BLKMBNTS

OF

STATIC ELECTRICITY

WITH

FULL DESCRIPTION OF THE HOLTZ AND TOPLER MACHINES
AND THEIR MODE OF OPERATING.

By PHILIP ATKINSON, A.M., Ph.D.

NEW YORK:
W. J. JOHNSTON, PUBLISHER,

168-177 POTTER BUILDING.
1887.

Copyright, 1886,
BY W. J. JOHNSTON.

INTRODUCTION.

IN this treatise the principles of electricity are
presented untrammeled, as far as possible, by mathe-
matical formulae, so as to meet the requirements of a
large class who have not the time or opportunity to
master the intricacies of formulae, which are usually so
perplexing to all but expert mathematicians.

This class includes those whose knowledge of
electricity is limited to .the practical details of teleg-
raphy, telephony, and electric lighting; also those
among the liberally educated, who desire to review
electric science in the light of recent investigation;
and those who wish to study its elementary principles,
preparatory to a more extended course, which shall
embrace all the details of electric measurement arid
electric engineering.

The original plan included dynamic as well as static
electricity, embracing its practical application to the
arts ; but it was subsequently thought best to confine
the present work to static electricity alone, to meet
the wants of those who are less familiar with its prin-
ciples than with those of dynamic electricity, and to

1 V IX TROD UC TWN.

reserve the consideration of the latter for a separate
volume.

Care has been taken to avoid the introduction of
new matter before the student was prepared for it ;
hence it was thought best that there should be a
thorough examination of elementary principles before
introducing complicated apparatus, the construction
and operation of which depends on those principles.

The theory assumed is, that electricity is one of
the forms in which energy manifests itself; that its
nature is not changed by the means emplo}Ted to
generate it, and that the various terms, positive,
negative, static, dynamic, express certain conditions and
relations in which this manifestation occurs, and
not different kinds of electricity.

The author takes pleasure in acknowledging his
obligations to Elisha Gray for the use of tables, giving
the results of observations on earth currents, made
under his direction on the Postal Telegraph line; also
to Ferguson, Gordon, Silvanus P. Thompson, Noad
uiicl Deschanel, from whose excellent works valuable
assistance has been obtained, though he has felt
compelled to dissent from some of their views.

The views here expressed are the result of many
years' experience in the class room, the lecture room,
and the laboratory, and were adopted only after the
most rigid test of actual and oft repeated experiment.
And some of the more important apparatus described

7,V TROD U(. 'TION. V

is of the author's own manufacture, constructed in
strict accordance with electric principles, verified by
his own experiments.

While humbly following the great pioneers in
electric science, who have hewed waymarks on the
rocks, the author will rest content if he has left some
foot-prints on the sands, which may serve to guide the
wayfarer till obliterated by the coming waves of
progress.

The impartial criticism of teachers and electricians
is especially requested, that faults and errors may be
corrected in future editions.

PHILIP ATKINSON.

CHICAGO, June, 1886.

CONTENTS.

CHAPTER L

PAGE

ATTRACTION AND REPULSION, 1

CONDUCTORS AND NON-CONDUCTORS, ... 4

QUANTITY AND INTENSITY, 6

STATIC ELECTRICITY DEFINED, 8

CHAPTER IT.
ELECTRIC POTENTIAL 10

CHAPTER III.
THE NATURE OF ELECTRICITY, 23

CHAPTER IV.
INDUCTION, 43

CHAPTER V.
ELECTRIC DISTRIBUTION AND CONDENSATION, . . 55

CHAPTER VI.
ACCUMULATORS, 72

Vlll CONTENTS.

CHAPTER VII.
ELECTRIC GENERATORS. —

THE ELECTROPHORUS AND FRICTIONAL MACHINE, 92

CHAPTER VIII.
ELECTRIC GENERATORS. —

THE HOLTZ AND TOPLER MACHINES, . . . 108

CHAPTER IX.
EXPERIMENTS WITH THE TOPLER MACHINE, . . . 125

CHAPTER X.
ELECTRIC TRANSMISSION IN VACUA, .... 146

CHAPTER XI.
ELECTROMETERS, 155

CHAPTER XII.
THE ELECTRICITY OF THE EARTH AND ATMOSPHERE. —

POTENTIAL AND EARTH CURRENTS, . . . 175

CHAPTER XIII.
THE ELECTRICITY OF THE EARTH AND ATMOSPHERE. —

THE AURORA, 190

CHAPTER XIV.
THE ELECTRICITY OF THE EARTH AND ATMOSPHERE. —

LIGHTNING AND THUNDER, 207

ELEMENTS

OF

STATIC ELECTRICITY,

CHAPTER I.

