 
which had to be removed for this purpose was always about 
015 mm. 

1 The total ionisation of the faster ray is, of course, greater than that 
of the slower. 



CATHODE RAYS 13 

There are many substances which afford striking and 
beautiful examples of the fluorescing ability of cathode 
rays. Among those which are useful in practice are barium 
platino-cyanide (a material of which fluorescent screens 
are usually made), the mineral willemite (a silicate of 
zinc), zinc blende (sulphide of zinc), and kunzite (a lithium 
felspar). 

The fluorescing power of a cathode ray increases with its 
velocity, and does not seem to be possessed by the very 
slowest rays. Rays as slow as those from a Wehnelt cathode 
are, however, capable of causing fluorescence. 

Magnetic Deflection of Cathode Rays. 

When a charged particle (mass ra, charge e) is projected 
along a line of magnetic force, it continues to move along 
it ; but if it is projected (with velocity v) at an angle to 
the magnetic field (of strength H), the deflecting force acting 
upon it is Hev sin 0. Hence, if 

p is the radius of curvature of ^^ y y 

the resulting path, 
Hev sin 6 = mv 2 /p 
mv 

p = H^re' 

which represents a helix. So F IG . 7. 

that, in general, the effect of 

a magnetic field on the path of a moving corpuscle is to 
twist it into a helix wound on a cylinder with the lines of 
force as axis. With the slow cathode rays given out by a 
Wehnelt cathode (p. 8), the helix can be beautifully 
demonstrated : with strong fields, the helix becomes so 
long and attenuated that the rays appear to follow the 
lines of force. 

In the particular case when the cathode ray is projected 
at right angles to the magnetic lines of force, sin 6= 1, and 

p = mv/He. 

Thus in a uniform magnetic field, the path of the particle 
is a circle in a plane at right angles to the magnetic force, 
i.e. the particle is bent away in a direction at right angles 




14 



X RAYS 



both to the field and to its former direction. 1 The extent 
to which cathode rays are bent by a magnetic field thus 
depends on the strength of the field, on the speed of the 
particles, and on the quantity denoted by e/m. As to the 
speed, that is related directly to the potential which is 
applied to the tube ; the velocity is, indeed, roughly pro- 
portional to the square root of the potential or alternative 
spark-length (see p. 96). 

Electric Deflection of Cathode Eays. 

If an electric field X is acting at right angles to the direction 
of projection of the cathode ray, the force on it is Xe in the 
direction of the field. Thus, as above, the radius of curva- 



ture is 



mv 



The corresponding expression for the magnetic deflection 
was 



FlllOre i 



Cathode 




FIG. 8. J. J. Thomson's apparatus for measuring efm of cathode rays. An 
electric and a magnetic field are contrived so that their effects on a beam of 
cathode rays balance each other exactly, in which case the fluorescent spot 
produced by the cathode rays remains undeviated. 



If we contrive things so that the magnetic and electric 
deflections are equal and opposite, we can, at once, from 
a knowledge of the fields, derive both the velocity and e/m 

1 The following mnemonical rule is convenient for remembering in which 
direction a cathode ray is deflected : If the magnetic field H (i.e. the 
direction in which a N-seeking pole would ^ move) is upwards towards 
the Head, and the cathode Ray is moving horizontally towards the Right 
hand, the mechanical Force on the ray is horizontally towards the Front. 



CATHODE RAYS 15 

for the cathode ray ; for v = X/H, and e/m = X/(pH 2 ). It 
was in this way that J. J. Thomson first arrived at the 
nature of the cathode rays (p. 8). (See Fig. 8.) 

Braun Tube. 

A practical application, due to Braun, of the bending of 
cathode rays under magnetic or electric force has come into 
use in electrical engineering for the purpose of studying the 
wave-form of rapidly changing alternating currents. In the 
Braun tube, a narrow pencil of cathode rays is received on 
a fluorescent screen, and is subjected en route to both a 
magnetic and an electric field. The two fields are at right 
angles, and are both actuated by the alternating current. 
The cathode rays, having practically no inertia, are able to 
follow the most rapid vagaries of the fields, and so trace 
out on the screen a pattern, from which the wave-form can 
be deduced. 

Magnetic Spectrum of Cathode Bays. 

Each interruption of the primary current in an induction 
coil produces a small train of strongly-damped oscillations 
in the discharge (p. 59). Thus the potential on the tube 
is intermittent, and the result is a stream of cathode rays 
with a variety of speeds, each peaklet on the oscillatory 
potential curve producing a group of uniformly fast cathode 
rays, of which the speed diminishes with successive oscilla- 
tions. 

Accordingly, if the cathode rays are subjected to a mag- 
netic field, the different groups are differently deviated the 
greater the speed, the less the magnetic deviation. Thus a 
slit of cathode rays, when allowed to fall on a fluorescent 
plate, produces in a magnetic field a number of bright lines 
or bands which go to make up a " magnetic spectrum " 
an appearance first noticed by Birkeland (C.R.) in 1896. 
The brightness of each band is a measure of the number of 
rays moving with the same speed : the displacement of the 
band from the undeflected position is inversely proportional 
to the velocity, and directly proportional to e/m and the 
strength of the field. 



16 X RAYS 

If the gas pressure in the discharge tube is not very low, 
the bands may be numerous (30 or more), but if the pressure 
is lowered, the oscillations are more strongly damped, and 
so the lines become fewer, group themselves more closely, 
are less deviated as a whole (the cathode rays being faster), 
while the least deviated line becomes the brightest ; thus 



Zero 
Position. 



Pressure O'OOS mm. 



Pressure 0*004 mm. 

FIG. 9. Examples of magnetic spectra of cathode rays. 

most of the cathode rays now possess the maximum velocity. 
Fig. 9 shows two magnetic spectra obtained by Birkeland 
at different pressures. 

Beatty (P.E.S. 1913) found that with a potential of about 
60,000 volts on the tube, the main stream of cathode rays 
had a velocity corresponding to about two-thirds of the 
potential as given by a spark-gap. 

A spectrum of rays can also be produced by an electric 
field, in which case the deviations are inversely proportional 
to the square of the velocity. 

A tube driven by a Wimshurst machine or a battery of 
cells does not yield a magnetic spectrum, but only a single 
bright line, which is evidence of the fact that all the cathode 
particles have the same velocity. 

Constants of Cathode Rays. 

Measurements of the speed of cathode rays have been 
made by various experimenters ; the rays in an ordinary 



CATHODE RAYS 17 

discharge tube have velocities Tanging from about 10 9 to 
rather more than 10 10 cms. per sec., i.e. from one-thirtieth 
to one- third of the speed of light. 

The latest determinations of e/m (see p. 14) for the 
slowest rays give a value of 1-77'xlO 7 expressed in electro- 
magnetic units. Measurements of the ionic charge make 
e=r57xlO~ 20 E.M.U., so that, for small velocities, m is 
8-8xlO~ 28 gramme. Theory indicates that electrons owe 
all their mass to their velocity, and that for cathode rays 
moving, for example, with one-third the speed of light m 
would have a value about 6 % greater than the above. 

Now, according to the best authorities, the mass of the 
hydrogen atom is 1*6 x 10~ 24 gramme, so that the number 
of electrons equal in mass to the hydrogen atom is about 
1830. 

Further constants will be found on p. xii-. 

The Ubiquity of Electrons. 

Since their discovery in so artificial a source as a vacuum 
tube, electrons have been found literally to pervade the 
universe. Relatively low-speed electrons are emitted in 
many chemical reactions and by metals when exposed to 
light, especially ultra-violet. High-speed electrons with 
velocities almost up to that of light constitute the /3 rays of 
radium and the radioactive substances : the alkaline metals 
(at any rate K and probably Rb) also emit corpuscles. They 
are ejected in abundance from hot bodies, markedly so 
from the alkaline earths (lime, baryta, etc.) with velocities 
and in amounts depending on the temperature. Without 
doubt they play a part in cosmical physics : the most 
recent explanations of the aurora or northern lights regard 
them as due to enormously fast electrons ejected by the 
sun, which are collected and guided in long spirals (see 
p. 13) to the polar latitudes by the earth's magnetic lines 
of force. 1 They there ionise and cause luminosity in the 

1 Aurora occur most abundantly not at the poles, but at about latitude 
68. This requires for the electrons a velocity closely approaching that 
of light a velocity even greater than that possessed by the fastest known 
(3 particles from radium. 

B 



18 X RAYS 

upper attenuated regions of the earth's atmosphere, just as 
they do in a vacuum tube. 

But whatever their origin, electrons have always been 
found to maintain their invariable and indivisible char- 
acter : they carry the unit of electricity, and can indeed 
be regarded as the ultimate fundamental carrier of negative 
electrification. 

The Electron Theory of Matter. 

Rutherford's theory of the constitution of matter, which 
satisfies a number of the main facts, regards an atom as 
built up of a minute nucleus of positive electricity x (no 
bigger than 10 ~ 12 cm. in diameter) surrounded by an inner 
cluster of negatively charged electrons which rotate round 
the nucleus, and an outer group of electrons which also 
rotate and are less rigidly attached. The total negative 
charge of the electrons is equal to the positive charge of 
the nucleus. The outer electrons, by their number and 
arrangement, are responsible for the chemical and physical 
properties of the atom : the inner electrons have influence 
only on the phenomena of radioactivity. This explains 
why physical and chemical behaviour do not go hand in 
hand with X and y-ray phenomena. Present theory indi- 
cates that the number of corpuscles in an atom is equal 
to either the atomic weight of the material or not more 
than a small multiple or sub multiple of it. As positive 
electricity has never been found to be associated with 
bodies less than atoms, it would appear that the atom 
owes most of its mass to its positive nucleus, which is 
capable of deflecting both a and /3 particles out of their 
paths. 

The electron theory of matter has been elaborated by 
Lorentz and others, and extended to many departments 
of physics. For instance, the phenomena of magneto- op tics 
show that electrons are intimately concerned with the 
spectrum lines. An electronic theory of magnetism has 
been developed by Larmor (1897) and Langevin (1905). 

1 Rutherford (P.M. 1911). See also Bohr (P.M. July, Sept. and Nov. 
1913), and J. J. Thomson, Engineering, March 21, 1913." 



POSITIVE RAYS 19 

In the case of solids, there are supposed to exist un- 
attached and wandering electrons interspersed between the 
molecules. These can be ejected by ultra-violet light or 
heat : they are the important agents in thermo-electricity, 
and in the conduction of electricity and heat ; a good con- 
ductor, for example, is one which contains many of these free 
electrons. The electron theory has, among other things, led 
to important deductions concerning the specific heats of 
metals at low temperatures a subject to which Nernst 
and others have lately given attention. 

For an account of the electron theory see Lorentz, Theory 
of Electrons, 1909, and N. R. Campbell, Modern Electrical 
Theory, 2nd ed., 1913. 



POSITIVE RAYS. 

Though the ordinary cathode rays are the most con- 
spicuous of the rays existing in a discharge tube, there are 
others also present. As long ago as 1886, Goldstein, by perfor- 
ating the cathode of a discharge tube, observed that a stream 
of rays travelled through the tube in the reverse direction 
to the cathode rays. To these rays he gave the general 
name of Canal-rays (Kanalstrahleri). Wien showed that the 
Canal rays were deflected in the opposite direction to cathode 
rays when subjected to a magnetic force, and he came to 
the conclusion that they must consist of positively charged 
particles. The deflection detected in this case, however, 
was small compared with that obtained with cathode rays, 1 
but the deflection in the electric field was of the same order 
of magnitude for both positive and negative particles. This 
pointed to the fact that the positive particle had greater 
mass than the cathode or negative particle ; Wien, in fact, 
found that the mass of the positive particle was of the 
same order of magnitude as that of a hydrogen molecule. 

The existence of the two kinds of rays in a discharge 
tube can very easily be shown by the different colours they 
produce in lithium chloride. This substance fluoresces blue 

1 Positive rays require, roughly speaking, magnetic fields of 1000 gauss or 
more, i.e. at least forty times as strong as are needed to deflect cathode rays. 



20 X RAYS 

under the action of cathode rays, and red under the action 
of positive rays, so that a small glass bead coated with 
lithium chloride and placed in a suitable position between 
the electrodes in the tube will appear red on the side towards 
the anode, and blue on the side facing the cathode. This 
bead can be utilised to explore the region between the 
electrodes in order to determine in what part of the tube the 
positive rays have their origin. Starting with the bead at 
the anode, we should find that it does not appear red on 
the side facing the anode until we arrive at the boundary 
between the negative glow and the Crookes dark-space 
(p. 2) ; it continues to fluoresce red throughout the 
Crookes dark-space. The amount of fluorescence in various 
parts of the dark-space, however, shows that most of the 
rays start from the boundary of the dark-space. 

The positive rays have strong ionising, fluorescing, and 
photographic actions. They cause soda glass to fluoresce 
a dull green ; willemite, a bright green : in both cases, the 
effects are much inferior to those produced by cathode rays. 
The positive rays show strong pulverising properties, and 
roughen or disintegrate any surface against which they strike. 

During the last few years the whole question of the 
electric discharge from the point of view of positive electricity 
has been taken up by Sir J. J. Thomson. He has shown 
that there exist in the tube high-speed atoms and molecules 
of the gases present, some positively charged, some nega- 
tively, and some uncharged. In no case has a positive 
ray been detected whose mass is smaller than that of the 
hydrogen atom ; a positive electron, if such exists, has 
hitherto eluded search. The velocities of these positive 
particles are in the neighbourhood of 10 8 cms. per second 
(the fastest have a speed of about 2 x 10 8 cms. per second) ; 
this is of the order of 1000 times the ordinary velocity of mole- 
cules as calculated from the kinetic theory of gases (p. 73). 

It is found that there are many more of these high-speed 
charged particles (or ions) moving towards the cathode than 
from it. 1 If a hole is made in the cathode, the positive 

1 These latter "retrograde rays" travel with and among the cathode 
rays, but can be detected when the cathode rays are removed by a magnet. 



POSITIVE RAYS 



21 



rays stream through it and form the Canal rays of Goldstein ; 
and in this region, where they are separated from the cathode 
rays, they can be independently investigated. 

Professor Thomson received a very fine pencil of these 
rays on a photographic plate, and en route subjected them 
simultaneously to the action of magnetic and electric fields, 
the magnetic field deflecting the rays at right angles to 



Places for 
Elecfr.cFiXd 



Anode 




PI Ate 



FIG. 10. J. J. Thomson's apparatus for measuring elm of positive rays 
(Kanahtrahleri). 

the deflection caused by the electric field (see Fig. 10). 1 
It is found that only a small portion of the beam is deflected, 
the main part goes on unaffected by the deflecting forces. 
If x and y are the deflections of a particle due to the 
action of the electric and magnetic fields respectively, and 
e, m, and v the charge, mass, and velocity of the particle, 

then we have e 

x = A . -, 



mv 



where A and B are constants depending upon the strengths 

1 In the corresponding cathode -ray experiment (p. 14) the magnetic 
force and electric force were parallel. 



22 



X RAYS 



of the electric and magnetic forces, and the distances the 
rays have to travel from the time they enter the field of 
force until they reach the photographic plate. 

The equation to the trace on the photographic plate 
becomes ^ 2 ^2 e 

x Am 

This is the equation of a parabola, so that we have on the 
plate a series of parabolas representing the loci of particles 

which have a constant value of e/m. 
(See Fig. 11.) Different particles 
register different loci on the plate, 
and for each locus ra has a definite 
value, so that the method affords 
a means of determining accurately 
the atomic weights of substances 
which are present in the tube. The 
system of curves obtained on the 
screen depends, of course, upon 
the nature of the gas in the tube. 
Most positive rays carry only one 
ionic charge (equal to that carried 
by the cathode rays), though with 
some elements 2, 3, and up to 
8 charges have been found. 

By this method, Professor 
Thomson has been able to de- 
termine the atomic weights of 
numerous elements, 1 and has, 
indeed, discovered substances as 
yet unknown to the chemist. 
When, for example, nitrogen is put into the tube there 
are seen on the photographic plate parabolas corresponding 
to N + (an atom of nitrogen with one charge), N ++ , N 2 + , 
N 3 + . A very interesting case is 'afforded by marsh gas 
(CHJ, which yields lines corresponding to the com- 
binations CH, CH 2 , CH 3 , and CH 4 . The great advantage 

1 e/m for the hydrogen molecule carrying a single charge is 10 4 electro- 
magnetic units. 




FIG. 11. Photograph obtained 
by J. J. Thomson of parabolic loci 
of positive particles subjected to 
magnetic and electric fields at right 
angles to each other. The differ- 
ent traces represent different sub- 
stances . In the figure, the magn etic 
deflections are vertical. The mag- 
netic field was reversed half-way 
through the exposure, so that both 
halves of each parabola are re- 
corded. 



POSITIVE RAYS 23 

of the method lies in the fact that only a very small 
quantity of the gas investigated is required in the tube ; 
and, in addition, a combination of atoms in. an unstable 
state has only to exist for a minute fraction of a second 
in the tube in order to allow it to be recorded on the 
photographic screen. 

For a fuller account of this ingenious method, see J. J. 
Thomson, Positive Rays (Longmans, 1913). 

The positive rays are important from the point of view 
of the X-ray worker, in that by their bombardment of the 
cathode, they liberate the cathode rays. Furthermore, the 
positive rays are responsible for the positive electrification 
which the inner surface of the glass walls of an X-ray bulb 
always assumes (p. 32), and which serves to heighten the 
fluorescence of the glass by attracting the secondary cathode 
rays from the anticathode. 

The positive rays doubtless also play a prominent part 
in the action of valve-tubes (p. 66). 



CHAPTER III. 
X BAYS. 

The Discovery of X Kays. 1 

We have dealt above in some detail with many of the 
features which cathode rays possess ; we have, however, 
made no more than mention of their most striking property 
of all that of generating X rays. In the autumn of 1895 
Professor Wilhelm Konrad Rontgen of Wiirzburg, Bavaria, 
discovered, it may be said almost accidentally, the rays 
which now bear his name. During the course of a search 
for invisible light rays, he turned on a low-pressure discharge 
tube, which for the purpose was completely enclosed in 
stout black paper, and to his surprise noticed that a fluores- 
cent screen lying on a table some 3 metres or so distant 
shone out brightly. The light-tight cover precluded any 
possibility of the effect being due to ordinary ultra-violet 
light ; there was evidently some curious radiation coming 
from the tube. If obstacles were interposed, Rontgen found 
that they cast shadows on the screen ; and in this way he 
traced back the unknown or " X " rays to their source, 
which proved to be the region of impact of the cathode 
rays on the glass walls of the tube. 

Further investigation revealed the fundamental fact that 
Rontgen or X rays are produced whenever and wherever 
cathode rays encounter matter. It was imagined by many 
that X rays were present in the original cathode ray beam, 
and were obtained by mere subtraction. But this was dis- 
proved by the discovery that when the cathode rays were 
magnetically deflected, the source of the X rays also moved. 
The experiment also put out of court the notion that X rays 
were due to the impact of particles of metal from the cathode. 

^ee also p. 217. 



DISCOVERY OF X KAYS 25 

But the fascinating feature^ of the new rays was their 
extraordinary ability to penetrate many substances quite 
opaque to light. The degree of penetration was found to 
depend on the density ; for example, bone is more absorbent 
than flesh, and if the hand is placed in the path of the rays, 
the bones stand out dark against the flesh in the shadow 
cast on a fluorescent screen. Rontgen at once appreciated 
the immense significance of his discovery to the surgical 
profession, and communicated his results to the Physico- 
Medical Society of Wurzburg in November 1895. 1 ^ 

It was soon ascertained that X rays affected a photo- 
graphic plate, 2 could not apparently be refracted or reflected 
(see p. 168), and, unlike cathode rays, were not bent by a 
magnetic or electric field, 3 a result which shows that the 
X rays do not carry a free electric charge.' In 1896, J. J. 
Thomson, Hurmuzescu, Benoist, Dufour, and others found 
that Rontgen rays shared with cathode rays (and ultra- 
violet light) the property of ionising or imparting temporary 
electrical conductivity to a gas, which ordinarily is a nearly 
perfect insulator. 

Before considering in any detail the advances that have 
been made in the various branches of the subject, it will 
probably be useful first to recount briefly the essential 
particulars of the working of a simple X-ray equipment. 

A BRIEF ACCOUNT OF THE PRODUCTION OF X RAYS. 

An X-ray Bulb. 

When a current of electricity from a Ruhmkorff induction 
coil is sent through an X-ray tube, a pencil of cathode rays 
from the concave cathode is focussed on the target or anti- 

x See also L>Edair Elect. 6. 241. 1896. For an account of Rontgen's 
later work, see Berl. Ber. 1897, and Ann. Phy. Chem. 1898. Rontgen's 
three memoirs are translated in the Electrician (Jan. 24, 1896 and April 
24, 1897) and A.Rt.R. (Feb. 1899). 

2 The inexplicable fogging of unopened packets of photographic plates 
in the neighbourhood of a Crookes tube was engaging the attention of 
more than one English physicist at the time of the discovery of the X rays. 

3 Walter (A.d.P. 1904) used magnetic fields up to 19,000 gauss. Paschen 
(P.Z. 1904) similarly exposed Ra y rays to fields of 30,000 gauss. 



26 X RAYS 

cathode, the surface of which is inclined at 45 to the rays, 
and is usually made of a metal of high atomic weight, such 
as platinum (Fig. 12). An additional anode is usually pro- 
vided, but is not indispensable. The anode and cathode 
are generally of aluminium. From the point of impact of 
the cathode rays on the anticathode, X rays are given out 
in all directions. The anticathode tends with continued 
use to become very hot, and is often either made massive 




Anode \ Anticathode / Cathode 



Fia. 12. A simple type of focus bulb showing the various electrodes. 

or cooled in some fashion. The pressure of the gas in an 
X-ray tube becomes lower with use, and a device for " soften- 
ing " the tube (i.e. raising the pressure) is therefore usually 
provided. 

The higher the pressure, the less the potential required 
to drive the tube and the less penetrating the X rays ; both 
the X rays and the tube are often termed " soft " if the 
pressure is high. The lower the pressure, the " harder " 
are the rays. In the X rays from any particular tube 
there are many qualities present ; this is shown by the fact 
that rays which have traversed one thickness of material 
are more penetrating to a second. 

X rays are invisible and do not make glass fluoresce ; the 
pale green hemisphere of fluorescence on the bulb is due to 
" reflected " cathode rays from the anticathode striking the 
glass walls. That this is so is shown by the distorting 
action of a magnet on the boundary of the fluorescence. 



PRODUCTION OF X RAYS 



27 



An Induction Coil. 

An induction coil is merely a device for transforming a 
low-potential current, such as is yielded by a battery of 
a few cells, into a high-potential current of the kind suitable 
for driving an X-ray bulb. An induction coil consists 
essentially of a cylindrical iron core round which is wound 
a coil of stout insulated wire ; this coil, which is known as 
the primary, consists of relatively few turns. Outside this 
is the secondary coil consisting of many thousands of turns 
of finer wire carefully insulated. Fig. 13 shows diagram- 




Primary 



Core 



Condenser 

FIG. 13. Diagrammatic representation of an induction coil. 

matically the various parts of a small coil. A hammer- 
break interrupter is shown in the primary circuit, and a 
condenser, usually mounted in the base of the coil, offers an 
alternative path to the break. The primary circuit is joined 
to a suitable battery ; and the object of the interrupter 
is to make and break the current in rapid succession. The 
consequence of this is at every " make " to induce in the 
secondary coil a momentary current, and at every " break " 
an equal momentary current in the opposite direction. 

But in X-ray work it is important that the current through 
the X-ray tube should be all in one direction, and herein lies 
the chief function of the condenser. When the circuit is 
made, the condenser takes and stores the first rush of current, 
which therefore grows relatively slowly and magnetises the 



28 X RAYS 

'core ; at break, however, the condenser discharges its elec- 
tricity through the primary circuit with great rapidity and 
demagnetises the core. The induced potentials in the 
secondary are accordingly much feebler at make than at 
break ; the currents resulting from the former are known 
as " reverse " or " inverse " currents, and in a good coil 
are nearly suppressed. Thus the sparks which pass between 
the terminals of the secondary circuit are due chiefly to the 
break and only pass one way. The power of a coil is often 
designated by the length of its longest spark, e.g. a 6-inch coil. 

The iron core serves to increase the number of lines of 
force through the coils. The condenser is made of alternate 
layers of tinfoil and paraffined paper. The hammer-break 
consists essentially of a steel strip on which is mounted a 
piece of soft iron ; this is attracted by the core when the 
current passes, and so breaks circuit between two platinum 
studs in the primary circuit. The spring is thus caused to 
vibrate backwards and forwards like the hammer of an 
electric bell, and so alternately makes and breaks the 
primary current. 

An extended account of the induction coil is given on 
p. 49 et seq. 



CHAPTER IV. 

AN X-RAY BULB. 

Early X-ray Tubes. 

The vacuum tube with which Rontgen made his famous 
discovery in 1895 was pear-shaped, with a flat disc for 
cathode mounted in the body of the bulb at its narrow 



CaJhode 




FIG. 14. Type of tube with which Rontgen discovered X rays. The cathode 
rays impinged on the broad end of the tube. 

end ; the anode was in a small side tube (Fig. 14). l The 
cathode rays impinged on the large end of the bulb, pro- 
ducing vivid fluorescence. This pattern of tube was widely 
copied, but it was soon found that it did not survive many 
of the prolonged exposures which were necessary to secure 
radiographs of any value. Moreover, owing to the large 
area of emission of the rays, the photographs were always 
blurred and somewhat indistinct. Experimenters set about 

1 In another early form of X-ray bulb used by Rontgen, the anode 
consisted of a large ring in the body of the bulb. 



30 X RAYS 

to find ways and means of prolonging the life of the tube, of 
shortening the exposure, and of improving the definition. 
Under the impression, then prevailing, that active fluores- 
cence was essential for the genesis of the X rays, 1 various 
workers, about 1897, constructed tubes of fluorescent glass 
(e.g. uranium and didymium glasses) with the idea of en- 
hancing the output of the tube ; it was, however, found 
later that the fluorescence was quite immaterial. 

Campbell- Swint on in 1896 modified Rontgen's design of 
tube by inserting a sheet of platinum obliquely in the path 
of the cathode rays. The improvement was considerable, 
though the radiographs were still lacking in sharpness, and 
the exposures unduly protracted. 




. FIG. 15. Tube used by Crookes to display the heating effects of focussed 
cathode rays. 

The same year Professor H. Jackson of King's College, 
London, turned to account a former discovery of Sir Wm. 
Crookes, and replaced the flat cathode by a concave one. 
Crookes had shown in 1874 that a hollowed-out cathode 
brought the cathode rays to a focus, and five years later 
actually constructed a tube with a plate of platinum at 
the focus to display the heating effects of the rays (Fig. 15). 
The tube must have given out X rays in abundance, but 
they remained unnoticed. Professor Jackson mounted the 
platinum target at 45 to the rays (Fig. 16) ; in essential 
respects his tube agreed with that of Crookes. The new 

1 It may be recalled that the late Henri Becquerel, at the suggestion of 
M. Poincare, was led to investigate whether X rays were an invariable 
accompaniment of phosphorescence in general. Among the substances 
he tried were uranium salts : the result was the discovery of radioactivity 
in 1896, two months after the discovery of X rays. 



AN X-RAY BULB 31 

focus tube was a vast improvement on its predecessors ; 
the exposures were shortened enormously, and, owing to 
the small area of emission, the resulting photographs showed 
wonderful sharpness and detail. 

It is remarkable how slight the subsequent changes have 
been ; many thousands of X-ray tubes have been made, 
but the design of the present-day model agrees essentially 
with that of fifteen years ago. 1 Indeed, it may fairly be 
said that the X-ray bulb has not kept pace with the very 
extensive improvements that have been made in the rest 
of the X-ray equipment. There is no gainsaying the fact 



Cathode 




Anode wid 
AnricaJtiode 

FIG. 16. Jackson's first focus tube, employing focussed cathode rays. 

that even now X-ray tubes are prone to be fickle, and it is 
scarcely possible to guarantee their behaviour. A bulb will 
be perfectly satisfactory one day, and yet refuse to work 
reliably the next ; and of two bulbs apparently precisely 
similar, one may work well for months, the other may break 
down within a few days. Many X-ray workers take the 
precaution of resting a favourite bulb occasionally ; a bulb 
is often improved by being allowed to lie idle for a few weeks. 

In general, a large bulb is better than a small for passing 
a heavy current (see p. 68). Some makers have accordingly 
constructed monster bulbs, which have, however, little to 
commend them on account of their unwieldiness and the 
greater thickness of the glass. 

In all the earliest tubes, the cathode was mounted in the 
body of the bulb, but by the end of 1896 it was withdrawn 
just within the neck of a side tube a design typical of all 
later makes, and one which conduces to greater steadiness 

1 See, however, p. 219. 



32 X RAYS 

and hardness l (see p. 69). In Jackson's bulb and its pre- 
decessors the target served also as anode ; we find an 
auxiliary anode introduced in a tube by Gundelach in 1896. 
Both forms persisted for some years, but nowadays the 
auxiliary anode always finds a place. 

About 1902, makers vainly sought to improve the 
efficiency of their tubes by coating some or all of the 
electrodes with radioactive material. Since then they have 
devoted most of their attention to the design and material 
of the anticathode. 



THE ELECTRODES OF AN X-RAY BULB. 

The electrodes are fixed to stout wires or rods which, for 
ease of manufacture and repair, are invariably mounted in 
side tubes projecting from the main bulb. It was found 
as early as 1896, that the discharge is materially steadied 
by completely sheathing the supporting wires with glass 
tubes. This largely serves to check the tendency for the 
discharge to pass along the walls of the tube, more especially 
at low pressures, and checks the " sputtering " 2 which is 
pronounced with wires and points. The glass walls become 
highly charged, negatively in the region of the cathode (p. 70) 
and positively in the main body of the tube (p. 23) : these 
charges, particularly with a blackened tube, may cause the 
focal-point to wander and lead to sparking along the glass. 

The passage of the discharge along the glass walls is not 
moreover confined to the inside of the tube. Alippi found 
in 1906 that if a large jet of steam were allowed to play on 
the bulb, the X-ray output and general fluorescence greatly 
increased. The effect is probably due to the removal of 
dust and the surface alkali in the glass with a consequent 
diminution of conductivity. Local surface electrification 
is probably responsible for the green wisp-like discharges 
which can often be seen playing over the inner surface of a 
bulb when in use. 

1 ,T. J. Thomson (P.M. 1912) finds that this position of the cathode is 
also the most favourable for the production of the positive " canal " rays. 

2 See p. 76. 



AN X-RAY BULB 33 

The part that surface electrification plays, and the control 
it possesses over the hardness and steadiness of a discharge 
is not, I think, generally appreciated. 



As remarked above, the modern X-ray bulb is always 
provided with an additional anode of aluminium which is 
joined externally to the anticathode (Fig. 12). The precise 
benefit of the anode is a little doubtful, though in some 
cases the result of disconnecting it from the anticathode is 
to soften the tube. C. E. S. Phillips (A.Rt.R. 1902) con- 
cluded that the auxiliary anode was helpful, probably by 
electrostatic action, in steadying the discharge. He remarks 
that the most advantageous position for the anode is behind 
the anticathode. The auxiliary anode is also probably bene- 
ficial during the passage of the inverse current which exists 
with all coil discharges : in these circumstances, the alu- 
minium anode, rather than the platinum anticathode, tends 
to act as a temporary cathode, and as aluminium exhibits 
much less cathodic sputtering (p. 78) than platinum, the 
walls of the tube are not blackened to the same extent. 

It is usually stated that the discharge is independent of 
the position of the anode. This is only true if the anode 
is outside the cathode dark-space : if the anode is within 
the dark-space, the discharge only passes with difficulty. 
Now, in an X-ray tube, working under ordinary conditions, 
the cathode dark-space is big enough to enclose the anti- 
cathode within its boundaries, and the presence of the 
anode, which is invariably inserted within a confined side 
tube, is therefore advantageous. 

There is this, too, to be remembered : the easiest direction 
for a discharge to cross an unsymmetrical tube is that which 
makes the less restricted electrode the cathode in other 
words, that direction which offers the cathode dark-space 
least obstruction ; the tube runs harder if the dark-space 
touches the walls. If the design of an X-ray bulb is borne 
in mind, it will be realised that this property (which is made 
use of in the various valve-tubes, p. 66) would, in the 

c 



34 X RAYS 

absence of the confined anode, result in facilitating rather 
than in retarding the passage of the inverse current. Most 
workers have experienced this tendency of X-ray bulbs to 
act as rectifiers, and their refusal, on occasion, to let through 
the *'" break " current at all. 



THE CATHODE. 

For the reasons given elsewhere (p. 78), the cathode 
is made of aluminium, and is mounted just within the neck 
of a side tube to the bulb. In a focus tube, the cathode 
is concave. Now, while the normal ejection of cathode rays 
holds for plane surfaces, it is not the case for concave cathodes 
except when the pressure is not very low. As the exhaus- 
tion proceeds, the focus of the rays recedes farther and 
farther from the cathode, and may reach a distance of 
something like four or five times the radius of curvature 
of the cathode : ordinarily the distance between the cathode 
and anticathode is some two or three times the radius of 
curvature. The focus may vary somewhat capriciously in 
practice, without any apparent alteration in the current or 
gas pressure. 

The correct disposition of anticathode and cathode is a 
matter of some nicety for the maker, who has to be guided 
mainly by his experience and the hardness at which the 
tube is to be run. The anticathode is usually mounted a 
shade out of focus to avoid its premature destruction by 
fusion, though for radiographic purposes this entails some 
loss in definition. Some of the earlier X-ray tubes were 
provided with devices for moving the anticathode to suit 
the conditions of use. 

Campbell-Swinton (P.R.S. 1897) found that, at moderate 
pressures, the cathode rays do not form a solid cone of rays, 
but are condensed into a hollow conical shell. At low 
pressures, however, the rays are chiefly concentrated along 
the axis of the cathode. Owing to the ionising effect on 
the residual gas, this bundle of rays is displayed as a luminous 
pencil which stretches from cathode to anticathode. The 
origin of the pencil of rays, which usually is readily dis- 



AN X-RAY BULB 35 

cemible in a soft X-ray bulb, is due to the repulsive effect 
of the electricity on the walls of the tube adjacent to the 
cathode. The same effect obtains also with plane cathodes 
(see p. 70). With a cathode made of a metal tube, a con- 
centrated pencil of rays emerges from each end along the 
axis ; such cathodes are sometimes convenient in experi- 
mental bulbs. 

It would appear from the work of some experimenters 
that to keep the cathode cool is of service in diminishing 
the tendency of the bulb to harden with use. In the Gaiffe- 
Barret tube, for instance, the cathode is cooled by directing 
an air blast on its back surface. 

One may mention here that cathodes made of the electro- 
positive metals conduce to smooth running of the discharge ; 
for example, an aluminium cathode faced with calcium 
metal permits a tube to be run with safety much harder 
than one with the plain aluminium cathode. This is prob- 
ably due to the comparative ease with which such metals 
emit electrons. See also the Coolidge tube (p. 219). 

THE ANTICATHODE. 

The desiderata in an anticathode intended for modern 
radiography are : 

(1)^A high atomic weight to secure a large quantity of 
rays. 

(2) A high melting point to permit sharp focussing of 

the cathode rays without fusing the target. 

(3) A high thermal conductivity to diminish local heat- 

ing. 

(4) A low vapour pressure at high temperatures to avoid 

thermal "sputtering" on the walls (see p. 81). 

The Atomic Weight of the Anticathode. ^/ 

It was known almost from the first that the heavier 
metals, or rather those of high atomic weight, make the 
most efficient anticathodes. Rontgen himself found in 1896 
that the rays from platinum are more intense than those 
from aluminium. Campbell-Swinton, Kaufmann, Roiti, Sir 
Oliver Lodge, S. P. Thompson, and Langer, all about 1897, 
did work connecting atomic weight and intensity of radiation. 



36 X RAYS 

These earlier workers used photographic or fluorescence 
methods of measuring intensities, and consequently most 
of their observations are of qualitative rather than quanti- 
tative interest. 

In some experiments made by Kaye in 1908 1 the metals 
used as anticathodes, some twenty in number, were mounted 
on a trolley inside the discharge tube (see Fig. 17). By 



loms&hon 
Chamber 




To Pum 



FlQ. 17. Apparatus for generating and measuring the X rays from different 
anticathode metals. 

means of a magnetic control, the trolley could be moved 
and any metal desired brought under the beam of cathode 
rays. The tube was provided with an aluminium window 
0-0065 cm. thick, and the emergent rays, which thus suffered 
but slight absorption, were measured by an ionisation method. 
^The discharge was maintained by an induction coil. 

The experiments showed that there are, in general, at 
least two classes of X rays given out by an anticathode 2 

1 Phil. Trans. Roy. Soc. A, 209, p. 123. 

2 The X rays from an anticathode will ordinarily be supplemented by 
at least two types of soft X rays one produced by the X rays in passing 
through the glass walls ; the other from the impact of " reflected " cathode 
rays against the glass and the residual gas molecules. / 



AN X-RAY BULB 



37 



heterogeneous " primary " or " independent " X rays, and 
homogeneous X rays characteristic of the metal. 

The quality and amount of the latter rays are controlled 
by the nature of the anticathode and the potential on the 
tube ; if the tube is soft, with many metals the X rays are 



125 



100 



o 75 



50 



25 











../ 

OPb 








rr 


^Tl 








/ 
/_ 


rr 




/ 


QSb 
^0Sn 

^gf d 






! 
r.- 


&: 

k5Fe 








/r 

/OC& 








Mc> AI 










1 Igv^ 












50 100 150 200 25 


Atomic Weight of R&difcJor 



FIG. 18. Graph connecting atomic weight of anticathode with intensity 
of " independent " X rays. (Pt = 100.) 

almost wholly characteristic (see p. 1 2 1 ) . In some cases, such 
rays are too soft to penetrate the glass walls of an ordinary 
tube. However, the aluminium window enabled their pre- 
sence to be readily detected. Their intensities did not follow 
any simple atomic weight order for example, the metals 
of the chromium- zinc group 1 emit radiations very rich in 
soft and ionising rays. 



1 Cr, Mn, Fe, Ni, Co, Cu, Zn. 



38 X RAYS 

To remove the characteristic rays, an aluminium screen 
2 or more mms. thick was used, and it was then found that 
the intensity of the remaining harder " independent " rays 
increased with the atomic weight of the anticathode ; the 
two are indeed roughly proportional. 1 Fig. 18 shows the 
relation for a potential of about 25,000 volts on the tube. 
Very little change was produced in the relative intensities 
by increasing the thickness of the aluminium screen the 
rays from all the metals were, under these conditions, very 
fairly homogeneous and of the same quality. Thick screens 
of other metals yielded much the same sort of curve, modified 
a little here and there. When the potential on the tube 
was raised the heavy-atomed anticathodes became slightly 
more efficient ; with a diminished potential the lighter 
elements somewhat increased their relative intensity values. 

Suitable Anticathode Metals: 

The list in Table I. gives the atomic weights, the radiation 
values, the melting points, and thermal conductivities (where 
known) of those elements which by reason of their refractori- 
ness may be regarded as suitable for the anticathode of a 
focus bulb. The radiation values are for hard rays and are 
taken from Kaye's experiments (p. 36) ; in some cases the 
numbers have been obtained by interpolation. The thermal 
conductivities quoted are at room temperatures ; most 
metals diminish in conductivity as the temperature rises. 
The remaining constants are from Kaye and Laby's Physical 
Constants. The properties of some of the metals are not 
convenient, and to others the scarcity and price are at 
present an insurmountable objection. 

Among the metals which have been commonly employed 
as anticathodes in radiography are osmium, iridium, tung- 
sten, tantalum, and, of course, platinum. Platinum, which 
is almost universally used, has a melting-point none too 
high for the purpose, sputters badly, and its price, steadily 
becoming exorbitant, is being instrumental in directing 
attention to the properties of tantalum and tungsten, metals 

1 Gray (P.B.S. 1911) obtained the same result for the 7 rays produced 
by the impact of radium ft rays on different elements. 



AN X-RAY BULB 



39 



whose chemistry has become familiar through their extensive 
employment in electric lamps. Neither metal sputters so 
badly as platinum, 1 both have a very much higher melting 
point, and but a slightly inferior radiation value, while 
tungsten has a superior thermal conductivity, thus permitting 
sharper focussing of the cathode rays. Tantalum was intro- 
duced into anti cathode work some years ago, but it is only 
recently that it has been possible to obtain forged pieces of 
pure, dense and malleable tungsten suitable for the purpose. 
It remains to be seen whether these metals prove as good 
as their promise. 

TABLE I. 



Metal. 


Atomic 
Weight. 


Density. 


Intensity of 
Radiation. 


Melting 
Point. 


Thermal 
Conductivity. 




(0=16) 


grms./c.c. 


(Pt=100) 


C. 


C.g.S. 


Uranium - 


238-5 


c. 18-7 


c. 125 








Thorium - 


2320 


11 3 


c. 120 








Gold 


197-2 


19-3 


101 


1064 


0-70 


Platinum - 


195-2 


21-5 


100 


1750 


0-17 


Iridium 


193-1 


224 


98 


2290 


0-17 


Osmium - 


190-9 


22-5/ 


97 


2700 


0-17 


Tungsten - 


184-0 


19-3 


91 


3200 


0-35 


Tantalum - 


181-0 


16-6 


90 


2900 


0-12 


Palladium 


1067 


11-4 


55 


1550 


0-17 


Rhodium - 


102-9 


12-4 


54 


c. 1900 





Ruthenium 


101-7 


123 


53 


1950? 





Molybdenum 


96-0 


8-6 


50 


2500 





Niobium - 


93-5 


12-7 


49 


2200? 





Zirconium 


90-6 


41 


47 


c. 1300 





Yttrium 


89-0 


3-8 ? 


46 








Copper 


63-6 


8-9 


33 


1084 


0-92 


Cobalt 


59-0 


8-6 


30 


1480 





Nickel :* 


58-7 


8-9 


30 


1450 


0-14 


Iron - 


55-9 


7-9 


27 


1530 


0-15 


Manganese 


54-9 


7-4 


26 


1260 





Chromium 


52-0 


6-5 


25 


1520 





Vanadium 


51-1 


5-5 


24 


1720 





Titanium - 


48-1 


3-5 


22 


1800 











i 





Iridium is even more expensive than platinum, but appears 
to behave satisfactorily if there is no oxygen in the X-ray 

1 It is important in the case of tungsten to get rid of water vapour or 
oxygen in the tube, if excessive sputtering is to be avoided. 



40 X KAYS 

tube. Osmium, which was introduced in the very early days 
of X rays by Sir James Mackenzie Davidson, while excellent 
as an antic athode, is very scarce and expensive. Rhodium 
would seem to have much to recommend it as a material 
for anticathodes ; it has a high atomic weight and low 
volatility. Bragg has moreover shown that the rhodium 
radiation from a soft tube is remarkably homogeneous (see 
also pp. 121 and 198). 

Platinised Nickel Anticathodes. 

It should be remarked that almost all the cheaper X-ray 
tubes are fitted with nickel anticathodes faced with very 




FIG. 19. Photomicrograph of a platinised nickel anticathode fused by a 
discharge. 



thin platinum sheet (about -j-J-g- mm. thick). The high price 
of platinum was responsible for the introduction (in 1897) 
of these composite anticathodes of which nowadays large 
numbers are turned out. There is no objection to the plan 
if the tube is intended only for moderate output ; but care 
should be taken that the platinum facing is not fused, as 
nickel is a greatly inferior radiator. Fig. 19 is a photo- 
micrograph (due to Mr. J. H. Gardiner) of a fused platinised 
nickel target. 

Design of the Anticathode. 

Some makers envelop the anticathode in a glass sleeve, 
others fit it within a porcelain ring. Both devices tend to 



AN X-RAY BULB 



41 



reduce the evil effects of the inverse current. In some cases, 
the anticathode is made trough-shaped or is surrounded by 
a hollow aluminium cylinder to do away with the X rays 
produced by the reflected cathode rays striking the glass 




FIG. 20. A Muller bulb, showing water-cooled anticathode and automatic 
softening device. 

walls : the definition is described as being improved. Kurl- 
baum in 1900 constructed an anticathode coated with 
platinum- black with the object of increasing the heat-loss 
by radiation. 

Nowadays, the very pronounced heating of the anti- 
cathode is overcome in many tubes by cooling the back 




FIG. 21. A Siemens bulb, showing massive anticathode and osmosis 
softening device: 

surface by water or a stream of air (Fig. 20). In some 
makes of tube, no attempt at cooling is made, the anti- 
cathode being designed for continuous use at a red heat. 
With other designs of target, the temperature is kept down 
by increasing the massiveness of the anticathode (Fig. 21) ; 



42 



X RAYS 



this is done by backing up the platinum plate with copper, 
nickel, or iron. In some cases, the support extends to the 




. 22. A Cossor bulb with automatic softening device and fin radiator 
for cooling anticathode. 

outside of the tube, and is there provided with fin radiators 
(Fig. 22). Muller has recently introduced a method of 




FIG. 23. A Muller bulb with tong method of cooling anticathode. 

cooling the anticathode by means of cooling-tongs (Fig.. 
23). A thin copper tube is closed at one end, to which is 



AN X-RAY BULB 43 

fastened the platinum target : the other end of the tube is 
fused to the glass through the intermediary of a platinum 
belt, and thus the inside of the copper tube is open to the 
air. Into the aperture can be introduced a pair of metal 
tongs, by means of which both the massiveness of the 
aiiticathode can be greatly increased and its temperature 
lowered. 1 

The point of impact of the cathode rays is generally not more 
than 1 or 2 mms. across, and with a heavy discharge the heat- 
ing is so intensely restricted and rapid that the anticathode 




Fid. 24. Photomicrograph of fused focus-spot in a tantalum anticathode. 

may be melted locally without damage to the rest of the plate. 
Fig. 24 is a photomicrograph (kindly lent me by Mr. J. H. 
Gardiner) of the focus spot of a tantalum anticathode sub- 
jected to a momentary heavy discharge. The metal was 
liquefied, and the pool of molten metal was blown away 
from the cathode into a mound, where it solidified on the 
cessation of the current. 

Gardiner (J.Rt.S. 1909), by the use of a small magnet to 
deflect the cathode rays to a new portion of the anticathode, 
showed that, without in any way impairing photographic 
definition, it is possible to prolong very greatly the life of 
an anticathode. 

1 Reference may be made to Muller's 1914 catalogue of X-ray bulbs 
which, besides being unusually well illustrated, deals comprehensively with 
many points of practical interest. 



44 X RAYS 

Obliquity of the Anticathode. 

The design of tube introduced by Prof. Jackson, in which 
the cathode rays are focussed on an anticathode inclined 
at 45 to the beam of cathode rays, has become the universal 
pattern. It has two disadvantages : 

(1) The obliquity of the anticathode to the cathode rays 

increases the area of emission of the X rays. This 
is to the detriment of definition in photographic work, 
though it must be conceded that in any case a point 
source of X rays is not feasible in practice. 

(2) It is impossible, with a coil discharge, to suppress 

entirely the reverse current at " make," during which 
time the cathode rays proceeding from the anti- 
cathode impinge on the glass walls, with the con- 
sequent risk of piercing the tube. 

Both objections could be met by mounting the anticathode 
parallel to the cathode and using normally incident cathode 
rays. 

The writer showed (P.E.S. 1909), in some preliminary 
experiments, that the output of a tube was almost inde- 
pendent of the obliquity of the anticathode. The fluores- 
cence of the bulb, which is due to the " reflected " cathode 
rays from the anticathode, increased very markedly as the 
angle of incidence (to the normal) of the cathode rays in- 
creased, but the X rays did not show any corresponding 
variation either in quality or quantity. Thus the 45 position 
enjoys no advantage over any other ; and probably a tube 
employing normal incidence would be found to possess useful 
features. 

Depth of Origin of X Rays in an Anticathode. 

Various observers have found that the mean depth at 
which Rontgen rays originate in an anticathode is directly 
proportional to the potential employed. Ham (P.E. 1910) 
found that with a potential of 21,500 volts the mean depth 
was 5-9 x 10 ~ 5 cm. in the case of a lead anticathode. The 
writer showed (P.C.P.S. 1909) that with spark-gaps of from 
1 mm. to 1 cm., a thickness of from 1 x 10 ~ 5 to 4 x 10 ~ 5 cm. 



AN X-RAY BULB 



45 



of gold, copper, or aluminium, was more than sufficient to 
generate X rays. 

These distances may be compared with the minimum 
thicknesses which have been found essential for complete 
" reflection " of cathode rays of various velocities. These 
are as follows : 

TABLE II. 



Potential. 


! 
Thickness of Metal. 


Authority. 


11,000 volts 


1 
5-3 x 10~ 5 cm. 


Al 


Warburg 1905 


16,500 


19-0 


Al 


> 


21,800 


24-4 


Al 




27,800 


<6-6 


Cu 


> 


90,000 


0-25 


Pb 


Ham 1910 



Distribution of the X Rays. 

The distribution of the X rays from a bulb of the ordinary 
type is not quite uniform. Ham (P.R. 1908), Bordier (1908), 
and Gardiner (J.Et.S. 1910) agree that in a plane determined 




100 80 60 40 



20 40 60 80 100 



FIG. 25. Graph showing distribution of X rays, the cathode rays being 
incident normally on anticathode. 



by the beam of cathode rays and the normal to the anti- 
cathode, the intensity reaches a maximum in a direction at 
about 60 from the normal. (Of. p. 112.) A distribution 
curve obtained by the writer (P.R.S. 1909) for normally 
incident cathode rays is given in Fig. 25, in which the length 
of the radius vector in any direction is proportional to the 



46 



X RAYS 



intensity. It would be interesting to obtain similar curves 
for very thin anticathodes in which the scattering of the 
cathode rays would not be complete. 

Thin Anticathodes. 

Some information on this point is afforded by the writer's 
experiments (P.C.P.S. 1909) on the emission of X rays in 
both backward and forward directions from anticathodes 
consisting of aluminium, copper, gold, or platinum leaf. The 
apparatus is shown in Fig. 26. The results indicate that 



Cathode 




FIG. 26. Apparatus for measuring X rays emitted from each side of a very 
thin anticathode. 

the forward or " emergence " X rays exceed the backward 
or " incidence " rays both in intensity and hardness. In 
other words, the X rays tend to proceed in the same direction 
as the cathode rays which produce them. This is most pro- 
nounced in the case of aluminium, where with leaf about 
O00001 cm. thick, and a spark-gap of 1 to 2 cms., the emer- 
gence rays were two or three times as intense as the incidence. 
Stark (P.Z. 1909), using a photographic method, has obtained 
similar results for a carbon anticathode. 

It would be of interest to test the homogeneity of the X 
rays from thin anticathodes. In many cases the proportion 
of characteristic radiation might be expected to be unusually 
large. 

<J 



CHAPTER V. 
HIGH-POTENTIAL GENERATORS. 

THE various means of exciting X-ray bulbs may be con- 
veniently grouped into : 

(1) Influence machines. 

(2) Induction coils. 

(3) Step-up transformers, 

INFLUENCE MACHINES, 

Influence machines, which are nowadays almost always 
of the Wimshurst type, have been largely used in France, 
Germany, and the States for the production of X rays, 
but, probably owing to climatic reasons, have found little 
favour in this country. Very few influence machines, sold 
as such, are really suitable for the purpose ; nearly all of 
them need redesigning both from a mechanical and an 
electrical point of view. If glass is chosen for the material 
of the revolving plates, it should be free from excess of 
alkali, which in damp weather makes the surface conduct- 
ing : ordinary window glass is quite unsuitable. Alkali- 
free glass is now procurable ; it is, for example, used in 
the Moscicki condenser. Such glass should not be coated 
with shellac varnish according to the usual custom ; shellac 
is slightly hygroscopic, and, although it is a better insulator 
than bad glass, it is not so good as the best glass. Care 
should be taken to avoid undue fingering of the plates. 

Ebonite plates have advantages over glass (see p. 226), 
certainly on the score of safety for high-speed machines. 



48 X RAYS 

With continued exposure, however, to the stray brush- 
discharges, the ebonite tends to deteriorate, 1 probably owing 
to the ozone, which is always generated in abundance, and 
which many workers find objectionable. 

For leads, massive or india-rubber sheathed wires free 
from points and sharp bends, and as short as possible, 
should be used, otherwise the leakage by brush- discharge, 
always considerable, will prove excessive. When an X-ray 
bulb is run by a machine, either two short spark-gaps or 
two Ley den jars should be put in series with the bulb, one 
on each side of it : this will prevent undue frittering away of 
the electricity. 

With a multiple-plate machine in good working order, a 
beautifully steady X-ray discharge can be obtained. The 
current is, moreover, unidirectional, and is found to be not 
so destructive to the anticathode as pulsating or alternating 
current. 

The voltage from a Wimshurst machine is proportional 
to the speed of the plates : there is no theoretical limit to 
the potential obtainable, except such as is imposed by 
leakage or disruptive discharge. A Wimshurst machine is 
peculiar in that the current obtained is almost entirely 
independent of the voltage. The current output can be 
raised by increasing the number of plates. The voltage is 
readily controlled by altering the tilt of the rod supporting 
the brushes : a needle-point spark-gap is useful in regulat- 
ing minor variations ot the potential. 

But, as has already been remarked, the idiosyncrasies and 
unreliability of influence machines have caused most workers 
to fight shy of them, at any rate for X-ray work. For 
instance, some machines refuse to work at all inside the 
glass cases provided for them ; yet, in their absence, the 
machines attract all the dust within reach and require con- 
tinual cleaning. It is a habit with nearly all machines to 
reverse their electrification if stopped and restarted : in at 
least one type, a device is provided to counteract this. 

1 A mixture of French chalk and methylated spirit is described as useful 
in restoring perished ebonite surfaces. The new insulator " bakelite " is 
said not to deteriorate with exposure to an electric discharge. 






INDUCTION COILS 49 

As an example of the successful large design of machine, 
one may mention that of Hulst in America. The plates, 
fifty in number and small in diameter, are constructed of 
compressed mica, and are motor-driven at a very high 
speed about a vertical axis. Such a machine will send a 
current of some 15 to 20 milliamperes 1 through an X-ray 
tube, and yield rays of an intensity such as would require 
double the current from a coil. The machine is, however, 
excessively noisy, and there is, of course, the danger atten- 
dant on the high speed of the whirling plates. 

Villard and Abraham (CM. 1911) describe a somewhat 
smaller 20-plate^Wimshurst machine, whose construction 
allowed speeds of from 1200 to 1400 revolutions per 
minute. The plates were of ebonite 70 cms. across. The 
maximum current obtained was 3 milliamperes, the highest 
voltage about 320,000 volts, and the longest spark-gap 
55 cms. / 

Some workers have been successful with Wimshursts, 
which work in air-tight cases into which air or carbonic 
acid is pumped under pressure. The idea is to kill the 
losses due to brush discharge ; but the working difficulties 
are so great that the latest designs of Wimshursts have 
reverted to the simple unenclosed pattern: 

INDUCTION COILS. 

It is only within the last few years that makers of in- 
duction coils have stirred themselves to meet the special 
requirements of the X-ray worker. The improvements in 
design and performance are doubtless not wholly uncon- 
nected with the competition offered by the various step-up 
transformers. The present-day coil offers improvements 
even on its predecessors of only five years ago ; standardis- 
ing of proportions proceeds, and any differences of design 
among the different coil makers depend more on individual 
predilections than on theoretical grounds. 

It is not generally realised that the same coil cannot be 
equally efficient for all purposes ; it cannot, for example, 

1 A milliampere = y^y ampere. 
D 



50 X RAYS 

prove equally satisfactory for hard and soft bulbs, or for 
all speeds of interrupters. 

While all the ambitious efforts of the early coil maker 
were directed towards phenomenally long sparks, nowadays, 
for X-ray work, he is content with a 10 to 12 inch spark, 
provided it is a " fat " one and as unidirectional as possible. 
A fat spark means heavy current and intense X rays, and 
that satisfies the radiographer, who requires short exposures 
for much of his work, and finds that very long sparks mean 
rays too penetrating for his purpose. 1 Some of the later 
coils will pass through an X-ray tube sustained secondary 
currents up to 60 milliamperes with relatively small primary 
currents and but little inverse current. It will not be un- 
profitable to consider in some detail the various parts of a 
modern coil, a brief account of which was given on p. 27. 

Core. 

The aim of the coil-maker is to magnetise the core slowly 
(at make) and demagnetise it rapidly (at break). The 
spark-length depends on how quickly the core can be 
demagnetised. On the other hand, the output or power of 
the coil depends largely on the degree of magnetisation. 
With modern high-frequency interrupters the core is never 
either fully magnetised or demagnetised. 

The ideal size of core depends on the size of the primary 
and the current in it, on the frequency and character of 
the break, and on the output required : the heavy dis- 
charge coil of to-day has a conspicuously large and stout iron 
core whose length is some five or six times the diameter. 

The chief objects kept in mind in core design are (1) to 
diminish the inverse current, and (2) to reduce the losses 
due to eddy-currents and hysteresis in the iron. The 
inverse current is lessened by packing as much iron as 
possible into the space available for the core. The 
hysteresis loss is diminished by using iron as soft as can be 
obtained. The eddy-currents are reduced by using, instead 
of a solid iron core, closely packed wires or plates varnished to 

1 A propos of long-spark coils, Carpentier showed in 1910 at Paris a 
monster coil capable of a 50-inch spark. 



INDUCTION COILS 51 

diminish the electrical contact between them. Laminated 
plates have a better " space factor " than wire in a cylin- 
drical core in other words, there is less space unoccupied 
by iron and accordingly plates are used for nearly all 
large coils. Iron with very high resistivity is now avail- 
able, and so fairly thick plates can be employed. 

Primary. 

The primary is usually wound in three layers, either as 
a simple winding, or in some form of adjustable winding 
to secure adaptability to prevailing conditions. There are 
in common use three different methods of winding primary 
coils which permit adjustment. In one, the connections 
are arranged so that each of the three layers can be put 
in series or parallel with its fellows ; in a second, a number 
of " tapping-off " wires permit connection to different 
parts of the primary circuit ; in a third, the primary is 
wound with several wires " abreast," so that these multiple 
windings can be put either in parallel or series at will. 

A heavy-discharge coil has a primary stout enough to 
permit direct coupling to the electric light supply of 100 
or 200 volts. Great care has to be paid to the insulation 
of the primary, owing to the induced E.M.F. from the 
secondary, of which all observers are well aware by reason 
of the shock which can be obtained from the primary of 
even a small coil in action. Nowadaj^s. if a fault develops 
in a coil, it is usually in the primary rather than in the 
secondary ; the defect is probably due to nitric acid 
formed by brush-discharges induced by the secondary. 

Condenser. 

It was Fizeau, nearly a century ago, who, by the addition 
of a condenser, revolutionised the induction coil and ob- 
tained sparks of lengths hitherto unheard of. But Lord 
Rayleigh demonstrated some years ago that if the primary 
current is interrupted with sufficient rapidity e.g. by 
severing a wire with a rifle bullet it is possible to dis- 
pense altogether with the condenser without impairing the 
length of the spark from the coil. Owing to the increasing 



52 X RAYS 

use of Wehnelt and high-frequency mercury breaks, the 
condenser, once paramount in importance, has become in 
such cases unessential. With the older patterns of breaks 
the condenser is, of course, still important. Its functions 
are three in number : it performs each of them with incom- 
plete success. 1 

(1) To increase the suddenness of the " break " and the 
slowness of the " make," and so to reduce the inverse 
current. 

(2) To suppress undue sparking and arcing at the inter- 
rupter. 

(3) To retard the formation of induced currents in the 
primary. 

It is important that the capacity of the condenser should 
be as nearly as possible adapted for the particular value 
of the inductance of the primary as well as for the magni- 
tude and frequency of the primary current. If the capacity 
is too large or too small, the secondary wave of potential 
will be neither so large nor so sudden. 2 The capacity 
required depends also very considerably on the type of 
break for instance, less capacity is required with a gas 
break than with an oil break and accordingly an adjust- 
able condenser should be used in the primary if a coil is 
required for a variety of purposes. But for coils restricted 
to X-ray work alone the invariable condenser is being 
increasingly fitted, on account of its simplicity. 

Condensers have improved out of all recognition during 
the last few years. With condensers of tin-foil and waxed- 
paper, this is chiefly due to a better knowledge of the 
hygroscopic properties of paraffin wax and of the impor- 
tance of manipulating it by machinery rather than by hand. 

Primary Tube. 

Between the primary and secondary coils comes the 
primary tube ; this is made of ebonite, micamte, or, less 

1 See W. H. Wilson, P.R.S. March 1912. 

2 See Jones and Roberts (P.M. Nov. 1911). In one instance, by reducing 
the capacity to one-fourth its value, the maximum potential was increased 
two and a half times. 



INDUCTION COILS 53 

commonly, porcelain. Ebonite has the advantage of being 
readily machined and worked, but inicanite, on account of 
its greater electric strength, is generally used in large coils, 
though it is inconvenient mechanically. 

Secondary. 

It is in the methods of winding the secondary that the 
greatest improvements have been effected in the modern 
coil. 

Simple winding is never used, partly because of the 
dangerous strain on the insulation owing to contiguous 

CenlYaJ 
EboniTePUre, 



Primary Tube 



1: 
: 
* 



FIG. 27. Diagrammatic representation of a bisectional winding of the 
secondary of an induction coil. 

layers being at very different potentials, and partly because 
one end of the wire finishes up at the innermost layer. An 
obvious way to avoid this, is to divide the secondary into 
two sections, wind each of them simply, mount them side 
by side, and connect the two innermost ends of the wires 
together at the adjacent faces (Fig. 27). This plan has 
several advantages. The electric strain on the primary tube 
is slight ; the tube may accordingly be very thin, so that 
the primary and secondary windings are close together, 
with a consequent gain in the efficiency and a diminution in 
the size and weight of the coil. The method is accordingly 
of special value for smaller and portable coils. Owing to 
the electric stress between the outermost points of the 



54 X RAYS 

adjacent faces of the two sections, the intermediate ebonite 
plate has to be made thick and protruding from the body 
of the coil (Fig. 28). 




FiG. 28. A Cox coil wound on the bisectional principle. 

For large coils (such as is shown in Fig. 29), some form 
of sectional winding is used, in which a large number of 




FIG. 29. A Butt coil wound on a multisectional principle 

circular flat sections, a few wires thick, are threaded side 
by side on the primary tube and separated by partitions 



INDUCTION COILS 55 

of waxed or varnished paper. In some cases, these sections 
are connected up in series by joining the innermost wire 
of the first section to the innermost of the second, the 
outermost wire of the second section to the outermost of 
the third, and so on (Fig. 30), as in the bisectional method ; 
in others, by joining the innermost wire of one section to 
the outermost of the next, and so on. Much ingenuity has 
been exercised in devising methods of winding. 1 It may 
be noted that the method of sectional winding requires a 
thick primary tube. 




IcvlD c: r.- cr r:| 
cj bk b apsl 
'.. *' ....' I '<.> | 

FIG. 30. Diagrammatic representation of a method of multiseet ional winding 
of the secondary of an induction coil. 

Whatever the method of winding, the secondary coiJ, 
when complete, is immersed in hot paraffin wax in vacuo. 
It is highly important to exclude air bubbles from the wax 
and the method of vacuum-exhaustion is absolutely essential, 
if a break-down in the secondary is to be avoided. 

Some Points in Coil Design. 

The chief objection to induction coils for X-ray work is 
the inverse current which all coils generate, chiefly at 
" make," but also to some extent at " break." The inverse 
current may be lessened 

(1) by making the number of turns in the primary as 
large as possible, 

1 See a paper by R. S. Wright (J.Rt.S. 1913) to which the writer is 
much indebted 



56 X RAYS 

(2) by reducing the magnetic leakage between the primary 
and secondary : this means paying attention to the 
core. 

The inverse current is augmented by irregular interruption, 
and care should therefore be taken to keep the break in 
good order. The inverse current also tends to increase if 
the X-ray bulb is softened. 

Sparking at the interrupter, with its attendant waste of 
energy, may be reduced 

(a) by increasing the self-induction of the primary, 

(b) by lowering the frequency of the interruptions. 

(1) and (a) are consonant, but they both imply a large 
secondary if the coil is to give long sparks. This is objec- 
tionable from the coil maker's point of view who, to obtain 
a heavy discharge, is very desirous of keeping down both 
the resistance and the number of turns in the secondary. 
It is, however, possible to obtain long sparks with a secondary 
of reasonable size, by increasing the rate of interruption. 
(b), however, requires a low-frequency break ; and, more- 
over, eddy-current losses become considerable with very 
high frequencies. 

If a heavy output is required from a coil, and the voltage 
available for the primary is only low, the self-induction of 
the primary should be kept down. This is inconsistent with 
but more important than (a). In such cases the output 
can often be materially improved by taking care that the 
leads from the battery to the coil are kept as short and 
straight as possible, the object being to diminish the self- 
induction in the circuit. 

The efficiency of even the best induction coils, considered 
as transformers, is not high in the region of 50 to 70 per 
cent. It could, of course, be increased by using a com- 
pletely closed (ring) core instead of a straight one, and so 
diminishing the magnetic leakage. But the difficulty 
hitherto has been that, with a closed core, demagnetisation 
does not occur with the intermittent current, which obtains 
in a coil discharge. The objection does not apply to true 
alternating current, in which there is a complete reversal, 



INDUCTION COILS 



57 



and for which, of course, closed-core transformers are 
always used. 

Enough has been said to indicate some of the problems 
which confront the coil designer. It is in reconciling 
necessarily antagonistic factors to suit the main purpose 
of the coil that his skill finds chief scope. 

The Wave-form of the Primary and Secondary Currents. 

The oscillograph l has been employed by a number of 
workers to investigate the shape of the waves of current 




FIG. 31. Oscillograph record of a make and break of the primary current 
of an induction coil. 

and potential generated by a coil at each make and break 
of the interrupter. Fig. 31 shows a typical record (due to 
Salomonson, J.Et.S. 1911) of a single make and break of 
the primary current in the case of a 13-inch coil giving a 
10-inch spark : a mercury-oil break was used. As soon as 
the circuit is completed, the current starts from zero and 
rapidly grows in strength until the moment at which the 
circuit is broken. The current then falls to zero in about 
1/1000 sec. In some cases, the curvature of the rising part 
of the curve is more marked than in Fig. 31. A close 

1 An oscillograph is essentially a low-resistance, moving-coil galvano- 
meter of few turns and with a very short time of swing. 



58 



X RAYS 



inspection will show that superposed on the main current 
are extremely rapid oscillations : these are produced by 
the condenser. W. H. Wilson (P.B.S. 1912) noted that much 
longer sparks could be obtained from a coil when these 
high-frequency oscillations were pronounced in the primary 

current. Fig. 32 illustrates them 
very well. The frequency of these 
rapid oscillations may reach many 
thousands a second. 

In regard to the secondary 
circuit, Duddell (J.Et.S. 1908) 
found that the discharge con- 
sisted of isolated groups of 
strongly-damped impulses very 
abrupt and short-lived. The in- 
terval between successive groups 
of waves was relatively long 
compared with the actual dura- 
tion of each group, which latter 
was of the order of 1/1000 sec. 
Fig. 33 gives a general notion of 
the state of things that obtains 
with a medium vacuum in the 
X-ray bulb. 1 The upper graph 
shows the current, the lower the 
potential. In the latter curve, 
the upper peak is the potential 
tending to send the current in 
the right direction through the 
tube : the smaller and broader 
inverted peak is due to the inverse 

potential, which in this case is conspicuous. The maximum 
direct potential is about 60,000 volts, the maximum inverse 
potential about 33,000 volts. The current curve is very 
similar to the potential curve : a small inverse current is 
detectable. 

In Fig. 34 a rectifying spark-gap is inserted in the circuit : 
its ability to suppress the obnoxious inverse pulses is well 

1 A graph showing greater detail is a good deal more complicated. 



FIG. 32 Oscillograph record of 
a primary current showing super- 
posed high-frequency oscillations. 






HIGH-TENSION TRANSFORMERS 



59 



displayed. The maximum direct potential now supplied to 
the bulb is 39/000 volts. Thus some 21,000 volts have been 




Current. 




Potential. 

FIG. 33. Oscillograph record of groups of impulses in the secondary circuit 
of an induction coil. 

used up in the spark-gap ; and the illustration serves to 
point out the loss of energy that occurs in a spark-gap, and 





Current. 



JLJl_J_JLJ 

Potential. 
FIG. 34. Conditions as in Fig. 33, but with rectifying spark-gap inserted. 

the desirability of avoiding its use by not generating the 
inverse current at all, if that were possible. 



HIGH-TENSION STEP-UP TRANSFORMERS. 

About 1908 the first high-tension transformer for X-ray 
work was introduced by Snook (Fig. 35), and since then 
transformers have been largely used in X-ray work, more 



60 



X RAYS 



especially in instantaneous radiography. The machine is 
essentially nothing more than an oil-immersed step-up trans- 
former, which is supplied with alternating current from an 
alternator. A rotating pole-changing switch rectifies the 
high potential alternating current from the secondary of the 




FIG. 35. Present design of Snook high-tension transformer. 

transformer. To secure the perfect synchronism which is 
essential for rectification, the commutator is mounted on. the 
same shaft as the alternator. The resulting current is, of 
course, not uniform, but pulsating as in B (Fig. 36) ; its 
amount can be varied at will from | to 100 milliamperes. 

The efficiency of the transformer, which is of the ring type, 
is considerably greater than that of an induction coil. The 



HIGH-TENSION TRANSFORMERS 61 

chief objections to such transformers are the high cost and 
large size, the excessive noise, and the attention which 
moving machinery requires. On the other hand, they are 
capable of enormous output and easy control, there is little 




/WWWV 

FIG. 36. A. Alternating current of sine form. B, Pulsating current pro- 
duced by rectification of A. 

or no inverse current, and no interrupter is needed. Recently, 
very considerable improvements in design and performance 
have been effected. 

It has been suggested that the sinusoidal current curve of 
the high-tension transformer is not quite as efficient, from 
an X-ray stand-point, as the long steep peaks of an induction 
coil (see p. 59), and that they are relatively more destruc- 
tive to X-ray tubes ; and doubtless there is something to 
be said for this point of view. 

In one direction it would appear that simplification is 
possible in the use of step-up transformers for X-ray work. 
Instead of sending into the primary of the transformer a 
sinusoidal current, use an alternator specially designed to 
give a very unsymmetrical wave form consisting of an 
abrupt high peak on one side and an almost suppressed 
loop l on the other. The necessity for the commutator thus 
disappears. Boas described such an arrangement in 1911, 
and found it to work well in practice. 

Cabot has recently designed a high-potential rotary 
converter, in which by the commutation of a symmetrical 
9-phase system, the voltage fluctuates no more than 1 to 
2 per cent. The maximum voltage attainable with the 
machine is 100,000 volts, and the output up to 15 kilowatts. 

1 Merely sufficient, in fact, to demagnetise the core after eacli reversal 
(see p. 56). 



62 



X RAYS 



BREAKS AND INTERRUPTERS. 

The Hammer Break. 

The hammer break (see p. 28), the accompaniment of 
most of the earlier coils, has been greatly improved recently. 
Attention has been paid to its period and its mechanical 
stoutness. Some of the later types compare favourably in 
steadiness with any kind of interrupter, when only a light 
output is required, as with a soft X-ray tube. 1 But, on 
a heavy load, the hammer break is useless : it cannot carry 
the current without excessive sparking and disintegration 
of the platinum. This does not contribute to steadiness 
and economy of working. 

The frequency of a hammer break never reaches more 
than about 200 (per sec.), and is usually much less : with a 
large coil it may be as low as 25 to 30. Accordingly a 
variety of other breaks have been introduced from time to 
time. These include the electrolytic interrupters, and the 
various kinds of motor-driven breaks which employ mercury. 

The Wehnelt Electrolytic Interrupter. 

Wehnelt in 1899, turning to account an earlier observa- 
tion of Violle in 1892, devised his 
interrupter, which now enjoys 
extensive popularity. It consists 
of two electrodes immersed in 
dilute sulphuric acid. 2 The ca- 
thode is a large lead plate, the 
anode consists of one or more 
platinum points. The amount of 
the anode exposed to the liquid 
can be adjusted by means of a 
porcelain sleeve round each of the 
platinum points (Fig. 37). 

For efficient interruption, the 

FlG. 37. Wehnelt interrupter with m 1T >TAnf rvma-f IIA Vk^vKxr^on ^Avfain 
single platinum anode (Siemens). 3tween Certain 

1 The parallelism and flatness of the contact-pieces should be seen to : 
a thin piece of flat wood faced on both sides with a fine grade of emery 
paper is useful for passing between the platinum studs. 

2 A density of T2 is suitable. Some workers add a little CuSO 4 . 




INTERRUPTERS 63 

limits ; if it is too small (below about 10 amperes) mere 
ordinary electrolysis occurs, if too great (say 40 amperes 
or more) the polarisation increases to such an extent that 
the current almost ceases and the anode becomes white- 
hot, and hisses and disintegrates in the liquid. With a 
suitable current the anode is normally surrounded by a 
violet light, and the interruptions are of an explosive and 
almost deafening character a very unpleasant feature of 
the break. 1 Electrolytic breaks will not work with voltages 
exceeding 80 to 120 volts ; they are capable of a larger 
output than any type of break, but the reverse current is 
considerable and the X-ray tubes suffer in consequence. 

Opinions are still very much divided as to the mode of 
action of the break : the usual explanation is that the inter- 
ruptions are brought about by the periodic sealing and 
unsealing of the anode by liberated bubbles of gas ; but 
this does not meet all the circumstances. There are many 
factors to take into account the size of the anode point, 
the current, the concentration and temperature of the 
acid, the inductance and capacity in the circuit : all these 
affect the interruptions. Compton (P.R. 1910) showed that- 
just as with the ordinary hammer break, the " break " is 
more sudden than the " make." 

The Wehnelt break usually requires a little humouring, 
and works rather better when the acid is warm, a state of 
things which soon results in practice ; indeed, for prolonged 
use, it is necessary for regular interruption to cool the acid, 
e.g. by means of a water-cooled worm of lead tubing. The 
interruptions are extremely rapid as high as 1500 to 2000 
per sec. when a very small anode point is used : even with 
very large currents the frequency may reach 200. The 
frequency is increased (1) by diminishing the size of the 
anode point, (2) by raising the temperature of the acid, (3) 
by diminishing the self-induction in circuit. Some self- 
induction is, however, essential or there will be no inter- 
ruptions. A condenser across an electrolytic break is not 
beneficial, and is, in fact, generally detrimental to the 
working of the break, which itself functions as a condenser. 

1 Many makers now fit silencers to the break. 



64 



X RAYS 




'in 



with perforated tube round 
anode (Schall). 



ipter 
lead 



The energy required is diminished by raising the tempera- 
ture and (slightly) by using stronger acid. It is found that 

to get the same spark-length, a 
more powerful coil is required with 
the Wehnelt than with any other 
break. An electrolytic break does 
not, in fact, conduce to the highest 
efficiency in the working of a coil. 
In another form of electrolytic 
break, both electrodes are of lead 
(Fig. 38), but one is surrounded 
by a porcelain cylinder pierced 
with a number of small holes, at 
which the bubbles of gas are 
formed. This type permits no 
control over the current, 1 but the 
reverse current is said to be 
smaller. This latter break is also 
suitable for alternating current, in which case it may be 
noted that the frequency is always equal to the frequency 
of the supply current, and is not 
affected by any of the controllable 
features of the break. 

Mercury Breaks. 

There are many ingenious forms 
of these breaks on the market, 
some of which are extensively used. 

They are invariably motor- 
driven. The early forms depended 
on the rapid dipping of a plunger 
into a trough of mercury ; in some 
of the later types a jet of mercury 
is pumped against a series of rapidly 
revolving metal vanes. To these 
and other types of breaks, the 
various makers' catalogues do full justice. Two varieties 
of mercury break are illustrated in Figs. 30 and 40. 

1 In the Caldwell-Swinton pattern, the cylinder is pierced with only one 
hole, the size of which can be regulated and the current thus varied. 




FIG. 39. Sanax mercury-paraffin 
break (Sanitas Co.). 






INTERRUPTERS 



65 



In the earlier forms the revolving system was immersed 
in paraffin oil or methylated spirit. With either liquid, but 
especially with the oil, the mercury emulsifies in most 
breaks, and the cleaning is a frequent and a disagreeable 
operation, besides being 
rather wasteful of mer- 
cury. A notable exception 
is the Sanax break, which, 
by reason of its ingenious 
design and mode of action, 
avoids all these difficulties. 

Coal gas or hydrogen at 
1 or 2 atmospheres is 
generally used nowadays 
in mercury breaks : the 
break needs less cleaning, 
and is usually more reliable 
and economical than with 
a liquid dielectric. Salo- 
monson has shown (J.Rt.S. 
1911), by means of the 
oscillograph, that stronger 
and more abrupt quench- 
ing of the spark is obtained 
with a gaseous dielectric 
than with a liquid in which 
a conducting charred track 
persists after each spark, 
required for a gas break than for an oil or spirit break. 

With most coils, these motor-driven breaks produce a 
heavier discharge current at the higher speeds. The 
mercury break is designed so that the current is " off ?? 
rather longer than " on " ; in this respect it is superior 
to the Wehnelt, in which the " on " period is equal to 
the " off," to the detriment of the demagnetisation of the 
core. 

Doubtless most workers would prefer a mercury break to 
any other kind for general use ; though for heavy instan- 
taneous work an electrolytic break is probably unequalled. 




FIG. 40. Cox mercury-gas break. 

Less condenser capacity is 



00 X RAYS 

A mercury break permits greater control, however, and the 
good types are not subject to current and voltage limits of 
working, such as obtain with an electrolytic break. 

RECTIFIERS AND VALVE- TUBES. 

The chief defect of the induction coil from the point of 
view of the X-ray worker is that it does not give unidirec- 
tional currents : the reverse current at " make " has a 
disastrous effect on the X-ray tube, and requires to be 
suppressed. 

For this purpose we may introduce into the circuit the 
simple point and plane spark-gap, which depends on the 
fact that the spark passes more readily when the point is 
positively charged than when it is negatively charged. The 
device is an old one, and is not always particularly efficient, 
more especially if the current is considerable. The greater 

the current which passes, the 
longer is the spark-gap required 
for rectification. For a current 
of about a milliampere, a spark- 
length of 1 cm. or more is suitable. 
Duddell (J.Bt.8. 1908) showed 

FIG. 41. The Duddell spark-gap. 

that with a point anode and a 

given 'spark-length, a cup-shaped cathode will rectify a 
larger current than a plane, and a plane a larger current 
than a sphere. Duddell has accordingly designed a recti- 
fying spark-gap, in which the point electrode is surrounded 
by a hollow sphere, through which the point enters by means 
of a glass tube in a cork (Fig. 41). Correctly disposed, one 
rectifier in series with the X-ray tube and a second (reversed) 
in parallel with the tube, the arrangement is described as 
extremely efficient. 

For most purposes, especially when electrolytic breaks are 
used, the various valve-tubes are more efficient than spark- 
gaps. These consist of a large aluminium cathode, often 
spiral in form, mounted in an exhausted bulb : the anode 
is small, and is contained in a restricted side tube (see p. 33). 
The design is due to Villard : in Sir Oliver Lodge's modi- 




RECTIFIERS 



67 



fie at ion (Fig. 42), the anode (of iron wire) is surrounded 
with a copper sheath, partly to prevent sputtering on the 
glass walls, and partly to increase the resistance of the tube 



Cathode 




Spiral of 

Aluminium Wire 



FIG. 42. Section of a Lodge valve-tube. 

for the reverse current (see p. 69). Owing to the use of a 
phosphorus method of completing the exhaustion, the Lodge 
valve is red in colour (see p. 225). The Lodge tube is said 
not to harden with use, but other types of valves should 
be fitted with some softening 
device, as they harden con- 
siderably with use and do 
not rectify well if the pressure 
becomes very low. A valve- 
tube is only efficient over a 
limited range of pressures. 

The Wehnelt valve-tube 
employs a hot-lime cathode 
such as is described on p. 8. 
Such a tube, in series with an 
X-ray bulb, will transmit, from the cathode to the anode, 
only the negative phase of the discharge from the coil. 
Miller's neat mica-disc valve should also be noticed. 

For heavy instantaneous work, a number of multiple 
valve-tubes (Fig. 43) are advisable, both in series and parallel 
with the X-ray bulb. 




FIG. 43. A multiple valve-tube. 



CHAPTER VI. 
THE HARDNESS OF AN X-RAY BULB. 

Factors controlling the Hardness of an X-ray Bulb. 

The hardness of the X rays produced by a bulb is mainly 
dependent on the maximum potential difference between the 
electrodes. There are a number of ways of controlling this 
potential difference : 

(1) The most generally recognised method is by varying 
the degree of exhaustion of the bulb. The lower the pressure, 
the higher the voltage required and the harder the X rays. 
The range of effective pressures for producing X rays is very 
wide. It is, however, possible to make use of other methods 
which do not involve any change in the gas pressure. 

(2) By inserting a spark-gap or valve-tube (p. 66) in 
series with the bulb, the tube is hardened. With very soft 
bulbs, Winkelmann (A.d.P. 1900) states that the spark-gap 
should be placed between the cathode and the coil. At 
lower pressures, the position of the spark-gap is immaterial. 

(3) By employing Tesla or other currents of extremely 
high potential, the tube runs harder. Tesla currents are 
obtained by transforming up the secondary current from a 
coil by means of a special transformer immersed in oil. 

(4) By bringing the electrodes nearer together, the tube 
may be hardened (see p. 33). 

(5) By altering the nature of the gas in the tube. For 
the same pressure, a tube runs harder in hydrogen and still 
harder in carbon dioxide than in air. In other words, in 
order to generate X rays of equal hardness, a tube filled 
with air must run at a lower pressure than one containing 
hydrogen or carbon dioxide. 



THE HARDNESS OF AN X-RAY BULB 



69 



(6) By increasing the current density through the tube. 
This can be done : 

(a) By increasing the current in the primary of the coil. 

(b) By diminishing the size of the cathode. A tube with 
a fine wire cathode runs 

harder than one with a 
cathode of moderate size. 

(c) By diminishing the 
size of the tube. Winkel- 
mann in 1900 experi- 
mented with various sizes 
of tubes, and found that 
with a tube 5 mms. in 
diameter, he could get 
X rays at as high a 

pressure as 10 mms. of mercury with air as the residual gas. 
In the case of hydrogen and a tube 10 mms. in diameter, 
he obtained X rays at the remarkably high pressure of 30 
mms. of mercury. If the tube is made too narrow, the 
hardening effect is spoilt. 




FIG. 44. The discharge is hardened by withdraw- 
ing the cathode from B to A. 




FIG. 45. A Cossor bulb of lithium -glass with recessed cathode. (See Fig. 44.) 

(d) By diminishing the clearance between the cathode and 
the surrounding tube. It was pointed out on p. 31 that 
if the space round the cathode is restricted, the discharge 
passes with difficulty, so that if the cathode is withdrawn 
from the bulb into a side tube, the discharge hardens accord- 
ingly (Figs. 44 and 45). Precisely the same effect is obtained 
with a plane as with a concave cathode, and, indeed, with 



70 X RAYS 

a tube in which the cathode is so inclosed the curvature of 
the cathode need only be very slight. A tube with a mov- 
able cathode employing this principle was described by 
Campbell- Swinton in 1897 (Electrician) ; the tube is in the 
Rontgen Society's collection of X-ray tubes in the South 
Kensington Museum. Swinton also employed an alterna- 
tive device consisting of a glass sleeve, a part of which was 
narrowed to slide along the glass rod which supported the 

cathode (Fig. 46). The 
remaining portion was 
widened so as to form a 
sheath round the cathode 
and project a varying dis- 
tance beyond it. Wehnelt 
(D.P.G.V. 1903) found 
that the arrangement 
allowed the alternative 
gap to be varied as much 
as eight times. Whid- 

. 40. Adjustable glass sleeve over the cathode dlllgton (P.C.P.8. 1913) 
for varying the hardness of the discharge. . . 

observed that, within 

limits, the distance the sheath projected beyond the cathode 
was proportional to the potential required to run the bulb. 

The hardening effect, as Goldstein remarked (D.P.G.V. 
1901), is due to the glass round the cathode becoming nega- 
tively charged owing to leakage from the cathode. The 
cathode rays accordingly retreat to the centre of the cathode, 
where they form a concentrated pencil. In this way, the 
current-density and effective resistance of the tube are 
increased, and the more markedly if the adjacent glass is 
coated with sputtered metal. 

This charging up of the glass is responsible for a well- 
known effect produced by touching the tube near the cathode 
while the discharge is passing. The glass under the finger 
becomes vividly fluorescent, and a bundle of cathode rays 
is deviated towards the hand. Maltezos (C.R. 1897) showed 
that if the finger is replaced by the knob of a Leyden jar, 
the jar becomes positively charged, a clear indication of 
the negative electrification within that part of the tube (see 




THE HARDENING OP AN X-RAY BULB 71 

p. 32). It is possible to vary the hardness of a tube by 
putting patches of tin-foil on the outside in suitable 
places. 

In the case of the hardening sleeve referred to above, 
Whiddington has shown that the tendency of the sleeve is 
to slide back into the side tube owing to electrostatic repul- 
sion by the cathode ; and, further, that if part of the sleeve 
is cut away, the cathode rays are bent away from the portion 
which remains. It can readily be demonstrated that a 
metal tube, if slid over the cathode inside the glass, will 
harden the discharge just like a glass tube. In fact, the 
cathode may be removed altogether and the cylinder alone 
used in its place ; a sharply defined pencil of rays will still 
proceed out along the axis of the cylinder (see p. 35). 

THE PROGRESSIVE HARDENING OF AN X-RAY BULB 
WITH USE. 

With a new discharge tube, the first effect of running the 
discharge is to cause an outburst of gas. The effect, which 
may persist for some time, is due largely to gas ejected from 
the cathode. Aluminium almost always contains large quan- 
tities of gas, chiefly carbon compounds. Such gas is more 
readily reabsorbed than air let into the tube. In an X-ray 
tube, the anticathode also gives out considerable amounts 
of gas : indeed, the method of bombardment by cathode 
rays is a most effective one for liberating the gas held by a 
metal. 

But, after some time, the gas-pressure becomes pro- 
gressively lower with continued running of the discharge. 
The cause of this has been a problem ever since the days 
of Pliicker (1858), and one to which a good deal of enquiry 
has been directed. The effect is undoubtedly not a simple 
one, and there appear to be several contributory causes. 
Formerly, the responsibility for the absorption was thrown 
largely on the metal electrodes, more particularly on the 
anode l ; and doubtless some such occlusion does take place, 
if only to a slight extent. 

1 See, for instance, Hodgson (P.Z. 1912). 



72 X RAYS 

But Hill (P.P.S. 1912) has recently shown that a marked 
absorption of gas occurs even with electrodeless discharges, 
and it would seem that it is to the glass walls of the 
tube we must look for the explanation. Campbell-Swinton 
(P.R.8. 1907 and 1908) concluded from his experiments 
that the gas is actually driven into the glass by the dis- 
charge. He found that when the glass was subsequently 
fused, such gas (which proved to be chiefly hydrogen) segre- 
gated into small bubbles 1 whose depth below the surface 
did not exceed about 0'015 mm. This thickness of glass 
is, as Swinton points out, the greatest that will transmit 
cathode rays to any appreciable extent. A propos of this, 
it may be remarked that the effect appeared to be inti- 
mately associated with the fluorescence-fatigue which glass 
displays when subjected to prolonged bombardment by 
cathode rays (see p. 12). If the gas-permeated region of 
the glass is removed by grinding, the glass recovers its 
usual fluorescing ability. Hill (loc. cit.) found a similar 
absorption-fatigue ; and it would be interesting to test 
whether such removal of the fatigued surface promoted 
vigorous gas-absorption on further running of the discharge. 

Hill agrees with Willows (P.M. 1901) in attributing the 
hardening of discharge tubes to chemical action between 
the gas and the glass. His experiments show that Jena 
glass gives the least absorption, lead glass coming next, 
while soda glass gives most of all. The greater stability 
of Jena glass is well known from its behaviour in other 
directions. Possibly fused silica or alkali-free glass would 
prove to be superior even to Jena glass. It would be 
interesting to subject an ordinary soda glass bulb to steam 
or boiling- water treatment before exhaustion, to see if the 
removal of the alkali affected the rate of hardening. 

Ramsay and Collie (N. 1912) discovered helium (and a 
trace of neon) along with hydrogen in the deeply stained 
glass of an old X-ray tube. 2 This is suggestive, for hydrogen 

1 The formation of bubbles in such circumstances was also noticed by 
Gouy (C.R. 1896) and Villard. 

2 Sir J. J. Thomson (P.R.S. 1913) finds, however, that nearly all sub- 
stances when subjected to prolonged bombardment by cathode rays emit 
hydrogen and helium. 



THE HARDENING OF AN X-RAY BULB 73 

and helium molecules have the highest speeds of all mole- 
cules. Under the electric discharge, these speeds may be 
increased a thousandfold, e.g. the average velocity of positive 
rays of hydrogen is 2 x 10 8 cms. /sec. (see p. 20). Gold- 
smith (P.R. July 1913) found that such high-speed mole- 
cules of hydrogen and helium can penetrate, for example, 
mica sheet from 0*001 to 0'006 mm. thick, though the slower 
air, argon, or C0 2 molecules cannot. But molecules which 
could penetrate so great a distance as 0'015 mm. of glass 
would have to be considerably faster. How fast, we may 
infer from the fact that a particles (helium atoms) from 
RaC have a range of 0*04 mm. in glass. Such particles 
have an initial speed of about 2 x 10 9 cms. /sec., i.e. ten times 
the above velocity. It has, of course, never been shown that 
sufficiently high instantaneous velocities are not possessed 
by individual hydrogen molecules in a discharge tube one 
can only measure average velocities. But, in any case, 
it is obvious that any explanation such as this could only 
be a partial one ; it does not, for instance, explain the marked 
difference in the behaviour of different kinds of glass. 

The absorption may be due in part to chemical activity 
excited in the gas by the discharge, such as has recently 
been found by Strutt to be the case with nitrogen. It may 
be, too, that the action is stimulated by a species of electro- 
lysis of the glass produced by the high-tension discharge 
playing over its surface. It is well known that glass may 
be readily electrolysed by quite moderate potentials, if the 
temperature of the glass is raised, and it is a matter of 
experience that the discharge seems to have an ageing effect 
on the glass, to the detriment of subsequent working in the 
blowpipe. Such electrolysis might have a marked effect on 
the gas film which glass and other solids can condense on 
their surfaces. Possibly in such circumstances the gas film 
is capable of taking up abnormal amounts of the residual gas 
in the bulb. 

The hardening of an X-ray tube is well known to be 
pronounced with tubes whose walls have become blackened 
by metal sputtered from the electrodes (see p. 76). The 
finely divided metal behaves like spongy platinum in its 



74 X RAYS 

absorptive properties for gases. 1 In most cases this is 
probably the right explanation of the hardening. 

To soften an X-ray Tube. 

It was early discovered that the resistance of a tube could 
be lowered by warming the bulb with a spirit lamp or gas 
burner, but the resulting benefit was only temporary, and 
various " softening " methods have been devised from time 
to time. Many of these methods involve the heating of 
some substance which has been inserted in the tube, e.g. 
sealing-wax, carbon, and red phosphorus have each been 
employed by various experimenters in the past : Sir William 
Crookes used caustic potash for this purpose as long ago as 
1879. 

In many X-ray bulbs, this occlusion method is arranged 
to work automatically. A small alternative discharge tube 

communicates with the 

main bulb (Fig. 22). When 
Pr or Pd Tube . ^ ! 

the resistance increases 

beyond a certain degree, 
the discharge chooses the 
alternative path, and in 
so doing heats up some 

absorbent material such as asbestos, sheets of mica, or 
glass-wool enclosed in the small tube (see Fig. 22). The 
consequent liberation of gas (largely C0 2 and water vapour) 
lowers the resistance of the bulb, and the discharge 
resumes its proper path. But, in time, such substances 
" fatigue," having yielded all their available gas ; and the 
only course is to open up the tube and renew the material. 

The plan often employed nowadays for softening bulbs 
is the " Osmosis " method, originated by Prof. Villard of 
Paris in 1898, and discovered independently by Profs. 
Winkelmann and Straubel of Jena in 1899. A small platinum 
or palladium tube closed at one end is sealed into the bulb, 
the unclosed end being open to the bulb (Figs. 47 and 21). 

1 Soddy and Mackenzie (P.R.S. 1907) showed that helium was absorbed 
by aluminium scattered from the cathode of a discharge tube. In such 
a case the gas may be mechanically trapped by a compact film of motel. 







TO SOFTEN AN X-RAY BULB 



75 



Adi&rubber 
Bulb 



By applying a flame to the tube a small quantity of hydrogen 
diffuses through the hot metal, and the pressure of the bulb 
can be .restored to the right amount. Palladium shows the 
effect so very markedly that care 
should be taken in the heating ; 
otherwise the result will be a bulb 
too soft for use. Indeed, this method 
should never be employed except 
when the discharge is running. 

The Bauer valve (J.Rt.S. Jan. 1907) 
is a more recent contrivance for 
letting minute quantities of air into 
Rontgen bulbs. The valve (see 
Fig. 48) consists of a small unglazed 
porcelain disc, through the pores of 
which air can pass. Ordinarily the 
disc is sealed by mercury, but by 
means of a pneumatic piston the 
disc can be laid bare for a moment 
by pushing the mercury away (page 
80). ^ ^ 

Interior of Bulb 

To harden an X-ray Bulb. 

If by any mischance a bulb be- 
comes too soft for use, the only thing 
possible, apart from drastic re-exhausting, is to try and 
harden it by prolonged running with as large a coil as can 
be got. Often it is beneficial to send this hardening dis- 
charge in the reverse direction, i.e. from cathode to anode, 
temporarily disconnecting the anticathode for the purpose. 



& 


f( 


C 

\ 


, 

y Porous 








Plug 






- - 


-^Filter 








> Porous 




i 


^ 


Plug 



FIG. 48. The Bauer valvi- 
for admitting air into an X-ray 
bulb. The filter is of gold leaf 
to absorb mercury vapour. 



CHAPTER VII. 
THE BLACKENING OF AN X-RAY BULB. 

WITH continued use, an X-ray bulb becomes blackened on 
its inner surface. The blackening is mischievous from several 
points of view. Firstly, the deposit not only tends greatly 
to increase the. resistance of the tube to the discharge, but 
accelerates the absorption of the residual gas ; secondly, 
the discharge is wont to spark irregularly along the walls 
of the tube instead of through the gas ; and thirdly, the 
film of metal arrests the softest X rays. 

Two main causes are answerable for the blackening : 

(1) The disintegration or " sputtering " of the antic at hode 
while acting as cathode during the 'inverse current ; and 
also of the cathode during the direct phase. 

(2) The volatilisation of the anticathode due to its high 
temperature under reduced pressure. 



CATHODIC SPUTTERING. 

Workers with discharge tubes have long been aware that 
when a high-potential current is passed through a vacuum 
tube provided with platinum electrodes, the glass adjacent 
to the cathode generally becomes coated with a mirror of 
platinum (Fig. 49). The anode, on the contrary, shows 
little or no such effect. This property of cathodic 
sputtering is common in greater or less degree to all 
metals. The effect was noticed in the very early days of 



THE BLACKENING OF AN X-RAY BULB 77 

vacuum tubes : both Geissler and Pliicker (1858) remarked 
on it. 

Thus, quite apart from the cathode rays and positive 
rays, there is a cathodic emission which consists of particles 
of disintegrated metal from the cathode. These particles 
appear to be projected normally (at any rate, very approxi- 
mately) from the surface of the cathode, and to travel in 
straight lines. The streams of metal are negatively charged, 




Cathode Anode 

FIQ. 49. Illustrating cathodic sputtering. (From the Chemical World.) 

and it is found that they deposit more readily on surfaces 
which are positive with respect to the cathode. The 
positive electrification which the inner surface of an X-ray 
bulb usually possesses, is thus favourable to cathodic 
deposition. 

It does not appear that, in ordinary circumstances, the 
disintegration of the cathode plays any appreciable part in 
the passage of the current. Unlike the cathode rays, the 
sputtered particles require strong magnetic fields (2000 gauss 
and upwards) before any deviation of their path can be 
detected. The inference would be, either that the particles 
are very fast moving or that they are relatively large aggre- 
gates of molecules ; the latter view is supported by other 
evidence. The lower the pressure in the tube and the 
higher the potential applied, the farther are the particles 
hurled. There is no deposition within the cathode dark- 
space. The sputtered metal does not appear to excite 
fluorescence when it strikes the glass walls of the tube. 

Cathodic disintegration is not a. simple phenomenon, and 
the exact mechanism of the production of the sputtered 
particles is doubtful. It appears, however, to be connected 
with the bombardment of the cathode by the positive 
rays, the pulverising properties of which we have already 
noticed (p. 20), 



78 



X RAYS 



Experiment shows that the amount of metal shot from a 
cathode depends on 

(1) The nature of the metal of the cathode. 

(2) The temperature of the cathode. 

(3) The nature of the gas in the tube. 

(4) The current through the tube. 

(5) The fall of potential at the cathode. 

(1) The Metal of the Cathode. 

Sir William Crookes (P.R.8. 1891) was the first to investi- 
gate systematically the relative sputtering of a number of 
metals under like conditions of discharge. The residual gas 
was air ; the pressure, that corresponding to a dark-space 
6 mms. thick (say -05 mm. Hg). A coil discharge was used, 
and in these circumstances the relative losses of weight at 
ordinary temperatures resulted as follows : 

TABLE III. CATHODIC SPUTTERING. 
(Palladium = 100.) 



Palladium - 


100 


Copper 


37 


Gold 


92 


Cadmium 


31 


Silver - 


76 


Nickel 


10 


Lead - 


69 


Iridium 


10 


Tin 


52 


Iron - 


5 


Brass - 


47 


Aluminium 





Platinum 


40 


Magnesium 






The order of these metals must not be regarded as in- 
violable. It is affected to some extent by a change in the 
pressure of the gas (which may, for instance, put platinum 
above gold), the nature of the gas, or the temperature of 
the cathode. Nevertheless, the sequence is of value to users 
of discharge tubes in general and of X-ray tubes in par- 
ticular. The reason for the invariable choice of aluminium 1 
for the cathode is as readily apparent as the need for sup- 
pressing the inverse current through a tube and so prevent- 
ing the platinum anticathode from officiating as cathode. It 

1 Geissler noticed that aluminium did not sputter appreciably. 



THE BLACKENING OF AN X-RAY BULB 79 

is not right, however, to assume that it is impossible to make 
aluminium sputter appreciably, as will be evident from a 
scrutiny of the cathode of an old X-ray bulb : a brown 
deposit may usually be found on the central area of the 
cathode as well as on the glass in the vicinity. 1 

Tantalum has also proved to be an excellent material for 
cathodes from the point of view of sputtering. I believe 
tungsten displays equally good properties. 

(2) The Temperature of the Cathode. 

Grookes showed that if the temperature of the cathode is 
raised appreciably, for instance by the passage of the dis- 
charge, the sputtering of many metals is markedly increased. 
The electrodes tend to get very hot if the tube is at all soft, 
as more current is then passed by the gas. The rise of 
temperature of the cathode is roughly proportional to the 
current. 

(3) The Nature of the Gas. 

The nature of the residual gas has a very marked effect 
both on the degree of sputtering that a metal exhibits and 
on the appearance of the deposit. Hydrogen, nitrogen, and 
carbon dioxide are in most cases unfavourable to the effect, 
while oxygen and especially the monatomic gases, mercury 
vapour, He, A, Ne, Kr, and Xe bring about pronounced 
disintegration of most metals. Helium shows the effect 
least of all these gases, but argon is particularly potent, and 
metals so varied as Al, Ag, Cd, Pt, and Au are all excited 
to a maximum activity in this gas. Aluminium shows only 
feeble sputtering in hydrogen or nitrogen, and but little 
more in oxygen. Iron sputters a little in hydrogen ; silver 
and lead sputter markedly in this gas. 

Systematic work is needed to find the most suitable gas 
for an X-ray tube. Unless precautions to the contrary have 
been taken, the gas will probably consist largely of hydrogen 
and carbon dioxide liberated from the electrodes. Pt and 
especially Al (and Mg) emit large quantities of gas when 
used as cathodes. The point is also of importance in 

1 See Kaye, P.P.S. Ap. 1913. 



80 



X RAYS 



connection with the various methods of controlling the 
hardness of bulbs (p. 74). The automatic devices introduce 
chiefly carbon dioxide, and, in some cases, a little water 
vapour ; the osmosis valves, hydrogen ; the Bauer valve, 
air. So far as sputtering goes, hydrogen and carbon dioxide 
would appear to have advantages, though there is some 
diversity of opinion on the point. On the other hand, it 
may be remarked that a tube rendered unsteady by the 
hardening effect of hydrogen may often be caused to run 
smoothly by letting in a little air. 

(4) The Current through the Tube. 

The disintegration of a cathode increases with the current 
through the tube, apparently either as the first power or 
the square of the current. 

(5) The Fall of Potential at the Cathode. 

The volatilisation of the cathode is augmented by in- 
creasing the potential on the tube, and such control is 

15 




mm . Hg. 0-5 

Pressure. 

FIG. 50. Relation between cathodic sputtering and pressure. 
(From the Chemical World.) 

readily obtained by lowering the pressure of the gas. Sput- 
tering is much more pronounced at low pressures than at 



THE BLACKENING OF AN X-RAY BULB 81 

high, though at the very low pressures of an X-ray tube 
the disintegration is not quite so marked as at rather higher 
pressures, when the tube runs more easily. Fig. 50 displays 
the relation between the pressure and cathodic disintegration 
of a number of metals. It is due to Granquist (1898). 

The potential that is applied to an X-ray tube is not 
distributed evenly between the electrodes. The greater part 
is used up close to the cathode ; there is a gentle potential 
gradient in the space between the electrodes, and the 
remaining fall of potential occurs close to the anode. The 
amount of sputtering depends on the cathode-fall of 
potential, and this increases as the pressure of the gas 
is lowered. It appears to be essential that the potential 
fall at the cathode shall exceed a certain minimum value 
before the metal becomes ionised and disintegrated to 
any appreciable extent. Holborn and Austin (1904) found 
that this critical potential was about 500 volts for a number 
of metals. 

VOLATILISATION OF THE ANTIC ATHODE. 

The high temperatures which anticathodes may attain in 
a focus tube are familiar enough, but the extent of the 
sublimation which most metals exhibit at temperatures well 
below their melting points may not have been brought home 
to many observers. A homely example of sublimation at 
low pressure is provided by the blackening which is a not 
uncommon feature of carbon and tungsten glow lamps. The 
subject has received attention at the hands of a number of 
workers, 1 and it appears that the degree of volatilisation 
is affected by : 

(1) The nature of the metal. 

(2) The temperature of the metal. 

(3) The nature of the surrounding gas. 

(4) The pressure of the gas. 

The disintegration of metals increases rapidly as the tem- 
perature rises. Of the platinum metals, platinum, rhodium, 
and iridium all disintegrate less as the pressure is reduced, 

1 See Kaye, Chemical World, June 1913. 
F 



82 



X RAYS 



and there is evidence to show i that in these cases the 
volatilisation is not a simple process, but is brought about 
by the formation of endothermic oxides more volatile than 
the metals themselves. It would seem that in order to 
reduce the sublimation of these metals to a minimum, the 
important thing is to ensure the absence of oxygen in the 
surrounding gas 2 a wise precaution, indeed, with most 
metals, as almost all observers agree. Hydrogen and 
nitrogen do not in general favour disintegration. 

With palladium and most other metals, a reduction of 
pressure is favourable to volatilisation as would be antici- 
pated in cases of true sublimation. 

Table IV. 3 gives, for a number of metals, data con- 
cerning the effect of pressure on the boiling point, as well 
as the temperatures at which appreciable vaporisation has 
been detected (mostly at low pressures). The correspond- 
ing melting points are added for the sake of comparison. 

TABLE IV. 



Metal. 


Boiling Point. 


Volatilisation 
detectable at 


Melting Point 
at 1 Atmos. 


At 1 Atmos. 


In Vacuo. 


Cadmium 


778 C. 


450 C. 


160C. 


321 C. 


Zinc 


918 


550 


180 


419 


Lead 


1525 


1150 


360 


327 


Silver 


1955 


1400 ? 


680 


961 


Copper - 


2310 


1600 ? 


400 


1084 


Tin 


2270 


1700 ? 


360 


232 


Gold 


2530 ? 


1800 ? 


1370 


1064 


Iron 


2450 





950 


1530 


Platinum 


2500 ? 





1200 


1750 


Osmium 








2300 


2200 


Iridium - 


2600 ? 





1400 


2290 


Tungsten 


3700 ? 





1800 


3000 



The table gives a notion of the extent to which volatili- 
sation occurs with metals, while still at temperatures well 

1 See Roberts, P.M. 1913. 

2 This is especially important in the case of iridium. 

3 See Kave and Ewen, P.R.S. 1913. 



THE BLACKENING OF AN X-RAY BULB 83 

below their melting points. There is scope for a good deal 
of systematic work on the volatility of platinum, tungsten, 
iridium, etc., when heated at low pressures in different gases. 
The results would be of great practical value to the user of 
X-ray bulbs. It is known that tungsten, for example, when 
heated, readily disintegrates and becomes brittle in the 
presence of oxygen or moisture. Irving Langmuir 1 has 
recently traced this to the formation of oxides. 

Coloration of the Glass of an X-ray Bulb with Use. 

The cathode rays "reflected" from the anticathode are 
responsible either directly or indirectly for the violet colour 
which the glass assumes in well used X-ray tubes. This 
coloration is most pronounced on the front side of the 
anticathode, and can be prevented by screening the glass 
with metal foil. Radium rays affect glass and quartz in 
the same way, though to a greater depth ; and cathode 
rays produce a similar colour in crystals of rocksalt or 
fluorspar. Possibly, therefore, the action is of the same 
nature in all these cases ; and may be the phenomenon is 
related to the violet permanganate coloration produced by 
ultra-violet light and sunlight in window glass. The violet 
colour is in all cases destroyed by heating. 

X-ray bulbs of lead glass become brown in colour rather 
than violet. Elster and Geitel (1898) have suggested that 
the various colorations are due to ultra-microscopic particles 
of reduced metal in the salt. 

1 Proc. Amer. Inst. Elect. Eng. Oct. 1013. 



CHAPTER VITL 
THE MEASUREMENT OF X RAYS. 

The International Radium Standard. 

The general desire to have a standard by which the output 
of an X-ray tube could be measured in a manner free from 
the defects of the usual methods, led the Rontgen Society 
in 1909 to appoint a Committee (with Dr. W. Deane Butcher 
as secretary) to consider the question. This Committee 
decided to initiate standards of radioactivity. These de-, 
pended on the y-ray activity of radium bromide and were 
prepared by Mr. C. E. S. Phillips. Largely owing to the 
efforts of Prof. Rutherford, the question was taken up by 
the Congress of Radiology at Brussels in September 1910. 
An International Committee was formed with Prof. Ruther- 
ford as President ; in March 1912 the Committee met at 
Paris and adopted as an International Radium Standard 
a specimen consisting of 21*99 milligrammes of pure 
radium chloride which had been prepared by Mme. Curie. 
The radium is contained in a thin- walled glass tube, and 
use is made of the y-ray ionisation. The International 
Standard is preserved at the Bureau International at Sovres 
near Paris. Secondary standards are obtainable by the 
various nations who require them. 

The British Radium Standard. 

The British Radium Standard, consisting of 2T10 milli- 
grammes of pure radium chloride, has been certified in 
terms of the International Standard, and is now deposited 
at the National Physical Laboratory at Teddington. The 
radium salt is contained in a small glass tube, through 
which a platinum wire is inserted to dissipate accumulated 



THE MEASUREMENT OF X RAYS 85 

electric charges (Fig. 51). The standard serves as a means 
of standardising radioactive preparations as well as the 
energy output of X-ray bulbs. 

In this connection it may be recalled that the average 
y rays of radium are something like thirty to forty times 
as penetrating to air as the X rays from a very hard bulb, 
while the /3 rays are a trifle more absorbable than very 
soft X rays. 






O I 2 3 cms. 

FIG. 51. The British Kadium Standard at the National Physical Laboratory. 

Standardisation of X-ray Bulbs. 1 

The difficulty of standardising the output of X-ray bulbs 
by means of such an ionisation standard is chiefly one of 
specifying and reproducing the working conditions of the 
bulbs. Possibly the various makers could be induced to 
work to standard dimensions, but few would assert that the 
design of an X-ray bulb has reached or even approached 
finality. Moreover, even if agreement in design were secured, 
the performance of a bulb is peculiarly susceptible to slight 
variations in the prevailing conditions (see p. 68), over 
some of which control is scarcely possible. It is a matter 
for urgent enquiry to find means of holding a bulb to con- 
ditions which have been specified ; until then, the stand- 
ardisation of an X-ray bulb can only be regarded as an 
assessment of the output which prevailed at the moment 
of test. 

From a practical point of view, the output from an X-ray 
bulb has to be specified with respect to both intensity and 
hardness, i.e. quantity per unit area and quality. 

METHODS OF MEASURING INTENSITY. 

The intensity of the X rays at a particular point is 
defined as the energy falling on one square centimetre of a 

1 For a full account of the various methods of measuring X rays (more 
especially for medical purposes), see Christen, Messung und Dosierung der 
Rontyenstrahlen. 



86 X RAYS 

receiving surface passing through the point and placed 
at right angles to the rays. Rontgen was able to show, ; 
and the fact has been amply confirmed by later workers, ] 
that the intensity of a beam of X rays from a focus-bulb 
falls off as the inverse square of the distance from the ; 
anticathode. 

General Remarks on Intensity Measurements. 

It may be noted that almost all the methods of intensity- 
measurement, as ordinarily practised, are unduly favourable 
to the soft rays when regarded from an energy standpoint. 
The ideal method of test would afford an exact comparison 
of the energy of a hard X ray with that of a soft ray ; 
but what almost always happens is that the hard rays are 
not wholly arrested by the testing instrument, and hence 
show up relatively badly. For instance, very hard X rays 
do not affect a photographic plate to the same extent as 
the softer rays ; and, again, soft rays have greater ionising 
power per centimetre than hard rays. In order to make a 
fair comparison between two bulbs, all the rays given out 
by both should be taken into account. The hard rays as 
well as the soft ones should be completely absorbed, in 
which case the measurements would give a fair estimate 
of the relative amounts of .energy emitted from the bulbs. 

(1) Current through the X-ray Tube. 

A rough notion of the intensity of the X rays from a bulb 
may be obtained by measuring (with a milliammeter) the 
current passing through the tube, provided the potential 
difference is kept constant. This method, which is often 
employed, would be a more reliable guide if all the current 
were carried solely by the cathode rays, and if all the 
cathode rays gave birth to X rays. But this is un- 
doubtedly not the case, 1 and, as Blythswood and Scoble 
(J.Rt.S. 1907) showed, a knowledge of the current in the 

1 Sir Oliver Lodge (P.M. 1911) maintains, indeed, that the current is 
mostly conveyed by positive rays, though Sir J. J. Thomson (P.M. 1912) 
inclines to the opposing view that the greater part of the current is carried 
by the cathode particles. 






THE MEASUREMENT OF INTENSITY 87 

.secondary circuit does not afford an accurate measure of 
the intensity of the X rays. It is important, however, 
in a set of comparative observations to keep the current 
in the primary constant, for an increase in the current 
through the primary not only augments the intensity of the 
rays, but hardens the tube and lengthens the alternative 
spark-gap. 

But, however constant the current in the primary is, it 
is difficult to estimate how any particular current- measurer 
will average up the peculiar pulsating current of a coil dis- 
charge (see p. 59). The interpretation to be put on the 
readings of the milliammeters ordinarily used for the pur- 
pose is dubious to a degree. Salomonson (J.Rt.S. 1912) has 
recently shown experimentally that both the form of the 
current and the frequency of the interruptions must be 
controlled in exact comparative measurements. To this end 
some form of electrostatic oscillograph would be useful. 

In usual practice, the methods for measuring intensity 
depend on one or other of the properties of the rays : heating, 
ionising, fluorescing, photographic, or chemical. 

(2) Thermal Methods of Measuring Intensity. 

The heat produced when X rays are completely absorbed 
by a metal was first measured by Dorn in 1897. Angerer 
(A.d.P. 1907) and Bumstead (P.M. 1908) have shown that 
the same amount of heat is generated by a stream of X rays, 
no matter what the absorbing metal a result unfavourable 
to the view formerly held that it was possible to unlock the 
internal stores of atomic energy by such means. The ex- 
periments are difficult, for the heating effects are minute, 
and can only be detected by instruments as sensitive as the 
radiomicrometer, bolometer, or radiometer. It will be seen 
that at present the method is only fitted for the research 
laboratory, and does not enter into the sphere of ordinary 
practice. 

(3) lonisation Methods of Measuring Intensity. 

The exact mechanism of ionisation is even now not fully 
comprehended, but the outcome is the formation of positively 



88 X RAYS 

and negatively electrified particles ions the presence of 
which imparts to the gas a conductivity that persists for 
some little time. The extent of the ionisation depends on 
the number of ions produced, and this is reflected in the 
degree of excellence with which the gas conducts. The 
generally accepted view of the formation of ions is that a 
negative nucleus (the electron) is broken off from the atom, 
leaving a positive nucleus ; each of these charged nuclei 
gathers round itself a cluster of gas molecules sometimes 
in considerable numbers and the resulting molecular aggre- 
gates constitute the gaseous ions, both positive and negative. 
At low pressures, the negative ion exists as the electron 
unencumbered by any attached molecules. 

An ionisation method of evaluating X rays thus resolves 
itself into the measurement of an electric current an opera- 
tion which can be carried out with such delicacy and con- 
venience that practically all recent workers have utilised 
this property of the rays. The ionised gas is subjected to 
an electric field which drives the two classes of ions 
positive and negative in opposite directions with velocities 
which depend on the strength of the field. The magnitude 
of the current generated by the motion of these charged 
particles depends to some extent on the potential difference 
of the surfaces between which the field is applied ; with 
small potentials, the two are roughly proportional, just as 
in cases of metallic conduction ; but with higher potentials 
the current responds less and less to the potential, and finally 
reaches a constant value called the saturation current (see 
Fig. 52). This is the current which should always be 
measured in practice, and care should accordingly be exer- 
cised that the potential difference applied to the surfaces 
is sufficient to give the saturation current The electric 
field necessary increases with the degree of ionisation, but 
for most cases likely to arise in X-ray work, 100 volts per 
cm. is adequate. 

The shape of the first part of the current-potential curve 
is explained by the liability of a charged particle to encounter 
and coalesce with another of opposite sign before reaching 
one of the bounding surfaces. But this tendency, which 



THE MEASUREMENT OF INTENSITY 



89 



militates, of course, against the growth of the current, will 
be lessened if the speed of the particles is increased by 
putting up the voltage between the surfaces. For the 
higher the speed, the shorter the time of passage, and the 
less likely are the chances of recombination. Finally, with 
the saturation voltage, all the ions reach the boundaries, and 
the number arriving exactly equals the number produced in 
the same time by the X rays passing through the gas. This 




Sparking t 

I 
Current" 



Applied Pofenh&l 

FIG. 52. Diagrammatic representation of the relation between current and 
potential for an ionised gas. 

is not the case with the lower voltages, and thus only from 
a knowledge of the saturation current can we infer the true 
degree of ionisatioii that the rays have produced. 

With still higher potentials, the current rapidly increases 
until the sparking point is reached. On this steep part, of 
the curve, both positive and negative ions acquire sufficient 
speed to produce fresh ions by colliding with the atoms of 
the gas. Thus, by working with potentials just insufficient 
to cause the passage of a spark, the original ionisation may 
be greatly increased a hundredfold or so. The plan has 
been adopted for the measurement of very feeble ionisations. 

Before adopting one or other of the various forms of 
ionisation chamber for any particular purpose, it is necessary 
to decide what we wish to. measure. If it is the total ionisa- 
tion that is desired, then we must arrange for the rays to 



90 



X RAYS 




be completely absorbed in the gas of the chamber, if necessary 
by contriving a suitably long path, or by increasing the 
pressure of the gas, or, again, by choosing a sufficiently 
dense gas 1 : the total ionisation, we have reason to believe, 
is the one measure of the energy in the rays and cannot be 
increased by reflection or any other device. If, on the 
other hand, we wish merely to ascertain the ionising power 
of a beam of rays at some particular point, then almost any 
form of ionisation chamber will suffice. 

One convenient design is shown in Fig. 53. A circular 
thin aluminium sheet is mounted midway between two 

similar sheets which are raised to 
f To Electrometer a potential of a few hundred volts 
by a battery of cells. The central 
sheet is carefully insulated and 
joined to an electrometer. It is 
easy to calculate the electric field 
with this shape of vessel, a state- 
ment that does not apply to the 
very common design made up of 
a cylinder provided with an in- 
sulated wire electrode along the 
axis. 2 In this latter form, the 
field, which is very strong near 
the wire, falls off a great deal 
towards the surface of the cylinder ; 
the applied potential must be very 
considerable to ensure a saturating 
field throughout the chamber. 

Ionisation currents produced by X rays are usually of 
the order of 10 ~ 10 to 10 ~ 15 ampere ; the exact amount varies 
a great deal according to the circumstances. For the larger 
currents, it is sometimes possible to use a sensitive galvano- 
meter 3 ; but in general it is much more convenient to 

1 E.g. sulphur dioxide or methyl iodide are very useful for the purpose. 

2 See, for instance, the comparison ionisation chamber in Fig. 71. 

3 The most sensitive galvanometers yet introduced are the Paschen and 
the Einthoven. The former, with a low resistance and a short period, will 
readily indicate 10~ 10 ampere. See Camb. Sci. Inst. Co.'s list. 



FIG. 53. An ionisation chamber, 
showing earthed guard-tube in the 
insulation. 



THE MEASUREMENT OF INTENSITY 91 

deduce the current from the change of potential as measured 
by means of a Dolezalek quadrant electrometer or some 
form of gold-leaf electroscope. With an electrometer and 
a suitable condenser, currents from 10 ~ 8 to 1C' 14 ampere 
can be measured. For smaller currents down to 10 ~ 17 
ampere an electroscope is better. 

Of the electroscopes, the C. T. R. Wilson tilted variety l 
is convenient and sensitive, and possesses a small capacity. 
Some observers use electroscopes provided with aluminium 
windows, the X rays being sent directly into the electro- 
scope instead of into a separate chamber. The leaf in this 
case is charged to a high potential, and its rate of leak to 
the outer case is measured. There are on the market 
several " direct reading " X-ray quant imeters of this type, 
which are convenient for comparative measurements but 
are not capable of accurate absolute work. All the various 
instruments require to be calibrated, and their capacity (as 
well as that of the ionisation vessel) determined, before the 
currents can be deduced from the potential measurements. 

The French workers largely employ the late Prof. Curie's 
piezo-electrique, in which the electricity generated by gradu- 
ally relieving the tension on a stretched quartz lamina is 
balanced against the ionisation current to be measured. 
The method requires considerable manipulative skill. 2 

(4) Photographic and Fluorescence Methods of Measuring Intensity. 
Practically all the earlier workers used photographic or 
fluorescence methods of measuring the intensity of their 
X rays, but nowadays these methods, at any rate for most 
purposes, have been displaced by ionisation methods. An 
ordinary photographic plate is incapable of arresting and 
recording the hardest kinds of X rays, and therefore, from 
an energy standpoint, the softer rays are given undue weight 
when a heterogeneous beam is used. We need, therefore, 
to exercise care in drawing conclusions from the density 
of the photographic image as to the intensity of the rays. 
Moreover, Barkla and Martyn (P.M. 1913) have shown that 

1 See, for example, the Camb. Sci. Inst. Co.'s list of electrometers. 

2 See Rutherford's Radioactive Substances, 1913. 



92 X RAYS 

if the X rays are just sufficiently hard to excite the 
radiations characteristic of silver or of bromine (the heaviest 
constituents of a photographic film), they are selectively 
absorbed and the photographic effect is greatly enhanced. 
X rays a little softer than this do not excite the charac- 
teristic rays, and are, therefore, recorded disadvantageously. 
Thus the photographic action is not proportional to the 
direct absorption of the X rays by the sensitive film. 

As far as practical difficulties are concerned, it should be 
remarked that the emulsion on an ordinary plate may vary 
in thickness by as much as 10 per cent., through want of 
fiatiiess of the glass backing. This can be reduced to the 
order of 5 per cent, by the use of patent plate glass and the 
exercise of special care in the coating. The slower fine 
grained plates are to be preferred for more precise work, 
and, of course, one should adhere to some standard de- 
veloper and method of development. In regard to the 
sensitiveness of different plates to X rays, Ely ths wood 
and Scoble (J.Rt.S. 1906) showed that but little guidance 
can be obtained from the speeds for light. In some cases 
the divergence amounted to as much as four times. 

To the worker with limited resources the photographic 
method of measuring intensity offers advantages because of 
its simplicity. Some form of opacity-meter for obtaining a 
measure of the density of the image is the chief requirement. 
The opacity meter measures the extent to which a standard 
beam of light is cut down by the photographic film whose 
density is required. If 7 is the intensity of the testing 
light which is incident on the developed film, and I t that 
of the transmitted light, then, if \x is the fraction of the 
energy which is absorbed by a very small thickness, x, of the 

film ' /,-#, 

where d is the thickness of the film 1 (see p. 100). The 
film is assumed equally dense throughout its thickness. 

For films of uniform thickness, d is constant, so that 
A is proportional to log (/ /^)- ^ * s called the absorp- 

1 More precisely, this assumes monochromatic light. A is different for 
different wave-lengths. 



THE MEASUREMENT OF INTENSITY 93 

tion coefficient ; (I /I t ) is known &s the opacity, 1 and equals 
the number of times the incident light is cut down. 
Log (lQ/I t ) is termed the opacity-logarithm. Now, by definition, 
A is proportional to the density of the image, i.e. to the 
amount of silver per unit area of film. Thus the ratio of 
two opacity-logarithms gives the ratio of the film densities, 
and therefore the ratio of the photographic energies in the 
two cases. The opacity meter is graduated to read directly 
in opacity-logarithms. 

In fluorescence methods the luminosity is matched against 
some standard fluorescence excited by a steady source of 
radiation such as radium. The drawback to such methods 
is that the fluorescing salt becomes " tired " under the action 
of the rays. The sensitivity of a screen may also vary con- 
siderably from point to point, so that it is difficult to make 
a fair comparison. Barium platinocyanide is the material 
commonly used to sensitise a fluorescent screen : recently, 
considerable improvements have been effected in the 
fluorescing ability of the salt. 

(5) Methods of Measuring Intensity used in Medicine. 

In the therapeutic use of X rays, various chemical re- 
actions brought about by the rays have been suggested 
and employed from time to time as aids to " dosage " ; 
for example, the discolouring of various alkaline salts (Holz- 
knecht, 1902) ; the liberation of iodine from a 2 per cent, 
solution of iodoform in chloroform 2 (Freund, 1904 ; Bordier 
and Galimard, 1906) ; the darkening of a photographic 
plate (Kienbock), see p. 92 ; the precipitation of calomel 
from a mixture of mercuric chloride and ammonium oxalate 
solutions 2 (Schwarz, 1907) ; and the change of colour of 
pastilles of compressed barium platinocyanide (Sabouraud- 
Noire and Bordier). X rays resemble light in their pro- 
perty of lowering the electrical resistance of selenium ; this 
property, if the pronounced fatiguing of the selenium could 
be overcome, would doubtless furnish the basis of a very 
convenient method of measurement. It must be admitted 

1 The transparency is the reciprocal of the opacity. 

- X rays share this property with Ra rays and ultra-violet light. 



94 X RAYS 

that most of these methods, if not all, provide nothing more 
than the roughest notion of the intensity of a beam of 
ordinary heterogeneous X rays. 

Of all the various intensity- measurers, the pastille finds 
most favour with medical men. The barium-platinocyanide 
discs are some 5 mms. in diameter, and their colour, initially 
a bright green, changes, when exposed to the rays, to a 
pale yellow, and finally to a deep orange. The pastille is 
placed at a specified distance from the anticathode of the 
bulb, and the colour is matched against one of a number 
of standard tints. The method is extremely easy in prac- 
tice, and is fairly reliable as a guide for short exposures, 
but it is not very trustworthy for times exceeding ten 
minutes or so. Possibly for long exposures, some of the 
other platinocyanides, all of which show similar colour 
changes when exposed to X rays, would be more reliable. 
The change of colour appears to be due to dehydration. 
If the pastille is put aside, rehydration subsequently takes 
place, especially in the presence of light, so that the pastille 
should not be exposed to full daylight during the X-ray 
treatment. Ultra-violet light and radium rays cause similar 
browning in such pastilles. 

The following /table gives an idea of the relation between 
the different scales : 

5H unitsr 1 (Holzknecht ; alkaline salt) 
= Tint B (Sabouraud-Noire ; pastille) 

= Tint 1 (Bordier ; varnished pastille) 

= 3 to 41 (Bordier and Galimard ; iodine solution) 

= 10X units (Kienbock ; photographic plate) 
= 3-5 Kaloms (Schwarz ; mercury solution) 
= Villard dose. 



METHODS OF MEASURING QUALITY OR HARDNESS. 

The range of qualities of X rays is very wide, as would 
be inferred from the fact that, while some rays are 
unable to penetrate more than a centimetre .or two of air 

1 Unit 1H = one-third of the radiation necessary to set up the first signs 
of reaction in the healthy skin of the face. 



THE MEASUREMENT OF QUALITY 95 

at atmospheric pressure, others have been detected at dis- 
tances of 100 metres or more. 

The hardness of a bulb is mainly dependent on the 
maximum potential difference between the electrodes : an 
account of the various methods of controlling this potential 
is given on p. 68. It is sufficient to repeat here that 
the X rays from a bulb may be divided into two main 
classes : 

(1) the heterogeneous "general" or "independent" 
radiation which depends in quality solely on the speed of 
the parent cathode rays ; 

(2) the homogeneous "characteristic" or "monochro- 
matic " radiations which are characteristic of the metal of 
the anticathode (p. 112). 

The proportion of these two classes depends on the con- 
ditions of discharge, and on the metal of the anticathode. 
The general radiation is always present, and has a range of 
hardnesses which depends on the range of speeds of the 
cathode rays. The characteristic radiations only appear 
when the cathode rays are sufficiently fast ; their hardness 
depends only on the material of the anticathode. 

(1) Wave-length. 

We have good reason now for believing jbhat X rays and 
light are identical, and that the hardness or penetrating 
power of an X ray is precisely defined by its wave-length : 
the shorter the wave-length, the harder the ray. The sub- 
ject is dealt with elsewhere (p. 186), but it has been shown 
by many observers that X rays are reflected by the invisible 
parallel planes of atoms in the interior of a crystal. From 
a knowledge of the distances separating the atoms, we can 
arrive at the wave-lengths of the X rays. If one compares 
corresponding radiations (i.e. in the same series) of different- 
elements, it is seen that the heavier atom gives the shorter 
wave : Prof. Bragg has, in fact, shown that the wave-length 
is inversely proportional to the square of the atomic weight 
of the radiating element (see p. 200). W. L. Bragg (P.E.8. 
1913) has calculated the atomic distances in the case of 
rock-salt (p. 197), and the following wave-lengths for the 



96 



X RAYS 



several characteristic radiations depend on his estimate. A 
more complete table will be found on p. 201. 

TABLE V. WAVE-LENGTHS OF SOME CHARACTERISTIC RADIATIONS. 



Platinum. 


Tungsten. 


Rhodium. 


Nickel. 


1-303 x 


10- 8 cm. 


1-25 x 10- 8 cm. 


0-607 x 10- s cm. 


l-66x 10- s om. 


1-109 


| strong 


L radiation, 


(strong). 


K radiation, 


1-091 


,, J L radn. 


(very soft). 


0-533 x 10- 8 cm. 


(very soft). 


0-948 


., 




(weak). 




0-918 


" 









Thus, it appears, we may regard the rays from an X-ray 
bulb as Consisting of a mixture of homogeneous radiations 
characteristic of the metal of the anticathode, together with 
a " background " of " white " rays (in other words, the 
analogue of white light). It is not yet settled whether 
these latter rays, which constitute the independent radia- 
tion referred to above, consist of a mixture of a number of 
characteristic rays of different hardnesses, or whether they 
represent a perfectly continuous spectrum of rays. 

(2) Potential Difference between Electrodes. 

A measure of the hardness of a beam of X rays is afforded 
by the potential difference between the terminals of the 
generating tube. On this potential difference depends the 
velocity of the cathode rays ; and since both the quality 
and energy of an X ray are related to the speed of the 
exciting cathode ray, it is important to be able to measure 
the potential with some precision. 

If E is the potential difference to which a cathode ray 
owes its velocity (v), then the two are connected by the 
energy equation i m tV * = E.e, 

where e and m are respectively the charge and the mass of 
the cathode ray. 

Taking efm = I'll x 10 7 , E in volts and v in cms. /sec., 

E=2'82v*. 10 ~ le 
or v*=5-95\/J5J. 10 7 . 



THE MEASUREMENT OF QUALITY 



97 



A series of values of cathode-ray velocities and potentials 
up to 200,000 volts is tabulated on p. 233. 

The potential difference on a tube may be measured by 
a high-potential electrostatic voltmeter, of which there are 
now one or two excellent examples on the market. Or, 
failing this, the length of the alternative spark-gap may be 
noted. The hardness of the X rays is roughly proportional 
to the square-root of the spark-gap, at any rate for the 
same bulb. It does not, however, by any means follow 
that two bulbs having the same equivalent spark-gap will 
give out rays of the same quality : usually they will not. 

TABLE VI. SPARKING POTENTIALS. 



Spark-gap. 


Diameter of Balls. 


Needle-pts. 


0-5 cm. 


1cm. 


2 cm. 


5 cm. 


cm. l inch. 


A.C. VOltS. D.C. VOltS. D.C. VOlts. 


D.C. volts. 


D.C. volts. 




(Max.) 






0-1 ! -04 


1,000 5,000 5,000 


5,000 


5,000 


0-2 


08 


2,000 8,000 8,000 


8,000 


9,000 


0-3 


12 


4,000 11,000 11,000 


11,000 12,000 


0-4 


16 


5,000 


14,000 14,000 


14,000 


15,000 


0-5 


20 


6,000 


16,000 17,000 


17,000 


18,000 


0-6 


24 


7,000 


17,000 


20,000 


20,000 


21,000 


0-7 


28 


8,000 


18,000 


22,000 


23,000 


24,000 


0-8 


31 


10,000 


19,000 


24,000 


26,000 


27,000 


0-9 -35 


11,000 


20,000 


26,000 


29,000 


30,000 


1 -39 


12,000 


21,000 


27,000 


31,000 


33,000 


2 


79 


24,000 


24,000 


36,000 


48,000 


57,000 


3 


1-18 


34,000 


26,000 


42,000 


58,000 


77,000 


4 


1-58 


42,000 


27,000 


45,000 


65,000 


93,000 


5 


1-97 


49,000 


Brush 


47,000 


71,000 


105,000 


6 


2-36 


55,000 


discharge 


Brush 


77,000 


116,000 


7 


2-76 


61,000 


usually 


discharge 


82,000 


125,000 


8 


3-15 


66,000 


occurs. usually 1 87,000 


133,000 


9 


3-54 


71,000 




occurs. 


91,000 


140,000 


10 


3-94 


76,000 






95,000 


145,000 


15 


5-91 


102,000 






Brush 


170,000 


20 


7-9 


122,000 






discharge 


190,000 


30 


11-8 


170,000 






usually 





40 


15-8 


220,000 






occurs. 






08 X RAYS 

Table VI. gives the approximate sparking voltages in airl 
at atmospheric pressure and room temperature. Too much 
reliance must not be placed on the figures, as the results 
of different experimenters do not agree well, probably owing 
to the difficulty of measuring the potential. The values for 
the needle-point electrodes are for alternating current of sine 
form, and are due to Steinmetz (Proc. Amer. Inst. Elec. Eng. 
1898). For alternating currents, the striking distance is 
most probably governed by the maximum voltage, which 
is accordingly given in the table rather than the effective 
(root-mean-square) value [ =(max.)/l*42]. 

For ball electrodes, the most recent and reliable measure- 
ments of the maximum spark-potentials for alternating 
current l come out about 5 per cent, smaller than those for 
direct current, the values for which are given in the remain- 
ing columns of the table. 2 These latter results refer to 
smooth polished metal balls of the same size. 

An inspection of the table shows that, in general, the 
spark passes more readily, the smaller the ball ; and that 
short spark-gaps require proportionately more potential 
than long. The measurements are taken in the absence 
of any visible brush-discharge, a condition essential for 
definite sparking. It is better in practice to use mode- 
rately large balls than small, as with the latter, brush- 
discharge tends to occur, more especially at the negative pole : 
such glow is, of course, a prominent feature with needle- 
point electrodes. The needle-point spark-gap often supplied 
with induction coils, while it enhances the apparent capa- 
bilities of the coil, is not suitable for measuring purposes. 
If unequal-sized balls are used, the smaller electrode controls 
the spark-gap for moderate lengths of spark : the larger ball 
should be made the negative electrode. 

Trowbridge (P.M. 1898) found a spark-length of 200 cms. 
with a potential of 3,000,000 volts. With very long sparks, 
the shape of the electrodes (if of moderate size) is im- 

1 See Kowalski and Rappel (P.M. 1909), who employed balls up to 
30 cms. diameter. 

2 Based largely on the results of Algermissen (A.d.P. 1906) and Topler 
(A.d.P. 1907). 



THE MEASUREMENT OF QUALITY 99 

material. For instance, with a potential of 240,000 volts, 
Jona obtained the same sparking distance (47 cms.), 
whether a point and plate or two balls (2 cms. diam.) 
were used. 

In the case of an X-ray tube, as the break-down voltage 

is higher than the running voltage, it is doubtful what 

precisely either voltmeter or spark-gap affords. With a 

j pulsating current, we need to know the shape and abruptness 

of the potential curve, as well as the proportion of time 

j between the impulses, before we can estimate the effective 

! potential (see p. 59). It is probable, however, that, at any 

j rate in the case of a hard tube, either instrument indicates 

J a value not very far from the maximum potential (see p. 98), 

\ and that the bulk of the X rays are generated by cathode 

j rays with a velocity which they owe to this maximum 

j potential rather than to a mean potential (see p. 16). 

In the case of the characteristic radiations, the quality 
can be defined rigorously in terms of the speed of the parent 
cathode rays. It is found that a certain minimum voltage 
: on the tube is required to excite a particular radiation. 
There is thus a critical cathode-ray velocity for each char- 
acteristic X ray : slower cathode rays can only excite inde- 
j pendent "rays"; faster cathode rays are, within limits, 
increasingly effective generators of the characteristic rays, 
but with very high-speed rays the " independent " radiation 
is once again generated. 

The subject is dealt with later (p. 126), but it may here 

be mentioned that the critical cathode-ray speed is pro- 

Iportional to the atomic weight of the anticathode. Not 

I only that, but the velocity of the secondary corpuscular rays, 

which such a radiation excites when it strikes matter, is 

equal to the velocity of the cathode rays which generated 

the radiation. 

(3) Absorption Coefficients. 

The customary way of specifying the character of X rays 
is to measure their absorption in a sheet of aluminium of 
definite thickness. Aluminium is not an ideal standard of 
reference, but it is chosen because it is readily procurable 



100 X RAYS 

in convenient form, and, so far as we know, does not, in 
the majority of cases, complicate matters unduly by super-! 
posing a characteristic radiation. 

Now it is found that if all the rays both entering and 
leaving a plate of material are homogeneous (that is, wholly; 
of the same quality), then the rays are absorbed exponentially 1 
by the plate, i.e. successive similar sheets of the material! 
absorb equal fractions of what they receive. In other words, 
if there is no " scattering " or transformation of the X rays, ' 
and if Xx is the fraction of the intensity which is absorbed 
when the rays pass normally through a very thin screen of^ 
thickness x (cm.), then for a plate of thickness d (cms.), 



in which 7 is the intensity of the beam when it enters, and 
/ that. of the beam when it leaves the screen. e( =2- 72) is 
the base of the hyperbolic system of logarithms. X is termed 
the linear absorption coefficient. 1 

2*3 

It follows that. X = (log 7 -log/) ; the logarithms are 
d 

to base 10. If in a set of observations with homogeneous 
rays, log I is plotted as ordinate against d, the graph is a 
straight line and X is 2- 3 times the slope of the line. 

The logarithmic curve of absorption for heterogeneous rays, 
such as are given out by an ordinary X-ray bulb, is not a 
straight line, but a curve which is steeper for thin screens 
than for thick. The general shape is like the heavy curve 
in Fig. 63. 

A large value of X corresponds to easity absorbed rays, 
and a small one to very penetrating rays. X also varies 
with the nature of the absorbing screen, so that it is necessary 
to specify the material used. For medical purposes, it 
has recently been suggested that water should be chosen as 
the standard absorbing medium, since the absorptive power 
of water agrees closely with that of animal tissue. 

It may be noticed that 1/X is the distance to which the 

1 The precise physical interpretation of an exponential law of absorption 
is not so simple as its compact and convenient mathematical expression 
would lead one to suppose. 



THE MEASUREMENT OF QUALITY;; 101 



rays penetrate before their intensity is reduced to 1/e of 
the original amount. Some workers prefer to think in 
terms of the thickness, D, which reduces the intensity to 
half value. D is connected with X by the expression 
D = 0'69/X. It follows that, if after traversing 1 cm. of a 
substance, the intensity is reduced by one-half, X =0-69 cm' 1 . 
A notion of the order of values of X may be got from the 
fact that an X-ray beam of average hardness has a X in Al 
of about 10. X for fatty tissue varies from about 0'4 for 
hard rays to 0'7 for medium rays. A table connecting 
I Q /I and \d is given on p. 232. 

A more fundamentally important constant is obtained by 
dividing the absorption coefficient (X) by the density (p) of 
the absorbing screen. 1 This quantity, X/p usually called 
the mass-absorption coefficient gives a measure of the absorp- 
tion per unit mass of the screen for a normally incident pencil 
of rays of unit cross section. Since it is mass alone that 
affects absorption, at any rate as determined by the usual 
methods of measurement, it is more profitable to use mass- 
coefficients than linear-coefficients.- 

If, as was at one time supposed, the absorbing powers of 
different materials were truly proportional to their densities, 
then for the same rays \/p would be a constant, no matter 
what the substance used as screen. In point of fact, dense 
substances are a good deal more absorptive, mass for mass, 
than light, and \/p increases rapidly with the atomic weight 
of the screen. 2 The increase is more noticeable with hard 
rays than with soft (see also p. 132). 

Benoist (J.d.P. 1901) was the first to examine systemati- 
cally the absorption of a beam of ordinary heterogeneous 
X rays in various absorbing elements. For our purposes 
it is convenient to translate his results into quantities pro- 
portional to absorption coefficients ; and, when this is done, 

1 For the standard material, aluminium, p =2'7. 

- A similar relation holds for the soft y rays from radium. For hard 
y rays, a density law holds, and A//3 is constant, except for the heaviest 
metals, which are a little more absorbent; In other words, these very 
penetrating rays almost entirely ignore atomic structure. For hard y 
rays, Xjp = 04 for all absorbing substances with an atomic weight less 
than 100. 



Fig. 54 is the result. It will be noticed that X/p increases 
steadily with atomic weight both for hard and soft X rays. 
For example, with hard rays, lead is twenty-five, silver 




50 100 ISO 200 

Atomic Weighr of Absorbing Element 



250 



FIG. 54. Graph (derived from Benoist's transparency curve) connecting 
absorption with atomic weight and displaying a region of selective absorption. 

eighteen, and copper eight times as absorbent as an equal 
mass of aluminium. 

There is a region of selective absorption round and about 
silver : the two beams of X rays, one hard, the other soft, 
are, moreover, absorbed almost equally by the silver group 
of elements. The explanation of this, as will be seen later 
(p. 135), is bound up with the amount of secondary radiation 
that silver emits under ordinary conditions. 



THE MEASUREMENT OF QUALITY 



103 



Absorption Coefficients of Heterogeneous Bays. 

But it may be urged that although characteristic rays 
have a perfectly definite A, the X rays from a coil-driven 
bulb are so very far from being homogeneous that the 
ub-orption coefficients as above defined are not particularly 
useful in X-ray practice. However, it should be pointed 
out that considerable guidance can be obtained from a 
knowledge of even an average value of X, calculated though 
it may be, on loose assumptions. And, moreover, while it 
is true that the X rays from a bulb are in general very 
heterogeneous, they become less so as the spark-gap is 
increased. 1 The rays from a bulb with a spark-gap of some 
centimetres, which are transmitted by an aluminium screen 
2 rnms. or more thick, are very fairly homogeneous. 

Eve and Day (P.M. 1912) have measured at various 
ranges (up to 100 metres) the absorption coefficients in air 
of the rays from X-ray bulbs of different degrees of hard- 
ness. Table VII. contains some of their results. 

TABLE VII. ABSORPTION COEFFICIENTS IN AIR. 





Distances from X-ray Bulb. 


Spark-gap. 


4 to 10 metres. 20 to 40 metres. 


40 to 60 metres. 




A 


A/p A 


A/p 


A ! A/p 


1-5 to 5 cm. 


JO-0010 


j'0-8 










(soft bulb) - 


to 


to 
















(0-0018 


(1-4 










11 cm. (med- 














ium bulb) - 


0-00040 


0-32 


0-00040 


0-32 


0-00029 


0-23 


30 cm. (hard 












bulb) 


0-00029 


0-23 


0-00027 


0-21 


0-00014(?)_ 0-11(?) 












i 



Eve and Day note that X= 0*0004 is a good value for 
radiographic work ; but rays whose X is O'OOOS are too 
penetrating for such a purpose. The above values may be 
compared with those of Chadwick (P.P.S. 1912) for the 

1 The explanation of this is probably bound up with the simpler character 
of the magnetic spectrum of the cathode rays at low pressures (see p. 16). 






104 X RAYS 

Ra 7 rays when absorbed by air. His values of X for air 
are 0-000062 and 0'000059 cm." 1 in the case of rays which 
have previously traversed 3 mms. and 10 mms. of lead 
respectively. 

The absorption coefficients for the various characteristic 
radiations are given on pp. 115 and 132. 

(4) The Benoist Penetrometer. 

Among medical men Benoist 's radiochromometer or perie- 
trometer enjoys extensive use as a measurer of hardness. 
It consists of a thin silver disc 0-11 mm. thick, surrounded 
by twelve numbered aluminium sectors from 1 to 12 mms. 
thick (Fig. 55). The X rays are sent through the instru- 
ment, and the observations consist 
merely in matching on a fluo- 
rescent screen or photographic 
plate the image cast by the silver 
disc against the images of the 
aluminium plates : the thickness 
of the matching sector increases 
with the hardness of the rays. 1 
Thus " Benoist 4 to 5 " is a 
good average hardness for cura- 
FIG. 55.- Henoist's penetrometer. tive work, while for general 

radiography No. 6 on Benoist's 

scale is useful. A notion of the discharge potential across 
a tube may be got from the very rough relation that the 
voltage is from 6,000 to 10,000 times the Benoist reading 
of the X rays. 

Salomonson, following the principle of the " light- wedges " 
emjiJbyed in photometry, uses two aluminium wedges, the 
one sliding over the other. A plate of aluminium of variable 
thickness down to ytro mm. is thus obtainable, a match being 
made against a silver plate O'll mm. thick. Other hard- 
ness measurers by Wehnelt (who also uses an aluminium 

1 Benoist (CM. 1902) based the theory of his instrument on the curves dis- 
played in Fig. 54, which go to show that while the transparency of aluminium 
alters a good deal with the quality of the X rays, silver is almost equally 
transparent to both hard and soft rays. As Table XI. shows, the assump- 
tion of a constant transparency for silver is by no means correct. 




THE ENERGY OF X RAYS 105 

wedge), Walter (a sequence of Pt discs), and Bauer (an 
electrostatic voltmeter) are in common use. The corre- 
sponding hardness-numbers are all much the same as 
Benoist's, except those of Wehnelt, which are 50 per cent, 
bigger for the same quality of rays. Christen has recently 
introduced a " half- value " meter, with water as the ultimate 
standard of absorption. 

THE " ENERGETICS " OF AN X-RAY BULB. 

When a stream of cathode rays strikes an anticathode, 
the different rays suffer a variety of fates. By far the 
greater number merely fritter away their energy until it 
becomes too small to render them distinguishable : the 
heat generated at the anticathode is ample proof that the 
energy of the cathode rays is mostly dissipated into heat. 

The remaining cathode rays either suffer conversion into 
X rays or are " reflected " by the anticathode in all directions 
against the glass walls of the tube with velocities which may 
be anything up to the original speed of the cathode rays. 
There is good evidence for believing that a cathode ray can 
pass through many atoms without being in any way deflected 
or transformed. The fate of the cathode ray is to some 
extent dependent on the material of the anticathode. The 
heavier the atom the more capable it is of swinging round 
a cathode ray which endeavours to pass it. Not only that, 
but the chances of the generation of X rays are also 
greater with the heavy atom. The density of the anti- 
cathode is of no consequence from this point of view ; it 
is only atomic weight which matters, for the cathode ray 
is never under the influence of more than one atom^&t a 
time. 

It seems certain from Whiddington's experiments (p. 125) 
that X rays are not formed from cathode rays unless the 
speed of the latter exceeds a certain critical value a value 
which increases with the atomic weight of the anticathode. 
It does not necessarily follow that even if it has the requisite 
speed, a cathode ray will ultimately come into suitable 
conflict with some atom and so generate an X ray. In 



106 X RAYS 

fact, the chances would appear to be against it, for the 
efficiency of the present methods of generation of X rays 
is very low : the X ray is merely a small bye-product in 
the energy transformations of a Rontgen tube. 

Wien, Angerer, and Carter (A.d.P. 1905 and 1906) have 
worked independently at the subject. They agree that the 
ratio of the energy of the X rays to that of the exciting 
(heterogeneous) cathode rays is of the order of T oVo ; Carter 
found that the efficiency increases with the hardness (is, in 
fact, proportional to the voltage on the tube), is independent 
of the current, but increases with the atomic weight of the 
anticathode. This value of the efficiency is not inconsistent 
with the estimate of Eve and Day (P.M. 1912), who remark 
that, of the energy supplied to an ordinary X-ray bulb, not 
more than about 1 in 20,000 is contained in the X rays as 
measured by their ionising ability. 

The efficiency of a soft bulb is probably even less than 
this, for more energy is converted into heat with low-speed 
cathode rays. 

Beatty's Experiments. 

Beatty (P.R.S. Nov. 1913) has recently evaluated the 
energy of X rays in terms of the velocity of the parent 
cathode rays and of the atomic weight of the anticathode. 
Homogeneous rays of known velocity were sifted from a 
cathode stream by a magnetic-spectrum method, and fell 
upon one of a number of anticathodes affixed to a sliding 
tray. The X rays so produced passed through a sheet of 
Al foil 0*0002 cm. thick, and were completely absorbed in 
an ionisation chamber consisting of a cylinder over a metre 
long filled with the vapour of methyl iodide. The resulting 
(total) ionisation was taken as measuring the energy of 
the X rays. The ionisation current was balanced against 
a fraction taken from the primary cathode -ray current by 
a variable shunt, so that a null deflection was obtained in 
the electroscope (see p. 128). Thus reliable readings could 
be obtained even when the cathode-ray current was very 
irregular. 

The ionisation which the cathode rays would have pro- 



THE ENERGY OF X RAYS 107 

duced in methyl iodide vapour was deduced from the work 
of GJasson (p. 12) and Whiddingtoii (p. 10) on the passage 
of cathode rays in air. 

The result finally established by Beatty was 



E c 
where E x = energy of the X rays, 

E c = energy of the parent cathode rays, 
A = atomic weight of anticathode, 
ft = velocity of the cathode rays expressed as a 
fraction of the velocity of light (i.e. 
3 x 10 10 cms./sec.). 

Thus, for an anticathode of platinum (with an atomic 
weight of 195) and a bulb of medium hardness, with cathode 
rays of speed, say 7*5 x 10 cms./sec., 73^0 of the cathode- 
ray energy reappears as X rays. With slower cathode rays, 
or an anticathode of lower atomic weight, the fraction would 
be smaller. 

These results refer only to the " independent " or primary 
X rays (p. 127). When characteristic rays are excited, their 
effects have to be added to those given by the above formula. 
A formula comprising both classes of radiation has not yet 
been obtained. 



CHAPTER IX. 
SECONDARY RAYS. 

WHEN X rays strike a substance, three classes of radiation 
are given off in general : scattered X rays, characteristic 
X rays, and corpuscular rays. Of these, the first two are 
X rays, the third negatively charged electrons. 

The proportions of the three classes depend both on the 
substance and on the quality of the primary rays. With 
materials of low atomic weight, by far the greater proportion 
of the X rays, if of a penetrating type, is merely scattered. 
With the elements of the chromium-zinc group, most of 
the emitted radiation is characteristic. The heavy elements 
give off both scattered and characteristic rays. As to the 
corpuscular radiation, there is in general a more copious 
emission, mass for mass, from the elements of high atomic 
weight. 

We may consider in some detail each of these secondary 
radiations. 

SCATTERED X RAYS. 

All substances, when exposed to a beam of X rays, them- 
selves give out X rays, some of which are identical with the 
primary rays in quality, and can, in fact, be conveniently 
regarded as so many unchanged primary rays which have 
been merely "scattered" or deviated in direction by the sub- 
stance. Such scattered radiation may be readily perceived 
experimentally by exposing a body of low density to X rays 
and viewing it with a fluorescent screen which is itself 
shielded from the direct action of the rays from the bulb. 



SCATTERED X RAYS 109 

Scattering produced by Different Elements. 

Scattering occurs at all depths, and increases in amount 
with the thickness traversed by the rays. 

The elements of low atomic weight (up to sulphur) scatter 
very much the same, mass for mass ; but the heavier elements 
scatter proportionally more than the light elements, though, 
owing to the greater absorbing power of the denser elements, 
it may happen that less of the scattered radiation actually 
escapes. With elements of quite low atomic weight, such 
as carbon, by far the greater proportion of the emerging rays 
is merely scattered radiation, more especially if the primary 
rays are of a penetrating type. The scattered radiation 
from aluminium is a good deal less in- amount than that 
from vegetable or animal matter, in which cases it may 
amount to 90 per cent, or more. 

With the copper group of elements, the scattered radiation 
is so small in amount (sometimes less than -n^-o of the total 
radiation) that it is, for most purposes, negligible. 

Barkla has introduced a coefficient of scattering, s, which 
is defined similarly to the absorption coefficient A (p. 100). 
For an absorbing screen of low atomic weight, s is pro- 
portional to the density, />, of the scattering substance ; and 
the mass-scattering coefficient, s/p, has a constant value of 
0*2, according to Barkla, no matter what the quality of the 
X rays. Crowther (P.R.S. 1912) has found, however, that 
the scattering coefficient increases considerably with the 
atomic weight 1 ; and that while, for example, s/p (as deter- 
mined by him) is 0-27 for filter paper and - 28 for aluminium, 
it increases to 0*9 for copper and nickel, and 1*5 for tin. 

s /A, the fraction of the absorption coefficient due to 
scattering, becomes important when hard rays are sent 
through light elements. In those instances where scattering 
accompanies absorption, it is necessary to subtract the 
scattering term from the total absorption to obtain the true 
absorption. For example, in the case of the absorption 
of a homogeneous radiation, the apparent \/p has to be 
amended to (A -s)/p to give the true mass-absorption 
coefficient. 

1 See footnote on p. 194. 



110 



X RAYS 



XR^ys 



Distribution of Scattered X Rays. 

The scattered X rays are distributed in all directions, 
though not uniformly ; more are to be found in the back- 
ward and forward directions of the original beam than at 
right angles. 

Barkla and Ayres (P.M. 1911) and Owen (P.C.P.S. 1911) 
have experimentally verified, over a considerable angular 

range, the approximate truth of 
the distribution formula derived 
from Sir J. J. Thomson's theory 
of scattering : 

/=/,,,( l+cos 2 0), 

where I e is the intensity of the 
scattered radiation along the direc- 
tion angle 6 to the primary beam. 
Thus the " fore and aft " intensity 
is roughly twice that at right 
angles. Experiment shows, how- 
ever, that the expression is inade- 
<l ua te for small values of fl-the 
calculated values are too small 
and' that, moreover, the forward intensity always exceeds 
the backward, sometimes largely. Fig. 56 shows the distri- 
bution obtained by Crowther (P.E.8. 1912) in the case of an 
aluminium plate. 

The scattering of X rays bears a strong resemblance to 
the scattering of light by fog (see p. 207). 

Polarisation of Scattered X Rays. 

In 1905, Barkla (P.T.) found that the scattered X rays 
from a plate bombarded by primary X rays do not distribute 
themselves quite uniformly in a plane at right angles to 
the line of flight of the primary rays, but tend to congregate 
in one particular plane passing through the line of flight of 
the primary rays. This plane of maximum intensity is at 
right angles to the path of the cathode rays in the generating 
X-ray tube. The intensity of the scattered rays falls off 
on either side of this plane, and reaches a minimum in a 




SCATTERED X RAYS 



111 



perpendicular plane (see Fig. 57). Thus, if X rays were 
visible, an observer, looking along the beam of primary 
X rays at the plate, would notice that the scattered rays 
would be brighter in two opposite quadrants than in the 
intervening quadrants. 

The distribution of the scattered rays thus reveals a 
peculiarity in the primary rays ; evidently they also pre- 
dominate in one plane, and hence may be said to be polarised. 
If the scattered rays themselves are allowed to fall on a 
second radiator, the asymmetry or " polarisation " is more 
complete in the resulting twice-scattered rays. 




Rd.dia.ror 



Min. 



FIG. 57. To explain polarisation of X rays. 

Prof. Barkla measured his rays by an ionisation method. 
His results have been confirmed and extended by Haga 
(A.d.P. 1907), Bassler (A.d.P. 1909), Herweg (A.d.P. 1909), 
and Vegard (P.E.S. 1910), some of whom employed photo- 
graphic methods. 

In experimental work, it is convenient to use radiators 
which do not possess a marked characteristic radiation. 
The characteristic radiations from materials of very low or 
very high atomic weight are either very soft or absent 
altogether ; and so carbon, paraffin wax, aluminium, lead, 
and metals of the platinum group are ordinarily to be 
preferred to metals of the chromium -zinc group, which 
possess pronounced characteristic radiations. The charac- 
teristic radiations are not polarised, at least not to any 
appreciable extent, and their presence only serves to mask 
the results. 

The polarisation in the case of carbon or wax amounts 






112 X RAYS 

to about 10 per cent., i.e. the maximum intensity is about 
W times the minimum. By filtering out the soft rays from 
the primary beam by the use of a suitable screen, the polari- 
sation can be doubled. Hardening the primary X-ray tube, 
however, apparently diminishes the effect. 

Ham (P.R. 1910) has also proved that primary X rays 
are polarised, by direct measurement of the intensity in 
different directions from an X-ray bulb. He found that 
the intensity reaches a maximum in a plane through the 
anticathode at right angles to the cathode stream. The 
intensity decreases symmetrically on either side of the 
maximum. Ham used a lead anticathode, and Miller (P.R. 
1911) confirmedShis results with a silver target. (Cf. p. 45.) 

CHARACTERISTIC OR "MONOCHROMATIC" X RAYS. 

The discovery by Barkla and Sadler in 1908 (P.M.) of 
the various characteristic radiations ranks as of the first 
importance. From an experimental point of view, the 
simplification brought about by the use of these radiations 
can hardly be overestimated. An ordinary X-ray tube 
generates a mixture of rays of many qualities, and the 
interpretation of the results obtained with such hetero- 
geneous rays is correspondingly difficult. But by allowing 
X rays to fall on different metals 1 copper, silver, iron, 
platinum, etc. characteristic X rays of uniform quality are 
given off which comprise a wide range of qualities. The 
quality of each of these radiations depends on the metal 
alone, and not at all on the exciting X rays. The only 
proviso is that the exciting rays shall be harder than the 
characteristic radiation : if the primary rays are too soft, 
no characteristic radiation is generated. 

When precautions are taken to eliminate the effects 
of the scattered and corpuscular rays, it is found that all 
the characteristic radiations are homogeneous and, unlike the 
scattered radiation, are uniformly distributed round the 
radiator. The penetrating power of a characteristic radia- 
tion increases with the atomic weight of the element from 

1 See, for example, Fig. 60, 



CHARACTERISTIC X RAYS 



113 



which it is emitted, so that the characteristic radiation of 
any atom can excite the corresponding radiation of a lighter 
atom, but not that of a heavier atom. 

K and L Series of Radiations. 

Experiment has shown that some elements give out at 
least two characteristic radiations under suitable conditions. 




Atomic Weighr of MetdJ emitting R&diaJion 

FIG. 58. Relation between mass-absorption coefficient (in Al) of a characteristic 
radiation and atomic weight of metal emitting radiation. The characteristic 
7 rays are included (see p. 118). 

Barkla has called these two types, series K and series L 
fluorescent radiations. For each metal, the K radiation is 
something like 300 times more penetrating than the L radia- 
tion. Both radiations become harder as the atomic weight 
of the radiator increases. With bodies of high atomic 
weight, the rays from an ordinary X-ray tube can only 

H 



114 



X RAYS 



excite the soft characteristic radiation L. From other 
substances, radiation K alone has been detected ; and from 
many light elements, neither radiation, up to the present. 
There is reason to believe, however, that all the elements 
give off both radiations, as well as other types not yet 
investigated. 

At the moment, series L Rontgen radiations have been 
got from the elements silver to uranium, comprising 
atomic weights from 108 to 239. It may here be remarked 
that if it is safe to extrapolate the Chapman formula 
(p. 116), it follows that no element with an atomic weight 
less than 48 can have an L radiation. Series K Rontgen 
radiations have been obtained over a range of elements 
from chromium to cerium (atomic weights 52 to 140). 




15 2-0 2-5 

Logarithm of Atomic Weigh!" of 

FIG. 59. Graph displaying Owen's 5th power relation between the log of 
the. mass-absorption coefficient (in Al) of a characteristic radiation, and the 
log of the atomic weight of the element emitting the radiation. (Of. Fig. 58.) 



CHARACTERISTIC X RAYS 



115 



i Owen's 5th-power Law connecting Quality of Characteristic 

Eadiation with Atomic Weight of Emitting Metal. 
In Table VIII. are given the qualities of the various 
characteristic radiations expressed as mass-absorption 
I coefficients in aluminium, together with the thickness 
of aluminium required to halve their intensity. Fig. 58 
displays graphically the relation between the quality of the 
radiation and the atomic weight of the emitting metal. 

TABLE VIII. MASS-ABSORPTION COEFFICIENTS (A//>) IN ALUMINIUM 
OF CHARACTERISTIC RADIATIONS. 

A. is defined by / I Q e~ xd (see p. 100) ; p is the density of aluminium 
(2-7) ; dj/2 is the thickness of aluminium required to reduce the 

radiation by one-half, and is calculated from the formula d = -1 , 

\/p 

which is derived from the above. The corresponding thickness of 
animal tissue (or of water) is about ten times that of Al. The values 
of \lp are due chiefly to Barkla, Sadler, Nicol, and Chapman. 



Element 
emitting 
characteristic 
Radiation. 


A/P in Al. 


*m 


Element 
emitting 
characteristic 
Radiation. 


A/P in Al. 


*//2 


Series A". 


Series/,. 


Series A". ISeries L. 


Series A". Series/,. 


Series A". Series L. 




cm. gm. 


cm. -.'in. 


cm. mi. 




cm. jfiii. cm. jftn. 


cm. cm. 


C (12) 








Rh (103) 






Na (23) 








Pd (107) 






Mg (24) 








Ag (108) 


2-5 700 


0-103 0-00037 


Al (27) 


580 


0-00044 


Cd (112) 


I 




Si (28) 








Sn (119) 


1-57 


0-164 


P (31) 








Sb (120) 


1-21 435 


0-212 0-00059 


S (32) 








I (127) 


0-92 300 


0-2S 0-00086 


Cl (35) 








Te (128) 






K (39) 








Ba (137) 


0-8 -2-24 


0-32 0-00115 


Ca (40) 


435 




0-00059 


Ce (140) 


0-6 


0-43 ! 


Ti (48) 








Ta (181) 






V (51) 








W (184) 30-0 


0-0086 


Cr (52) 


136 




0-0019 


Os (191) ! 




Mn (55) 


100 


0-0026 


Ir (193) 




Fe (56) 


88-5 


0-0029 


Pt (195) 22-2 


0-0116 


Co (59) 


71-6 


0-0036 


Au (197) i 21-6 


0-0119 


Ni (59) 


59-1 0-0043 


Hg (200) 




Cu (64) 


47-7 1 


0-0054 


Tl (204) 




Zn (65) 


39-4 




0-0065 


Pb (207) 17-4 


0-0148 


As (75) 


22-5 


0-0114 


Bi (208) 16-1 


0-016 


Se (79) 


18-5 


0-0139 


RaB (214) ' 14-7 


0-0175 


Br (80) 


16-3 


0-0157 


RaC (214) ' 0-042 6-1 


Kb (85) 
Sr (88) 


10-9 
9-4 




0-023."> 
0-027 


lo (230) ' 
Th (232) 


8-35 
8-0 


0-031 
0-032 


Zr (91) 








U (238) 


7-5 


0-034 


Mo (96) 


4-8 




0-053 








Rii (102) 















*X rays (see p. 118). 



116 X RAYS 

The general resemblance between the K and L curves 
will be remarked. If the logarithms of both coordinates 
of Fig. 58 are plotted, the result is two straight lines 
(Fig. 59), the slope of which indicates that the pene- 
trability is roughly proportional to the 5th power of the 
atomic weight of the radiator. This result was first estab- 
lished by E. A. Owen (P.R.S. 1912) l from his experiments 
on the absorption of characteristic rays in light gases (see 
p. 136). If similar logarithmic curves were plotted for any 
other absorber than aluminium, the resulting straight lines 
would be parallel to the aluminium lines, except in the 
regions of selective absorption (see p. 129). 

Whatever the physical significance of Owen's remarkable 
relation may prove to be, there is no doubt as to its utility 
in inferring the absorption coefficients of radiations as yet 
undiscovered. 

Relation between K and L Series. 

The characteristic radiations obviously correspond to the 
lines in an optical spectrum ; and the well-known series 
relations between the wave-lengths of associated spectral 
lines suggested the probability of some such relation between 
the hard (K) and soft (L) series of X radiations. Whidding- 
ton (N. 1911) was able to derive a simple empirical relation 
connecting the penetrating powers of the two radiations 
with the atomic weight. ^If an element of atomic weight 
A L possesses a soft (L) radiation of a certain hardness, then 
the atomic weight (A K ) of the particular element whose K 
radiation is of the same hardness, is given by 



Chapman (P.R.S. 1912), having investigated the L radia- 
tions from the metals of high atomic weight, derived the 
expression 



which fits in more closely with the observed results. 

As an example of the application of this formula, bismuth 
with an atomic weight of 208 is found to have a soft radiation 

1 See also Kaufmann (P. Z. 1913). 






CHARACTERISTIC X RAYS 



117 



which is of the same penetrating power as the hard radiation 
from bromine (with an atomic weight of 80[ = |(208 -48)]). 

Characteristic Radiations from the Heavy Elements. 

\Yith elements of high atomic weight, the scattered radia- 
tion may be so excessive as to mask or even swamp the 
characteristic radiation. Chapman (P.R.S. 1012), in an 
investigation of the char- 
acteristic rays of the 
heavy metals (from tung- 
sten and platinum to ^ ,'i ^ Lea.d 
uranium), attacked the g j | E, > 
difficulty by choosing an 
exceedingly penetrating 
beam of X rays, so that 
the scattered radiation 
was very much harder 
than the characteristic 
radiation. The hetero- 
geneous mixture of char- 
acteristic radiation and 
the superposed scattered 
radiation was examined 
in the usual way by a 
series of aluminium ab- 
sorption-screens of in- 
creasing thickness, the 
result being gradually to remove the characteristic radiation. 
Ultimately, the residual rays consisted almost wholly of 
scattered radiation ; its amount was thus revealed, and 
could be applied as a correction to the earlier observations 
with the thinner screens. In this way, the absorption 
curve of the characteristic radiation was ascertained ; it 
revealed the homogeneity common to all such radiations. 

Chapman (whose apparatus is shown in Fig. 60) worked 
with very thin radiators, and by so doing minimised the 
scattered radiation. For the homogeneous radiation, being 
soft, emerges only from a small depth : if this thickness is 
exceeded the result is merely to increase the proportion of 




,"Tesh'ng Screen 
(Aluminium) 



E I ec h-oscope 



/ /Comparison 
" ^"""^ E I ec h-o s cope 



FIG. 60. Chapman's apparatus for investigat- 
ing the characteristic radiations of the heavy 
metals. 



118 X RAYS 

scattered radiation which is able to emerge from deeper 
layers. 

Radiations other than K and L. 

It has long been suspected that the K and L radiations 
are not, by any means, the only homogeneous radiations 
that an element can emit ; and recently, evidence of the 
truth of this has been forthcoming. Bragg (P.R.S. 191 3), 
in the course of his crystal-reflection experiments (see 
p. 189), has shown that, for instance, platinum gives out 
at least three homogeneous components, having \/p in 
aluminium equal to 35*5, 23*7, and (approx.) 11 respectively. 
Moseley and Darwin (P.M. 1913) similarly found for 
platinum, values 34, 22, 19, and 16, as well as a fifth 
weaker component. The second of these tallies with the IA 
radiation (\/p = 22' 2) from platinum. 

In more recent work (pp. 198-201) Moseley and Bragg have; 
obtained similar results for a variety of metals ; and Laub 
(P.Z. Oct. 1913) has described experiments dealing with' 
what he calls the " / " radiations, of which A//o A1 in the case^ 
of the iron radiation is 43'9, copper 23'8, and zinc 18'5. 

Characteristic y Radiations from Radioactive Elements. 

Rutherford has recently brought forward evidence that 
the many and various groups of homogeneous y rays which 
radium emits can be regarded as so many characteristic! 
X radiations produced by the expulsion of p particles. Some 
of these groups correspond to the K and L radiations, others 
are much harder, and others, yet again, are softer. For 
example, the y rays from RaC are homogeneous and have 
a value of \/p in Al = 0*0424, which finds a place among 
the K series corresponding to the atomic weight, 214, of j 
RaC. Again, Rutherford and Richardson (P.M. 1913) have 
shown that among the various groups of y rays from RaB 
are three which have values of V/o A1 =85, 14*7, and - 188, 
the second of which appears to be the L radiation for an] 
atomic weight of 214. Further, Chadwick and Russell 
(P.R.S. 1913) found that among the y rays from ionium 
(atomic weight 230) are homogeneous components having 



CHARACTERISTIC X RAYS 119 

8-35, and 0*15 respectively. The second of 
these corresponds to the L series for a metal with an atomic 
weight of 230. A summary of the various characteristic 
7 rays is given on p. 234. 

Incidentally, we may record Gray's observation (P.B.S. 
1912) that the y rays of RaE are capable of exciting the 
K radiations of a number of metals including silver, lead, 
and barium. 

Very soft X Rays. 

Ordinary X rays are usually produced by potentials of 
between say 10,000 to 100,000 volts or more, but it is possible 
to generate X rays with much lower voltages. Such X rays 
are soft and have only a short range in air, but they have 
much the same general properties as harder rays. 

Dember in 191 1 gradually increased, by means of an electric 
field, the speed of the photo-electrons liberated by ultra-violet 
light, and found that they were capable of exciting X rays 
from a platinum anticathode when the speed reached 
that equivalent to 250 volts. From more recent work 
(D.P.G.V. July 1913) Dember lowers this voltage to 18'7, 
and calculates that the X rays produced have a wave-length 
of 7 x 10 ~ cm., which is just beyond the ultra-violet. 

Seitz (P.Z. 1912) got X rays from Pt with voltages ranging 
from 400 to 900 volts. Wehnelt and Trinkle (Sitz. Phys. 
Med. Soc. Erlangen, 37, 1905) generated soft X rays by the 
use of slow cathode rays from hot-lime cathodes (p. 8) 
excited by voltages of from 400 to 1000 ; while Whiddington 
(P.C.P.S. 1912), using similar means and potentials from 
130 to 220 volts, was able to detect the X rays from the 
residual gas in the discharge tube. 

Characteristic Light Rays. 

When a variety of qualities of X rays is allowed to 
penetrate a substance, it is found that the absorption 
becomes abnormally large for the particular qualities of ray 
which are capable of exciting the characteristic radiations of 
the substance (see p. 129). Such selective absorption is accom- 
panied by a large increase in the corpuscular emission. This 



120 X KAYS 

selective X-ray effect is on all fours with the selective photo- 
electric effect discovered by Pohl and Pringsheim (D.P.G. V. 
1911 and 1912) in the case of ultra-violet light. They have 
ascertained that the emission of electrons produced by the 
impact of ultra-violet light on metals reaches a maximum 
for a particular wave-length of the light : the positions of 
these " absorption bands " for the alkali metals are approxi- 
mately as follows : 

Wave-length. 

Li 2800 A.u. 1 

Na 3400 

K 4400 

Rb 4800 

Thus these several ultra-violet radiations may be regarded 
as soft characteristic X rays of the different metals. 

It is worth while adding the remark here that Pohl and 
Pringsheim have succeeded in carrying the photoelectric 
effect well into the infra-red (10,000 A.U.). 

Characteristic X Eays are independent of Chemical Combination. 

Chapman and Guest (P.C.P.S. 1911) showed that the 
intensity of the characteristic X radiation from a metal was 
the same, no matter whether the metal was combined or 
not. For example, a given weight of tin continued to give 
the same quantity of characteristic tin rays after it was 
converted into the nitrate. Thus, in common with all X-ray 
phenomena, the effect is a purely atomic one. 

Chapman (P.M. 1911) found further that the vapours of 
methyl iodide and ethyl bromide gave out strong radiations 
characteristic of iodine and bromine respectively when struck 
by hard X rays. 

Glasson (P.C.P.S. 1910) noticed that the quality of the 
characteristic radiation from, say, iron was independent of 
whether the iron was free or combined ; in the latter case, 
neither the valency nor the position of the ion was material, 
e.g. FeSO 4 , Fe 3 O 4 , Fe 2 3 , K 4 Fe(CN) , all excited iron rays 
of the same quality. 

1 One A.U. (Angstrom Unit) is 10~ 8 cm. 



I 

CHARACTERISTIC X RAYS 1:M 

The Direct Generation of Characteristic Rays. 

By the ordinary " reflection " method, using a radiator 
at 45 to the primary beam of X rays, 1 the amount of 
energy which is transformed into characteristic radiation 
does not at the most reach 50 per cent. ; and of this, only a 
fraction, say TO, manages to escape from the surface of the 
radiator. Thus the arrangement is very inefficient as a 
source of characteristic rays. The writer showed, however, 
in 1908 (P.T.) (see p. 36) that a large proportion of the 
radiation from the anticathode of an X-ray bulb may consist 
of the characteristic radiation of the metal of the anticathode, 
more especially if the bulb is soft. By the employment of 
screens of the same metal as the anticathode. the other radia- 
tions present are either absorbed or transformed into the char- 
acteristic radiation, the result being an intense and almost pure 
beam of characteristic rays . The potential on the tube should 
not be too high, otherwise the proportion of heterogeneous 
primary rays in the emitted beam will increase in amount. 

Figs. 61 to 64, taken from the above paper, give the log- 
absorption curves for three such different metals as Al, Cu, 
and Pt. The homogeneity of much of the radiation, when 
screen and anticathode are alike, will be apparent. With 
all three metals, there is a superposed softer homogeneous 
radiation, which is removed by quite thin screens. In the 
case of copper, the K radiation shows up prominently. 

Figs. 63 and 64 show the way in which the radiation from 
a platinum anticathode is absorbed by aluminium and 
platinum screens respectively. In the former case, the 
(thick) absorption curve betrays no apparent homogeneity 
in the rays. It is, however, possible to analyse the curve 
into three homogeneous components, having \/p =5'6, 23' 7. 
and 70 respectively. These are represented both in amount 
and hardness by the three thin lines. The hardest is probably 
independent radiation, the second proves to be the character- 
istic L radiation of platinum. With the platinum screen, 
the independent radiation has disappeared, and the absorp- 
tion curve shows that the X rays transmitted by a screen 
O'OOOo cm. thick are almost entirely homogeneous L rays. 

1 See, for example, Fig. 60. 



122 



X RAYS 



Al Anhc&thode , Al Screen 
7mm Sp<i.rk, 23000 voirs. ' 




o 01 cm o 02 o-oa 

Thickness of Screen 



0-0+ 



005 



FlU. 61. The heavy curve is the log-absorption curve, for an Al screen, of 
the X rays from an Al anticathode. The curve can be resolved into two 
homogeneous components (indicated in amount and absorbability by the 
two thin straight lines). 



2-0 



O I-O 



<d 

oo 

o , 




Cu A nl~ic^l"hode,Cu Screen 

7mm. Spa.rk ,23000 volrs. 



\ 



(K 




looy. 
eo% 



50% 



2O% 



10% c 
0) 



5% 



z% 



Q 0-OOfiCm 0-OO4 O-O06 0-OOP O-OIO 

Thickness of Screen 

FIG. 62. The heavy curve is the log-absorption curve, for a Cu screen, of 
the X rays from a Cu anticathode. The homogeneity of much of the radia- 
tion is apparent. The curve can be resolved into two homogeneous com- 
ponents (indicated in amount and absorbability by the two thin straight 
lines), one of which is the K radiation of Cu. 



CHARACTERISTIC X RAYS 



123 



of Inrens i t~y 

i ro 

> w o 


V 


Pt Anhcfclhode , Al Screen 

7mm.Spa.rk, 23OOO volrs. 


8O% 
50% 

30% 
20% 

to 

C 

c 

57 
2% 


V] 


N~ 








| 

1. 

O" 

.3*5 

1 

th 


-A X = 2 

\ Ve 

\ 


^ 

OO(evpprox.) 
- 70 (4>pprox 


\^^^ 






>\ 

\ ^ 


Vp=23-7 (L 


RcvdioJMon) 


3 0-02 cm. 0-04- o-oe o of* o-io 
Thickness of Screen 

FIG. 63. The heavy curve is the log-absorption curve, for an Al screen, of 
e X rays from a Pt anticathode. The curve can be resolved into three 



homogeneous components (indicated in amount and absorbability by the three 
thin straight lines)., one of which is the L radiation of Pt. 





\ 


Pr AnMco^rhode , Pr Screen 


80% 


w 

c 


\Vh 


7mm 5pArk,23OOO volrs 


50% 

3O7 
20% 


\ 


V 








c 
"o i-o 

J , 


\ 


X 








IO% 




< 
= 6500 


\ 






T 

XVI 

Jo-s 


X e =300 


^^X- 2300 
\^Vp-l07(LRAd. A hon) 


5 % 
27. 




\ 




N 


X 



OOOO4CIT1 O OOO8 OOIZ 

Thickness of Screen 



00020 



FIG. 64. Conditions as in Fig. 63, except that the Pt X rays are absorbed 
by a Pt screen instead of Al . The homogeneous L radiation now predominates 
in the absorption curve, which is resolvable into the L radiation and a second 
softer component. 



124 



X RAYS 



Quality of Characteristic Rays in terms of Parent Cathode Ray 

Velocity. 

The writer in 1909 (see J.Rt.8. 1913) attempted to associate 
the hardness of the characteristic radiation emitted by an 
anticathode, with the speed of the cathode ray required to 
excite the radiation. The underlying notion was that unless 
the cathode rays possessed a velocity greater than a certain 

critical value, no charac- 
teristic rays would be 
generated. If this were 
so, the X rays could, so 
to speak, be labelled in 
terms of the speed of the 
exciting cathode rays. 

Obviously a first sim- 
plification was to work 
with cathode rays of uni- 
form speed. This can be 
done by the use of either 
(1) an influence machine 
or (2) a magnetic-spectrum 
method applied to a coil 
discharge. In the latter 
plan, the cathode ray 
energy is, at suitable 
pressures, largely con- 
centrated in the fastest 

cathode rays (see p. 16) ; and the method had other 
obvious advantages which led to its adoption. 

The apparatus is indicated in Fig. 65. The cathode 
rays from C were spread by a magnetic field into a mag- 
netic spectrum, plainly visible along the plate anode, AS, 
which was coated with willemite. By varying the 
strength of the field, any part of the spectrum could be 
brought over the slit, 8. The pencil of cathode rays which 
passed through S impinged on the anticathode, T, below, and 
a bundle of X rays passed out through the thin aluminium 
window, W, and was measured by an ionisatiori method. 
Some half-dozen anticathodes were mounted on a trolley as 




FIG. C,~>. Apparatus for showing production of 
X rays with cathode rays of varying speed. 



CHARACTERISTIC X RAYS 



125 



described on p. 36. The additional cathode K was pro- 
vided to bombard the anticathodes so as to liberate the 
occluded gas, which otherwise, by its continued emission, 
softens the tube during the actual measurements. 

The experiments, which were arrested soon after their com- 
mencement, served, however, to show the extreme inefficiency 
of the slowest cathode rays as producers of X rays. As the 
different parts of the cathode spectrum were passed over 
the slit, and faster and faster cathode rays were brought 
into action, the rapid gain in the intensity of the X rays 
was very noticeable. The increase in intensity came in quite 
suddenly for some one speed of the cathode rays which did 
not appear to be the same for the different anticathodes 
employed. 

Whiddington's Experiments. 

In 1010 Whiddington carried out a research on somewhat 
similar lines, and obtained quantitative measurements of 



To E^rfh 



Br^ss Cylinder 



Solenoid for 
Magnetic 
Field 




WVWV jljl 



TbEwrh 



To Electroscope ^ 



FIG. 66. Whiddington's apparatus for connecting the speed of cathode rays 
with the quality of various characteristic radiations. 

great importance (P.R.S. 1911) for the K radiations of a 
number of elements. His final apparatus is shown in Fig. 
06. The cathode-ray spectrum was produced by a solenoid 



126 X RAYS 

which yielded a uniform and calculable magnetic field. The 
anticathode was of silver, and the generated X rays struck 
a secondary radiator. The speed of the cathode rays was 
increased (by the hardening device described on p. 70) 
until the secondary radiator emitted its characteristic radia- 
tion, which of course was duly indicated in the ionisation 
chamber. Below this critical value of the velocity, there 
was little effect in the chamber ; above it, the ionisation 
current grew very rapidly. Thus the cathode ray in the 
X-ray tube must possess a minimum velocity if it is to 
excite an X ray of given quality. Different radiators were 
tried, and the critical velocity was found to be roughly pro- 
portional to the atomic weight of the radiator : in point of 
fact, the speed in cms. per sec. was 100 million (10 8 ) times 
the atomic weight. Beatty (see next page) has since 
shown that the same result is true if the metal, instead of 
being used as a secondary radiator, is employed as an anti- 
cathode, as in Kaye's arrangement. 

Thus, to recapitulate, if V R is the critical velocity of the 
cathode rays in cms. per sec., and A is the atomic weight 
of the anticathode, then in the case of the K series of radia- 
tions, the empirical relation 

V K =A . 10 8 

is approximately satisfied for a range of elements from Al 
to Se. 

By combining this expression with Chapman's formula 
(p. 116), it follows that for the L series 



In Table IX., Whiddington's experimental values for the 
K radiations are given in heavy type in columns 3 and 5. 
The values for the other K radiations and the whole of the 
L radiations are calculated by the formulae above. It must 
be understood that many of these radiations have not yet 
been discovered (see p. 1)5). 



CHARACTERISTIC X RAYS 



127 



TABLE IX. MINIMUM SPEED or CATHODE RAYS REQUIRED TO 
EXCITE CHARACTERISTIC RADIATIONS. 



Radiator. 


Atomic Weight 
(0 = 16). 


Critical Velocity of 
Cathode Rays to excite 


Requisite Potential to 
impart Critical Speed 
to Cathode Rays. 1 


K radiation. 


L radiation; 


K radn. 


L radn. 




cm./sec. 


cm./sec. 


volts. 


volts. 


Hydrogen - 


1-01 1-0 xlO 8 





3 





Carbon 12-0 1-2 x 10 9 





410 





Aluminium 27-1 


206 





1200 





Chromium - 52-0 


509 ., 


2-0 x 10 8 


7320 


11 


Iron - 


55-8 5 83 


3-9 


9600 


43 


Nickel 


58 7 617 


54 


10,750 


80 


Copper 


636 626 


78 11,080 


170 


Zinc - 


654 632 


87 11,280 


210 


Selenium 


79-2 


738 


1-56 xlO 9 15,400 


690 


Rhodium - 


102-9 


03 x 10 10 


27 29,900 


2,100 


Silver 


107-9 


08 


3-0 33,000 


2,500 


Tin - 


119-0 


19 


3-6 40,000 


3,600 


Tungsten 184-0 


84 


6-8 95,000 


13,000 


Platinum - 195-0 


95 


7-4 108,000 


15,000 


Lead - 207-1 


2-07 


8-0 120,000 


18,000 


Uranium 


238-5 


2-38 


9-5 160,000 


26,000 



Energy of an X Ray. 

By slightly modifying the arrangement, and putting the 
ionisation chamber in place of the secondary radiator, 2 
Whiddington was able to correlate the energy of the X rays 
with the velocity of the parent cathode rays, and so to 
establish the truth of a relation deduced theoretically by 
Sir J. J. Thomson in 1907, that the energy of an X ray is 
proportional to the fourth power of the velocity of the 
exciting cathode ray. Beatty (P.R.8. 1913) has recently 
proved that this relation is only true for " independent " 
X rays : if characteristic rays are generated, the expression 
no longer holds (see p. 107). 

Beatty's Experiments. 

Beatty (P.R.S. 1912) has shown that the bulk of the 
characteristic rays generated in Kaye's experiments (p. 121), 

1 See p. 96 for relation between cathode-ray speed and potential. 

2 See Fig. 66. 



128 



X RAYS 



900 



is due to a direct transformation of the cathode radiation 
into characteristic radiation ; and that only a small re- 
mainder owes its origin, 
as one would perhaps 
infer, to the indirect 
action of primary X rays 
in emerging from beneath 
the surface of the anti- 
cathode. Beatty obtained 
cathode rays of uniform 
speed by means of the mag- 
netic-spectrum method, 
and was able to show 
that the direct and in- 
direct effects occur simul- 
taneously as soon as the 
speed of the cathode rays 
exceeds the critical value 
(see p. 126). Fig. 67 shows 
for the case of a copper 
antic athode, the relative 
amounts of characteristic 
copper radiation generated 
directly and indirectly by 
cathode rays of different 
velocities. Both effects 
disappear if the speed 

falls below 6*25 xlO 9 cm. /sec., a value which agrees very 
closely with Whiddington's critical speed for copper. 

To overcome the difficulties of measuring the X rays due 
to the vagaries of a coil discharge, an ingenious null method 
was devised which consisted in balancing the current in the 
ionisation chamber against part of that carried by the 
cathode-ray discharge. Fig. 68 shows the connections. 
The interior of the antic athode tube A was lined with 
aluminium and joined to the anticathode. The greater part 
of the cathode ray current passed to earth through the 
variable resistance P. A smaller fraction passed through 
the high resistance Q to the ionisation chamber. The 




6x10* 7*IO 9 

Speed ofCd^rhode Po-hcles 
in cms. pffr sec. 

FIG. 67. Showing relative amounts of charac- 
teristic X rays generated directly and indirectly 
from a copper anticathode by cathode rays of 
a variety of speeds. 



CHARACTERISTIC X RAYS 



129 



resistance P was altered until the latter current just 
neutralised the leak in the ionisation chamber. Of the 
current leaving A, P/(P-\-Q) goes to the chamber. Since 



CAfhode Kays 
of known Spec 



AnhcAt-hode 




To Electroscope 



FIG. 68. Beatty's apparatus for measuring the characteristic X rays generated 
directly and indirectly by cathode rays of various speeds (see Fig. 67). 

was very large of the order of 10 12 ohms we may 
ite this, P/Q. Thus the relative intensity of the X rays 
evaluated by determining P in each case. 



ABSORPTION OF CHARACTERISTIC RADIATIONS. 

;tive Absorption of Certain Qualities of Radiation by a 
Particular Element. 

It is found that an element exhibits a maximum trans- 
>arency for X rays of a quality identical with that of either 
of its own characteristic radiations ; and, further, the 
absorption becomes abnormally large for X rays which 
have a penetrating power just greater than that of either 
of the characteristic radiations. For example, if the absorp- 
tion in copper is measured for a variety of homogeneous 
X radiations from, say, calcium rays to cerium rays, then 
if we start with the soft Ca rays and pass up the series, 
the absorption in copper steadily diminishes in normal 
fashion as the ravs are hardened. But as soon as the 



130 



X RAYS 



stage is reached when the X rays have become as pene- 
trating as the Cu radiation, the absorption slows up, and 
reaches a minimum. With slightly harder rays the Cu (K) 
radiation is excited, and the absorption then rapidly in- 
creases. As the incident rays are hardened still further, 
the absorption begins once more to diminish, and eventu- 
ally reassumes the normal type, steadily lessening as the 
hardness increases, but now more rapidly than was the 
case before the " loop." 



C <D 

.2 b 
a_S 



L Absorption 




.$*** ! of Element 



of Element 



QuaJify of X FUy ( >/? in Alj 
> Increasing W&ve-lengl'h 

FIG. 69. Diagrammatic representation of the absorption by a particular 
element of a range of qualities 9f X rays. The absorption reaches a minimum 
for rays identical in quality with either of the characteristic radiations : for 
rays a little harder than these, the absorption is abnormally high. Thus, 
in these regions there is selective absorption, but elsewhere the absorption 
is normal. 



These various phenomena are displayed in Fig. 69, which 
shows, for a range of qualities of incident X rays, the two 
loops in the absorption curve of an element which possesses 
both K and L radiations. It must be emphasised that 
Fig. 69 is purely diagrammatic and not at all to scale. As 
yet, the complete absorption curve for any single element 
has not been obtained. The corresponding curve for an 
absorbing metal of lower atomic weight would have the 
loops shifted to the right ; for one of higher atomic weight, 
to the left. 

This selective transmission of the various characteristic 



CHARACTERISTIC X RAYS 



131 



radiations was well displayed in Kaye's experiments x on 
the direct generation of characteristic rays from anticathodes 
of X-ray bulbs. Table X. shows the effect of interposing 
the same metal screen in turn in the path of the X rays 
from various anticathodes. It will be noticed that, in most 
cases, the intensity of the transmitted rays is greatly aug- 
mented when the anticathode is of the same material as 
the screen. To provide a basis of comparison for the results 
for each metal screen, the intensity of the transmitted 
radiation from the Al anticathode is called 100 in the table. 

TABLE X. SHOWING SELECTIVE TRANSMISSION OF VARIOUS 
RADIATIONS. (Al radiation = 100.) 

Screen of 



-V rays ironi auucauioae 01 


Al 


Fe 


Ni Cu 


Pt 


Aluminium (27) - 


100 


100 


100 


100 


100 


Iron (56) 


160 


600 


340 


380 


160 


Nickel (59) 


180 


200 


740 


570 


220 


Copper (64) 


210 


210 


810 


740 


270 


Platinum (195) - 


530 


450 


480 


480 


670 



The selective absorption phenomena illustrated in Fig. 

69 are further revealed from a scrutiny of Table XI., or, 

better still, from Table XII. In Table XI. are put out 

the mass-absorption coefficients of a number of characteristic 

radiations in various absorbers, while in Table XII. the 

values are, in every case, relative to the absorption in 

! aluminium. Table XII. is, of course, immediately derivable 

ifrom Table XI. Barkla and Collier (P.M. 1912) have 

, pointed out that the shape of either of the absorption loops, 

i indicated roughly in Fig. 69 for some particular absorber, 

i is not only similar to, but identical. with, the corresponding 

I loop for any other absorber, provided proper choice is made 

i of the scales of coordinates for each absorption curve. This 

i may be secured by arranging that the particular ordinate 

1 corresponding to the characteristic radiation of the absorber, 

both occupies the same position and has the same length 

1 See p. 121, and P.C.P.S. May 1907. 



132 



X RAYS 



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iO CO CO <N <N 



A 



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I CO I-H <M (N CO CO 



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^ OS l> O 



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I I> l> Oi t> CO O 
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CO (M rH r-H 



i-HOOO 





CHARACTERISTIC X RAYS 



133 



CO O CO 00 
I "* FH CO I> l> CO 

1 




134 X RAYS 

in all the different curves. In other words, by arranging 
the scales of absorption and wave-length so that the absorp- 
tion of Fe (K) radiation in Fe, of Cu (K) radiation in Cu, 
etc., are all represented by the one point (^4) on the graph, 
the various absorption curves in the region of the K loop 
will coincide if superimposed. And similarly for the L 
loop. 

Barkla and Sadler's Relation for Normal Absorption. 

The constant absorption-ratio that exists, no matter what 
the hardness of the X ray, for each absorber outside the 
range of selective absorption, is well displayed in Table XII. 
Some of the departures from proportionality with the harder 
types of rays, in what should be regions of normal absorp- 
tion, are due to the fact that the coefficients for many of 
the elements given in Table XI. have not been corrected 
for scattering. If we make the proper scattering correction 
(p. 109), it is found that the ratio of the absorptions of a 
radiation in any two particular elements is approximately 
constant, and does not depend on the quality of the radiation, 
provided only that such radiation does not excite the 
characteristic radiations of either element. This important 
relation was first pointed out by Barkla and Sadler. 

To take an example, Barkla and Collier have shown that 
in the case of carbon, the absorption values relative to 
aluminium which rise in Table XII. from O'll for soft 
rays to 0*41 for hard rays become, when corrected for 
scattering, a steady value of 0*11 for all types of X ray. 

Selective Absorption of a Particular Eadiation by Certain Elements. 

The selective absorption of X rays under certain con- 
ditions (to which we have referred above), is perhaps better 
brought out by Fig. 70 (taken from Barkla and Collier's 
paper, P.M. 1912), which exhibits the way in which the 
absorption of Ni (K) radiation in a number of elements 
varies with the atomic weight of the absorber. 

It will be noticed that, if we start with the light elements, 
the absorption of the Ni rays increases steadily with the 
atomic weight of the absorbing substance, so long as all 



CHARACTERISTIC X RAYS 



135 



the various characteristic radiations (K, L, . . .) of the 
absorber are excited. But as soon as the atomic weight 
of the absorber becomes so high that its K characteristic 
ceases to be excited (though the others remain), then the 
absorption suddenly drops. With higher atomic weights, 
the Ni radiation can only excite the L, M, . . . radiations, 
and so the absorption steadily increases until the stage when 
the Ni radiation no longer excites the L radiation, and the 



800 

2 

c 

Q) 

V 600 
UJ 

/J 

3 
O 

> 400 

C 

* 

c 

.2 200 

1_ 
Q. 

L 


<o 

jQ 

< n 




















A 
/ \ 






3 


A 


/ 


7 \ 


J> 


/ 

f" 


/ 


\W 

IK Ra.dia.hon 




^**r: 

L R&di&fion 



Q 40 8O I2O I6O 200 

Atomic Weighr of Absorbing Elemenf 

FIG. 70. Graph showing relation between the absorption of Ni (K) radiation 
by various elements and the atomic weight of the absorbing element. The 
absorption passes through a minimum for a screen of Ni and also for one 
of atomic weight of about 164 (whose L radiation is identical with the Ni (K) 
rays). Compare Fig. 54. 

absorption falls once more. In other words, the trans- 
parency reaches a maximum with a nickel screen and also 
with one whose L characteristic radiation is identical with 
the nickel K radiation. Thus the regular curve of increas- 
ing \/p with atomic weight of the absorber, is modified 
by the addition of sudden drops at as many regions as 
there are elements having one or other of their characteristic 
radiations identical with the X rays which are being 
absorbed. 

Similar curves are obtained for any other characteristic 
radiation : if the radiation is harder, all the maxima and 



136 



X RAYS 



minima are displaced to the right, and, if softer, to the 
left. 

It will be remarked that Fig. 70 is the analogue of Benoist's 
curve (p. 102) for heterogeneous X rays. Evidently, Be- 
noist's rays were rich in components approximating to the 
characteristic radiation of silver, and this explains the absorp- 
tion " loop " which is prominent in his curve for soft X rays 
and less pronounced in that for the harder rays. 

Absorption of Characteristic Radiations in Gases. 

E. A. Owen (P.E.8. 1912) measured the absorption of a 
number of characteristic radiations in light gases. To get 



To Electro 



Com pA.rison 
I onis^hon 
Chamber 




To Electroscope 




Silver Window 
AnHc*|-hode 
(wa^rer-cooled) 



FIG. 71. Owen's apparatus for measuring the absorption coefficients of 
characteristic X rays in gnsos. 

over the difficulty of working with the feeble radiations 
generated by the ordinary method of placing the radiator 
at an angle to the path of primary X rays, Owen employed 
the ingenious device of mounting a thin silver anticathode 



CHARACTERISTIC X RAYS 137 

as a window in the discharge tube. The various radiators 
were placed near the outside of the window, and, by this 
means, intense characteristic radiations were obtained. 
Fig. 71 shows the apparatus. The anticathode was soldered 
to the glass, through the intermediary of an electrolytic 
deposit of copper ; and to prevent the fusion of the anti- 
cathode by the cathode rays, it was watercooled. 

A pencil of characteristic (K) rays from a series of radiators 
ranging from iron to molybdenum entered an ionisation 
chamber through a parchment window. The pencil was 
sufficiently narrow to prevent it striking the electrodes, 
which, together with the whole of the inner surface of the 
chamber, were coated with paper. Both the nature and 
pressure of the gas in the chamber could be altered. A 
comparison chamber enabled the vagaries of the coil dis- 
charge to be overcome. 

In some cases, e.g. SO 2 , the absorption at atmospheric 
pressure was very great for the softer rays, and measurements 
had to be made at lower pressures. Owen first showed that 
the absorption coefficients of any of the characteristic radia- 
tions in a gas varied directly with the pressure, as, of course, 
would be anticipated for a homogeneous beam. 

Further, the absorption coefficients of the different radia- 
tions in a particular gas proved to be proportional to the 
corresponding absorption coefficients in air, which goes to 
show that Barkla and Sadler's generalisation (p. 134) can be 
extended to gases. Owen was led to take the logarithms 
of the mass-absorption coefficients of the various radiations 
in a particular gas, and plot them against the logarithms 
of the atomic weights of the radiating metals. He found 
not only that the various observations all lay on a straight 
line, but that the different straight lines for the various 
gasas were all parallel to each other. The slope of these 
lines showed that the absorption coefficient of a radiation is 
approximately inversely proportional to the fifth power of 
the atomic weight of the radiator. Owen's fifth-power law 
is exemplified for an aluminium absorber on p. 114. 

Barkla and Collier (P.M. 1912) have also worked at absorp- 
tion in gases. Some of their results, as well as those of 



138 



X RAYS 









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SECONDARY CORPUSCULAR RAYS 139 

Owen, are incorporated in Table XIII., which gives the 
absorption coefficients (in cm.-gm. units) for air, carbon 
dioxide, and sulphur dioxide, ethyl bromide, and methyl 
iodide vapours. 

CORPUSCULAR RAYS. 

Curie and Sagnac (C.R. 1900) first showed that when a 
plate was struck by X rays, part of the secondary radiation 
was negatively charged. 

The detection of these high-speed corpuscles as they 
have proved to be is an easy matter, since the bombarded 
plate will charge up positively if it is insulated in a vacuum. 
By measuring the rate of charging, it would be possible to 
determine the number of corpuscles expelled per second ; 
while, by applying an electric force of suitable direction and 
magnitude, we could ascertain their speed. In actual prac- 
tice, this method of measuring velocities, while convenient 
for slow-speed electrons such as are liberated, for example, 
by ultra-violet light, is not very practicable in the case of 
X rays, owing to the magnitude of the potentials required. 

The magnetic-deflection method of determining high speeds 
is readily available if the stream of electrons is sufficiently 
intense ; and this method was actually employed by Innes 
and others in the case of the secondary X-ray corpuscles 
(see below). 

Distribution of Corpuscular Rays. 

The intensity of the corpuscular emission increases with the 
atomic weight. The corpuscles preponderate in a direction 
at right angles to the beam of X rays, which accounts for the 
fact (shown by Laub. A.d.P. 1908) that there are more cor- 
puscles liberated by a glancing beam of X rays than by a 
normal one. 

Furthermore, when X rays are sent through a thin metal 
plate more corpuscles are given off on the far or " emergence " 
side than on the near or " incidence " side. This is the more 
marked with hard X rays and with elements of low atomic 
weight. For example, Beatty (P.C.P.8. 1910) found with 
silver leaf, that while the excess amounted to no more than 



140 X RAYS 

2 per cent, in the case of Fe X rays x (which are very soft), 
it attained 30 or 40 per cent, with the harder tin and alu- 
minium rays. The speed is equally fast on both sides of 
the plate. On the other hand, Cooksey (P.M. July 1912) 
found that when the absorption of the exciting X rays in 
the layer of emitting metal is allowed for, the ratio of 
emergence to incidence corpuscular rays shows no certain 
variation either with the metal or the X ray. The excess 
emergence rays were of the order of 20 per cent, for both 
gold and silver plates, and for a range of rays extending 
from Cr X rays to Sn X rays providing an eighty-fold 
variation of penetrating power. 

It may be added that Kleeman (PM.S. 1910) and Stuhl- 
mann (P.M. 1911) found much the same value for corre- 
sponding experiments with ultra-violet light. 

Velocity of the Corpuscular Rays. 

Dorn in 1900 first measured the velocity of these negative 
rays, and in 1907 Innes (P.R.S.) made a more complete 
examination. In his experiments, the X rays fell on a 
metal sheet in a vacuum, and the corpuscles emitted passed 
in succession through a couple of slits in lead sheets and 
were recorded on a photographic plate. The whole of the 
apparatus was exhausted for the reason that the corpuscles 
are scattered at ordinary pressures, and their paths are too 
short to be followed. The velocity of the particles was 
ascertained from their deflection in a uniform magnetic 
field (produced by a pair of Helmholtz coils). Innes was 
able to establish the fact that the velocity of the corpuscle 
is independent of the distance of the X-ray bulb from the 
emitting plate a most important result. The distance was 
varied some eight or nine-fold without discernible effect on 
the speed obtained. Nor did alteration of the frequency 
or magnitude of the current through the X-ray bulb produce 
any change in the speed. 

But an increase in the spark-gap, i.e. in the potential 
applied to the bulb, evidenced itself at once by a speeding 
up of the corpuscles. This is plainly shown in Table XIV. 

1 I.e. the characteristic X rays from iron. 



SECONDARY CORPUSCULAR RAYS 



141 



below. To derive absolute velocities from the measure- 
ments, it is necessary to assume a value for e/m for the 
corpuscles ; this was taken to be T7 x 10 7 E.M.U. 

TABLE XIV. VARIATION OF SPEED OF CORPUSCLES WITH LENGTH 

OF SPARK-GAP. 



Metal emitting 
Corpuscles. 


Atomic 
Weight, 


Spark-gap. 


Velocity of Corpuscles. 


Zinc 
Silver - 


65 
108 


39 cm. 
3-9 


6-0 
3-0 


to 
to 


6-4 x 10'- 1 cm. /sec. 

7-2 


19-0 


61 


to 


8-0 






Platinum - 195 


32 


6-1 


to 


7-4 






ty 




14-0 


6-5 


to 


8-0 






Gold 


197 3-4 


6-1 


to 


7-5 






Lead 


15-0 
207 5-1 


6-2 
6-3 


to 

to 


8-1 

7-8 












16-0 


6-4 


to 


8-3 







It is apparent from the table that the nature of the 
material exerts little, if any, certain influence on the velocity 
of the corpuscles ; but a more definite pronouncement is 
possible from the later experiments of Beatty (P.M. 1910) 
and of Sadler (P.M. 1910), both of whom used characteristic 
X rays. 

Absorption of Corpuscular Rays by Gases. 

Sadler has shown that the corpuscular radiation excited by 
characteristic rays is of uniform quality for any particular 
metal and follows an exponential law of absorption. This 
is true no matter what the metal or the characteristic rays. 
Thus, in each case, the corpuscular rays have a definite 
absorption coefficient, the value of which Sadler found to 
be proportional to the atomic weight of the metal whose 
characteristic rays were being employed. 

Both Sadler and Beat by employed a method in which two 
parallel plates were mounted opposite each other with a 
saturating electric field between them. The high-potential 
plate was bombarded with characteristic X rays, and the 
resulting stream of corpuscles proceeded towards the 



142 X RAYS 

insulated plate, ionising the intervening gas. This ionisation 
was measured. Sadler cut down the corpuscular rays by 
moving one electrode and so lengthening their path in the 
gas ; Beatty, on the other hand, raised the effective path 
by increasing the pressure of the gas, a method probably 
superior in accuracy. 

The results of Beatty and Sadler are given in Table XV. 

In spite of conflicting data for some of the elements, the 
table shows plainly that the absorption coefficient (and thus 
the velocity) of the corpuscular rays is constant throughout 
for the same X ray and the same absorber, 110 matter what 
the nature of the atom from which the corpuscles are emitted. 

The velocity, however, does depend very greatly on the 
quality of the characteristic X ray. Sadler concluded from 
his experiments that the corpuscular rays are an invariable 
accompaniment when characteristic rays, and possibly also 
scattered X rays, are produced. The corpuscular rays from 
a substance increase very markedly as soon as the substance 
begins to emit its characteristic X radiation, and possibly the 
former rays are conditioned by the presence of the latter. 

Whiddington (P.B.S. 1912) has shown that Beatty's 
results conform to the expression 

\A 4 = const., 

where A is the absorption coefficient and A the atomic 
weight of the metal supplying the characteristic X rays. 

It follows, as a deduction from the fourth power absorp- 
tion formula for cathode rays (p. 10), that 

Xv 4 = const., 

where V Q is the velocity of a particle at the moment of pro- 
jection from the plate. 

Combining these expressions, we conclude that 

V O GC A ; 
and, in fact, Beatty's measurements show that 

v ^W 8 .A. 

But this is the value of the critical velocity which cathode 
rays must possess to generate a characteristic radiation 
(p. 126), and, therefore, it follows that the secondary corpus- 



SECONDARY CORPUSCULAR RAYS 143 



O M 





g 


* 

-* ~H t> * t> CO 

1C ^ CO O OS CO 


* 




s 


co co **o co co co 







SI 


^ -H * 00 
00 op op t>- 
ob ob ob ob oo 






5 


OS 00 CO 

.122 12 




I 




lO O >O ^* >O 

i-^ r ( ^H 14 i 1 




1 





* 
O iO 00 ^ <N 

O ^ O CO ^ 
(M CQ <N ^H W 




1 


k 


1 I II 




g Characte 


.I 


* 

OS 1 -0 
C^ CO CO v^ CO 




.5 

1 

H 


.i 


00 (N ?-^ 

8 g ' 55 


* 
t^ 




si 


IS 12 


* 

OS 




si 


OS 

1 * II 






SI 


II IS 

00 


1 

I-H 


tt 


. 






3 


is 


i> co ^ os oo 
<N iO CO I> O 


oo 




i 


51 


< 5 % 


5 


i 


II 




W" 


< 




5 " 





144 X RAYS 

cular rays from X rays have the same velocity as the origina 
generating cathode rays in the X-ray bulb a result of grea 
theoretical importance. 

Fatigue Effect in Production of Secondary X Bays. 

A number of experimenters, among them More (P.M 
1907), Gowdy (P.R. 1910), and Rieman (P.R. 1911), have 
obtained results which indicate that the output of secondary 
X rays from a metal diminishes with time, and that the 
metal exhibits a fatigue effect under the action of the 
rays. It is now, however, pretty generally accepted tha 
the effect is due largely, if not wholly, to chemical change 
of the surface by the action of the surrounding gas. A 
freshly prepared metal surface, obtained for instance by 
distillation in vacuo, shows little or no fatigue if the vacuum 
is continually maintained and any action due to gases 
thereby prevented. 




CHAPTER X. 
FURTHER PROPERTIES OF X RAYS. 

lONISATION BY X RAYS. 

r HEN X rays pass through a substance they here and there 
ttrude into an atom, and are able to expel from it a cor- 
mscle and so ionise the atom. But the proportion of atoms 
bhus affected is extremely minute. Even in favourable 
cases, only one atom in a billion (10 12 ) or more is ionised : 
the rest are passed over, presumably without receiving any 
energy or being influenced in any appreciable way. 

lonisation and Pressure. 

The degree of ionisation depends greatly on the nature 
of the gas and on its pressure, as well as on the quality of 
the X rays. The ionisation produced in a gas by the passage 
of X rays should, in the absence of any secondary radiation, 

1 be proportional to the mass of the gas, that is, to the pres- 
sure (at constant temperature). But usually, secondary 
radiation is generated : if it is sufficiently penetrating to 
reach the electrodes, the ionisation it produces will be pro- 
portional to the square of the pressure. If the secondary 

; radiation were absorbed before reaching the electrodes, the 
ionisation would be simply proportional to the pressure. 

But numerous observers agree that in the great majority 
of cases where the pressure is not very low, the ionisation- 
pressure curve follows a straight line, 1 and the inference 

1 For example, Crowther (P.R.S. 1908) using ordinary X rays ; and 
' Owen (P.R.S. 1912) with homogeneous X rays. The conditions must be 
such that no characteristic radiations are generated. 

K 



146 X RAYS 

would be that the ionisation is due either to the direct 
action of the X rays or to the easily absorbed corpuscular 
rays. 

X Rays ionise indirectly. 

It is now generally accepted that the ionisation produced 
by X rays is an indirect one and due solely to the corpus- 
cular rays ; in other words, the secondary corpuscles carry 
all the ionising power but not all the energy. Prof. Bragg 
was the first to insist on the fact that X rays spend little 
energy in their flight, and that they can therefore have 
little ionising action. 

Beatty's results (P.E.8. 1911) on the ionisation of heavy 
gases SeH 2 , AsH 3 , and Ni(CO) 4 gave great support to 
Prof. Bragg's theory. Beatty found that the quantity of 
the corpuscular rays was the same whether the substance 
was in the gaseous or solid condition. But indisputable 
proof of the correctness of Bragg's notion was given by 
C. T. R. Wilson from his condensation experiments (see 
p. 151). 

More recent work by Barkla and Philpot (P.M. 1913) has 
established the additional fact that the relative ionisations 
produced by equal absorptions of X rays in gases are the 
same as those produced by the corpuscular rays. 

Relative Ionisation in Various Gases. 

Most of the early experiments on gaseous ionisation were 
vitiated by the corpuscles released from the impact of the 
X rays against the electrodes or the surface of the ionisation 
chamber. Crowther (P.R.8. 1908), in an investigation of 
the ionisation produced by heterogeneous X rays in a large 
number of gases, took steps to avoid this difficulty. Some 
of Crowther 's results are given in Table XVI. 

With the exception of hydrogen, and possibly ethyl bro- 
mide, the degree of ionisation is evidently not affected much 
by the quality of the X rays. Owen (P.B.S. 1912) * and 
Barkla and Philpot (P.M. 1913), working with a series of 
homogeneous X rays of a great range of quality, have 

1 Owen's apparatus is shown on p. 1 36. 



IONISATION BY X RAYS 



147 



TABLE XVI. RELATIVE IONISATION PRODUCED IN VARIOUS 
CASES BY HETEROGENEOUS X RAYS. 







Ion i sat ion relative to Air=l. 




Density 




Gas or Vapour. 


relative to 


j 




Air-1. 


Soft X Rays Hard X Rays 






(6 mm. spark). (27 mm. spark). 


Hydrogen, H 2 


007 


0-01 


0-18 


Carbon dioxide, CO 2 - 


1-53 


1-57 


1-49 


Ethyl chloride, C 2 H 5 C1 


2-24 


18-0 


17-3 


Carbon tetrachloride, CC1 4 - 


5-35 


67 


71 


Nickel carbonyl, Ni(CO) 4 


5-90 


89 


97 


Ethyl bromide, C 2 H 5 Br 


3-78 


72 


118 


Methyl iodide, CH 3 I - 


4-96 


145 


125 


Mercury methyl, Hg(CH 3 ) 2 - 


7-93 


425 






TABLE XVII. RELATIVE IONISATION PRODUCED IN VARIOUS CASES 
BY HOMOGENEOUS X RAYS. 1 



Element emitting 
Characteristic 
K Radiation. 


lonisation relative to Air=l. 


Ho 

(Beatty). 


0. 
(B. & P.). 


CO 2 

(Owen). 


SO- 
(Owen). 


C,H 5 Br 

<B:& p.). 


CHsI 
(B.&P.). 


Fe 


00571 


1-37 


1-58 


11-3 


41-2 




Ni. 




35 


1-55 


11-6 





162 


Cu 


00573 


38 


1-55 


11-8 


42 


152 


Zii 


00570 


42 


1-54 


11-5 


41-6 




As 


00573 


27 


1-51 


11-7 


42-2 


158 


Se 




31 


1-53 


11-8 


41-7 




Sr 




28 


1-53 


11-8 


153 




Mo 




28 


1-54 


115 


213 


188 


Ag 
Sii 


04 


32 
29 






272 
. 335 


198 
205 


Sb 




28 











I 













211 


Ba 











251 



B. & P., Barkla and Philpot. 

1 All the secondary radiations except scattered X rays were completely 
absorbed in the gas. With ethyl bromide and methyl iodide, however, 
this was not the case with the X rays hard enough to excite the char- 
acteristic radiations of bromine and iodine respectively. 






148 



X RAYS 



established the fact that the ionisation is independent of 
the hardness of the X rays. The one proviso is that no 
characteristic radiations shall be excited in the gas. If a 
characteristic radiation is generated, both the ionisation 
and absorption are usually increased. These results are 
apparent from Table XVII. 

Total Ionisation in Various Gases. 

Owen derived also the important result that the total 
number of ions produced by the complete absorption of a 
beam of homogeneous X rays in a gas is the same no matter 
what the hardness of the rays or the nature of the gas, so 
long as the characteristic radiations are not excited. This 
result was extended and confirmed by Barkla and Philpot, 
who also showed the parallel effect for corpuscular rays, 
and that the total ionisation produced by a beam of homo- 
geneous corpuscular rays is independent both of the velocity 
of the corpuscles and of the nature of the absorbing gas. 
In regard to mixed gases the conditions are complicated, 

and the results are diffi- 
cult of interpretation. It 
appears probable that an 
additive law does not hold 
(Barkla and Philpot, P. M. 
1913), and that when a 
mixture of two gases is 
traversed by X rays the 
observed ionisation differs 
considerably from the sum 
of the ionisations that 
would be observed if the 
pencil of X rays went 
through the gases sepa- 
rately. 



.ys 



ToPu 




FIG. 72. Crowther's apparatus for showing 
that the degree of ionisation by X rays in a gas 
is independent of the temperature. The guard- 
ring device will be noticed. The apparatus is 
here shown as arranged for liquid-air tempera- 
ture. 



Ionisation and Temperature. 

In 1909Crowther(P.J?.^.) 

showed that the ionisation 

produced by X rays was 



IONISATION BY X RAYS 149 

independent of the temperature, provided the density was 
kept constant. In these experiments, Crowther used a range 
of temperature from about -180 to +184 C., and took 
especial care that the X rays did not strike the testing 
electrodes. His apparatus as arranged for liquid air tem- 
peratures is shown in Fig. 72. 

C. T. E. Wilson's Condensation Experiments. 

C.T.R. Wilson (P.R.S. 1912), in a series of remarkable 
experiments, has recently succeeded in rendering visible and 
photographing the tracks of the charged ions which are 
produced when a beam of X rays (or radium rays) passes 
through a gas. The method is based on the fact that 
supersaturated water vapour deposits on ions just as it 
does on dust particles and forms tiny drops. Thus the 
trail of a beam of X rays, itself invisible, becomes marked 
by a crowded line of cloud. 

Wilson has been able to take instantaneous photographs 
of these condensation nuclei in the positions which they 
occupied immediately after their liberation by the X rays. 

Fig. 73 shows the apparatus. The air within the shallow 
condensation chamber was kept completely saturated with 
moisture by means of water in the bottom of the vessel. 
Supersaturation was produced by suddenly increasing the 
volume of the chamber by exposing the under side of the 
movable bottom to a vacuum chamber. This was effected 
by a sharp pull (to the left) on the cord shown (in Fig. 73), 
which opened the valve below the condensation apparatus. 
After the release of the valve, the cord pulled up with a 
jerk, the heavy weight attached to it was thus suddenly 
arrested, and the fine thread below it carrying a steel ball, 
snapped and the steel ball fell. In its descent, the ball 
passed in succession through two spark-gaps. The first 
passage caused a Leyden jar flash through the X-ray bulb ; 

(the second similarly excited the illuminating spark. 
The arrangements were such that a horizontal beam of 
X rays crossed the centre of the chamber ; the illuminating 
spark flashed a pencil of light at right angles to the beam 
of X rays, and horizontally, or nearly so ; and the camera 



150 



X RAYS 



was usually mounted horizontally on the opposite side of 
the chamber to the illuminating spark. An electric field 
was maintained between the upper and lower faces of the 
expansion chamber. 

Illuminating Spa^rK 



X Ra,y 
Bulb 




Illuminating Sp^rk 



FIG. 73. Diagrammatic representation of C. T. R. Wilson's apparatus for 
photographing the track of a beam of X rays in moist air. 

The order of events in an experiment was, therefore, (1] 
expansion producing supersaturation, (2) X-ray discharge 
producing ionisation in the cloud chamber, (3) condensation 
of water on the ions, (4) passage of the spark for photo- 
graphing the cloud tracks. 

Wilson's instantaneous photographs (see Figs. 1 and 74] 



IONISATION BY X RAYS 151 

show the tracks of corpuscles starting within the beam of 
X rays and extending for some distance beyond it. There 
is no indication of any activity on the part of the X rays 
other than the production of corpuscles : and the track of 
the X ray is not distinguishable otherwise than as being 
the region in which corpuscles have their origin. The cloud 







FIG. 74. Photograph obtained by C. T. R. Wilson of the path of a beam of 
X rays in air supersaturated with moisture (see p. 149). The beam of rays. 
about 2 mm. in diameter, traversed the air (from left to right of the picture) 
immediately after the expansion which produced the supersaturation. The 
axis of the camera was horizontal, and the magnification of the photograph 
is 6 diameters. 

trails show that the corpuscles start in all directions from 
within the path of the primary beam : they do not appear 
to exhibit preference for any particular direction. 

The result is striking confirmation of the view which 
Prof. Bragg has advocated for some years that the X ray 
is completely inoffensive and innocuous during its life, and 
that only on its disappearance does the effective agent 
the corpuscle oome to life. lonisation by X rays appears, 
therefore, to be entirely a secondary process. 

Fig. 2 shows a pencil of X rays passing obliquely through 
a copper plate. The transmitted beam, though much less 



152 X RAYS 

dense than the initial beam, can be plainly seen. From 
the copper issue corpuscular rays in all directions ; these, 
which are responsible for the " halo " round the sheet, 
prove to be mostly of relatively long range. The char- 
acteristic copper radiation also excites corpuscular rays in 
the air, the majority of which have only a range of about 
1 mm. at atmospheric pressure, and are to be found scattered 
throughout the vessel. This may be compared with the 
1 cm. to 3 cm. tracks of corpuscles from the primary X-ray 
beam. If silver is used instead of copper, the secondary 
corpuscles have a much longer path. The clear space shown 
on both sides of the copper sheet in Fig. 2 is due merely to 
the heat of absorption of the X rays and the consequent 
formation of a region of air which is not saturated. 

Wilson attempted to display the crystal-reflection of X 
rays (see p. 186) by means of the above apparatus, but, for 
some reason, the reflected beam was ineffective in producing 
ions, and the plan did not succeed. 

VELOCITY OF X RAYS. 

In 1906, Marx in Germany published the results of an 
ingenious and elaborate investigation on the speed of the 
Rontgen rays. He excited an X-ray bulb by means of 
electric waves from an electrical-wire system ; these waves 
also charged to a varying potential an insulated plate on 
which the X rays fell. The secondary corpuscles emitted 
from this plate were collected by a Faraday cylinder con- 
nected to an electrometer : the amount was obviously con- 
trolled by the phase-relation between the potenial of the 
plate and that of the cathode of the Rontgen-ray bulb. 
If the various distances and the connecting wire lengths 
were adjusted so that the charge received by the Faraday 
cylinder was (say) a maximum, then it was found that if 
the distance of the X-ray bulb from the insulated plate 
was increased by a certain amount, the wire along which 
the waves travelled to the plate had to be lengthened by 
the same amount to restore the maximum. Thus, according 
to Marx, the Rontgen rays travel with the same velocity as 



VELOCITY OF X RAYS 153 

electric waves along wires, and, therefore, with the velocity 
of light, at any rate to within 5 per cent. 

Marx's experimental arrangements were subjected to 
severe criticism by Pranck and Pohl, who, having repeated 
the experiments, doubted the validity of the method. In 
reply, Marx (A.d.P. 1910 et seq.) has since carried out a new 
series of experiments which, he claims, support his original 
result, but which nevertheless do not appear to have satisfied 
his critics (A.d.P. 1911). 

All this work was carried out before the nature of the 
X rays was known ; and there is now no reason for believing 
that X rays travel with a velocity other than that of light. 



CHAPTER XI. 
PRACTICAL APPLICATIONS OF X RAYS. 

RADIOGRAPHY. 

AN extended treatment of this most important branch of 
the subject can be found in a number of existing works ; 
it does not form part of the scope of this volume. A few 
points may, however, be noticed. A radiograph is, of 
course, nothing but a shadow picture, and naturally care 
must be taken to place the subject symmetrically with 
regard to the bulb, so as to avoid unnecessary distortion 
of the image. For perfectly sharp images, the X rays 
should obviously proceed from a single point on the anti- 
cathode, but this, as has been remarked, is impracticable, 
and so it is usually beneficial to stop down the rays as 
much as is feasible. For this purpose, lead tube diaphragms 
are often employed, and can, in some medical cases, be 
made to serve a double purpose for example, the kidneys, 
which are in continual periodic motion, can be arrested 
temporarily, for radiographic purposes, by pressing down 
such a tube tightly into the abdomen. 

The greater the distance of the bulb from the fluorescent 
screen or photographic plate, the more correct the picture ; 
in practice the distance is usually from 12 to 24 inches. 
The spark-gap should not exceed 10 to 12 inches. With 
longer spark-gaps, rays too hard for radiographic purposes 
result. 1 

1 In the same way, radiographs obtained by radium y rays show only 
slight contrast between substances of different density. 



RADIOGRAPHY 



155 




FIG. 75. Radiograph of the hip-joint 




FIG. 76. Radiograph of the shoulder. 






156 



X RAYS 



Photographic exposures naturally vary enormously with 
the coil, break, and tube used. With a hammer break, a 
10-inch spark, and a tube in average condition, some 5 to 




FIG. 77. Microradiograph (magnified 17 diameters) of legs of a tiny lizard 
(Seps Tridactylus). By Pierre Goby. 

10 seconds is suitable for the hand ; 20 to 30 seconds for 
the ankle ; and a minute or more for the thicker parts of 
the body. The latitude in X-ray photographic exposure 
is large, though it is important to avoid under exposure. 

The photographic plates are placed with the film towards 
the bulb, and most photographers agree that slow develop- 



RADIOGRAPHY 157 

meiit is useful for work such as this, where full detail is 
required. Two examples of modern radiography are shown 
in Figs. 75 and 76. 

As examples of uses other than medical for the X rays, 
one may notice their former employment in the Ceylon 
pearl-fishing industry to locate pearls in oysters without 
opening the shells. The degree of transparency to X rays 
serves as a means of differentiation between paste and real 
diamonds : the heavy lead glass is much more opaque than 
the natural gem. 

By the use of extremely soft X rays, radiographs have 
recently been obtained of, for example, the soft tissues of 
the body (showing the veins and nerves), the wings of 
insects, the venation of leaves, and the structure of flowers. 
Radiomicrography of tiny objects forms one of the latest 
achievements of X-ray manipulation, an example of which 
by M. Goby (A.Rt.R. Dec. 1913) is shown in Fig. 77. 

Bismuth Radiography. 

The alimentary system may be radiographed by rendering 
the required part temporarily opaque through the adminis- 
tration of bismuth salts or emulsions with the food. This 
increases the contrast in the photograph. Fig. 78 shows 
a good illustration of the method. Thorium oxide and 
barium sulphate are also used. A word of caution should 
be added, for the pronounced and very soft secondary rays 
that bismuth and other heavy metals emit, may actually 
be injurious. Chemical combination does not affect the 
rays from the constituent elements, so that bismuth salts 
give off secondary rays just like those from bismuth metal. 

Stereoscopic Radiography. 

In this work, two distinct pictures are taken in turn by 
moving the X-ray tube, between the exposures, 2 or 3 inches 
parallel to the surface of the plate, the distance between 
tube and subject being about 20 inches. The resulting 
photographs are examined in a stereoscope. The method 
affords a means of ascertaining the depth of a foreign 
substance in the body, and is often of great assistance in 



158 



X RAYS 




FIG. 78. Bismuth radiograph of the intestines. The black circular sp 
near the centre of the picture is produced by a metal disc which is placed c 
the umbilicus as a " landmark." 



diagnosis. There are other types of localisers, some of which 
display much ingenuity of design ; they can be found fully 
described in the makers' catalogues. 



RADIOGRAPHY 159 

Instantaneous Radiography. 

It is a far cry from the prolonged exposures in the early 
days of X rays to the instantaneous work that is possible 
with modern apparatus. Nowadays, snapshots can be taken 
through any part of the body, and almost any of the moving 
organs can be radiographed. The worker who requires 
exposures short and frequent enough for, say, cinemato- 
graph films now experiences no difficulties out of the ordinary. 

If a single rapid photograph is all that is required, it is 
possible to secure it by comparatively simple means, and 
to send through an X-ray tube momentary currents of a 
magnitude undreamt of a few years ago. One method for 
obtaining practically instantaneous radiographs is to join 
the primary of a modern heavy current induction coil, or 
other high-tension transformer, straight to the direct-current 
town-lighting mains with the usual fuses in circuit. When 
the current is switched on, the fuses are immediately blown, 
and the consequent interruption of the current produces a 
powerful discharge through the secondary winding and the 
Rontgen tube in circuit with it. For such rapid exposures 
a simple X-ray tube without cooling and regulating devices 
suffices. Dessauer in 1909, by using a type of explosive 
fuse for the break, was able to take single flash radiographs 
with exposures of the order of T ^o sec. The momentary 
current through tin. tube was some 200 milliamperes or 
more, and the alternative discharge in air consisted of a 
broad band of flame 40 to 50 cms. long. 

Sir James Mackenzie Davidson has recently succeeded in 
radiographing a bullet leaving the muzzle of a revolver. 
The bullet in its flight over the surface of a photographic 
plate broke the primary circuit of a coil somewhat after 
the fashion employed by Mr. Boys some twenty years ago 
in his flying-bullet photography. The resulting flash through 
a suitably disposed X-ray tube in the secondary circuit gave 
a shadow photograph of the bullet. 

Perhaps even more remarkable are Dr. WorralTs recent 
experiments with a monster coil having a core weighing 
some 3 hundredweights. With a primary current of from 
40 to 80 amperes at 240 volts, and the use of an explosion 






160 X RAYS 

break, flash currents of the order of T4 amperes lasting for 
an interval of from ^^ to roVo second were sent through 
an X-ray tube. The intensity of the discharge was such 
as to be capable of chiselling out a piece of metal from the 
anticathode and leaving a pit behind. Dr. Worrall has 
obtained very beautiful instantaneous radiographs by means 
of his apparatus. 

The possibilities of the extension of such experiments as 
these are far from being exhausted. A transformer which 
weighs about half a ton was referred to by Mr. Duddell in 
his Presidential Address to the Institution of Electrical 
Engineers (1912). Given a closer co-operation between the 
medical profession and the electrical engineer, mammoth 
apparatus and extraordinary results may be looked for in 
the future. 

Intensifying Screens. 

But by the aid of intensifying screens (a device which 
dates back to 1897), instantaneous radiography is possible 
with a much less formidable equipment. The recent im- 
provements in such screens have removed the defects of 
grain, etc., which formerly militated against their extensive 
employment. The Sunic screen, for example, is coated with 
a tungstate of calcium, which, fluorescing as it does with 
a very actinic bluish light, is capable of reducing an exposure 
twentyfold. The screen is placed in close contact with the 
film of the plate and the X rays are sent through the screen 
before reaching the plate. Owing to the after-luminescence, 
which persists for some minutes, the screen should either be 
removed immediately after the exposure or not be disturbed 
for some little time. 

X-ray Photographic Plates. 

The large demand for photographic plates in radiography 
has brought about the introduction, by several firms, of 
plates specially coated for X-ray work. Dr. Kenneth Mees 
is responsible for a photographic plate * which presents some 
novel features. The plate is coated with an unusually thick 

J The Wratten and Wainwright X-ray plate. 



THERAPEUTIC USE OF X RAYS 161 

emulsion containing a heavy metal along with the silver. 
The emulsion is thus rendered dense enough to arrest and 
record most of the incident rays, and the confusing secondary 
radiation from the glass backing is avoided. The result is 
a gain in definition and detail without any sacrifice in speed 
and contrast. 

Plastic Prints. 

On account of the pictorial beauty of the results, this 
method of printing deserves mention. From the original 
negative, a positive is printed on a lantern plate. The 
positive and negative, which should be equally dense, are 
mounted in accurate register, glass sides together. A print 
is then taken by means of light incident at an angle of 
about 45, and a picture thus obtained which shows pseudo- 
relief. Fig. 79 shows an example of plastic printing. 

PHYSIOLOGICAL APPLICATIONS or X RAYS. 

X-ray "Burns." 

The dangers of indiscriminate exposure to X rays are 
now common knowledge, but some of the pioneers in X-ray 
work bought their experience at the price of their lives. 
Undue exposure results in severe dermatitis or skin disease, 
followed in chronic cases by large and cancerous ulceration, 
scaling and shedding of the nails. Unfortunately, the ex- 
tremely painful progress of the disease does not appear to 
be arrested by avoiding further exposure to the rays. Nor 
is there any known means of hastening recovery, though, 
according to Sir James Mackenzie Davidson, some relief 
and improvement has been obtained in superficial cases 
by the application of radium to the affected part in " doses " 
of some minutes at a time. 

Protective Devices. 

It is now known that X-ray " burns " are mainly due 
to the absorption by the skin of the very soft rays ; such 
rays are easily arrested by screening. The various pro- 
tective devices (gloves, spectacles, aprons, etc.), now always 

L 



162 



X RAYS 



employed for the safety of workers, rely on the absorptive 
properties of lead or lead salts in some form or other. 




C. Thurstan Holland. 



FIG. 79. Plastic print of hand, showing fracture of heads of fourth and fifth 
metacarpal bones. 

Impregnated rubber is often used, and Droit (C.R. 1912) 
has recently succeeded in heavily loading silk tissue with 
phospho-stannate of lead (up to about 68 per cent.), and 



THERAPEUTIC USE OF X RAYS 163 

so producing a material which, while extremely light and 
supple, affords adequate protection against the rays. 

The Rontgen bulb is fitted with a lead glass sheath, or, 
in some instances, the bulb itself is made of lead glass 
provided with a window of soda or lithia glass to allow 
the rays to get out. Fluorescent screens used for examina- 
tion work should be faced with lead glass (not less than 
5 to 10 mm. thick) on the side remote from the bulb. 

For long continued exposures, the German Rontgen 
Society advocates as a protection for the body the following 
thickness of screen : 

Lead 2 mm. 

Lead-impregnated rubber - 8 mm. 

Lead-glass 10 to 20 mm. 

Physiological and Curative Action of X Rays. 1 

It might be anticipated that an agency possessing such 
vital characteristics would, under control, find a wide field 
of application in the treatment of disease. This has proved 
to be the case, and, as their technique is being improved, 
the X rays are finding a sphere of activity quite distinct 
from that of radiography. The method of " dosage " is 
usually that of the pastille (see p. 94) assisted by a tacheo- 
meter (or speed-counter) on the mercury break. 

In many skin diseases, the action of the rays has been 
turned to account and has proved to be of notable service. 
The effects are not, however, confined to the skin ; some 
of the internal organs, notably the spleen, are found to be 
even more susceptible. Happily the nervous system gene- 
rally is not at all sensitive to the rays. One of the most 
striking physiological effects of Rontgen rays is their action 
on the growing cells of the young ; the growth of young 
animals is greatly stunted by the rays ; the adult animal 
shows a greater capacity for resistance. Sweat glands and 
hair follicles are attacked and ultimately destroyed a pro- 
perty which provides a signal cure for ringworm, and, with 
prolonged exposure, is capable of producing total baldness. 

1 The writer is indebted to a lecture by Sir James Mackenzie Davidson, 
at the Royal Institution in 1912, for much of this section. 



164 X RAYS 

The white corpuscles of the blood are affected by X rays, 
but the red corpuscles are very resistant. The treatment 
has been largely and successfully employed for rodent ulcers, 
but experience has shown that it does not provide a cure 
for malignant tumours and large cancerous growths, though 
it may arrest their rapidity of growth : this is equally true 
of radium treatment, though in this case the outlook seems 
more hopeful. Apparent success has resulted from the em- 
ployment of X rays in cases of tuberculosis of bones and 
joints. Curiously enough, R.ontgen rays seem to have little 
or no- action on bacteria, and, in this respect, stand out in 
marked contrast to ultra-violet light, which is most destruc- 
tive to all forms of bacteria. X rays (and 7 rays) induce 
a sensation of luminosity in the retina, so that the shape 
of interposed obstacles can be made out by a blind-folded 
normal eye or even by a cataract-affected eye. A totally 
colour-blind eye may be abnormally sensitive to X rays. 

Suitable Rays for Therapeutics. 

In the therapeutic use of X rays, the one essential is 
that the rays shall be sufficiently hard to reach and be 
absorbed by the diseased tissue. In treatment of the skin 
the very softest rays are the useful ones, but to do any 
good to more deep-seated parts harder rays are required. 
In this case the less penetrating rays should be removed to 
avoid their prejudicial action on the skin. An aluminium 
screen J mm. thick is generally sufficiently thick for the 
purpose. One difficulty in treating deep-seated tissue is 
that the harder rays are mainly scattered instead of being 
absorbed by the tissue. This small energy absorption means 
that the curative effects must be feeble. They could, of 
course, be enhanced artificially in some cases by bismuth 
treatment or the like. 

It may be added that the therapeutic effect of X rays 
often evidences itself pronouncedly in the proximity of 
bones ; this is probably due in part to the characteristic 
radiation emitted by the calcium of the bone. Similarly, 
zinc and other metallic ointments might be employed to 
augment the effect in superficial treatment. 



THERAPEUTIC USE OF X RAYS 165 

The practice, which is often followed, of employing the 
same X-ray bulb for both curative and radiographic work 
is, of course, wasteful. Rays which are useful in thera- 
peutics are obviously unsuitable for radiography, as in the 
latter case the essential thing is that the rays should not 
be absorbed, but should reach the photographic plate. 1 As 
was remarked on p. 37, the iron, nickel, copper group of 
metals, when used as anticathodes, emit radiations very 
rich in soft rays, such as are suitable for curative work. 
There is no necessity for using a point source of X rays 
in therapeutic work, and, in fact, the anticathode can 
advantageously be put out of focus, or, if necessary, a 
plane cathode used. 

Glasses specially Transparent to Soft X Bays. 

A bulb intended for skin treatment should be either 
made of a glass specially transparent to X rays or provided 
with a window of such glass. Schott in 1899 was the first 
to make up a glass of this kind a silico-borate of soda 
and alumina as the result of experiments on the trans- 
parency of various oxides and carbonates to X rays. His 
list reads in order of diminishing transparency Li, B, Na, 
Mg, Al, Si, K, Cu, Mn, As, Ba, and Pb a sequence which 
is that of atomic weight. Schott 's glass was never put 
on the market, as at that time the radiographic properties 
of the X rays were the only ones considered, and in this 
respect the glass possesses no appreciable advantage over 
soda glass. C. E. S. Phillips' conducting glass (P.R.S.E. 
1906), which is a mixture of silicate of soda and borax with 
a little lead glass, is also very transparent to X rays. Its 
coefficient of expansion is unusually high, but by the use 
of intermediate glasses, windows of it could probably be 
fused into X-ray tubes. Lindemann (1911) has recently 
constructed focus bulbs provided with windows of a glass 
of lithium borate, which, of all the glasses ever made, is 
probably the most transparent to soft X rays. This glass 

1 The same thing occurs in the use of radium. The highly penetrating 
y rays have no medical value ; it is the softer y and the a and ft rays 
which are arrested by the body. 



166 X RAYS 

is not very permanent, however, but Messrs. Cossor have 
recently brought out an improved lithium glass which can 
be worked and permits joints with platinum, so that X-ray 
bulbs can be constructed entirely of it (Fig. 45). 

Therapeutic Use of Characteristic Eadiations. 

It has been suggested that the various characteristic 
radiations p. 112) would find application and lead to greater 
precision and efficiency in curative X-ray work. These 
radiations are each of uniform quality, and it is, therefore, 
only a question of choosing a suitably hard radiation for 
the purpose in hand. But characteristic X rays, as ordi- 
narily generated, are so feeble that hours of exposure are 
required in place of the minutes necessary with primary 
X rays from a bulb. The writer showed, however, in 1908 
(p. 121), that, with a soft bulb, a considerable proportion of 
the rays from an anticathode may consist of its characteristic 
radiation. By this means, an intense beam could be 
obtained from a tube provided with a suitable metal for 
anticathode and a window of thin glass or aluminium. It 
is further advantageous to use a thin filtering screen of the 
same metal as the anticathode. Better still, perhaps, in 
some respects, would be to make the window itself of the 
metal whose radiation is desired and to use the window 
also as anticathode. Such a tube with a window soldered 
to the glass was, in fact, used by Owen (see p. 136). 

Therapeutic Use of Cathode Rays. 

IL as Prof. Bragg has long maintained, and, as is now 
generally believed, the X ray is in itself ineffective and 
owes all its activity physical and chemical to the electrons 
which it produces when arrested, then the only purpose 
the X ray serves in therapeutics is to plant the action 
deeper in the body. To produce therapeutic action at any 
particular point, there must first of all be transformation 
of the X rays into corpuscular rays, and then absorption 
of these corpuscular rays. If cathode rays themselves were 
simply discharged at the skin by means, say of a Lenard 
tube (p. 5), they could not penetrate more than about 



THERAPEUTIC USE OF X RAYS 107 

-V mm., i.e. about the thickness of a cigarette paper. 1 
Possibly such a treatment might be valuable for some 
surface ailments, more especially as the radiation would 
certainly be accompanied by an abundance of very soft 
X rays from the aluminium window. 



1 The j3 rays of radium with their higher velocities penetrate, of course, 
much farther. 






CHAPTER XII. 
INTERFERENCE AND REFLECTION OF X RAYS. 

Early Attempts to diffract X Rays. 

From time to time, a good deal of ingenuity has been 
exercised by various experimenters in testing whether there 
are, on the boundaries of the shadows cast by small obstacles, 
variations in the intensity of the X rays corresponding to 
optical diffraction fringes. Rontgen (1898) could not satisfy 
himself on the point. Haga and Wind (Wied. Ann. 1899- 
1901) experimented with a V-shaped slit, a few thousandths 
of a millimetre broad at its widest point, and obtained, in 
their photographs of the slit, broadenings of the narrow 
part of the image : if the effect were due to diffraction, the 
same amount of broadening with light would be associated 
with a wave-length of about l - 3 xlO~ 8 cm. 

It must be confessed that the result is in accordance 
with those recently obtained by crystal-reflection methods 
(p. 201), but Walter and Pohl (A.d.P. 1908), who repeated 
Haga and Wind's experiments, found that the width of 
the image of the slit was largely affected by secondary 
effects in the photographic plate depending on the amount 
of energy sent through the slit, with the result that different 
times of exposure gave rise to images of different widths. 
They concluded that the diffraction effect was not proven, 
and that their own experiments went to show that the 
wave-length of an X ray does not exceed 10 ~ 9 cm. 

Attempts to refract X Bays. 

Many attempts have also been made to refract X rays. 
Rontgen, for example, tried prisms of a variety of material 



INTERFERENCE AND REFLECTION 169 

such as ebonite, aluminium, and water. He also attempted 
to concentrate the rays by lenses of glass and ebonite. 
Chapman (P.C.P.S. 1912) experimented with a prism of 
ethyl bromide vapour a substance which is strongly ionised 
by X rays. Two distinct experiments were conducted, in 
which the conditions might have been expected to favour 
a positive result. In one, the X rays were such as to stimu- 
late markedly the radiation characteristic of bromine (p. 
132) ; in the other, the rays were of a type that was selec- 
tively absorbed by the vapour. In neither case, however, 
could any trace of refraction be discovered. 

Eeflection Experiments. 

Many fruitless efforts have also been made to reflect 
X rays. We now know that the obstacle in the way of 
success to such experiments was the extreme shortness of 
the wave-length of the X rays. The specular reflection 
of ordinary light waves is rendered possible by the fact 
that the irregularities remaining in a polished surface are 
small compared with the wave-length of light. But irre- 
gularities negligible for light waves become all important 
with X rays, and a reflecting surface, such as mercury or 
plate glass, deals with X rays in much the same way as a 
surface covered with innumerable facets scatters light rays 
in all directions with no trace of regular reflection as a 
whole. 

It was Prof. M. Laue of Munich who, believing that X 
rays were short light rays with wave-lengths of an atomic 
order of magnitude, 1 conceived in 1912 the notion that the 
regular grouping of the atoms in a crystal, which modern 
crystallography affirms, should be capable of producing inter- 
ference effects with the X rays, in a way analogous to that 
in which diffraction gratings deal with light waves. Laue's 
theory was at once put to the test and triumphantly justified 
by Friedrich and Knipping (A.d.P. 1913). Later, W. L. 
Bragg, at Cambridge, showed that X rays were regularly 

1 Planck's theory of radiation had led Wien in 1907 and Stark in 1908 
to values of 0-7 x 10~ 8 and 0-6 x 10~ 8 cm. respectively for the wave-length 
of an X ray. 



170 



X RAYS 



reflected by cleavage planes of crystals, and could apparently 
be focussed by bent sheets of mica. These experiments and 
their later developments we may now consider in some detail. 

Laue's Theory. 

Crystallographers have gradually developed the theory 
introduced by Bravais in 1850, which contemplates the 
atoms of a crystal as residing at the angular points of a 
" space-lattice." In a crystal, like atoms are regarded as 
forming a perfectly regular system of points in space, each 
and every kind of atom present in the crystal conforming to 
its own independent system. These different point-systems, 

of course, interpenetrate, the 
result being a parallel net-like 
arrangement of points, to 
which the term " space-lattice " 
is applied. Thus the crystal 
naturally divides itself up into 
a large number of precisely 
identical elements, in all of 
which the same relative posi- 
tions of the atoms are main- 
tained. This elementary vol- 
ume is, in a sense, the brick 
from which the crystal pattern 
is built up everywhere after 
the same plan. 

The several atoms thus repeat themselves at definite 
intervals, and Laue's notion was that the resulting regular 
avenues of atoms should be capable of acting as a three- 
dimensional diffraction grating for rays of suitably short 
wave-lengths. 

Laue first considered the case of a simple cubic crystal, 
and assumed that the atoms were arranged at the corners 
of little elementary cubes this being the simplest cubic 
point-system possible. As the incident X rays pass through 
the crystal, they influence the atoms en route, and a secondary 
wavelet spreads from each atom as a wave passes over it. 
Let us take for convenience axes of reference parallel to 




FIG. 80. llepresentation of the diffrac- 
tion of X rays by the atoms at the corners 
of an elementary cube of a cubic crystal. 



INTERFERENCE AND REFLECTION 171 

the sides of a cube and an origin at the centre of one of 
the atoms, O. (Fig. 80 shows the atoms in the xz and yz 
planes of the lattice.) For simplicity, consider a beam of 
X rays to enter the cube in the direction of the z axis. Let 
us ascertain the conditions which will ensure that the wave- 
lets from all the various atoms in the lattice shall co-operate 
or " be in phase " in some particular direction OP, whose 
direction cosines are a, /3, and y. 1 

It is sufficient for the purpose to take the cases of the 
nearest atoms to O on the axis, viz. A, B, and C, and express 
the conditions that the wavelets from these atoms shall be 
in phase with that from O. These conditions are 

aa ^h^ . I,} 

>O 7 T I / 1 \ 

a(l-y)=hl.l,l 

where a is the distance between neighbouring atoms (i.e. 
one side of the cube), I is the wave-length of the X rays, 
and h v h 2 , and h 3 are integers representing the number of 
complete wave-lengths that the waves from A, B, and C 
respectively are ahead of the wave from 0. 

From (1) we obtain 

a _ ft _ (1 - y) _ I 
h l h.j h% a 

and therefore a, /3, and (1 7) ought to be in a simple 
numerical ratio. 

From a consideration of the other cubes grouped round 
the z axis, it is apparent that there is a number of other 
points of maximum intensity situated precisely like P with 
reference to the z axis, so that if a photographic plate is 
placed to receive the transmitted X rays, there should 
appear, where the waves co-operate, a group of spots of 
fourfold symmetry. 

The Experiments of Friedrich and Knipping. 

Laue's theory was put to the test of experiment at Laue's 
request by Friedrich and Knipping (A.d.P. 1913). All that 

1 That is, a, (3, and y are the cosines of the angles which OP makes with 
the axes of x, y, and z respectively. 



172 



X RAYS 



was required was to arrange that a parallel beam of X rays 
should, after traversing a crystal, be received on a photo- 
graphic plate, so that any directions showing " interference 
maxima " would be registered as spots. The apparatus 
used is shown in Fig. 81. 

The X rays emitted from the bulb were cut down by 
lead stops, so that a narrow pencil of rays fell on the crystal, 
behind which a photographic plate was placed a few cms. 
distant. The first crystal that was tried gave the result 
anticipated from the theory. The photographic plate showed 
an intense undeflected spot round which was grouped a 



/Phorogra-phic 




FIG. 8] . Friedrich and Knipping's apparatus for showing diffraction of X rays 
by transmitting them through a crystal. 

number of diffracted spots, some of which were deviated by 
as much as 40 from the original direction of the rays (see 
Figs. 82 and 84). If the crystal were moved parallel to 
itself, the grouping of the spots was unaffected. By altering 
the distance of the photographic plate from the crystal, 
the spots, while showing but little alteration in size, in- 
creased or diminished their displacement from the centre. 
Further, if the crystal was rotated so as to make a different 
angle with the primary beam, the pattern on the plate 
was affected : by careful adjustment, it was possible to 
obtain positions in which the spots grouped themselves 
quite symmetrically round the centre spot. 

The results were generalised for a number of different 



INTERFERENCE AND REFLECTION 



173 




FIG. 82. Pattern of Laue spots obtained by Friedrich and Knipping when 
X rays are diffracted by a zinc-blende crystal. The incident rays are parallel 
to a cubic axis of the crystal. 







FIG. 83. W. L. Bragg's construction to explain the position of the Laue 
spots shown in Fig. 82 (see p. 181). (From the Proceedings of the Cambridge 
Philosophical Society.) 



174 X RAYS 

crystals. It was found that exposures of some hours were 
necessary to obtain good results, since by far the greater 
proportion of the rays was unaffected and undeviated by the 
crystal. Shorter exposures, however, sufficed to reveal the 
more intense spots. 

Laue's Results for Zinc-blende. 

Figs. 82 and 84 are reproductions of the results obtained 
in the case of zinc-blende when the rays travel along "two 
different axes of symmetry in the crystal. Knowing the 
coordinates of any spot on the photographic plate relative 
to rectangular axes having their origin at the point where 
the primary beam strikes the crystal, we can get at once 
the direction cosines, a, /3, and (1 7) of the ray which 
gives rise to that particular spot, and hence we can deduce 
the values of the parameters h lt h 2 , and h s . Now, as re- 
marked above, since H 1} h 2 , and h s are whole numbers, 
these values of a, /3, and (1 7) should be in a simple numeri- 
cal ratio. This was actually found to be the case in all 
the photographs. In no instance was it necessary to assume 
a number for h l9 h 2 , or h a greater than 10 to give the values 
of a, /3, and (1 7) a whole number ratio. This in itself 
is strong confirmation of the theory that the spots are due 
to interference. 

Each spot has its own values of h lt h 2 , and h B . These 
have to conform to equations (1). The associated values 
of a, /3, and 7 have further to obey the relation 



and so it follows that there is only one value which I/a can 
have to satisfy all the equations for each spot. Thus every 
spot gives a different wave-length, since the values of h lt 
h 2 , and h s are different for the different spots. It is here 
that an important distinction arises between a crystal grating 
and a line-grating. In a line-grating an interference maxi- 
mum is always possible, no matter what the wave-length ; 
that is to say, the grating yields a continuous spectrum 
with incident white light. But in the case of a three- 
dimensional grating, certain wave-lengths only are eligible 



INTERFERENCE AND REFLECTION 175 




FIG. 84. Pattern of Lane spots obtained by Friedrich and Knipping, when 
X rays are diffracted by a zinc-blende crystal. The incident rays are parallel 
to ;i trigonal axfc of the cubic crystal (i.e. diagonal -wise through centre of cube). 







FiQ. 85. W. L. Bragg's method of stenographic projection, applied to the case 
in Fig. 84 (see p. 182). 



176 X RAYS 

to form interference maxima, so that a continuous spectrum 
is impossible. A similar effect may be imitated by mounting 
half-silvered parallel plates in front of an ordinary line- 
grating. If white light is now thrown on the grating, the 
former continuous spectrum will be replaced by a line 
spectrum representing a series of definite wave-lengths. 

The Laue photographs seem to show that while, in general, 
the larger the values of the integers h l9 h 2 , and h 3 , the fainter 
are the spots to which they correspond, yet, at the same 
time, the smallest integers do not represent the most intense 
spots as one would be led to infer by analogy with a diffrac- 
tion grating, for which the low-order spectra are generally 
the brightest. Not only that, but certain spots associated 
with simple values of h lt h 2 , and h 3 are absent altogether. 
But if the pattern were the most general possible, then all 
values of the integers, at any rate up to a certain limit, 
should be represented on the plate. 

A satisfactory theory must account for these anomalies, 
and Laue sought to explain them by assuming that the 
primary beam was made up of a limited number of inde- 
pendent homogeneous constituents, the absence on the 
plate of a spot with simple parameters being ascribed to 
the absence of the particular wave-length, which alone is 
capable of forming the spot in question. It was pointed 
out above that any fixed values of h l} h 2 , and h s gave a 
definite value for I /a, but it is evident that if we took the 
same multiples of all these values, say, nh lf nh 2 , and nh 3 , 
the equations (1) on p. 171 would still be satisfied, but now 
by a wave-length l/n instead of I. By adjusting the values 
of h lt h 2 , and h 3 in this way, Laue was able to account for 
all the spots in the photographs by assuming the existence 
of only five different wave-lengths in the incident beam. 

The explanation is not, however, entirely satisfactory, 
because these five wave-lengths should 'give many other 
spots which do not appear in the photographs. 

The Laue Spots for a Zinc-blende Crystal. 

A zinc-blende crystal belongs to the cubic system, and 
crystallography distinguishes between three elementary 



INTERFERENCE AND REFLECTION 



177 



^1 


j 


r 

D 


T 

L I 


E* 


it 

P 

f 


& 


D 


/"A 



point systems of cubic symmetry, namely those con- 
taining : 

(1) points at each corner of the elementary cube, 

(2) points at each corner and one at the centre of the 

cube, and 

(3) points at the corners and at the centres of the cube 

faces. 

Laue assumed that zinc -blende belongs to the first system, 
but in point of fact it almost certainly belongs to the third, as 
Pope and Barlow have shown 
from other considerations. W. 
L. Bragg (P.C.P.S. 1912) was 
led to examine the Laue spots 
of zinc-blende from this point 
of view. 

Adopting this view of the 
structure, Bragg supposed, as 
before, that axes are taken with 
origin at an atom (Fig. 86 
shows the atoms in the xz and 
yz planes), and that when the 
various atoms are stimulated 
by the X rays (incident along 
the z axis), O emits a wavelet 
which in the direction OP is h 
wave-lengths behind that from atom A on the x axis, and 
so on. The equations (1) on p. 171 ensure that all the 
corner atoms (including that at the origin) shall emit 
wavelets which are in phase along OP. It is necessary to 
obtain the corresponding conditions for the centre-face atoms 
(such as D and E), so that their wavelets also shall be in 
phase with those from the corner atoms. 

The difference in phase between the wavelets from D 

and O will be \- -^) wave-lengths, since D is situated in 

the middle of the face of the cube. This must be a whole 
number of wave-lengths to give an interference maximum 
along OP ; and it follows that h and h 3 must either be both 

M 



t 



XFUys 



FIG. 86. Representation of the diffrac- 
tion of X rays by the atoms at the corners 
and face -centres of an elementary cube 
of a cubic crystal. 



178 X RAYS 

odd or both even. The same must also hold for h 2 and 
h s . This at once explains why the complete series of values 
of h lt h 2 , and h 3 for the Laue spots is not represented on 
the photograph. 

Consider first of all the set of spots in the appropriate 
Laue photograph of zinc-blende (Fig. 82), which have 
^ 3 = unity. The corresponding wave-lengths prove to have 
every possible value greater than a limiting wave-length of 
I = 0'034a, where a is the length of the side of an elementary 
cube. The sets of values corresponding to wave-lengths 
approaching I = 0- 06a are responsible for the two very 
intense spots in the inner square of the pattern ; all other 
wave-lengths smaller or greater than 0*06# give fainter 
spots until, for the limiting wave-length 0*034a. they are 
barely visible. Bragg accordingly concluded that the X 
rays utilised in this particular Laue pattern formed a con- 
tinuous spectrum, with a maximum intensity in the region 
of Z = O06a. 

Exactly similar results are obtained for the sets of numbers 
having h 3 = 2. There are two very intense spots which 
form the outer square, and, in addition, a few others con- 
siderably fainter. Similarly for h 3 = 3, in which series there 
are still fewer spots. 

In Table XVIII. 1 is displayed a typical set of values of 
I/a for the different spots corresponding to h 3 - 1. 

The table is very interesting because of its completeness ; 
within a certain range of wave-lengths, every spot anti- 
cipated from theory is registered on the photographic 
plate. 

Thus Bragg's results afford strong support to the atomic 
grouping which Pope and Barlow claim for the zinc-blende 
space-lattice. In later work, Bragg has shown that the 
zinc-blende diffraction pattern is due almost entirely to 
the heavier zinc atoms. The sulphur atoms are situated 
on a similar parallel lattice, which may be reached by 
stepping along one quarter of the diagonal of the elementary 
cube of the zinc -lattice. 

1 See W. L. Bragg's paper in Science Progress, January 1913, to which 
the writer is much indebted. 



INTERFERENCE AND REFLECTION 



179 



TABLE XVIII. 

Zinc-blende crystal ; incident X rays parallel to a cubic axis. 
Values of wave-length for h s =l. 



Value 


Values of I/a, for &= 1. 

/ 7 o 


of h.>. 


*, = ! 


1 = 3 


* l = 6 


h l = l 


V-9 


1 


(off the 


0-178 (m) 


0-073 (v) 


0-039 (v) 


0-024 




photograph) 








(invisible) 


3 


0-178 (m) 


0-104 (v) 


0-057 (v) 


0-034 (/) 


0-022 












(invisible) 


5 


0-073 (v) 


0-057 (v) 


0-039 (m) 


0-027 













(invisible) 




7 


0-039 (/) 


0-034 (/) 


0-027 














(invisible) 






9 


0-024 


0-022 













(invisible) 


(invisible) 









[The letters v, m, and/ indicate the intensity of the spots " v " signify- 
ing very intense, " m " moderately intense, and "/" faint.] 

W. L. Bragg's Theory of the Laue Spots. 

Bragg was led to bring forward an alternative explanation 
of the Laue interference phenomena from the point of view 
of the parallel and equidistant planes of atoms which can 
be pictured in a crystal. Many systems of planes can, of 
course, be chosen, but we can confine the choice to the 
relatively few systems in which the planes are rich in atoms. 

Contrary to the view of Laue, Bragg (as mentioned above) 
supposed that the incident beam of X rays contained (like 
white light) every possible wave-length over a wide range, 
and thus formed a continuous spectrum of rays. Imagine 
then that such a beam falls on a crystal, and let us assume 
that when it strikes a system of parallel planes of atoms a 
small amount of energy is reflected by each plane. The 
wave front of the reflected beam from a particular plane is 
formed by the wavelets sent out by the individual atoms 
in the plane. If the distance between successive planes is 
d, and the glancing angle of the rays is 0, the train of 
waves reflected from the different planes in the system 



180 X RAYS 

will follow each other at intervals of 2d sin ; and if the 
wave-length is such that this distance is equal to a whole 
number of wave-lengths, the waves will reinforce each other, 
and we shall get an interference maximum in that direction. 
Hence in this case, when the incident beam contains every 
possible wave-length, a particular system of planes in the 
crystal picks out, so to speak, the right wave-lengths, and 
the result of the simultaneous working of all the various 
systems of planes is to resolve the beam into its constituents. 
If the angle of incidence is altered, then different wave- 
lengths will in general be selected to form the interference 
maxima. 

On this view, the different intensities which the various 
spots exhibit might be due either to an unequal distribution 
of the energy in the spectrum of the incident X rays or to 
a difference in the closeness of packing of the atoms in the 
various reflecting planes. 

Bragg's method of regarding the interference is, of course, 
analytically equivalent to that of Laue. The reflection- 
method has the great advantage of being more readily 
pictured, especially in considering what happens when a 
crystal is rotated, in which event the pattern of spots is 
distorted exactly as it would be if the spots were reflections 
in plane mirrors. By changing the angle of incidence, we 
alter the phase difference (2d sin 0) between waves from 
successive planes ; and so a spot produced initially by a 
certain wave-length continues to represent without break a 
sequence of the wave-lengths present in the incident beam. 
If, as Laue imagined, certain wave-lengths only were pre- 
sent in the incident X rays, then as the crystal was slowly 
tilted spots would suddenly appear and disappear on the 
plate ; but, on the contrary, when the experiment is tried, 
the same spots can be traced continuously across the plate. 
It is also interesting to notice that some spots are very 
much changed in intensity as the crystal is tilted. One 
spot, for instance, which is barely visible in the symmetrical 
pattern, becomes, in another position of the crystal, the most 
intense of all, because its new wave-length now coincides 
with the maximum in the spectrum of the X rays. 



INTERFERENCE AND REFLECTION 



181 



The elliptical shape of the Laue spots is a direct con- 
sequence of the fact that the incident pencil of X rays is 
not strictly parallel but slightly conical, and so the reflected 
pencil, which is obliquely received on the photographic 
plate, shows an elliptical section. 

Elliptical Loci of Laue Spots. 

With prolonged exposures, many more spots appear on 
the photographic plate than can be detected in Fig. 82. 
As Fig. 83 shows, the various spots group themselves natu- 
rally on ellipses of various sizes, all of which pass through the 




X Rays 



FiQ. 87. Construction demonstrating elliptical loci of Laue spots. 

central spot. These ellipses, which are nearly circular, are 
sections of circular cones having the incident beam as a 
generator. The elliptical locus is a consequence of the fact 
that the various systems of parallel planes which can be 
selected in a crystal may have all manner of orientations : 
the atoms are grouped on parallel straight lines as well as 
on parallel planes, and each of these rows has a set 01 planes 
parallel to it. For example, suppose as before that a beam 



182 X RAYS 

of X rays travels along the z axis, and consider a closely 
packed plane of atoms passing through the line OS (Fig. 
87), 8 being a point in the xz plane. If this plane of atoms 
contains the y axis, then the reflected beam will pass along 
OP. But there is a family or " zone " of planes of atoms 
which can also be selected as passing through OS, and as 
we pass in rotation from one of these planes to another, the 
reflected beam OP will sweep out a circular cone with OS 
as " zone-axis." The trace on the photographic plate (which 
is at right angles to the z axis) will accordingly be an ellipse 
which passes through Oz and touches the yz plane. 

Similarly, if the plane is rotated about a zone- axis which 
is in the yz plane and passes through the origin, the ellipse 
passes through the z axis and touches the xz plane. Now 
suppose that there is a plane of atoms in the crystal which 
contains both these zone-axes, then the reflected beam 
from this plane will give a spot at the intersection of the 
two ellipses obtained as above. 

We can in this way, by drawing ellipses corresponding to 
rotations about various axes through the origin, locate 
almost all the Laue spots. This is done in Fig. 83, which 
graphically displays a key to the spots for zinc-blende 
when the incident rays traverse a cubic axis of the 
crystal. 

The ellipses are marked in each case with the co-ordinates 
of the atom nearest the origin through which the axis of 
rotation passes. The scales of co-ordinates are measured 
in terms of a unit equal to half the distance between con- 
secutive points along the axes. This unit is chosen because 
the only system competent to account for all the Laue 
spots in the case of zinc-blende, is that in which there are 
points both at the corners and face centres of the elementary 
cube (see p. 177). 

Stereographic Projection of Laue Spots. 

In representing a Laue pattern diagrammatically, it is 
tedious and inconvenient to draw the various elliptical loci. 
A much easier method is, however, possible without unduly 
distorting the pattern. 



INTERFERENCE AND REFLECTION 



183 



Suppose the X-ray beam AO (Fig. 88) traverses the 
crystal at O, the undeviated beam striking the photographic 
plate ZD at Z. Let OS be a " zone-axis "; the rays reflected 
in the family of planes which pass through this zone-axis 
lie on a circular cone, of which OS is the axis and OZ and 
OP are two generators. This cone cuts the sphere of which 
OZ is a radius, in a circle of which ZB is a diameter. The 
projection of this circle on the plane ZDhom the " pole " A 
is also a circle, of which ZP' is a diameter and 8 is the centre. 



PlaJ-e 




FIG. 88. Geometrical construction to explain stereographic projection of Laue spots. 

Thus, if we consider the Laue pattern which is formed 
on the surface of the sphere ZBA, and then project this 
pattern on the plane ZD from the pole A, we shall have 
a new projection in which the ellipse with ZP as major 
axis is replaced by the circle on ZP' . The distortion of 
the pattern of spots by the transformation is very small 
except in the regions remote from the centre ; and we 
now have the convenience of drawing circles instead of 
ellipses. It is easy to calculate the positions of the centres 
of the circles, such as $, from the dimensions of the pattern 
when the crystal is symmetrically placed. 



184 X KAYS 

An application of this method of projection to the case 
of zinc-blende is shown in Fig. 85. It will be observed 
how closely the diagram follows the corresponding photo- 
graph obtained by Friedrich and Knipping (Fig. 84). 

Display of Laue Spots by Fluorescent Screen. 

Terada (Proc. Maih.-Phys. 8oc. Tokyo, Ap. 1913) found 
that by the use of a sufficiently transparent crystal and a 
not too narrow beam of X rays, 1 he could detect the Laue 
spots visually by means of a fluorescent screen. 

On rotating the crystal, the elliptical loci of spots referred 
to above are strikingly displayed. 

The fluorescent method is likely to be especially useful 
for a rapid initial examination of a crystal. It is also of 
value for watching the progressive behaviour of a crystal 
which is being subjected to physical or chemical treatment. 
For instance, it was found that on heating a crystal of 
borax, the spots remained visible until the crystal was 
almost entirely melted. 

Interference by Metallic Crystals. 

Little work has been done so far on metallic crystals. 
Keene (P.M. Oct. 1913) found that if a beam of X rays 
was passed through freshly rolled metal sheets, a sym- 
metrical pattern was formed on a photographic plate placed 
behind the sheet. The axis of symmetry of the pattern 
was parallel to the direction in which the sheet had been 
rolled. A rotation of the sheet in its own plane produced 
a corresponding rotation of the spots in the pattern. If 
the sheet were heated and allowed to cool, the pattern 
was replaced by a number of radial streaks arranged in a 
circular band around the undeflectecl spot. A very old 
specimen of metal gave the same result. These radial 
streaks are undoubtedly due to reflection from small crystals 
formed in the one case by age and in the other by annealing. 

The subject was also investigated by Owen and Blake 
(N. Feb. 19, 1914), who adopted a reflection method. A 
piece of copper was cut in two and one of the pieces was 

1 5 to 10 mm. diameter. 



INTERFERENCE AND REFLECTION 185 

annealed, while the other was untreated. A beam of X 
rays was allowed to fall in turn on each of the samples, 
with the result that a large number of spots were obtained 
on a photographic plate in the case of the annealed specimen, 
while no effect was produced with the other specimen. The 
difference is, in the one case, to be accounted for by the 
presence of innumerable small crystals variously oriented, 
each of which reflects its quotum of the original beam, 
while in the unannealed specimen there is an absence of 
crystalline structure and regular atomic grouping. The 
experiments answered equally well if the surfaces were 
highly polished or badly tarnished. 

Owen and Blake further obtained the Laue spots for a 
metallic crystal. Owing, however, to the difficulty of secur- 
ing a single crystal, the resulting patterns were not sym- 
metrical. Crystals of antimony, zinc, and an aluminium 
alloy (50 per cent. Al, 50 per cent. Cu) were tried. 

Interference Phenomena with y Rays. 

We have every reason to believe that the great pene- 
trating power of most y rays is due to the shortness of 
their wave-length as compared with that of X rays. It 
follows that for pronounced reflection of y rays the parallel 
planes of atoms in a crystal should be very close together. 
It might be anticipated that crystals which show marked 
X-ray reflection would not be so successful with the hardest 
y rays, and this, together with the fact that a beam of 
y rays is always very weak, explains the very long exposures 
which have been found to be necessary to give even faint 
photographic impressions of the reflected beam. Prob- 
ably better results could be obtained with dense metal 
crystals, in which the atomic planes are nearer together 
than in most crystalline salts, or by using crystals of great 
thickness. 

Shaw (P.M. July 1913) found that by exposing, for 
several weeks, a plate of mica to y rays, at almost grazing 
incidence, he obtained, on a photographic plate, a series of 
lines which were, however, too faint to allow accurate 
measurement. 



186 X RAYS 

Rutherford and Andrade (N. Oct. 30, 1913) have recently 
carried out a similar investigation, using as the source of 
their 7 rays a thin-walled a ray tube which contained 100 
millicuries 1 of radium emanation. The y rays are given 
off by the products of the emanation, viz. RaB and RaC. 
A pencil of rays was allowed to fall on a crystal of rock-salt, 
and the reflected beam was examined photographically. 
Now, as was remarked on p. 118, among the groups of 
homogeneous y rays from RaB is a strong characteristic 
L radiation, whose \/p in Al is 14- 7. 

From the known data of the crystal it was calculated that 
this L radiation should be reflected at a grazing angle of 
9, and on examination the photographic plate was found 
to show a group of fine lines between 8 and 10. A similar 
group of lines was obtained with a grazing angle of 2 : it 
is possible that these are first-order lines, of which the second 
or third order are in the 9 position. Similar results were 
obtained for a crystal of potassium ferrocyanide. 

THE " REFLECTION" OF X RAYS. 

It will have been remarked how completely and satis- 
factorily the various Laue phenomena are interpreted on 
W. L. Bragg's view of reflection from planes of atoms. 
Bragg was led, at the suggestion of C. T. R. Wilson, to 
ascertain whether X rays were regularly reflected from 
cleavage planes in crystals : such planes are known to be 
very rich in atoms. Mica at once suggested itself, and the 
experiment, when tried, proved immediately successful, only 
a few minutes' exposure being required to give a visible 
impression on a photographic plate. 

A fuller investigation of the reflection phenomena was 
carried out by Prof. W. H. Bragg and his son, W. L. Bragg. 
Their work, which is of great importance, is described in a 
series of papers in the Proceedings of the Royal Society for 
1913 and 1914. Experiments on similar lines were con- 
ducted at the same time by Moseley and Darwin (P.M. 

1 A millicurie is the amount of emanation in equilibrium with 1 milli- 
gramme of radium. 



INTERFERENCE AND REFLECTION 



187 



1913) at Manchester. In both investigations, an X-ray 
spectrometer was used in which an ionisation chamber was 
substituted for the customary telescope, while a lead tube 
with slits acted as the collimator and ensured a fine pencil 
of rays. 1 Bragg's apparatus is illustrated in Fig. 89. 



K 




FIG. 89. Photograph of Bragg's X-ray spectrometer. B is a box containing 
an X-ray bulb. Si and S-> are adjustable slits which direct a beam of X rays 
on to the face of the crystal C. The reflected beam passes through the slit 
S 3 into the ionisation chamber I, where it is recorded by the tilted electro- 
scope in the metal box E. K is an earthing key ; M, a mirror for illuminating 
the electroscope. C and I can each be rotated about the axis of the spectro- 
meter. 



Of the pencil of X rays which falls on the face of the 
crystal, only a small fraction is reflected. In order to 
increase the effect in the ionisation chamber, the Braggs 
filled it with a heavy gas or vapour, usually sulphur dioxide. 

1 In more recent work Moseley (P.M. Dec. 1913) has employed a photo- 
graphic method (see p. 199). 



188 X RAYS 

Moseley and Darwin used helium and augmented the effect 
by an ingenious method depending on ionisation by collision, 
such as is described on p. 89. The crystal was mounted 
as a reflector on the central revolving table. There is no 
need to polish the surface of the crystal, as the underlying 
planes of atoms are the effective agents : in many cases, 
Prof. Bragg merely rubs the surface on sand-paper. The 
ordinary method of fracturing a crystal so as to provide a 
cleavage surface is apt to produce incipient if invisible 
fissures in other cleavage planes in the crystal ; these, 
together with the natural faults which occur in many cry- 
stals, are probably the cause of the multiple images or 
ghosts which have worried many experimenters. The X-ray 
bulb should be very " soft " in many of Bragg's experi- 
ments the cathode stream (see p. 34) was visible over its 
whole length. 

While the reflected beam has all the properties of X rays, 
and is, in general, similar to the incident beam, it differs 
in one important particular, that of penetrating power. 
This is due to the fact that the different constituents of the 
incident beam are not reflected equally by the crystal, with 
the result that the two differ in their average hardness. 
The reflected beam may indeed be considerably harder than 
the incident beam. 

All the experiments, however, go to show that both 
incident and reflected radiations consist essentially of the 

same constituents, but that these 
constituents are present in differ- 
ent proportions in the two beams. 
Fig. 90 shows graphically the 
way in which the ionisation varies 
as the chamber is moved across 
the region of the reflected beam. 
No matter what the angle of 

Angle of Reflection . ., 

incidence, the position of maxi- 

FIG ' 9 - J -XV 

mum ionisation corresponds with 

great precision to the angle of optical reflection, although 
the magnitude of the maximum ionisation varies greatly 
with the crystal and its position. 




INTERFERENCE AND REFLECTION 



189 



If for any particular crystal-face a series of measurements 
is made at varying angles of incidence, and the strength 
of the reflected beam is plotted against the glancing angle, 
then, while there is a general reflection of the rays at all 
angles, the reflection increases very greatly as grazing inci- 
dence is approached. 

Superposed on this curve, there is, at certain very sharply 
defined angles, a sudden and large increase in the intensity, 



OS 
CD 

DQ 



T3 
0) 



0) 

DC 



I ^Order 



Platinum Anric^t"hode, 
Rock-SaJf Reflector 




5 10 15 20 25 30 35 40 
Glancing Angles of Incidence of X Rays 

FIG. 91. Showing intensity-distribution of spectrum of X rays from platinum. 
There are three main spectrum lines, and a large proportion of " white " or 
general X rays. 

which shows itself as marked peaks on the curve. Fig. 91 
is the curve obtained by the Braggs for the rays from a 
platinum anticathode. The reflector in this instance was 
a rock-salt crystal, though the general form and relative 
proportions were found to be the same for all the crystals 
examined. 1 The curve shows three prominent peaks (marked 



1 Zinc-blende, potassium ferrocyanide, potassium bichromate, quartz, 
calcite, and sodium ammonium tartrate. 



190 X RAYS 

A, B, and C in the figure) thrice repeated. The rays corre- 
sponding to each of these peaks are found to be homogeneous 
when tested by the usual absorption method. Correspond- 
ing peaks in the different series prove to be closely related : 
not only are the absorption coefficients of the rays producing, 
for instance, B lt J5 2 , and B 3 identical, but the sines of their 
reflection angles are in simple ratio. For example, the 
several angles of reflection of the B peaks are 

ll-55, 23-65, and 36-65. 
The sines of these angles are 

0-200, 0-401, and 0'597, 

which are very nearly in the ratio of 1, 2, and 3. 

There -can be little doubt as to the interpretation of these 
results. The peaks A, B, and C represent three different sets 
of homogeneous rays which appear as first, second, and third 
order spectra. Rays of a definite quality are reflected from 
a crystal when, and only when, the crystal is set at the correct 
angle. The three groups of rays are not manufactured in 
the crystal, for their properties prove to be the same, no 
matter what crystal is used. The incident X rays consist, 
in fact, of " independent " rays of all wave-lengths with an 
admixture of homogeneous radiations characteristic of the 
platinum antic athode (compare Kaye and Beatty's results 
on pp. 36 and 127). 

Moseley and Darwin (loc. cit.), using rather more refined 
apparatus, similarly detected five homogeneous constituents 
in the platinum radiation : peaks B and C are, in fact, close 
doublets (see p. 201). The proportions of these constituents 
appeared to depend on the state of the X-ray bulb. 

Barkla and Marty n (P.P.S. 1913), using a divergent pencil 
of X rays, demonstrated somewhat similar results photo- 
graphically ; and Owen and Blake (N. April 10, 1913), by 
using narrow pencils of rays, obtained photographic spectra 
of lines so well defined as to permit great accuracy of 
measurement. To increase the separation between the 
lines, the plate was mounted very obliquely to the reflected 
beam. The angles of reflection agreed very closely with 
those obtained by Moseley and Darwin. 



INTERFERENCE AND REFLECTION 191 

The Two Methods of Analysis. 

To recapitulate, there are then two distinct methods of 
crystal- analysis depending on X rays. 

(1) The Laue transmission method, which uses the inde- 
pendent, heterogeneous (or " white ") X rays that commonly 
constitute the greater part of the output from an ordinary 
bulb. The crystal plays a part somewhat like that of a 
" crossed " transmission grating, and the structure of the 
crystal controls the pattern of the diffracted spots. 

(2) The Bragg spectrometer method, which employs the 
homogeneous X radiations and uses the crystal as a re- 
flection grating. The structure of the crystal evinces itself 
in the distribution and intensity of the spectrum lines 
among the various orders. The Bragg method gives the 
data by which the dimensions of the lattices of crystals 
can be compared, and the X-ray spectrometer has already 
proved itself a powerful instrument for examining crystal- 
structure. The Laue method, on the other hand, can only 
supply information concerning the nature of the lattices, 
and that in a limited degree. 

CRYSTAL- STRUCTURE . 

We must now refer > if only briefly, to the recent important 
work which has been done by W. L. Bragg (P.R.S. 1913) 
on the structure of crystals. 

In order to arrive at the structure of a crystal and the 
dimensions of its space-lattice, we require to determine 

(1) to what point system it belongs ; 

(2) what are the several distances separating the atomic 

planes parallel to the different crystal faces. 

On these lines, Bragg attacked, first of all, the problem 
of the structure of the halogen salts of the alkaline metals, 
all of which form cubic crystals. 

(1) Nature of the Space-Lattice. 

The salts dealt with were the chlorides of sodium and 
potassium, and the bromide and iodide of potassium. The 



192 X RAYS 

Laue patterns were obtained for each of these when the 
X rays fell normally on a plate cut parallel to a cube-face. 
On a priori grounds, we might anticipate that these chemi- 
cally similar bodies would all belong to the same point 
system and give identical Laue patterns. On the contrary, 
when the tests were made it was found that 

(1) potassium chloride gave rise to a simple pattern such 

as would be produced by a simple cubic lattice of 
the first kind (p. 177) ; 

(2) potassium bromide and iodide each produced the 

pattern characteristic of the face-centred lattice of 
the third kind (p. 177) ; 

(3) sodium chloride gave a third pattern more complex 

than either of the others, and apparently intermediate 
between them. 

The most obvious and plausible explanation of this dis- 
similarity is that the relative masses of the two constituents 
affect the diffracting ability of the molecule. It is reason- 
able to infer that a heavy atom would form a better diffract- 
ing centre than a light one, and, if we assume this to be the 
case, the apparently strange results become rational. In 
the case of potassium chloride, the atomic weights of potas- 
sium and chlorine (39'1 and 35' 5 respectively) are sufficiently 
close for the atoms to be equally efficient as diffracting 
centres. The disparity in the atomic weights of sodium 
and chlorine (23 and 35' 5) complicates the rock-salt pattern, 
while with potassium iodide (39- 1 and 127) and bromide 
(39- 1 and 79' 9) one atom is so much heavier than the other 
that the system consists in effect of only one kind of atom 
on a simple space-lattice. 

We have now to conceive a grouping of metal and halogen 
which, though common to all four salts, will bring out the 
points of difference. Following Bragg's notation, let us 
distinguish between the two kinds of diffracting centres in 
a salt by calling one black and the other white. Then the 
points must be arranged in such a way that 

(1) there are equal numbers of black and white ; 



INTERFERENCE AND REFLECTION 193 

(2) the arrangement of points, black and white, taken 

all together is that of the first cubic space-lattice, 
viz. points at the corners only of each elementary 
cube (p. 177) ; 

(3) the arrangement of blacks alone or of whites alone 

is that of the third cubic space-lattice, viz. points 
at the corners and face-centres (p. 177). 

An arrangement which gives this result is shown in Fig. 
92. The space-lattice formed by the whites is the same 
as that formed by the blacks, each of the two being the third 




FlQ. 92. Representation of two types of diffracting- centres in a cubic crystal. 

system. If black and white centres become identical, as 
in KC1, the lattice becomes the simple cubic one of the first 
type. 

The evidence for this arrangement of the atoms in these 
halogen salts seems very weighty, but we are still unable 
to say with certainty whether the diffracting unit at each 
point of the system contains only one* atom or more. Bragg 
has, however, brought strong support in favour of the view 
that single atoms are associated with each centre. Laue 
photographs were obtained for three such different sub- 
stances as zinc-blende (ZnS), fluorspar (CaF 2 ), and calcite 
(CaCo 3 ) ; in each case the X rays traversed the crystals 

N 



194 X RAYS 

along the trigonal axes. 1 The patterns proved to be identi- 
cal, which points to the fact that the diffracting centres are 
arranged on precisely similar space-lattices in all three cases. 

But, since a space-lattice is an arrangement in which 
each point is situated relatively to its neighbours in exactly 
the same way as every other point, it would be impossible 
to arrange complex molecules in a space-lattice unless only 
one point in each molecule were effective. We are led to 
infer that, at any rate in the above cases, each molecule 
acts as a single point, by reason of the fact that it contains 
one atom much heavier than the rest ; and that it is the 
lattice of the heaviest atom in the molecule which is re- 
sponsible for the diffraction pattern observed. 2 

Thus far, well and good, but to complete the argument 
in support of the view that each diffracting centre contains 
only one atom, we need to know the comparative dimen- 
sions of the lattice of the several crystals. This we can 
get by the X-ray spectrometer, as we will now proceed to 
indicate. 

(2) Separation-Distances of Atomic Planes. 

A knowledge of the angles at which the various X-ray 
" peaks " are reflected in the spectrum of an element, 
enables us to find the distance between the planes of the 
reflecting system in terms of the wave-length of the X rays 
concerned. By measuring the first- order value of the 
glancing angle in l = 2d gin Q 

for the reflection curves from the three primary planes of 

the crystal, we can derive the several values of > > 

I I' I 

for the three principal directions. We can thus deduce 
both the form of the elementary parallelepiped and the 

value of 1 2 3 or , where F is the volume of the parallelo- 
l I 

piped. Now, if p is the density of the crystal, the mass 

1 I.e. diagonal-wise through the centre of the cube. 

2 Bragg's later work leads him to the conclusion that the scattering 
power of an atom is proportional to its atomic weight, 



INTERFERENCE AND REFLECTION 



195 



associated with each parallelepiped, and so presumably with 
each diffracting centre, is Vp. If M is the molecular weight 
of the substance, the number of molecules associated with 

each centre is t-, which we may write Z 3 ( S-J. If, in a 

series of comparative experiments, I is kept constant, then 
the expression within the brackets is proportional to the 
number of molecules per centre, and can, moreover, be 
evaluated by experiment. Bragg proceeded to do this for 
a number of different crystals, each of which contained one 
heavy atom, viz. zinc-blende (ZnS), fluorspar (CaF 2 ), calcite 



TABLE XIX. 1 



Crystal. 


Lattice. 


Density 
P 


Mol. Wt. 
M 


Face. 


e 


d 
I 


V 

P 


VP 

m 
























o 








ylvine, KC1 


Simple cubic 


1-97 


74-5 


(100) 


10-2 


2-86 


23-4 


0-605 










(111) 


18-0 


1-62 


22-2 




lock-salt, 


Face -centred 


2-15 


58-5 


(100) 


11-4 


2-53 


32-5 


1-22 


NaCl 


cubic 






(110) 


16-0 


1-82 


33-9 










(111) 


9-8 


2-95 


33-5 




inc -blende, Face -centred 


4-06 


97-0 


(110) 


16-5 


1-76 


30-8 


1-28 


ZnS cubic 
















'luorspar, 


Face -centred 


3-18 


78-0 


(100) 


11-7 


2-46 


29-8 


1-18 


CaF 2 


cubic 






(111) 


10-3 


2-79 


28-3 




lalcite, 


Rhombo- 


2-71 


100-0 


(100) 


10-5 


2-74 


44-8 


1-22 


CaCO 3 


hedral 






(111) 


11-2 


2-60! 




ron pyrites, 


Face -centred 


5-03 


120-0 


(100) 


12-1 


2-39 


27-3 


1-15 


FeS 2 


cubic 

















1 In this table 

6 = glancing angle of Pt B peak, first order. 
I = wave-length of Pt B peak. 

d = distance between planes parallel to the face investigated. 
V = volume of elementary parallelepiped, calculated from this value of 
d and a knowledge of the nature of the lattice. 



196 X RAYS 

(CaC0 3 ), iron pyrites (FeS a ), and rock-salt (Nad). Using 
in every case the homogeneous rays of the B peak of Pt, 
he found that the value of the quantity was, within a few 
per cent., the same for all substances. His results are put 
out in Table XIX. 

Thus the number of molecules associated with each diffract- 
ing centre is the same, and if we take into consideration 
the very different constitution of these crystals, this fact 
seems to point to the association of one molecule, and one 
alone, with each diffracting centre. By combining this 
result with the deductions on p. 194, it would seem that, 
since there is only one heavy atom in each molecule, the 
pattern obtained with the various crystals is due to a space- 
lattice formed by the association of only one heavy atom 
with each centre. 

It will be noticed that potassium chloride gives a value 
for Vp/l 3 M equal to half that for the other crystals, the 
explanation being that the two atoms, being of nearly the 
same weight, are equally effective as diffracting centres, 
and that a parallelepiped with half the side is now the 
crystal unit. 

The above argument is obviously not a complete proof 
of this important point, but the probability of the truth 
of the assumption that each centre represents a single atom 
has been strengthened by each and every one of the many 
varieties of crystals subsequently examined. 

In later work, Bragg has been able to allocate the 
positions of both the light and heavy atoms in many types 
of crystals. 



DIMENSIONS OF SPACE-LATTICE AND WAVE-LENGTH or 

X RAYS. 

If the arrangement assigned to the alkaline salts is correct, 
we are now in a position to calculate the wave-length of 
the B peak, Pt radiation, for 



M 



INTERFERENCE AND REFLECTION 197 

Take the case of rock-salt (Nad), 

Molecular weight, M =.58- 5 x 1'64 x 10 ~ 24 grammes. 
Density, - p = 2-15 gm./c.c. 

V/l 3 = 33' 3 (experimentally determined). 
/. Z 3 (33'3x2-15) - 58-5 x 1-64 xlO" 24 ; 
whence Z 3 = 1'34 xlO' 24 

and Z = l- 10x10-* cm., 

which gives us the wave-length of the homogeneous radiation 
of the B peak of platinum. 

By means of the values of d/l given in Table XIX., we 
can calculate the lattice-constants for any of the crystals 
investigated. 

Platinum L Radiation. 

The mass-absorption coefficient in Al of the rays con- 
stituting the B peak of platinum was measured by Bragg 
and found to be 23*7. From Fig. 59 this value corresponds 
either to a K characteristic radiation from an element of 
atomic weight 72- 5 or an. L characteristic radiation from 
one of atomic weight 198. The atomic weight of platinum is 
195 : the agreement is too close to be fortuitous, and there 
can be little doubt that the B peak is due to the L radiation. 

We have the means of deriving further relations. From 
Whiddington's rule for K radiations (p. 126) we can cal- 
culate that the cathode-ray energy necessary to excite the 
K radiation from an atom of weight 7 2- 5 is about 2 xlO~ 8 
ergs. This energy should be equal to the energy of the 
X ray excited, which, if Planck's radiation formula holds 
in this connection, is given by hv l or hV/L h is Planck's 
constant (6*55 xlO~ 27 erg sec.), v is the frequency of the 
radiation, and V is the velocity of light. Now we have 
just shown that Pt L radiation has a wave-length of 
I'lO X 10~ 8 cm., and therefore 

hV_ _6-55xlQ- 27 x3xl0 10 

l MOxicr 8 

= 1-78 xlO~ s ergs 
which is in fair agreement with the calculated value. 

1 hv is the energy of a quantum, according to Planck's theory. 



198 



X KAYS 



X-ray Spectra. 

In later work (P.R.S. 1913 and 1914) the Braggs have 
conducted further experiments using a variety of anti- 
cathodes. The three allied metals, osmium, iridium and 
platinum, yield X-ray spectra with certain common char- 
acteristics. Each contains three main groups of homo- 
geneous rays, together with a good proportion of general 



0) 

cr 

<+- 

o 

-c 



0) 



l sr Order 




Rhodium Anh'c^fhoole 
Rock-5*!r Reflector 




5 10 15 

Glancing Angles of Incidence of X Rays 

FIG. 93. Spectrum showing distribution of energy in X rays from rhodium. 
There is a large proportion of monochromatic radiation, and very little " white " 
or general radiation. 

radiation. The spectra of palladium and rhodium are very 
similar to each other ; each is very nearly homogeneous, 
at any rate with a soft bulb (see p. 121), and contains little 
general radiation. On this account, both radiations have 
been employed a great deal by the Braggs in their later 
crystal experiments. Fig. 93 shows the rhodium spectrum. 
It needs to be pointed out that Figs. 91 and 93 are examples 
of X-ray spectra of which the general form depends on the 
circumstances. While it is true that the spectral lines 



INTERFERENCE AND REFLECTION 



199 



themselves are invariable in position, their relative intensity 
and that of the general radiation are modified by such factors 
as the hardness of the X-ray bulb, the presence of any 
filtering screens, the type of discharge, and, of course, on 
the resolving power of the spectrometer. The chemical 
nature of the crystal also exerts a marked effect on the 
distribution of the energy. Bragg has shown that this is 
due to the selective absorption by the atoms of the crystals 
of the various components of the X rays. 

Moseley's Experiments. 

Moseley (P.M. Dec. 1913) has recently examined photo- 




Increasing Wave Length 



FIG. 94. Moseley's photographs of the X-ray or high-frequency spectra 
from a number of metallic anticathodes. The spectra, which are in the third 
order, are placed approximately in register in the figure. The wave-lengths 
are given on p. 201. For each metal, the more intense line, with the longer 
wave-length, is the K characteristic radiation. The brass shows the Zn and 
Cu lines ; the cobalt contained both nickel and iron as impurities. 



200 X RAYS 

graphically the spectrum of X rays produced by a crystal 
of potassium ferrocyanide used as a reflection grating. The 
method has the advantage of doing away with the necessity 
of keeping the source of radiation constant. Moseley em- 
ployed Kaye's apparatus for generating characteristic X 
rays (p. 121), and found that for a range of elements ex- 
tending from calcium to zinc, the spectrum consisted in 
each case of two sharply defined lines, of which the longer 
wave-length was the more intense ; this latter is doubtless 
the K radiation. The various spectra are placed in approxi- 
mate register in Fig. 94. It will be noticed that the wave- 
length increases as the atomic weight diminishes. The 
presence of any impurities is clearly shown in the spectra ; 
the simplicity of the spectra suggests a powerful method 
of chemical analysis. The characteristic radiations are ap- 
parently very homogeneous, to judge by the photographs. 
Moseley has made important deductions from his results, 
bearing on Rutherford and Bohr's theories of the structure 
of the atom (see p. 18). 

A series of wave-lengths of the various " lines " in the 
X-ray spectra is given in Table XX. 

Kelation of Wave-length to Atomic Weight. 

From Whiddington's results (p. 125) the energy of a 
characteristic X ray is roughly proportional to the square 
of the atomic weight ; and, at the same time, according 
to Planck's quantum theory of radiation, the energy of an 
X ray is inversely proportional to its wave-length. We 
have an opportunity of testing this in Table XX., where 
the wave-lengths of the K radiations of twelve metals are 
given. Taking one of the intermediate metals (cobalt) as a 
standard of reference, the relative values come out as follows : 

Ca. Ti. V. Or. Mi). Fe. Ni.* Co. Cu. Zn. Eh. Pd. 

(Atomic weight) 2 - 46 66 75 78 86 90 99 100 116 123 304 328 
l/(Wave-length) - 53 65 72 78 85 92 108 100 116 124 298 314 

Thus the wave-lengths are approximately proportional 
to the reciprocals of the squares of the atomic weights, over 
a considerable range of atomic weights. 

* From an X-ray point of view, Ni behaves like an element with an 
atomic weight of about 61 '5. 



INTERFERENCE AND REFLECTION 



201 



TABLE XX. WAVE-LENGTHS OF MONOCHROMATIC OR HOMOGENEOUS 

X RAYS. 

The values below are due to Moseley, Moseley and Darwin, and 
Bragg. They are given to the nearest 0-005 A.U. All the measure- 
ments depend on W. L. Bragg's estimate of the atomic distances 
in the case of rock-salt (p. 197). See p. 235 for later values. 



Element. 


Atomic 
Weight. 


Wave -length. 


Remarks. 


Calcium 


40-1 


3-36 x 10~ 8 cm. 


Strong K radiation. 






3-09 


Weak 


Titanium 


48-1 


2-76 


Strong K 






2-525 


Weak 


Vanadium 


51-1 


2-52 


Strong K 






2-30 


Weak 


Cliromi urn 


52-0 


2-30 


Strong K 






2-09 


Weak 


Manganese 


54-9 


2-11 


Strong K 


Iron 


55-9 


945> [| 


Weak 
Strong K 






765 


Weak 


Cobalt - 


59-0 


so x ,, 


Strong K 






63 


Weak 


Nickel - 


58-7 


66 


Strong K 






505 


Weak 


Copper - 


63-6 


55 


Strong K 






40 


Weak 


Zinc 


65-4 


445 


Strong K 






305 


Weak 


Rhodium 


102-9 


0-605 


Strong K ,, 






0-53 


Weak 


Palladium 


108-7 


0-575 


Strong K ,, 






0-51 


Weak 


Tantalum . - 


181-0 


1-525 


Strong L 






1-33 


Weak 






29 


Weak 


Tungsten 


184-0 


25 


L ,,\ 


Platinum 


195-2 


305 








11 \ 


7, 






09 ' J 


J-J y y 






0-95 








0-92 





202 X KAYS 

It may be added that C. G. Darwin (P.M. Feb. 1914) 
remarks, from a scrutiny of Moseley's wave-lengths and 
Barkla's absorption coefficients (p. 115) for a considerable 
range of qualities of homogeneous X rays, that if A is the 
absorption coefficient and / is the wave-length, then 

I* oc X almost exactly. 
But from the above, 



(atomic weight) 



and therefore 1 

X K 



(atomic weight) 5 
which is Owen's fifth power law, referred to on p. 116. 

Wide Range of Electromagnetic Waves. 

With the addition of X rays to the list of electromagnetic 
waves already known, the table of wave-lengths is extended 
greatly in one direction. At the other end of the scale are 
the waves which were originally discovered by Hertz and 
are now used in wireless telegraphy. The longest wave- 
length generated up to the present is about 15,000 metres, 
or a little over nine miles : the shortest is a few milli- 
metres. The waves ordinarily used in " wireless " are a 
few thousand metres long, e.g. the wave-length of the wire- 
less time signals from the Eiffel Tower is 2000 metres ; of 
the Navy signals, from 600 to 1800 metres ; of trans- 
Atlantic signals, 7000 metres or more. 

Next to Hertzian waves, in order of magnitude, come 
the infra-red or heat rays, the greatest wave-length yet 
observed being xV mm. We pass from these right through 
the visible spectrum to the ultra-violet rays, which have 
been explored as far as wave-length 10 ~ 5 cm. ; these are 
examined photographically. An extreme form of ultra- 
violet rays is probably represented by the " Entlad- 
ungstrahlen " which are emitted from electric sparks, 
or the negative glow in a discharge tube (see Laird, 
P.E. 1911). 



INTERFERENCE AND REFLECTION 



203 



Next come X rays with wave-lengths of the order of 10 8 
cm., and beyond them the most penetrating of all the y 
rays of whose wave-lengths little is yet known. 

The various wave-lengths are summarised in Table XXI. ; 
they cover, as will be seen, the amazing range of about one 
thousand million million fold. 



TABLE XXI. WAVE -LENGTHS OF ELECTROMAGNETIC RADIATIONS. 



Kind of Wave. 



Hertzian waves 
Infra-red rays 
Visible light rays - 
Ultra-violet rays - 
Eiitladungstrahleii 
X rays - 
y rays - 



Wave-length in cms. 



10" to 0-4 

0-013 to 7-7 x 10 

7-7 x 10~ 5 to 3-6 x 10 
3-6 x 10~ 5 to 10~ 5 
about 10-(?) 
about 10~ 8 
about 10~ 9 (?) 



CHAPTER XIII. 
THE NATURE OF THE X RAYS. 

THE discovery of the X rays by Rontgen, and their im- 
mediate application in surgery, excited the popular interest, 
to an astonishing degree. Geissler tubes, no matter what 
their suitability, were in immediate demand by a strangely 
interested public. The scientific journals of 1896 bear wit- 
ness to the many workers, who turned, if only for a time, 
from their usual pursuits, eager to test the extraordinary 
properties of the new rays. Naturally enough, among such 
an army of enthusiasts, speculation as to the nature of the 
rays was not marked by any great restraint ; a few of the 
responsible suggestions may be briefly recalled. 

Rontgen, Boltzmann, and others regarded the rays as 
longitudinal ether- vibrations of short period and great wave- 
length : Jaumann added to this a transverse component : 
Goldhammer, Sagnac, and many others believed that the 
new rays were extremely short transverse ether- waves akin 
to ultra-violet light ; on the other hand, Re took the view 
that the wave-length, far from being short, was infinitely 
long : Sutherland considered X rays to be due to internal 
vibrations of the electrons within the atom : other workers 
held that the rays were a manifestation of the breaking up 
of molecules into atoms at the target : Michelson suggested 
that Rontgen rays were ether vortices : Stokes put forward 
a theory of irregular pulses in the ether ; and finally, many 
physicists, including at one time, Rontgen himself, and 
more recently Prof. Bragg, inclined to the view that the 
rays were flights of material particles which resembled 



THE NATURE OF THE X RAYS 205 

strongly, and were possibly an extreme though electrically- 
neutral form of, the parent cathode rays. 

It is only within the last year or so that controversy 
has been stilled by the discovery that X rays can be re- 
flected and diffracted by crystals. There can scarcely be 
any doubt now that X rays are identical with ultra-violet 
light of extremely short wave-lengths ; wave-lengths, in 
fact, of the order of the diameter of the atom. 

Yet it is not quite all plain sailing, for while it seems 
certain from the extreme precision observed in the reflection 
experiments that X rays are regular light waves and occur 
in trains of great length, yet the difficulty is that in many 
of their properties the rays behave strangely like streams 
of discrete entities, the effects of which are localised in space 
in much the same way as are the effects of rifle bullets. 
The difficulty is not, however, unique ; it is now known 
to be common to all forms of radiation. The Newtonian 
laws implying perfect continuity and infinite divisibility of 
time and space have, until recently, found complete cred- 
ence ; but in the very nature of things they do not seem 
to be reconcilable with modern experiment, which suggests 
that energy radiation is essentially discontinuous and must 
take place by finite '* jumps." As to the mechanism by 
which this is accomplished, it is at present obscure and 
still a matter for speculation. 1 

To meet the difficulty, J. J. Thomson, in his nucleated 
pulse theory, has suggested that all the various light radia- 
tions consist of concentrated and localised electromagnetic 
impulses which travel with the speed of light in some one 
direction through the ether (see p. 213). Planck's quantum 
theory, as developed by Einstein and Stark (P.Z. 1909 and 
1910) similarly argues that X radiation (in common with all 
radiation) is made up of definite and indivisible increments 
which can travel without loss or alteration of form, the 
energy of these " bundles " being proportional to the fre- 
quency of the radiation. The same difficulty was felt by 
Prof. Bragg when he put forward his corpuscular or neutral- 

1 For an excellent treatment of this subject see N. R. Campbell, Modern 
Electrical Theory, 2nd ed. 1913. Also Rep. Brit. Assoc. Sect. A, 1913. 



206 X RAYS 

pair theory of the X ray. This theory, which regarded an 
X ray as a neutral corpuscle, was conspicuously successful 
in predicting and interpreting the energy transfers met with 
in the inter-relations of the cathode rays and X rays. 

On the other hand, almost all the well established results 
of the undulatory theory of light seem to be irreconcilable 
with entity views such as these. Nucleated light does not 
appear to conform to the marvellous explanation of inter- 
ference and diffraction, which Young and Fresnel founded on 
a theory of spreading waves, nor does it obviously lead to 
the general laws of reflection and refraction which are 
apparently obeyed by all waves, from the shortest X rays to 
the longest Hertzian. 

Points of Resemblance between Light Eays and X Rays. 

The essential identity of X rays and light rays cannot 
be denied, in view of the work on crystal-reflection, but, 
notwithstanding, it will be useful and not without interest 
to summarise the remaining points of resemblance which 
experiment has revealed between X rays, 7 rays, and light 
rays : in many cases it would be anticipated that the effects 
would differ only in degree. For example, the ionising effect 
of ultra-violet light on gases (first established by Lenard in 
1900, and more recently and completely by Hughes, P.C.P.S. 
1911) is relatively feeble when contrasted with the more 
vigorous activity of X rays. 

Again, all three agencies cause the ejection of corpuscles 
from metals, and experiment has shown that : 

(1) The intensity of the incident rays does not affect the 

speed, but merely the number of the ejected corpuscles 
(p. 140). 

(2) The speed is controlled by the quality of the incident 

rays, 1 but not at all by the metal (p. 141). [With 

1 In the case of ultra-violet light, Hughes (P.T. 1912) finds experimentally 
that the energy rather than the speed of the fastest electrons is proportional 
to the frequency of the light. This confirms a deduction from Planck's 
quantum theory (p. 205), which regards the photoelectric effect as due to 
a quantum handing over its energy to an electron. Hughes found the 
velocity of emission to vary from metal to metal. 



THE NATURE OF THE X RAYS 207 

ultra-violet light the range of speeds is from about 
10 7 to 10 8 cms. per sec. ; with X rays, 10 9 to 10 10 ; with 
7 rays, 10 10 to 2*99 x 10 10 .] 

(3) The secondary corpuscles tend to continue in the line 

of flight of the original rays (p. 139). 

(4) There is a selective emission of corpuscles for certain 

wave-lengths of the rays (pp. 119 and 142). 

These results are common to all three rays. 

There are further points of resemblance. As was noticed 
on p. 108, if an element is exposed to X rays, then, in general, 
two different classes of X rays are given out by the sub- 
stance. Of these, one is identical in nature with the incident 
rays, and is nothing more than so much scattered radiation ; 
the other is a radiation characteristic of the element, and 
does not depend at all on the nature of the exciting rays, 
provided only that they are harder than the characteristic 
radiation. This latter feature at once recalls Stokes' law of 
fluorescence. Apart from some exceptions, Stokes' law 
that the frequency of the exciting light is always greater than 
that of the fluorescent light holds generally for light rays. 
The analogy with X rays is complete. 

An even more striking similarity is presented if the dis- 
tribution of the two secondary X radiations is compared 
with the distribution of light in kindred circumstances. 
When light is allowed to fall on minute particles in sus- 
pension, as in a fog, it is found that the scattered light 
is not uniformly distributed, but varies in amount in 
different directions. The scattered light emitted parallel 
to the original beam is double that at right angles ; 
the intermediate intensities are proportional to (l-fcos 2 $), 
where is the angle measured from the original beam. 
But if the particles emit fluorescent light as well as 
scattered, the fluorescent light is equally intense in all 
directions. 

In just the same way, it is found (p. 110) that the intensity 
of the scattered X rays obeys, at any rate approximately, 
a (1 -J-cos 2 0) law over a considerable angular range ; and that 
the " fore and aft " intensity is very roughly twice that at 



208 X RAYS 

right angles. And, to complete the parallel, the character- 
istic X radiations show a uniform distribution just as 
fluorescent light does. 

Other points of resemblance between X rays and light 
rays have been noticed from time to time in the preceding 
pages. One point of difference is provided by the pheno- 
mena of absorption. In the case of light, it is known that 
many of the dark lines in the absorption spectrum of a body 
are in the same position as the bright lines in its emission 
spectrum : in other words, a body, under suitable con- 
ditions, is capable of absorbing strongly its characteristic 
light radiations. But, with X rays, ^on the contrary, we 
find that an element is especially transparent to its char- 
acteristic X radiations (see p. 129), and it is only for rather 
harder rays than these that the absorption becomes abnor- 
mally large. 

We may now consider the case for the restricted entity 
hypothesis. It will be convenient first of all to recall the 
main features of Stokes' famous theory of the X rays. 

THE ETHER-PULSE THEORY OF STOKES. 

Sir George Stokes promulgated the pulse theory of the 
X rays in the Wilde Lecture before the Manchester Literary 
and Philosophical Society, on July 2, 1897. He considered 
that " when the charged molecules 1 from the cathode strike 
the target, it is exceedingly probable that by virtue of their 
charge they produce some sort of disturbance in the ether. 
This non-periodic disturbance or ' pulse ' would spread in 
all directions, so that, on this view, the Rontgen emanation 
consists of a vast and irregular succession of isolated and 
independent pulses starting from the points and at the 
times at which the individual charged molecules impinge 
on the target. We know of no reason beforehand forbidding 
us to attribute an excessive thinness to the pulses " ; and 
to the narrowness of these pulses Stokes attributed some of 
the differences between ordinary light and X rays, which, 

1 This was in the days when the cathode rays were thought to be mole- 
cules. 



THE NATURE OF THE X RAYS 



209 



apart from this, resemble each other closely x : both consist 
of electric and magnetic forces at right angles to each other 
and to the direction of propagation, but in the X rays there 
is not that regular periodic character occurring in trains of 
waves of uniform wave-length. 

Thus a Rontgen pulse on Stokes' theory is somewhat 
analogous to the crack of a whip when it is suddenly stopped, 
or the flash of flame when a 
projectile strikes a target. 
Briefly, the theory claims 
that the energy of an X ray 
is contained within a thin 
spherical shell which travels 
outwards with the speed of 
light in all directions, from 
the place where the speed of 
a cathode ray is suddenly 
changed. The faster the 
cathode ray and the more 
abrupt the change in its 
speed, the thinner and more 
energetic the pulse. By the 
laws of electrodynamics, such 
pulses of intense electric and 
magnetic forces are inevitable when rapidly moving elec- 
trified particles are suddenly stopped or started. 

The Polarisation of X Rays. 

The polarisation of X rays (p. 110) follows as an immediate 
deduction from the pulse theory. The theory contemplates 
secondary radiation as owing its origin to the disturbances 
produced in the corpuscles when the primary X rays pass 
over them. While the X rays are thus accelerating the 
corpuscles, each gives out a pulse of electric and magnetic 

1 It should here be mentioned that, as a result of the work of Rayleigh, 
Schuster, and others, our notions of the nature of white light have been 
modified in recent years, and it is now generally accepted that white light 
(like " independent " X rays) consists of irregular pulses which are capable 
of being transformed into trains of sine-waves by the various diffracting 
or refracting instruments. 

O 




FIG. 95. Representation of spreading 
pulse, showing kink in line of force OPQR 
attached to the charged particle O, the 
velocitj' of which has been suddenly altered. 



210 X RAYS 

force the secondary Rontgen pulse. A single primary 
pulse may produce a great number of secondary pulses with 
properties which depend on the grouping, etc., of the cor- 
puscles. Thus, on this point of view, there is, to use Sir 
J. J. Thomson's apt comparison, much the same difference 
between the primary and secondary rays as there is between 
the sharp crack of lightning and the reverberations of 
thunder. 

The argument in the polarisation experiments is that since 
in an X-ray tube the cathode rays are all travelling in the 
same direction, then in the resulting pulses the electric 
forces (which are at right angles to the direction of motion 
of the cathode rays) will lie in planes passing through that 
direction, and not at right angles to it (see Fig. 57). In 
other words, the particular pencil of X rays which is employed 
will be concentrated in the plane which contains both X rays 
and cathode rays. 

Hence the motion of the excited corpuscles in the radiator 
will also be mainly in this plane, and so the intensity of 
the secondary radiation will be a minimum in this plane, and 
a maximum at right angles to it, a result which agrees with 
that actually found. The fact that the X rays are only 
partially polarised, may be ascribed to the fact that the 
cathode rays in the X-ray tube are not stopped in a single 
collision, but describe many directions before finally coming 
to rest. 

Modification of Spreading-pulse Theory necessary. 

But experiment has clearly established that the theory 

of the spreading pulse needs extensive modification. It 

will be profitable to review the trend of the results (to 

some of which we have already referred), that have led to 

the theory in its modified form. Categorically these are : 

(1) When X rays encounter a gas, only an exceedingly 

small fraction less than one in a billion of its 

molecules become ionised x (p. 145). 

1 The same difficulty occurs in understanding why, when ultra-violet 
light falls on metals which show photoelectric properties, such a very 
small proportion of the particles are liberated. 



THE NATURE OF THE X RAYS 211 

(2) The extent of this ionisation is unaffected by tempera- 

ture (p. 148). 

(3) When X rays encounter a metal, the corpuscles ejected 

have a velocity which 

(a) does not depend on the intensity of the X rays, 

and so is independent of the distance of the 
metal from the X-ray bulb (p. 140), 

(b) increases continuously with the hardness of the 

X rays (p. 140), 

(c) does not depend on the nature of the metal 

(p. 141), 

(d) is equal to the velocity of the cathode rays in 

the X-ray bulb (p. 142). 

(4) These ejected corpuscles are not evenly distributed, 

but tend to pursue in the main the original direction 
of the X rays. The effect is most pronounced with 
metals of small atomic weight and hard X rays (p. 139). 

In considering the first result, we may recall that according 
to the ether-pulse theory in its original form, all the molecules 
of a gas are equally exposed to the X rays, and we are led 
to infer that those few which become ionised must have been 
in a state very far removed from the average. Their ab- 
normal condition cannot be attributed to an exceptionally 
high kinetic energy, for the kinetic theory of gases would 
then require that the ionisation should vary rapidly with 
the temperature and we are immediately confronted with 
result (2). 

We may, however, claim that the ionisation is controlled 
by the internal conditions of the different atoms, rather than 
by their kinetic energy. The phenomena of radioactivity 
lead us to believe that atoms possess large stores of internal 
energy which are not readily unlocked by outside agencies ; 
and if it should be the case that the possession of an excep- 
tional amount of internal energy means weakened stability, 
then it might easily happen that only abnormal atoms 
would be ionised by X rays. Or, again, it might be that an 
atom is capable of collecting energy from many X rays 
until it has enough for one electron. On either view, the 



212 X RAYS 

ejection of corpuscles from a metal subjected to X rays is 
interpreted as the outward sign of a sort of radioactive 
explosion of some of the atoms rendered temporarily un- 
stable. The X ray thus acts merely as a trigger to start the 
explosion ; the corpuscles come from the atom, and owe 
their energy to it alone. That their velocity should be 
independent of the intensity of the X rays follows at once, 
and is in accord with result (3a). 

But we have now to explain why the speed of the corpuscles 
is not independent of the quality of the X rays (result (4)). 
Why should the velocity be greater when the X rays are 
hardened, if their only effect is that of a trigger action ? 
and further, why should the path of a corpuscle be in- 
fluenced by the direction of the X ray, if the latter merely 
precipitates the disintegration of the atom ? On the 
explosion theory, neither result could be anticipated ; nor 
should we be unreasonable in expecting that the disintegra- 
tion of different metals would lead to very different velocities 
of the ejected corpuscles. The reverse is the case. We are, 
in fact, left with no alternative but to suppose that the 
energy of the corpuscle is derived from that of the X ray. 

Now, result (3a) remarks that the energy of the corpuscle 
is independent of the distance of the X-ray tube. But, 
for reasons similar to the above, the X ray must derive its 
energy directly from the parent cathode ray, and, according 
to the pulse theory, it distributes this energy over an ever- 
enlarging surface. The argument is fatal to the spreadingX 
pulse theory. The energy of the X ray must, it is evidelit, 
be confined within very narrow bounds which do not widen 
as the X ray travels. 1 Combining this result with (3d,) 
above, we are led to conclude that the X ray is a minute 
entity whose energy is not frittered away along its track, 
but is handed over completely to one corpuscle and no 
more on suitable encounter with an atom. 

This is a statement of the case for the entity hypothesis, 
and the difficulty remains of reconciling it with the ordinary 
electromagnetic theory of Maxwell. Of the attempts which 

1 Sommerfeld (1911) maintains that the pulse theory is competent to 
explain part, at any rate, of this localisation of energy. 



THE NATURE OF THE X RAYS 213 

have been made, we may here refer briefly to the broad 
outlines of Sir J. J. Thomson's nucleated pulse theory. 



THE NUCLEATED OR LOCALISED PULSE THEORY OF 
J. J. THOMSON. 

Sir J. J. Thomson's theory of the X ray assumes a fibrous 
structure in the ether, and pictures a corpuscle as the seat 
of a tube of force which stretches out into space. When a 
cathode ray has its velocity altered, the radiated energy runs 
along this tube, as a kink runs along a stretched wire. 
The energy is confined to the region of the kink, and it is 
not given up until it strikes a corpuscle, to which it can 
then transfer its energy without waste. The nucleated pulse 
is equivalent to Stokes' pulse, with the exception that instead 
of spreading out uniformly in all directions, it is confined to 
one direction only. 

Professor Thomson further believes that the energy of light 
is distributed in analogous fashion ; that individual light 
waves are not continuous, but correspond to a collection of 
wires along which the various disturbances travel ; and that 
if a wave-front could be made visible we should get, not con- 
tinuous illumination, but a series of bright specks on a 
dark ground. The energy is not, therefore, uniformly dis- 
tributed throughout the whole volume of the waves, but is 
concentrated in " bundles." 

The rays diminish in intensity with increasing distance 
owing to the greater separation of the " batches," and not 
to the enfeeblement of individual units. The distribution 
of energy thus resembles that contemplated by the New- 
tonian emission theory of light, according to which the 
energy was located on moving particles sparsely dissemi- 
nated throughout space. 

In the case of X rays the phenomena are sharply defined, 
but with light rays they are much more involved. The dis- 
continuous wave-front theory, in fact, regards X and y rays 
as light in its ultimate simplicity. This agrees with experi- 
ment : the general laws covering the behaviour of X rays 
are obeyed with fewer exceptions than is the case with light. 



214 X RAYS 

" Fluctuation " Experiments with y Eays. 

Experimenters have naturally sought to establish by direct 
means the presumed discrete nature of light rays and 
X rays. 

As is well known, the spinthariscope of Sir Wm. Crookes 
exhibits for the a rays of radium fluctuations both in time 
and space. Similarly, the effects predicable for /3 rays have 
been observed ; and since 1910 a number of workers, among 
them von Schweidler (1910), E. Meyer (1910), Laby and Bur- 
bidge (1912), and Burbidge (1913), have endeavoured to 
detect corresponding fluctuations in the ionisation produced 
by y rays in a gas. For this kind of work, a steady source 
of rays is absolutely essential, and so y rays have been worked 
at rather than X rays. 

Laby and Burbidge (P.E.S. 1912) used two ionisation 
chambers, identical in all respects, and disposed them sym- 
metrically about and equidistant from the radium emitting 
the y rays. If the y radiation has a spherical wave surface, 
then the ionisations in the two chambers will have a con- 
stant ratio. If, on the other hand, the y rays are circum- 
scribed entities, emitted in random directions, as a rays are, 
then the number entering each chamber in a given time will 
fluctuate. ' There is one outstanding difficulty : if Prof. 
Bragg's view as to the indirect process of ionisation by y rays 
is correct (p. 146), the fluctuations might be produced by a 
variation in the number of /3 rays generated by each y ray. 
The fluctuations which Laby and Burbidge actually observed 
in their experiments cannot, therefore, be interpreted with 
certainty. 

More recently E. Meyer (A.d.P. March 1912), using some- 
what similar apparatus, finally concluded that a single y ray 
can produce ionisation in more than one direction and on 
more than one occasion : the numbers of y rays emitted by 
the same source in two different directions do not appear to 
be independent. Meyer's experimental arrangements have 
been criticised by Burbidge (P.R.S. 1913). Meyer's results 
are, however, in accord with those of Rutherford (P.M. Oct. 
1912), who has recently found reasons for supposing that a 
swift /3 ray may give rise to several y rays in escaping from 



THE NATURE OF THE X RAYS 215 

an atom, and still retain part of its original energy. One 
may here refer to the work of Chadwick (P.M. 1912), who 
has obtained evidence that a rays, like /3 rays, are able to 
excite y rays when they fall on ordinary matter. This would 
suggest that it is kinetic energy rather than velocity which 
is the determining factor. 

" Fluctuation " Experiments with Light Rays. 

N. R. Campbell (P.C.P.S. 1909, 1910) attacked the 
problem of light emission by the " fluctuation " method, 
with the object of discriminating between the ordinary and 
entity light hypotheses. Unfortunately the difficulty of 
finding a source of light which is very intense and also 
extremely constant proved insurmountable. 

Taylor (P.C.P.S. 1909) approached the problem from a 
different standpoint. All ordinary optical phenomena are 
average effects, and are therefore incapable of differentiating 
between the usual electromagnetic theory of light and a 
restricted entity type. If, however, the intensity of light in 
a diffraction pattern were so greatly reduced that only a 
few of the indivisible bundles of energy could occur at once 
on a zone, the ordinary phenomena of diffraction would be 
modified. The method of attack was to photograph the 
shadow of a needle under various illuminations, and with 
exposures chosen such that the total energy supplied was 
constant. Exposures ranging from a few seconds to three 
months were employed, but no variation in the sharpness of 
the diffraction pattern could be detected in the different 
photographs. 

Thus the more direct attempts to confirm the " discon- 
tinuous " nature of light and of X rays have not met with 
success. 

The Outstanding Problem of Radiation. 

It will be apparent that the problem of the nature of the 
X ray cannot yet be dismissed. We have succeeded in estab- 
lishing the essential identity of X rays and light rays, and 
the interest has, accordingly, shifted its ground. The diffi- 
culties, conspicuous with Rontgen rays, have merged into 



216 X RAYS 

those which all classes of electromagnetic waves are found 
to present in greater or less degree. The full secret of the 
nature of X rays will doubtless be revealed when we find 
the key to the overshadowing problem of the mechanism of 
radiation in general. We are left confronted with the riddle 
of modern physics. 



APPENDIX I. 

IN connection with Ront gen's discovery, Sir James Mac- 
kenzie Davidson has been kind enough to write down for 
me his recollections of an interview with Prof. Rontgen not 
very long after the discovery of the X rays. 

" While travelling on the Continent in 1896 I made a 
pilgrimage to Wiirzburg, and called at Professor Rontgen' s 
house in the evening, and was kindly granted an appoint- 
ment for the folio whig morning. I presented myself about 
11 a.m., and was shown into a laboratory which contained 
a coil and a small cylindrical-shaped X-ray tube. Pro- 
fessor Rontgen, a tall man with dark bushy hair, a long 
beard, and very kindly and expressive eyes, received me 
cordially. He could not speak much English ; I was still 
worse at German. However, by means of English and 
some Latin we made ourselves intelligible to one another. 
He excited the tube and showed me various shadows on a 
fluorescent screen. On each of the terminals of his coil 
he had a small aluminium ball, 1 cm. in diameter, which 
he told me he used as an alternative spark-gap to test the 
hardness of the tube. He incidentally remarked that he 
found a tube had its maximum photographic effect when 
it was working just at 2' 5 cms. alternative spark a fact 
which I have always found to be correct. I asked some 
blunt questions as follows : 

Q. " * What were you doing with the Hittorf tube when 
you made the discovery of the X rays ? ' 

A. " ' I was looking for invisible rays.' 

Q. "'What made you use a barium platino-cyanide 
screen ? ' 



218 X RAYS 

A. " ' In Germany we use it to reveal the invisible rays 
of the spectrum, and I thought it a suitable substance to 
use to detect any invisible rays a tube might give off.' 

{( He then detailed how he made the discovery. He said 
he had covered up the Hittorf tube with black paper so 
as to exclude all light, and had the screen (which was simply 
a piece of cardboard with some crystals of barium platino- 
cyanide deposited on it) lying on a table 3 or 4 metres from 
where the covered tube was situated, ready to be used. 
He excited the tube to ascertain if all light was excluded. 
This was so, but to his intense surprise he found the distant 
screen shining brightly ! 

"I asked him, 'What did you think ? ' He said very 
simply, ' I did not think, I investigated.' 

" Incidentally, he told me how he had taken a photograph 
through a pine door which separated two of his labora- 
tories. On developing the negative, he found a white band 
across it, which, he ascertained, corresponded to the beading 
on one of the door panels. He stripped the beading off, 
arid found the band of shadow was due not to the increased 
thickness of wood but to the ' plumbum ' (white lead 
really) the doormaker had employed in attaching the strip 
of wood. 

" He seemed amused at my remonstrating with him about 
keeping the ' screen ' lying about in his laboratory. I 
told him it was a ' historical screen,' and should be pre- 
served in a glass case ! I hope he has carried out this 
suggestion. For the sudden shining of that ' screen ' un- 
doubtedly led to one of the greatest discoveries in modern 
times." 

J. M. D. 



APPENDIX II. 



THE COOLIDGE X-RAY TUBE. 

DR. W. COOLIDGE l (P.R. Dec. 1913) has designed a new 
X-ray bulb which marks an important step in the progress 
of radiography and radiotherapy. The cathode and anti- 
cathode are mounted parallel to and facing each other, 
some 2 cms. apart, in the centre of the bulb. The chief 
novelty lies in the cathode. This consists of a small flat 
spiral of tungsten wire, surrounding which is a molybdenum 



Hor C&fhode 



Tungsten AnricJ"hode 





Cooling Rings 
CATHODE 



Hor 

Tungsfen 

SpiraJ 



Molybdenum 

focussing Tube 
FIG. 96. Coolidge X-ray tube. 

tube, the two being electrically connected. The tungsten 
spiral is heated by a subsidiary electric current (as with a 

1 Of the General Electric Co.'s Research Laboratory, Schenectady, New 
York. 



220 X RAYS 

Wehnelt cathode, p. 8), and so becomes a source of electrons 
(or cathode rays) to an extent which increases rapidly with 
the temperature. The molybdenum tube serves to focus 
the stream of electrons (see p. 71) on the anticathode, 
which is of tungsten and unusually heavy. There is no 
additional anode. 

The vacuum within the tube is extremely high about 
1000 times that of an ordinary X-ray tube with the result 
that unless the cathode is heated, it is impossible to send 
a discharge through the tube. Furthermore, the greatest 
care is taken in freeing the electrodes and the glass walls 
from gas, before sealing off, after exhaustion. 

The intensity of the X rays is precisely and readily con- 
trolled by adjusting the temperature of the cathode. At 
high temperatures (2300 C. or so) an enormous output of 
X rays is possible. For example, such a tube has been 
run, on a 7 cm. alternative spark-gap, for hours at a time, 
with a steady current of 25 milliamperes passing through 
the tube continuously. 

The penetrating power of the rays depends, as in the 
ordinary tube, on the potential difference between the 
electrodes. Either direct or alternating potential can be 
used to excite the tube, for the hot cathode allows the 
current to pass only in one direction, except indeed when 
the anticathode also becomes very hot. As a result, with 
a coil discharge, the " inverse " current is entirely 
abolished. 

Owing to the low pressure, positive rays do not play an 
essential role, and there is in consequence no evidence of 
cathodic sputtering. There are slight traces of blackening 
due to vaporisation of the tungsten. 

By reason of the care taken in exhausting the tube, there 
is no appreciable change in the vacuum, and, therefore, in 
the output of X rays, even after a run of many hours. The 
focal spot does not wander but remains perfectly steady. 

The starting and running voltages are the same, and the 
tube is remarkable in showing no fluorescence of the glass 
as in the ordinary X-ray tube, so that its appearance affords 
little notion of the output or indeed of any activity at all. 



APPENDIX II 221 

The apparatus appears to indicate a very great step for- 
ward, and once the technique is mastered, we are justified 
in expecting revolutionary changes in X-ray dosage and 
radiography. 

THE LlLIENFELD X-RAY TtTBE. 

In the Lilienfeld tube, the disposition and design of both 
anticathode and cathode are as in the usual X-ray tube. Two 
additional " priming " electrodes are provided ; one of these, 
which is in close proximity to the anticathode, is raised to 
incandescence. The other, which is an aluminium cylinder, 
forms the anode of this priming circuit. As in the Coolidge 
tube, the vacuum is extremely low, but it suffices to apply 
about 500 volts to the priming circuit, in order to enable 
the main discharge to pass. 

The tube permits ready and accurate adjustment of the 
output, and exhibits no hardening effects. One of them 
has been in steady use for a considerable time at Leipzig 
For further particulars see a paper by Dr. Rosenthal in the 
Fortschritte aufdem Gebiete der EontgenstraJilen for June 1913, 
an abstract of which is given in A.Rt.R. for Feb. 1914. 



APPENDIX III. 



THE PRODUCTION OF HIGH VACUA. 

A BRIEF notice may be taken of the present methods of 
exhausting vacuum tubes. The very highest vacua can be 
got by making use of the extraordinary absorptive powers 
for gases of charcoal (e.g. cocoanut char- 
coal) when immersed in liquid air a 
remarkably quick and effective method 
we owe to Prof. Dewar. Oxygen, nitro- 
gen, water vapour, etc., are absorbed to 
large extents, hydrogen rather less so, 
helium and neon least of all. It is 
essential that the charcoal should be 
previously heated in situ x and the 
emitted gas pumped off before applying 
the liquid air. Angerer (A.d.P. 1911) 
records a pressure of 8xlO~ 7 mm. Hg 
by the use of charcoal and liquid air. 

With one exception, all the various 
mechanical pumps for the production of 
vacua employ the plan of repeatedly 
abstracting and isolating, by means of 
a solid or liquid " piston," a certain 
fraction of gas from the vessel to be exhausted, and 
delivering it elsewhere. The exception is a strikingly 
ingenious " molecular pump " recently introduced by Gaede. 2 

1 As a practical precaution, a plug of glass-wool should be inserted above 
the charcoal, to stop the small carbon particles, which are expelled when 
the charcoal is heated, from passing over into the apparatus. 

2 See Engineering, Sept. 20, 1913. 




FIG. 97. Tube contain- 
ing charcoal immersed in 
liquid air. 



APPENDIX III 223 

It depends for its success on the viscous dragging of gas 
by the surface of a cylinder rotating within a second 
cylinder at a speed comparable with the velocities of the 
molecules of the gas, which are accordingly impoverished in 
one direction and accumulated in the other. The pump is 
extremely rapid in action, but requires the initial pressure 
to be reduced to a few mms. of mercury by an auxiliary 
pump. There is no piston, but always free communication, 
through the molecular pump, between the vessel to be ex- 
hausted and the auxiliary pump. With a speed of rotation 
of 12,000 revs, per min., and an initial vacuum of ^V mm., 
Gaede records the remarkably low pressure of 2 xlO~ 7 mm. 
It is very interesting to note the susceptibility of the pump 
to the molecular velocity of the gas present. For the same 
velocity of the cylinder, a lower pressure is attainable with 
air (molecular velocity 500 metres/sec.) than with hydrogen 
(molecular velocity 1800 metres/sec.), as may be shown by 
scavenging the vacuum with one or the other gas. The 
pump shares with the cooled-charcoal method the advantage 
of not requiring any drying agent vapours are sucked 
away as readily as gases. For such a rapid type of pump 
the connecting tube must not be restricted in bore ; the 
remark, indeed, applies to all pumps. 

Still more recently (1914), Gaede has put on the market 
a new hand-driven piston pump, which can produce very 
high vacua with great rapidity, and is capable of dealing 
with water- vapour. 

Next come the various types of mercury pumps the 
Topler and Sprengel in a variety of modifications, some 
of them designed to be automatic in action. The rotary 
mercury pumps, such as the Gaede, have come into extensive 
use, and possess the great advantage that they can be motor- 
driven a feature commending itself to all who have worked 
with the hand-manipulated Topler. In regard to mercury 
(and oil) pumps it is well to remember that they will not 
pump vapours, 1 and that the vapour pressure of mercury 

1 To obtain high vacua it is, therefore, necessary to remove water vapour 
by means of a drying agent such as phosphorus pentoxide. Other vapours 
can be frozen out by means of liquid air. 



224 X RAYS 

at ordinary temperatures is about TTTOTF mm. mercury 
a fact which does not always tally with the claims sometimes 
advanced on behalf of this or that pump. 1 Mercury pumps 
ought not to be set the task of exhausting from a high 
initial pressure they work best as finishing-off pumps. 

For the earlier stages of exhausting there is available a 
variety of oil-pumps which can be motor-driven, and some 
of which can deal with large quantities of gas. Ordinary 
heavy engine-oil works well in these pumps and has a low 
vapour pressure. A drying chamber should be used in con- 
junction with an oil-pump, or the oil may emulsify and the 
efficiency of the pump will suffer. 

Gas held by Walls of Tube. 

A great deal of gas mostly hydrogen and moisture is 
held by the electrodes and the walls of a vacuum tube. To 
liberate the gas, the discharge should be run for some time 
to suit the conditions under which the tube is intended to 
be used. 2 This, of course, ought to be done by the maker 
before the tube is sealed off from the pump. The walls of 
the tube hold this surface gas tenaciously it appears to be 
largely moisture which is held bound as a condensed surface 
layer. To get rid of it, the tube has to be heated to between 
300 and 400 C., at which stage there is a great evolution of 
gas. If this is pumped off while the tube is hot, the vacuum 
will be found to improve greatly when the tube is cooled, 
and will not deteriorate with time so much as it otherwise 
would. 

" Finishing-off " Processes. 

There are one or two " finishing-off " processes to follow 
a pump, which are well known to research workers. Cocoa- 
nut charcoal, when used as anode, or the liquid alloy of 
potassium and sodium, when used as cathode, absorb 

1 It is, however, possible for a pump to exhaust somewhat lower than the 
vapour pressure of the liquid used. A really good water injector (filter) 
pump will exhaust to about 7 mms. of mercury, whereas the vapour pressure 
of water at atmospheric temperatures is some 12 to 15 mms. 

2 At higher pressures, more current can be passed through the tube, and 
the electrodes can be made hotter than at very low pressures. 



APPENDIX III 225 

ordinary gases, if the discharge is not too heavy. Yellow 
phosphorus is converted to red by bombardment with cathode 
rays ; the change is accompanied by a diminution in pressure, 
due partly to the lower vapour pressure of the red allo trope, 
and partly to the fact that under the discharge the red 
phosphorus combines with any oxygen, nitrogen or hydrogen 
present, forming compounds with very small vapour pres- 
sures. This latter method is used in exhausting the Lodge 
vacuum valve (p. 67) : the presence of the phosphorus 
compounds is further useful in regulating the vacuum 
during the subsequent use of the valve. 

Table XXII. gives a notion of the capabilities of various 
pumps. 

TABLE XXII. 



Pump. j Attainable Vacuum. 

mm. Hg. 

Gaede molecular - 0-000,000,2 

Gaede rotary (mercury) 0-000,01 

Improved Topler (mercury) 0-000,01 

Gaede piston 0*000,05 

Geryk (oil) - 0-000,2 

Sprengel (mercury) 0*001 

Injector (water) - 7 



APPENDIX IV. 
ELECTRICAL INSULATORS. 

Or the available insulators, ebonite, sulphur, amber, sealing 
wax and fused silica are at present the only ones at all 
suitable for electroscopic work. With all of these, care 
should be taken to avoid fingering grease is fatal to insula- 
tion. In testing insulation, it should be remembered that 
a delicate electroscope may indicate signs of surface electri- 
fication for some hours after new insulation has been put 
in. Such electrification may be dissipated by means of a 
spirit lamp, or, better, by placing some uranium oxide near 
the insulator. To reduce the absorption of the electric 
charge which occurs to a greater or less extent with all 
insulators, 1 the size of insulating supports should be kept 
as small as possible in electrometer work. 

Ebonite that is really good is difficult to obtain nowadays ; 
it seems to be regarded by most rubber manufacturers as a 
convenient means of using up rubber refuse unfit for any- 
thing else. Some of its defects are occasionally due to the 
materials used in polishing. Modern ebonite ages with some 
rapidity in sunlight, and on damp days may, owing to the 
film of sulphuric acid which forms on its surface, almost play 
the role of a conductor. In a room which gets much sun- 
shine most modern ebonite usually turns a dirty yellow 
colour in a few weeks, though some of the ebonite made ten 
or twenty years ago will exhibit no signs of deterioration. 
Notwithstanding its defects, ebonite which has had its 

1 Paraffin wax, which is an excellent insulator, shows this soaking effect 
to a marked and objectionable degree. 



APPENDIX IV 227 

surface recently renewed is an excellent insulator. Ebonite 
offers the great advantage of being easily workable. 

Sulphur is convenient, in that it can be cast to shape. 
In this operation the vessels (glass or porcelain) and sulphur 
used should be clean, and the temperature should be raised 
but slightly above the liquefying point of the sulphur. In 
this limpid condition it can, for example, be poured or 
sucked into clean warmed glass ! tubing, if sulphur plugs 
are required. The tubing can be readily slipped off later 
by slightly warming the outside. For some hours after 
solidification, sulphur can be turned to size or pared to shape 
with great ease. The insulating properties improve for some 
time after setting. There is no better insulator than sulphur, 
but, after a few months, especially in a room which gets 
much sunshine, its insulating qualities generally fall off to 
a considerable extent. 

Amber is an excellent insulator, and is almost always 
reliable. It can, of course, be obtained in the form of pipe 
stems, which can be mounted in position with sealing wax. 
The Amberite and Ambroid companies supply amber pressed 
to convenient shapes and sizes. Amber has the disadvan- 
tages of being somewhat brittle and rather expensive. 

Sealing Wax is particularly useful in that it combines the 
qualities of an insulator and an air-tight cement with a very 
low vapour pressure. The insulating properties depend very 
much on the quality of the wax. One of the most reliable 
is " Bank of England." The usual care must be taken to 
avoid indiscriminate fingering. The insulating ability of 
the wax will be impaired, if in its manipulation it is allowed 
to catch fire and carbonise, or if a luminous flame is used. 
As shellac is hygroscopic, sealing wax as an insulator is 
somewhat susceptible to damp weather. 

Fused Silica yields place to none in its insulating qualities. 
Its specific resistance has been determined at the National 
Physical Laboratory to be greater than 2 x 10 14 ohm cms. 
at 16 C. Fused silica is practically independent of atmos- 
pheric humidity, and in the form of rod or tubing is par- 
ticularly convenient as an insulating material. It is the only 

1 Not metal, unless lined, say, with paper. 



228 X BAYS 

high-class insulator which is unimpaired by moderate heat ; 
it is, however, spoilt if subjected to very high temperatures. 
Fused silica is now very cheap, but unfortunately the 
modern furnace methods of production cannot be relied upon 
to yield a product which [possesses the insulating properties 
of the more expensive silica made by the oxy hydrogen flame. 
This remark applies alike to the clear transparent variety 
and the air-streaked satin-like kind. The furnace silica 
seems to be contaminated in some way, possibly by carbon 
from the electric furnace. Silica intended for insulation 
purposes should, of course, be alkali-free. 



APPENDIX V. 



TABLE XXIII. 



THE ELEMENTS IN THE ORDER OF ATOMIC 
WEIGHTS. 



International atomic weights for 1914; O =16. The international 
atomic weights are fixed yearly by an international committee of 
chemists, consisting at present of Profs. F. W. Clarke (U.S.A.), W. 
Ostwald (Germany), T. E. Thorpe (Gt. Britain), and G. Urbain 
(France). The list below comprises 83 elements. 



Atomic 
Weight. 
= 16. 


Element. 


Sym- 
bol. 


First isolated by 


Date. 


1-008 


Hydrogen 


H 


Cavendish 


1766 


3-99 


Helium - 


He 


Ramsay and Cleve * 


1895 


6-94 


Lithium - 


Li 


Arfvedson 


1817 


9-1 


Beryllium 










(Glucinum) - 


Be 


Wohler and Bussy 


1828 


11-0 


Boron 


B 


Gay-Lussac & Thenard 


1808 


12-00 


Carbon - 


C 





Prehistoric 


14-01 


Nitrogen - 


N 


Rutherford 


1772 


16-00 


Oxygen - 





Priestley and Scheele 


1774 


19-0 


Fluorine 


F 


Moissan 


1886 


20-2 


Neon 


Ne 


Ramsay and Travers 


1898 


23-00 


Sodium - 


Na 


Davy 


1807 


24-32 


Magnesium 


Mg 


Liebig and Bussy 


1830 


27-1 


Aluminium 


Al 


Wohler 


1827 


28-3 


Silicon - 


Si 


Berzelius 


1823 


31-04 


Phosphorus 


P 


Brand 


1674 


32-07 


Sulphur - 


S 





Prehistoric 


35-46 


Chlorine 


Cl 


Scheele 


1774 


39-10 


Potassium 


K 


Davy 


1807 


39-88 


Argon 


A 


Rayleigh & Ramsay 


1894 


40-07 


Calcium - 


Ca 


Davy 


1808 


44-1 


Scandium 


Sc 


Nilson and Cleve 


1879 


48-1 


Titanium 


Ti 


Gregor 


1789 


51-0 


Vanadium 


V 


Berzelius 


1831 


52-0 


Chromium 


Cr 


Vauquelin 


1797 


54-93 


Manganese 


Mn 


Gahn 


1774 


55-84 


Iron 


Fe 





Prehistoric 


58-68 


Nickel - 


Ni 


Cronstedt 


1751 


58-97 


Cobalt - 


Co 


Brand 


1735 


63-57 


Copper - 


Cu 





Prehistoric 


65-37 


Zinc 


Zn 


Mentd. by B. Valentine 


15 centy. 


69-9 


Gallium - 


Ga 


L. de Boisbaudran 


1875 


72-5 


Germanium - 


Ge 


Winkler 


1886 


74-96 


Arsenic - 


As 


Albertus Magnus 


13 centy. 


79-2 


Selenium 


Se 


Berzelius 


1817 


79-92 


Bromine 


Br 


Balard 


1826 


82-92 


Krypton 


Kr 


Ramsay and Travers 


1898 


85-45 


Rubidium 


Rb 


Bunsen and Kirchhoff 


1861 



* Janssen and Lockyer (in sun), 1868. 



230 X RAYS 

THE ELEMENTS IN THE ORDER OF ATOMIC WEIGHTS Continued. 



Atomic 
Weight. 
= 16. 


Element. 


Sym- 
bol. 


First isolated by 


Date. 


87-63 


Strontium 


Sr 


Davy 


1808 


89-0 


Yttrium 


Y 


Wohler 


1828 


90-6 


Zirconium 


Zr 


Berzelius 


1825 


93-5 


Niobium 










(Columbium) 


Nb 


Hatchett 


1801 


96-0 


Molybdenum - 


Mo 


Hjelm 


1790 


101-7 


Ruthenium 


Ru 


Glaus 


1845 


102-9 


Rhodium 


Rh 


Wollaston 


1803 


106-7 


Palladium 


Pd 


Wollaston 


1803 


107-88 


Silver - 


Ag 





Prehistoric 


112-40 


Cadmium 


Cd 


Stromeyer 


1817 


114-8 


Indium - 


In 


Reich and Richter 


1863 


119-0 


Tin 


Sn 





Prehistoric 


120-2 


Antimony 


Sb 


Basil Valentine 


15 centy. 


126-92 


Iodine 


I 


Courtois 


1811 


127-5 


Tellurium 


Te 


v. Reichenstein 


1782 


130-2 


Xenon - 


Xe 


Ramsay and Travers 


1898 


132-81 


Caesium - 


Cs 


Bunsen and Kirchhoff 


1861 


137-37 


Barium - 


Ba 


Davy 


1808 


139-0 


Lanthanum - 


La 


Mosander 


1839 


140-25 


Cerium - 


Ce 


Mosander 


1839 


140-6 


Praseodymium 


Pr 


Auer von Welsbach 


1885 


144-3 


Neodymium - 


Nd 


Auer von Welsbach 


1885 


150-4 


Samarium 


Sa 


L. de Boisbaudran 


1879 


152-0 


Europium 


Eu 


Demarcay 


1901 


157-3 


Gadolinium 


Gd 


Marignac 


1886 


159-2 


Terbium 


Tb 


Mosander 


1843 


162-5 


Dysprosium - 


Dy 


Urbain & Demenitroux 


1907 


163-5 


Holmium 


Ho 


L. de Boisbaudran 


1886 


167-7 


Erbium - 


Er 


Mosander 


1843 


168-5 


Thulium 


Tm 


Cleve 


1879 


172-0 


Ytterbium 










(Neo, Yb) - 


Yb 


Marignac 


1878 


174-0 


Lutecium 


Lu 


Urbain 


1908 


181-5 


Tantalum 


Ta 


Eckeberg 


1802 


184-0 


Tungsten 


W 


Bros. d'Elhujar 


1783 


190-9 


Osmium 


Os 


Smithson Tennant 


1804 


193-1 


Iridium - 


Ir 


Smithson Tennant 


1804 


195-2 


Platinum 


Pt 





16 centy. 


197-2 


Gold 


Au 





Prehistoric 


200-6 


Mercury 


Hg 


Mtd. by Theophrastus 


300 B.C. 


204-0 


Thallium 


Tl 


Crookes 


1861 


207-10 


Lead 


Pb 


Mentd. by Pliny 


Prehistoric 


208-0 


Bismuth 


Bi 


Mtd. by B. Valentine 


15 centy. 


222-4 


Radium Eman- 










ation (Niton) 


Nt 


M. and Mme. Curie 


1900 


226-4 


Radium - 


Ra 


Curies and Bemont 


1898 


232-4 


Thorium 


Th 


Berzelius 


1828 


238-5 


Uranium 


U 


Peligot 


1841 



APPENDIX V 



231 



ATOMIC WEIGHTS OF THE RADIOACTIVE ELEMENTS. 



Element. 


At. Wt. 


Element. 


At. Wt. 


Uranium 1 
2 - 
X, Y 

Ionium 
Radium - 
Ra Emanation 
Radium A 
,, B, G! - 
2 , D, E - 
F (Polonium) * - 


238-5 
234-5 
230-5 
230-5 
226-4 
222-4 
218-4 
214-4 
210-4 
210-4 


Thorium 
Meso- thorium 
Radio -thorium 
Thorium X - 
Emanation 
A - 
B, G v (7 2 - 


232-4 
228-4 
228-4 
2244 
220-4 
216-4 
212-4 
208-4 


* Probably converted into Pb. 
t Bi. 



The atomic weight of actinium is probably about 230. 
TABLE XXIV. ATOMIC WEIGHTS AND DENSITIES OF THE ELEMENTS. 



Element. 


Atomic 
Weight. 


Density. 


Element. 


Atomic 
Weight. 


Density. 


Element. 


Atomic 
Weight. 


Density. 


Al 


27 


2-70 


He 


4 


0-178$ 


Rb 


85 


1-532 


Sb 


120 


6-62 


H 


1 


0-08987J 


Ru 


102 


12-3 


A 


40 


1-78 J 


In 


115 


7-12 


Sa 


150 


7-8 


As 


75 


5-73 


I 


127 


4-95 


Sc 


44 





Ba 


137 


3-75 


Ir 


193 


22-41 


Se 


79 


4-5 


Be 


9 


1-93 


Fe 


56 


7-86 


Si 


28 


2-3 


Bi 


208 


9-80 


Kr 


83 


3-708 J 


Ag 


108 


10-5 


B 


11 


2-5 ? 


La 


139 


6-12 


Na 


23 


0-971 


Br 


80 


3-10 


Pb 


207 


11-37 


Sr 


88 


2-54 


Cd 


112 


8-64 


Li 


7 


0-534 


S 


32 


2-07 


Cs 


133 


1-87 


Lu 


174 





Ta 


181 


16-6 


Ca 


40 


1-55 


Mg 


24 


1-74 


Te 


127 


6-25 


C 


^ 




Mn 


55 


7-39 


Tb 


159 





Diamond 1 12 3-52 


Hg 


200 


13-56 


Tl 


204 


11-9 


Graphite j 
Gas carbon J 


2-3 
1-9 


Mo 
Nd 


96 
144 


10-0 
6-96 


Th 
Tm 


232 

168 


11-3 


Ce 


140 6-92 


Ne 


20 


0-9002 $ 


Sn 


119 


7-29 


Cl 


35 


3-23 J 


Ni 


59 


8-9 


Ti 


48 


4-50 


Cr 


52 


6-50 


Nb' 


93 


12-75 


W 


184 


18-8 


Co 


59 


8-6 


N 


14 


1-2507 J 


U 


238 


18-7 


Cu 


64 


8-93 


Os 


191 


22-5 


V 


51 


5-5 


Dy 


162 




O 


16 


1-429| 


Xe 


130 


5-851 J 


Er 


167 


4-77 ? 


Pd 


107 


11-4 


Yb 


172 





Eu 


152 




P 


31 


22 


Y 


89 


3-8? 


F 


19 


1-69 J 


Pt 


195 


21-5 


Zn 


65 


7-1 


Gd 


157 




K 


39 


0-862 

64 O 


Zr 


91 


4-15 


Ga 


70 


5-95 


Pr 


141 


48 








Ge 


72 


5-47 


Ra 


226 


? 


Paper 





1-0 


Au 


197 


19-32 


Rh 


103 


12-44 









J Grms. per litre at C- and 760 mm. 



232 



X RAYS 



TABLE XXV. I =I t) e'^ d . TABLE CONNECTING I/I Q AND Ad. 
E.g. if Xd = -893, I/I,, = -5. e = 2-71828. (See p. 100.) 
(From Kaye & Laby's Physical Constants.) 



For values of A^ from -0000 to '0999. 


Subtract Differences. 


\d 





001 


002 


003 


004 
9960 


005 
9950 


006 '007 


008 


009 


0001 2 3 4 


5 


6789 


oo 


1-000 


9990 


9980 


9970 


9940 


9930 


9920 


9910 


1234 


5 


6789 


01 

02 
03 

04 


9900 
9802 
9704 
9608 


9891 
9792 
9695 
9598 


9881 
9782 
9685 
9589 


9871 
9773 
9675 
9579 


9861 
9763 
9666 
9570 


9851 
9753 
9656 
9560 


9841 
9743 
9646 
9550 


9831 
9734 
9637 
9541 


9822 
9724 
9627 
9531 


9812 
9714 
9618 
9522 


1234 
1234 
1234 
1234 


5 

5 
5 
r> 


6789 
6789 
6789 
6789 


'05 


9512 


9502 


9493 


9484 


9474 


9465 


9455 


9446 


9436 


9427 


1234 


5 


6789 


06 
07 
08 
09 


9418 
9324 
9231 
9139 


9408 
9315 
9222 
9130 


9399 
9305 
9213 
9121 


9389 
9296 
9204 
9112 


9380 
9287 
9194 
9103 


9371 
9277 
9185 
9094 


9361 

9268 
9176 
9085 


9352 
9259 
9167 
9076 


9343 
9250 
9158 
9066 


9333 
9240 
9148 
9057 


1234 
1234 
1234 
1234 


5 
6 
5 
5 


6789 
6788 
6778 
6678 


For values of AtZ from 100 to 2 -999. 


Subtract Differences. 


\d 





01 


02 

8869 
8025 
7261 
6570 


03 

8781 
7945 
7189 
6505 


04 


05 


06 


07 


08 


09 


001 234 


5 

43 

39 
25 
32 


6789 


1 

2 
3 
4 


9048 
8187 
7408 
6703 


8958 
8106 
7334 
6637 


8694 
7866 
7118 
6440 


8607 
7788 
7047 
6376 


8521 
7711 
6977 
6313 


8437 
7634 
6907 
6250 


8353 

7558 
6839 
6188 


8270 
7483 
6771 
6126 


9 17 26 34 
8162331 
7142128 
6131926 


52 60 69 77 
47 55 62 70 
42 49 56 63 
384551 57 


5 


6065 


6005 


5945 


5886 


5827 


5769 


5712 


5655 


5599 


5543 


6 12 17 23 


29 


35 40 46 52 


6 
7 
8 
9 


5488 
4966 
4493 
4066 


5434 
4916 
4449 
4025 


5379 
4868 
4404 
3985 


5326 
4819 
4360 
3946 


5273 
4771 
4317 
3906 


5220 
4724 
4274 
3867 


5169 
4677 
4232 
3829 


5117 
4630 
4190 
3791 


5066 
4584 
4148 
3753 


5016 
4538 
4107 
3716 


5101621 
5 91419 
4 91317 
4 81215 


26 
2i 
21 
19 


31 37 42 47 
28 33 38 43 
26 30 34 38 
23 27 31 35 


i-o 


3679 


3642 


8606 


3570 


3535 


3499 


3465 


3430 


3396 


3362 


4 7 11 14 


18 


21 25 28 82 


11 
12 
13 
1'4 


3329 
3012 
2725 
2466 


3296 
2982 
2698 
2441 


3263 
2952 
2671 
2417 


3230 
2923 
2645 
2393 


3198 
2894 
2618 
2369 


3166 
2865 
2592 
2346 


3135 

2837 
2567 
2322 


3104 
2808 
2541 
2299 


3073 
2780 
2516 
2276 


3042 
2753 
2491 
2254 


3 6 913 
3 6 911 

3 5 810 
2579 


16 
14 
13 
12 


19 22 25 29 
17 20 23 26 
16182123 
14161921 


1-5 


2231 


2209 


2187 


2165 


2144 


2122 


2101 


2080 


2060 


2039 


2468 


11 


13151719 


1'6 
1'7 
1'8 
1-9 


2019 
1827 
1653 
1496 


1999 
1809 
1637 
1481 


1979 
1791 
1620 
1466 


1959 
1773 
1604 
1451 


1940 
1755 
1588 
1437 


1920 
1738 
1572 
1423 


1901 
1720 
1557 
1409 


1882 
1703 
1541 
1395 


1864 
1686 
1526 
1381 


1845 
1670 
1511 
1367 


2468 
2357 
2356 
1346 


10 
9 

8 

7 


12131517 
10121416 
9 11 13 14 
91011 13 


20 


1353 


1340 


1327 


1313 


1300 


1287 


1275 


1262 


1249 


1237 


1345 


6 


8 91012 


21 
2-2 
2'3 
2'4 


1225 
1108 
1003 
0907 


1212 
1097 
0993 
0898 


1200 
1086 
0983 
0889 


1188 
1075 
0973 
0880 


1177 
1065 
0963 
0872 


1165 
1054 
0954 
0863 


1153 
1044 
0944 
0854 


1142 
1033 
0935 
0846 


1130 
1023 
0926 
0837 


1119 
1013 
0916 
0829 


1245 
1234 
1234 
1233 


6 
5 
5 
4 


7 8 911 
6789 
6789 
5678 


2'5 


0821 


0813 


0805 


0797 


0789 


0781 


0773 


0765 


0758 


0750 


1223 


4 


5567 


26 
2-7 
28 
29 


0743 
0672 
0608 
0550 


0735 
0665 
0602 
0545 


0728 
0659 
0596 
0539 


0721 
0652 
0590 
0534 


0714 
0646 
0584 
0529 


0707 
0639 
0578 
0523 


0699 
0633 
0573 
0518 


0693 
0627 
0567 
0513 


0686 
0620 
0561 
0508 


0679 
0614 
0556 
0503 


1123 
1123 
1122 
1122 


4 
3 
3 
3 


4566 
4456 
3455 
3445 


For values of \d from 3'0 to 8'9. 


Subtract Differences. 


\d 





1 


2 


3 


4 


5 


6 


7 

0247 
0091 
0033 
0012 
0005 
0002 


8 


9 


Mean differences no longer 
sufficiently accurate. 


3 

4 
5 
6 
7 
8 


0498 
0183 
0067 
0025 
0009 
0003 


0450 
0166 
0061 
0022 
0008 
0003 


0408 
0150 
0055 
0020 
0007 
0003 


0368 
0136 
0050 
0018 
0007 
0002 


0334 
0123 
0045 
0017 
0006 
0002 


0302 
0111 
0041 
0015 
0006 
0002 


0273 
0101 
0037 
0014 
0005 
0002 


0224 
0082 
0030 
0011 
0004 
0002 


0202 
0074 
0027 
0010 
0004 
0001 



APPENDIX V 



233 



TABLE XXVI. CATHODE-BAY VELOCITY AND POTENTIAL. 

Cathode-ray velocities in cms./sec., corresponding to various voltages. 
The values are calculated by the formula, V = 5 95 JE . 10 7 , 
where V is the velocity and E the voltage (see p. 96). 



For voltages from to 990. 








10 


20 


30 


40 


50 


60 


70 


80 


90 


xl09 




x!0 
0-188 


x!Q9 

0-266 


x!09 
0-326 


x!09 
0-376 


x!09 
0-421 


x!09 
0-461 


x!09 
0-498 


x!09 

0-533 


xlO 

0-565 


100 
200 
300 
400 


0-595 
0-842 
1-031 
1-191 


0-624 
0-862 
1-048 
1-205 


0-652 
0-883 
1-065 
1-220 


0-678 
0-902 
1-081 
1-234 


0-704 
0-922 
1-097 
1-248 


0-729 
0-941 
1-113 
1-263 


0-753 
0-960 
1-129 
1-277 


0-776 
0-978 
1-145 
1-290 


0-798 
0-996 
1-160 
1-304 


0-820 
1-014 
1-176 
1-318 


500 


1-331 


1-344 


1-357 


1-370 


1-383 


1-396 


1-409 


1-421 


1-434 


1-446 


600 
700 
800 
900 


1-458 
1-575 
1-683 
1-786 


1-470 
1-586 
1-694 
1-796 


1-482 
1-597 
1-705 
1-806 


1-494 
1-608 
1-715 
1-815 


1-506 
1-619 
1-725 
1-825 


1-517 
1-630 
1-735 
1834 


1-529 
1-641 
1-745 
1-844 


1-541 
1-651 
1-755 
1-854 


1-552 
1-662 
1-766 
1-864 


1-564 
1-673 
1-776 
1-873 


For voltages from 1000 to 9900. 


1000 
2000 
3000 
4000 





100 


200 


300 


.400 


500 


600 


700 


800 


900 


x!0 
1-88 
2-66 
3-26 
3-76 


xlQ9 

1-97 
2-73 
3-31 
3-81 


x!09 
2-06 
2-79 
3-37 
3-86 


xlO 

2-15 
2-85 
3-42 
3-90 


x!09 
2-23 
2-92 
3-47 
3-95 


x!0 
2-31 
2-98 
3-52 
3-99 


xlO 
2-38 
3-04 
3-57 
4-04 


x!09 
2-45 
3-09 
3-62 
4-08 


xlQ9 
2-52 
3-15 
3-67 
4-12 


x!09 

2-59 
3-21 
3-72 
4-17 


5000 


4-21 


4-25 


4-29 


4-33 


4-37 


4-41 


4-45 


4-49 


4-53 


4-57 


6000 
7000 
8000 
9000 


4-61 
4-98 
5-33 
5-65 


4-65 
5-01 
5-36 
5-68 


4-69 
5-05 
5-39 
5-71 


4-72 

5-08 
5-42 
5-74 


4-76 
5-12 
5-45 
5-T7 


4-80 
5-15 
5-49 
5-80 


4-84 
5-19 
5-52 
5-83 


4-87 
5-22 
5-55 
5-86 


4-91 
5-26 
5-59 
5-89 


4-94 
5-29 
5-62 
5-92 


For voltages from 10,000 to 199,000. 


10,000 
20,000 
30,000 
40,000 





1000 


2000 3000 4000 


5000 


6000 7000 8000 9000 


x!09 
5-95 
8-42 
10-31 
11-91 


x!09 

6-24 
8-62 
10-48 
12-05 


x 109 x 109 x 109 
6-52 6-78 7-04 
8-83 9-02 9-22 
10-65 10-81 10-97 
12-20 12-34 12-48 


x!09 
7-29 
9-41 
11-13 
12-63 


x!09 x!09 x!09 xlO 

7-53 7-76 7-98 8-20 
9-60 9-78 9-96 10-14 
11-1I9 11-45 11-60 11-76 
12-77 12-90 13-04 13-18 


50,000 


13-31 


13-44 


13-57 


13-70 13-83 


13-96 


14-09 14-21 


14-34 


14-46 


60,000 
70,000 
80,000 
90,000 


14-58 
15-75 
16-83 
17-86 


14-70 
15-86 
16-94 
17-96 


14-82 14-94 
15-97 16-08 
17-05 17-15 
18-06 18-15 


15-06 
16-19 
17-25 
18-25 


15-17 
16-30 
17-35 
18-34 


15-29 15-41 15-52 
16-41 16-51 lfi-62 
17-45 17-55 17-66 
18-44 18-54 18-64 


15-64 
16-73 
17-76 
18-73 


100,000 


18-8 


18-9 


19-0 


19-1 19-2 


19-3 


19-4 !l9-5 19-6 


19-7 


110,000 
120,000 
130,000 
140,000 


19-7 
20-6 
21-5 

22-3 


19-8 
20-7 
21-6 
22-4 


19-9 
20-8 
21-7 
22-5 


20-0 
20-9 
21-7 
22-5 


20-1 
20-9 
21-8 
22-6 


20-2 
21-0 
21-9 
22-7 


20-3 20-4 20-4 20-5 
21-1 21-2 21-3 21-4 
22-0 22-1 22-1 22-2 
22-8 22-8 22-9 i 23-0 


150,000 


23-1 


23-1 


23-2 


23-3 


23-4 


23-4 


23-5 23-6 i 23-7 


23-8 


160,000 
170,000 
180,000 
190,000 


23-8 
24-5 
25-2 
25-9 


23-9 
24-6 
25-3 
26-0 


24-0 
24-6 
25-4 
26-1 


24-1 
24-7 
25-4 
26-1 


24-1 

24-8 
1 25-5 
26-2 


24-2 
24-9 
25-6 
26-3 


24-3 24-3 ! 24-4 
24-9 25-0 25-1 
25-6 25-7 25-8 
26 3 26-4 26-5 


24-5 
25-1 
25-9 
26-5 



234 



X RAYS 



TABLE XXVII. CHARACTERISTIC y RAYS. 

Rutherford and Richardson, P.M. 1913 and 1914 (see p. 118). 
The absorption coefficients in aluminium may be contrasted with 
those for X rays on p. 115. 



Element. 


Atomic 
Weight. 


Absorption Coef. in Al. 


Remarks. 


\ 


A/p 






cm.- 1 


cm. -gm. units 




Uranium X - 


230 


24 


8-9 


L radiation. 






0-70 


0-26 


Hard rays. 


V, X 2 - 


230 


0-140 


0-052 


Very hard rays. 


Ionium 


230 


1080 


400 


Extremely soft rays. 






22-5 


8-35 


L radiation. 






0-41 


0-15 


Very hard rays. 


Radium B 


214 


230 


85 


Softer than ordinary 










X rays. 






40 


14-7 


L radiation. 






0-51 


0-188 


Very hard rays. 


c 


214 


0-115 


0-0424 


K radiation. 


D 
E 


1 210 


(45 

\0-99 


16-5(?) 
.0-38 


L 
Hard rays. 


Mesothorium 2 


228 


26 


9-5 


L radiation. 






0-116 


0-043 


K 


Thorium B 


212 


160 


59 


Softer than ordinary 










X rays. 






32 


11-8 


L radiation. 






0-36 


0-13 


Very hard rays. 


D 


208 


0-096 


0-035 


Hardest y rays 










known. 


Radioactinium 


228(?) 


25 


9-2 


L radiation. 






0-190 


0-070(?) 


K 


Actinium B 


212(?) 


120 


44 


Softer than ordinary 










X rays. 






31 


11-4 


L radiation. 






0-45 


0-165 


Very hard rays. 


D - 


208(?) 


0-198 


0-073 


K radiation. 



APPENDIX V 



235 



TABLE XXVIII. WAVE-LENGTHS OF LINES IN X-RAY SPECTRA. 

The values below are due to Moseley (P.M. April 1914). They 
include some of those given on p. 201. The most intense line is 
called a, the next (3. There are other lines present in both the 
K and L series. Reference should be made to Moseley's papers 
(see p. 199). 





K Series. 




L Series. 


a 


ft 


a 


ft 


Al 


x 10-8 cm . 

8-364 


x 10-8 cm . 
7-912 


Zr 


x 10-8 cm . 
6-091 


x 10-8 cm . 


Si 


7-142 


6-729 


Nb 


5-749 


5-507 


Cl 


4-750 





Mo 


5-423 


5-187 


K 


3-759 


3-463 


Ru 


4-861 


4-660 


Ca 


3-368 


3-094 


Rh 


4-622 





Ti 


2-758 


2-524 


Pd 


4-385 


4-168 


V 2-519 
Cr 2-301 


2-297 
2-093 


Ag 
Sn 


4-170 
3-619 





Mn 2-111 


1-818 


Sb 


3-458 


3-245 


Fe 1-946 


1-765 


La 


2676 


2-471 


Co 1-798 


1-629 


Ce 


2-567 


2-360 


Ni 1-662 


1-506 


Pr 


2-471 


2-265 


Cu 1-549 


1-402 


Nd 


2-382 


2-175 


Zn 1-445 


1-306 


Sa 


2-208 


2-008 


Y 0838 





Eu 


2-130 


925 


Zr 0-794 





Gd 


2-057 


853 


Nb 0-750 





Ho 


1-914 


711 


Mo 0-721 





Er 


1-790 


591 


Ru 0-638 





Ta 


525 


330 


Pd 0-584 
Ag 0-560 


= 


W 
Os 
Ir 


486 
397 
354 


201 
155 











Pt 


316 


121 











Au 


287 


092 



INDEX. 

PAGE 

Abraham and Villard, influence machine - 49 

Absorption of cathode rays - - 10 

,, ,, 4th power law of 10 

,, ,, characteristic radiations - 129 

,, ,, ,, ,, in gases - 136 

,, ,, curves of - - 122 

,, ,, corpuscular rays in gases - 141 

y rays - - 101, 104, 234 

X rays - 99 

,, by photographic film - 92 

selective - - 102, 129, 134 

Absorption-coefficients of characteristic radiations in Al - - 115 

,, ,, ,, ,, gases - - 138 

,, ,, ,, ,, various substances 

132 

y rays 118, 234 

,, ,, corpuscular rays in gases - - 143 

definition of - - 100 

,, and intensity, table connecting - 232 

of y rays - - 101, 104, 234 

X rays - 101, 103 

Algermissen, sparking voltages - 98 

Alkali -halogen salts, space-lattices of - 192 

u-rays, constants of - - xii 

medical uses of - 1 65 

Amber, Ambroid, Amberite, insulating properties of - 227 

Andrade and Rutherford, reflection of y rays - 186 

Angerer, charcoal and liquid-air vacua - 222 

energy of X rays - - 106 

heat generated by absorption of X rays - 87 

Anode of X-ray bulb - - 33 

glow - 2 

Anticathode, atomic weight and efficiency of - 35 

,, characteristic radiation from - * 36, 121 

,, depth of origin of X rays in - 44 

,, design of - 40 

,, distribution of X rays from - - 45 

,, general radiation from - - 36 

,, metals used for 38 

,, methods of cooling - - 41 

obliquity of - 44 



INDEX 237 

PAGE 

Anticathode, thin - - 46 

of X-ray bulb - - 35 

Aston, cathode dark-space 

Atomic distances in crystals - - 194 

,, weight and efficiency of anticathode - 35 

weights of elements, table of -229 

Aurora 

Austin and Holborn, cathodic sputtering 

Automatic methods of softening X-ray bulb 

Ayres and Barkla, scattering of X rays 110 

Bakelite, for influence machines 
Barium platinocyanide, fluorescence of 

pastilles of - 

,, screens of - 93 

Barkla, polarisation of X rays 
Barkla and Ayres, scattering of X rays 

,, Collier, absorption of characteristic radiations in gases - 137' 
,, ,, shape of absorption curves for characteristic 

radiations 

Martyn, reflection of X rays - - 190 

selective action of X rays on photographic plates 91 

Philpot, ionisation independent of quality of X rays 
X-ray and corpuscular ionisation in gases - 146 

Sadler, absorption-relation for characteristic radiations 134 
discovery of characteristic X rays 

Bassler, polarisation of X rays 

Bauer, penetrometer - -.105 

valve - 75 

Beatty, absorption of corpuscular rays in gases 

characteristic X rays and cathode rays - 127 

distribution of corpuscular rays 

energy of X rays - - 106 

ionisation in gases, table of - 

ionisation in heavy gases - - 146 

,, velocity of cathode rays and potential applied to tube - 16 
Becquerel, discovery of radioactivity 
Benoist, absorption of X rays 

,, discovery of ionisation by X rays - 25 

penetrometer - 104 

/?-rays, constants of - *ii 

4th-power law of scattering - 

medical uses of 165, 167 

See also " Cathode rays." 
Birkeland, magnetic spectrum of cathode rays 
Bisectional winding of secondary of induction coil - 

Bismuth radiography - 157 

Blackening of X-ray bulb - 76 

Blake and Owen, diffraction of X rays by metallic crystals - 

spectra of X rays - 190 

Blythswood and Scoble, current through X-ray bulb 

photographic plates and X rays 

Boas, alternator 
Bohr, theory of the atom - 1 8, 200 



238 X RAYS 

PAGE 

Boltzmann, nature of X rays 204 

Bordier, distribution of X rays from bulb - - 45 

,, scale of X-ray dosage 93 

Bordier and Galimard, scale of X-ray dosage 93 

Bragg, W. H. (Prof.), characteristic X rays from platinum - 118, 189 

,, corpuscular theory of X rays - - 205 

,, indirect action of X-ray ionisation 146 

reflection of X rays 186 

wave-length of X rays - - 201 

X-ray spectrometer - - 187 

Bragg, W. L., atomic distances 194-197 

,, characteristic X rays from platinum - 118 

,, crystal structure - 191 

experiments on diffraction by zinc-blende crystals - 176 

,, theory of diffraction of X rays by crystals 179 

,, reflection ,, ,, ,, ,, 179 

Braun tube - 15 

-Bravais, theory of crystal-structure - 170 

Breaks, hammer 62 

mercury 64 

Wehnelt electrolytic 62 

British Radium Standard - 84 

Bumstead, heat generated by absorption of X rays - 87 

Burbidge, y-ray fluctuation experiments - 214 

Burbidge and Laby, y-ray fluctuation experiments - 214 

" Burns," X-ray ' 161 

Butcher, Deane, standardisation of X rays - 84 

Butt, induction coil - 54 

Cabot, rotary converter 61 

Caldwell-Swinton, electrolytic break - 64 

Campbell, N. R., discontinuity of radiation - 205 

fluctuation experiments with light 215 
Campbell-Swinton, see Swinton. 

Canal rays - 19 

Carpentier, induction coil - 50 

Carter, energy of X rays - 106 

Cathode, cylindrical - 71, 35 

,, dark-space - 2 

,, glow 2 

,, lime 9 

,, position of, in X-ray bulb - 69 

, sputtering of 76 

Wehnelt 8 

of X-ray bulb 34 

Cathode-rays, absorption and transmission of 10 

,, ,, and characteristic rays, table connecting - - 127 

,, ,, constants of - 16, xiii 

., ,, early work on - 4 

,, ,, electrical deflection of 14 

,, ,, e/m of - 17, xii 

fluorescence produced by 12 

,, focussing distance - 34 

,, fourth-power law of absorption of 10 



INDEX 239 

PAGE 

Cathode-rays, furnace 11 

,, heating effects of 11 

,, ionisati on produced by 11 

,, ,, magnetic deflection of 13 

,, ,, magnetic spectrum of - 16 

,, ,, mnemonic for deflection of 14 

,, nature of - 45 

therapeutic use of 166 

,, thickness of metal required to rofleot - - 45 

,, ,, velocity of - 17, xii 

,, velocity and potential, table of - 233 

Cathodic disintegration or sputtering 76 

Chad wick, absorption of y rays in air 103 

., y rays from a rays - 215 

and Russell, characteristic y rays - 118 

Chapman, characteristic radiations from heavy elements 117 

, vapours 120 

mass-absorption coefficients - 115 

refraction of X rays - 169 

Chapman and Guest, characteristic rays from salts - - 120 

Characteristic y rays - 118, 234 

light rays 119 

Xrays 112 

absorption of - 129 

in gases - - 136 

and cathode-ray velocity 124 

table of 127 

direct generation of - - 121 

from heavy elements - 117 

,, independent of chemical combination - - 120 

,, ,, selective absorption of - 129, 134 

,, tables of absorption in aluminium - 115 

gases - - 138 

elements, various - 132 

therapeutic uses of - 166 

wave-lengths of 197, 235 

Christen, penetrometer - 105 

Cloud experiments of C. T. R. Wilson 149 

Coefficient of absorption, definition of - 100 

,, scattering, ,, - 109 

Coil, induction, condenser of - 51 

,, core of - 50 

design of - - 55 

,, detailed account of - - 49 

elementary account of - 27 

,, ,, primary winding of - 51 

,, primary tube of - 52 

,, secondary winding of - 53 

Collie and Ramsay, gas in glass of X-ray bulb 

Collier and Barkla, absorption of characteristic radiations in gases - 137 
shape of absorption curves for characteristic 

radiations - 131 
Coloration (violet) of X-ray bulb 

Compton, electrolytic break - - 63 



240 



X RAYS 



Condensation experiments of C. T. R. Wilson 
Condenser of induction coil - - 

Moscicki - ... 

Conductivity, thermal, of elements - 
Cooksey, distribution of corpuscular rays 
Coolidge, X-ray bulb - ... 

Cooling of anticathodes 

Core of induction coil .... 

Corpuscles - - 

Corpuscular rays (secondary), absorption of. in gases 
Curie and Sagnac - 
distribution of - - 

ionisation in gases 
at right angles to X rays - 
table of absorption-coefficients in gases 

velocities of 
therapeutic uses of 
velocity of 
Corpuscular theory of X rays 
Cossor, lithium glass - - - 

X-ray bulbs - ... 

Cox, break - ... 

induction coil 

Crookes, cathode rays - .... 

,, cathodic sputtering of metals 

dark-space - ... 

focus-tube - ... 

fluorescence in glass, fatigue of 
,, method of softening discharge tube 
nature of cathode rays 
,, radiometer - - 

tube 

Crowther, ionisation (X-ray) and pressure 
scattering of X rays 
X-ray ionisation in gases - 
? ,, and temperature 

Crystals, Bragg's theory of diffraction by 

Lane's 

space-lattice of (definition) - 

theory of structure of - 

Curative action of characteristic rays 

,, ,, X rays - - 

Curie, Mme., International Radium Standard 
P., piezo-electrique 
,, and Sagnac, corpuscular rays - 
Current through tube and X-ray intensity - 

,, wave-form of, in primary and secondary of induction coil 
Cylindrical cathode - - - . - 71, 

Dark-space, Crookes, or cathode 

Faraday - 

Darwin, wave-lengths of monochromatic X rays 
Darwin and Moseley, characteristic rays from platinum 
reflection of X rays 



PAGK 

149 
51 
47 
39 

140 

- 219 

41 

50 

8 

141 
139 
139 
148 
139 
143 
141 
166 
140 
205 
166 

- 42, 69 

65 

54 

4 

78 
2 

30 

12 

74 

6 

5 

: xvii 

145 

109 

146 

148 

179 

170 

170 

191 

166 

163 

84 

91 

139 

86 

57 

35, 219 

2 
1 

202 
118 
I si; 



INDEX 



241 



PAGE 

Davidson, J. Mackenzie, curative properties of X rays - 163 

instantaneous radiograph of bullet - 159 

,, ,, interview with Prof. Rontgen 217 

radium-treatment for X-ray burns 161 

Day and Eve, absorption of X rays in air - 103 

energy of X rays - 106 

8 rays ' - x ii 

Dember, very soft X rays 119 

Densities of elements - 231 

Depth of origin of X rays in anticathode - .44 

Dermatitis, X-ray - 161 

Design of induction coil -_ 55 

Dewar, charcoal and liquid air vacua - 222 

Diffraction of X rays 168 

by metallic crystals - 184 

zinc-blende 174 

Direct generation of characteristic X rays 121 

Discontinuity of radiation - - 205 

Discovery of X rays - - 24 

Disintegration of anticathode - 81 

cathode - 76 

Distribution of corpuscular rays - 139 

,, photoelectrons ~ -" 140 

scattered X rays - - 110 

X rays from bulb - 45, 112 

,, ,, thin anticathode 46 

Dorn, heat generated by X rays - 87 

velocity of corpuscular rays 140 

Dosage, scales of - 94 

Droit, protective silk for X-ray work - 162 

Duddell, instantaneous radiography - - 160 

spark-gap 66 

,, wave-form of secondary currents - - 58 

Dufour, discovery of ionisation by X rays - - 25 

Ebonite, for influence machines - - - 47 

insulating properties of - - 226 

Eiffel Tower, wireless waves from - - 202 

Einstein, quantum theory of X rays - 205 

Einthoven galvanometer - - - 90 

Electrical insulators - - - 226 

Electrification on surface of X-ray bulb 23, 32, 70 

Electrodes of X-ray bulb - - 32 

Electrolytic interrupter - 62 

Electromagnetic waves, table of - 203 

Electrometers _ - - - - - - - -91 

Electron - _ _ _ . g 

number in atom - - - - 17 

positive, not known - 20 

theory of magnetism - 18 

matter - 18 

ubiquity of- - - .- -17 

Electroscopes - - - - . -91 

Elements, table of atomic weights of - 229 



242 X RAYS 

PAGE 

Elements, table of densities of - 231 

,, ,, discovery of 229 

Elliptical loci of Laue diffraction spots 181 

,, shape ,, ,, ,, - 181 

Elster and Geitel, coloration of X-ray bulb - - 83 

e/m of cathode rays - 14 

,, positive rays - 20 

Energetics of an X-ray bulb - - 105 

Energy of a cathode ray 1 1 

an X ray - 105, 127 

Entladungstrahlen 202 

Ether-pulse theory of Stokes - 208 

Eve and Day, absorption of X rays in air 103 

,, ,, ,, energy of X rays 106 

Ewen and Kaye, volatilisation of metals 82 

Faraday cylinder 

,, dark-space - 1 

Fatigue of fluorescence in glass 1 2 

in production of secondary rays - 144 

Fizeau, condenser of induction coil - 51 

Fluctuation experiments with y rays - - 214 

,, light rays - 215 

Fluorescence by cathode rays 12 

in glass, fatigue of 12 

of glass at moderately high pressures - 3 

low 12 

of lithium chloride by cathode rays and positive rays - 19 

methods of measuring intensity of X rays - 93 

Fluorescent screen, 93 

display of Laue diffraction spots - 184 

Focus tube, Crookes' - 30 

,, Jackson's 30 

,, ,, modern types of - 41 

Focussing distance of cathode rays - - 34 

Fog experiments of C. T. R. Wilson - 149 

Formula connecting K and L radiations - 116 

Franck and Pohl, velocity of X rays 153 
Freund, scale of X-ray dosage 

Friedrich and Knipping, X rays and crystals 169 
Furnace, cathode-ray 

Fused silica, insulating properties of - - 227 

Gaede, mercury pump 

molecular 

piston 

,, rotary 

Galimard and Bordier, scale of X-ray dosage 93 

y-rays, absorption of - -101, 104, 234 

,, fluctuation experiments 

,, from a rays - - 215 

interference of 

,, reflection - 186 

Gardiner, distribution of X rays from bulb - - 45 



INDEX 243 

PAGE 

Gardiner, magnetic displacement of anticathode focus-spot - 43 

photomicrograph of anticathode - 40, 43 

Gases, absorption of characteristic rays in - - 136 

,, corpuscular ,, - 141 

,, y rays in - 104 

X rays in - 103 

held by glass surfaces - - 224 

Geissler, discharge through gases - - xvii 

,, cathodic sputtering - 77 

Geitel and Elster, coloration of X-ray bulb - - 83 

Glass, conducting - - 165 

,, for influence machines - 47 

,, insulating properties of - 47 

lithium - 165 

,, transparency of, for X rays - 163, 165 

Glasson, characteristic X rays from salts - 120 

ionisation by cathode rays - 12 

Goby, radiomicrography - 157 

Goldhammer, nature of X rays - 204 

Goldsmith, penetration of mica by positive rays 73 

Goldstein, cathode rays 4 

,, electrification of glass round cathode 70 

Gouy, gas bubbles in glass walls of X-ray tube 72 

Gowdy, fatigue of production of secondary rays - - 144 

Granquist, cathodic sputtering - 81 

Gray, characteristic y rays - 38, 119 

Guest and Chapman, characteristic X rays from salts - 120 

Haga, polarisation of X rays - - 1 1 1 

,, and Wind, diffraction of X rays - 168 

Half- value thicknesses for absorption, table of - 115 

Ham, depth of origin of X rays in anticathode - 44 

,, distribution of X rays from bulb - 45. 

polarisation of X rays - - 112 

,, thickness required for complete reflection of cathode rays - 45 

Hammer break - 62 

Hardness of an X-ray bulb - - 68 

,, methods of measuring - 94 

,, ,, ,, ,, ,, ,, varying - 74 

Heat generated by absorption of X rays - - 87 

,, cathode rays - 11 

Hehl, cathode-glow - 2 

Hertz, nature of cathode rays 5 

Hertzian waves, wave-length of - 202 

Herweg, polarisation of X rays - 111 

High-frequency oscillations in primary circuit - 58 

,, spectra of metals - - 199 

High-tension transformers - - 59 

Hill, hardening of discharge tube - 72 

Hittorf, cathode rays 4 

,, tube - xvii 

Hodgson, gas absorption by anode 7 1 

Holborn and Austin, cathodic sputtering - - 81 

Holzknecht, scale of X-ray dosage - - 93 

Q2 



244 X RAYS 

PAGE 

Homogeneous X rays - 112, 190, 235 

Hughes, energy of photoelectrons - - 206 

ionisation by ultra-violet light - 206 

Hulst, influence machines - 49 

Hurmuzescu, discovery of ionisation by X rays - 25 

"I" radiation - - 118 

Induction coil, condenser of - 51 

,, ,, core of - 50 

,, detailed account of - 49 

design of 55 

,, elementary account of - 27 

primary tube of 52 

,, ,, primary winding of - 51 

,, secondary winding of 53 

Influence machines - - 47 

Innes, velocity of corpuscular rays - 140 

Instantaneous radiography - 159 

Insulators, electrical - - 226 

Intensifying screens - - 160 

Intensity of X rays, fluorescence methods of measuring - 91 

,, ionisation ,, 87 

,, ,, ,, methods of measuring used in medicine - 93 

,, photographic methods of measuring 91 

thermal ., 87 

Interference of y rays - 185 

X rays - 168 

International Radium Standard - 84 

Interrupter, sparking at - 56 

types used on induction coils - 62 

Wehnelt electrolytic, design of - 62 

Inverse currents, - - 28 

as affected by coil design - - 55 

-square law for X-ray intensity - - 85 

Ionisation chambers - - 90 

,, by collision 89 

,, in heavy gases - 146 

independent of quality of X rays - - 146 

,, methods of measuring X-ray intensity - 87 

and pressure - 145 

,, produced by cathode rays 11 

,, and temperature - - 148 

by ultra-violet light - 206 

X rays - 145 

,, indirect action of - 146 

,, ,, in mixed gases - 148 

Ions - 20, xii 

Jackson's focus bulb - 31 

Jaumann, nature of X rays - - 204 

Jona, sparking voltages - 99 

Jones and Roberts, function of condenser of induction coil - - 52 

K characteristic radiations - - - - - -113 



INDEX 245 

PAGE 

Kanalstrahlen 19 

Kathodenstrahlen - 4 

Kaufmann, fifth-power law of characteristic radiation - 116 

,, various anticathodes - - 35 

Keene, diffraction of X rays by metallic crystals - - 184 

Kienbock, scale of X-ray dosage - - 93 

Kleeman, distribution of photoelectrons - - 140 

Knipping and Friedrich, crystals and X rays 169, 171 

Kunzite, fluorescence of 13 

Kurlbaum, anticathode of X-ray tube 41 

L characteristic radiations - 113 

Laby and Burbidge, y-ray fluctuation experiments - 214 

Laird, Entladungstrahlen - - 202 

Langer, various anticathodes 35 

Langevin, electron theory of magnetism - 18 

Larmor, electron theory of magnetism 18 

Lattice, space-, definition of - - 170 

Laub, characteristic / radiation - 118 

distribution of corpuscular rays - 139 

Laue, diffraction spots, displayed by fluorescent screen - 184 

elliptical loci of - 181 

elliptical shape of - - 181 

produced by zinc-blende - - 174 

,, interference of X rays - - 169 

theory of crystal diffraction of X rays - - 170 

Lenard, ionisation by ultra-violet light - 206 

law of absorption of cathode rays - 10 

rays 5 

Light rays and X rays, resemblance between - 206 

Lilienfeld, X-ray bulb - 221 

Lime cathode 8 

Lindemann, lithium glass - - 165 

Lithium chloride, fluorescence of, by cathode and positive rays 19 

glass - 165 

Localised pulse theory - 213 

Localisers - 158 

Lodge, Sir Oliver, current through X-ray bulb - 86 

valve-tube - 67 

various anticathodes - 35 

Lorentz, electron-theory of matter - 18 

Machines, influence - - 47 

Wimshurst - - 48 

Mackenzie and Soddy, absorption of gas by sputtered metal 74 

Magnetic deflection of cathode rays - 13 

spectrum ,. 15 

Magnetism, electron theory of 18 

Maltezos, electrification of glass round cathode 70 

Martyn and Barkla, reflection of X rays - 190 

selective action of X rays on photographic plate 91 

Marx, velocity of X rays - - 152 

Mass-absorption coefficients, see Absorption coefficients. 

Matter, electron theory of 18 



246 X RAYS 

PAGE 

Mees, Kenneth, photographic plates for X ray work - 160 

Melting points of elements - 39 

Mercury- breaks 64 

Metallic crystals, diffraction of X rays by - 184 

Metals used as anticathodes - 38 

Meyer, y-ray fluctuation experiments 214 

Michelson, vortex theory of X rays - 204 

Micro-radiography - 157 

Miller, polarisation of X rays 112 

,, Leslie, valve - 67 

Mixed gases, ionisation in 148 

Monochromatic X rays 112 

More, fatigue in production of secondary X rays 144 

Moscicki condenser - - 46 

Moseley, X-ray spectra 199, 235 

Moseley and Darwin, characteristic rays from platinum - 118, 201 

,, ,, reflection of X rays - 186 

Muller, X-ray bulbs - 41 

Multisectional winding of secondary of induction coil - 55 

National Physical Laboratory, Radium Standard - 84 

Negative glow 1 

,, ion xii 

Neutral-pair theory of the X ray - 205 

Newton, corpuscular theory of light - 8, 213 

Nickel, platinised, use as anticathode 40 

Nicol, mass-absorption coefficients - . 115 

Noir6 and Sabouraud, scale of X-ray dosage 93 

Northern lights 17 

Nucleated pulse theory of X rays - 213 

Obliquity of anticathode - 44 

Occlusion methods of softening X-ray bulb - 74 

Oil-pumps - 224 

Opacity, definition of 93 

Opacity-logarithm - 93 

Oscillograph records of primary and secondary currents - - 57 

Osmosis methods of softening X-ray bulb - 74 

Owen, absorption of characteristic radiations in gases - 136 

5th-power law of absorption - - 115, 137, 202 

ionisation (X-ray) and pressure - 145 

metallic window used as anticathode - 136 

relative X-ray ionisation in gases - ~'-'- 146 

scattering of X rays - 110 

total X-ray ionisation in gases - 148 

Owen and Blake, diffraction of X rays by metallic crystals - 184 

,, ,, ,, spectra of X rays - 190 

Palladium, use as anticathode - 198 

Paschen, attempt to deflect y rays magnetically - - 25 

galvanometer 90 

Pastille method of dosage - - 94 

Penetrometers - - - - - - - 104 



INDEX 247 

PAGE 

Perrin, negative charge on cathode ray 7 

Phillips, C. E. S., conducting glass - 165 

,. ,, function of anode - - 33 

,, radium standard - - 84 

Philpot and Barkla, ionisation independent of quality of X rays - 146 

,, X-ray and corpuscular ionisation in gases - 146 

Photoelectric effect, selective - 120 

Photoelectrons, distribution of 140 

energy of - - 206 

speed of 207 

Photographic film, absorption by - 92 

,, methods of measuring intensity of X rays - 91 

,, plate, selective action of X rays on - 91 

,, plates for radiography - 160 

Photographs of X-ray tracks - 149 

Physiological action of characteristic rays - - 166 

Xrays - 163 

Planck, quantum theory of X rays - 205, 206 

theory of radiation - - 169 

universal constant - 197 

Plastic printing - 161 

Platinised nickel anti cathode 40 

Platinocyanide of barium, fluorescence of 13 

pastilles of 94 

,, ,, ,, screens of - 93 

Platinum radiation - 189, 197 

wave-length of - 197, 201 

spectrum of X rays from - 1 89 

use as anticathode - - 38 

Pliicker, cathode rays 4 

,, cathodic sputtering - 77 

,, hardening of discharge tube 71 

tube - - xvii 

Pohl and Franck, velocity of X rays 153 

,, ,, Pringsheim, characteristic light rays 120 

Walter, diffraction of X rays 168 

Point and plane spark-gap - - 66 

Polarisation of X-rays 110 

theory of 209 

Positive column 1 

electron, absence of - 20 

ion - - xii 

rays 19 

e/moi - 21 

striae 2 

Potassium-sodium alloy, absorption of gases by 224 

Pressure and ionisation (X-ray) in gases - 145 

Primary tube of induction coil - 52 

winding ,, - 51 

Pringsheim and Pohl, characteristic light rays - 120 

Production of high vacua - - 222 

Progressive hardening of X-ray bulb 71 

Protective devices against X-ray burns - 161 

Pulse- theory of X rays, Stokes .... 208 



248 X RAYS 

PAGE 

Pulse-theory of X rays, modification necessary - 210 

,, ,, nucleated - - 213 

Puluj, cathode rays - 4 

Quality of X rays, methods of measuring - 94 

,, and potential on tube - - 96 

Quantum theory of X rays 205, 206 

Quartz (fused), insulating properties of - 227 

Radiation, discontinuity of - , 205 

Planck's theory of 169 

Radiochromometer - 104 

Radiography - 154 

,, bismuth - 157 

,, instantaneous - - 159 

,, micro- - 157 

,, stereoscopic - 157 

Radiometer, Crookes' 5 

Radio-micrography - - 157 

Radium y rays 154, 165 

Standard, International and British - 84 

,, treatment of X-ray burns - 161 

Ramsay and Collie, gas in glass of X-ray bulb 72 

Rayleigh, condenser of induction coil 51 

Re, nature of X rays - 204 

Recoil atoms - xii 

Rectifiers - 66 

Rectifying properties of X-ray bulb - 34 

References to Journals xviii 

" Reflection " of cathode rays 45 

Xrays 186 

y rays 185 

Refraction of X rays - - 168 

" Relief " photographs of X rays - 161 

" Reverse " currents (see also inverse currents) - 28 

" Revolution of the corpuscle " - xiii 

Rhodium, use as anticathode - 40 

Richardson and Rutherford, characteristic y rays 118, 234 

Rieman, fatigue in production of secondary X rays - 144 

Roberts, volatilisation of metals - - 82 

Roberts and Jones, function of condenser of induction coil - - 52 

Roberts-Austen, interdiffusion of metals - 6 

Rock-salt, atomic distances in 197 

,, crystal structure of - 191 

Roiti, various anticathodes - - 35 

Rontgen, diffraction of X rays - 168 

,, discovery of X rays 24, 217 

nature of X rays - - 204 

refraction of X rays 168 

Rosenthal, X-ray tube - 221 

Russell and Chadwick, characteristic y rays - - 118 

Rutherford, y rays from ft rays 118, 214 

,, International Radium Standard - 84 

theory of the atom -18, 200 



INDEX 249 

PAGE 

Rutherford and Andrade, reflection of y rays - 186 

Rutherford and Richardson, characteristic y rays 118, 234 

Sabouraud and Noire, scale of X-ray dosage - 93 

Sadler, absorption of corpuscular rays in gases 141 

and Barkla, absorption relation for characteristic radiations 134 

discovery of characteristic radiations - 112 

Sagnac and Curie, corpuscular rays - 139 

nature of X rays - - 204 

Salomonson, current through X-ray bulb - 87 

influence of dielectric in mercury breaks 65 

,, penetrometer -. 104 

,, wave-form of current in primary of induction coil - 57 

Sanax break 65 

Saturation current - 88 

Scattered X rays - 108, 194 

,, ,, distribution of 110 

polarisation of - 110 

scattering-coefficient - 109 

X rays and light, resemblance between - - 207 

Schott, glass specially transparent to X rays 165 

Schwarz, scale of X-ray dosage 93 

Schweidler, y-ray fluctuation experiments - - 214 

Scoble and Blythswood, current through X-ray bulb 86 

,, photographic plates and X rays 92 

Screens of barium platinocyanide - 93 

,, intensifying - 160 

protective - 161 

Sealing-wax, insulating properties of 227 

Secondary winding of induction coil - 53 

X rays - 108 

Seitz, very soft X rays - 119 

Selective absorption - 102 

,, of characteristic radiations - 129, 134 

action of X rays on photographic plates - 91 

,, photoelectric effect - -120 

Selenium, effect of X rays on - 93 

Separation distances of atomic planes in crystals - 194 

Shaw, interference of y rays - - 185 

Siemens, X-ray bulb - - 41 

Silica, fused, insulating properties of *- 227 

Silk, protective, for X-ray w r ork '<*?. ] 62 

Snook, high-tension transformer - - 59 

Soddy and Mackenzie, absorption of gases by sputtered metal - 74 

Sodium-potassium alloy, absorption of gases by - - - 224 

Soft X rays - ..-119 

Softening of an X-ray bulb - .74 

Sommerfeld, spreading-pulse theory of X rays 212 

Space-lattice, definition of - - 170 

,, ,, dimensions of - -196 

of alkali-halogen salts - - 191 

Spark-gap, point and plane - 66 

Sparking at interrupter ' - - , - 56 

voltages, table of - - - - . - .- . 97 



250 X RAYS 

PAGE 

Spectrometer for X rays - 187 

Spectrum of cathode rays - 15 

X rays - 198, 235 

Speed of X rays - 152 

Sprengel pump . 223 

Sputtering of cathode - 76 

,, thermal - - 81 

Standard of radioactivity - - 84 

Standardisation of X rays - - - - 85 

Stark, distribution of X rays from thin anticathodes - 46 

,, wave-lengths of X rays - 169 

,, quantum theory of X rays - - 205 

Steinmetz, sparking voltages - - 98 

Stereographic projection of Laue diffraction spots 182 

Stereoscopic radiography - - 157 

Stokes, ether-pulse theory of X rays - 208 

,, law of fluorescence - - 207 

Stoney, Johnstone, use of term " electron '' - 8 

Straubel, osmosis valve - 74 

Striae in discharge tube - - 2 

Strutt, active nitrogen - 73 

Stuhlmann, distribution of photo-electrons - - 140 

Sublimation of metals - - - 81 

Sulphur, insulating properties of - - 227 

Sunic screen - - 160 

Sutherland, nature of X rays - 204 

Swinton, Campbell-, adjustable cathode - 70 

,, ,, fluorescence in glass, fatigue of 12 

,, ,, hardening of discharge tube - - 72 

,, ,, historical X-ray bulb - - 30 

,, ,, shape of beam of cathode rays 34 

,, ,, various anticathodes - 35 

Tantalum, use as anticathode - - 38 

Taylor, fluctuation experiments with light - - 215 

Temperature and X-ray ionisation - - 148 

Terada, fluorescent screen observations of Laue spots - 184 

Therapeutics, action of X rays - 163 

,, suitable rays for - 164 

,, use of corpuscular rays in 166 

,, ,, characteristic rays in - - 166 

Thermal conductivities of elements - - 39 

,, methods of measuring intensities of X rays - 87 

Thompson, S. P., various anticathodes - 35 

Thomson, J. J., current through X-ray bulb - - 86 

,, discovery of ionisation by X rays - - 25 

,, electric deflection of cathode rays - 14 

,, e/m of cathode rays - 14 

,, e/m of positive rays - - 21 

fluorescence of walls of discharge tube 2 

,, 4th-power absorption law for cathode rays - 10 

,, gas expelled by cathode-ray bombardment - 72 

nature of cathode rays 7 

negative charge on cathode rays - 7 



INDEX 251 

PAGB 

Thomson, J. J., nucleated or localised pulse theory of X rays 

,, scattering law for X rays - - 110 
theory of the atom - 

Tiede, cathode-ray furnace - 

Topler, pump 

sparking voltages - 98 

Total ionisation in gases 89, 148 

Transformers, high tension - - 59 

Transmission of cathode rays - 

Transparency, definition of 

of substances to X rays - 101 

Trinkle and Wehnelt, very soft X rays - 119 

Trowbridge, long-spark voltages 98 

Tungsten, use as anticathode 38, 219 

cathode - - 219 

Ultra-violet light, ionisation by 206 

Vacua, production of - 

Valve, Bauer - 75 

osmosis 74 

tubes - 66 

Varley 6 

Vegard, polarisation of X rays 111 

Velocity of cathode rays - 16 

and potential, table of - 233 

,, corpuscular rays - - 140 

Wehnelt cathode rays - 9 

Xrays - - 152 

Villard, gas bubbles in wall of X-ray tube - 72 

,, osmosis valve 74 

valve tube - - 66 

Villard and Abraham, influence machines - 49 

Violet coloration of X-ray bulb - 83 

Violle, interrupter - 62 

Volatilisation of metals 82 

Walter, attempt to deflect X rays magnetically 25 

,, penetrometer - - 105 

Walter and Pohl, diffraction of X rays 168 

Warburg, thickness required for complete reflection of cathode rays 45 

Wartenburg, cathode-ray furnace 11 

Wave-form of primary and secondary currents in induction coil - 57 

Wave-length of various electromagnetic waves - 202 

X rays 95, 196 

,, ,, relation to atomic weight - - 200 

table of 201, 235 

Wehnelt, adjustable cathode - 70 

,, interrupter - 62 

lime cathode 8 

,, penetrometer 104 

valve tube - - 67 

Wehnelt and Trinkle, very soft X rays - 119 

Whiddington, adjustable cathode - 70 



252 X RAYS 

PAGE 

Whiddington, characteristic X rays and cathode- ray velocity ' 125 

,, formula connecting K and L radiations 116 

fourth-power law of absorption of cathode rays 10 

soft X rays from Wehnelt cathode 119 

Wien, energy of X rays 106 

,, positive rays - 19 

,, wave-length of X rays 169 

Willemite, fluorescence of - 13 

Willows, hardening of discharge tube 72 

Wilson, C. T. R., condensation experiments - - 149 

reflection of X rays 186 

Wilson, W. H., function of condenser of induction coil 52 

,, ,, high-frequency oscillations in primary circuit of 

induction coil 58 

Wimshurst machine - 48 

Wind and Haga, diffraction of X rays 168 

Winding of primary of induction coil 51 

,, ,, secondary ,, - 53 

Window, metallic, use as anticathode -. 136 

Winkelmann, hardness of X-ray tube 68 

osmosis valve - 74 

" Wireless " waves, wave-length of - - 202 

Worrall, instantaneous radiography - 159 

Wratten and Wainwright, X-ray plate 160 

Wright, R. S., induction coil - 55 

Zinc-blende, crystalline structure of - 174 

fluorescence of - - 13 



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