ATTRACTION AND REPULSION — CONDUCTORS AND
NON-CONDUCTORS — QUANTITY AND INTENSITY-
STATIC ELECTRICITY DEFINED.

ATTRACTION AND REPULSION. — Amber, called in
Greek rfaxroov, was known to the ancients to acquire,
when rubbed, the power of attracting light bodies;
hence this property, now known to belong to all sub-
stances, has received the name of electricity. The
earliest conception of electricity, then, was that of
force, and the latest discoveries sustain this view.

Electricity may be generated by various simple
methods, as follows: — Let a spoon be balanced on the
edge of a cup, and an ebonite (hard rubber) knife-
handle, rubbed on a woolen or silk fabric, be held
near it, and the spoon will be attracted. Substitute for
the knife-handle a stick of sealing-wax, a lamp-chim-
ney, or a paraffin wax-candle, rubbed in the same way,
and the spoon will be attracted by each of them.

These different substances may be multiplied, and
different rubbers used, but it will be found that the

Z ELEMENTS OF STATIC ELECTRICITY.

attractive force, though variable in intensity, is com-
mon to all.

The balanced rod, represented in Fig. 1, will be
found more convenient for these experiments than the
balanced spoon. It consists of a round wooden rod,
about twenty inches long, and half an inch in diameter,
with the ends rounded or terminating in balls. It is
pivoted at the center on a point, and may be mounted
on a stand, oryon a bottle with a pin through the cork,
and made to revolve rapidly by the force of attrac-
tion, following any of the electrified bodies already
mentioned wrhen held near it, as represented.

Fig. 1— The Balanced Rod.

A more sensitiveinstrument for investigations of this
class is represented in Fig. 2, and known as the pith-
ball electroscope; the name electroscope being derived
from the Greek cxoTteco, to see, ifaxrQov,. electricity. It
is constructed as follows : A small brass rod, bent at
right angles, has its short arm inserted into an ebonite
stem attached to a wooden base, giving the instrument
a vertical height of about Hi inches. The horizontal
arm is about 8 inches long, and terminates 'in a small
brass ball. From this arm two pith balls, each about
half an inch in diameter, are suspended by silk threads.

ATTRACTION AND REPULSION. 3

Let the pith balls be separated at the points of sus-
pension, so that when they hang vertically a consider-
able space shall intervene between them, and let a
stick of sealing-wax, previously electrified by friction,
be brought near one of them ; the ball will.be attracted
to the wax, and, after a momentary contact, repelled.
Follow it with the wax, and it continues to recede as if
pushed back by some invisible barrier.

Now let the other pith

ball be moved near this / »v

one, and they will be ' '•

attracted to each other,
and, after contact, re-
pelled ; the lines of sus-
pension showing diver-
gence in each direction
as represented.

Let the electrified wax
be again brought near,
and each ball is repelled
by it, so that when it is Fi«' 2~T"e Pk"-Ba!1 Electroscope,
placed between them, they are driven further apart ;
but let any non-electrified body be brought near and
they are attracted to it.

If each of the balls be separately electrified by the
wax, and they are then brought near each other, they
will show mutual repulsion without previous attraction.
"From this series of phenomena we learn, first, that
electrified bodies not only attract non-electrified bodies,
as already shown, but communicate electricity to them
by contact ; and, secondly, that bodies electrified, either
by each other or from the same source, show mutual
repulsion.

4 ELEMENTS OF STATIC ELECTRICITY.

The first fact was shown when the pith ball, after
contact with the wax, attracted and electrified the
other pith ball ; and the second fact by the repulsion
of the pith ball from the wax after contact ; then of
ther two pith balls from each other and from the wax,
after contact ; and finally by the mutual repulsion of the
balls, without previous attraction, after being separately
electrified by the wax.

This series of phenomena may be produced by using
a glass or ebonite rod, or any of the substances already
mentioned, as well as by the sealing-wax ; showing that
repulsion as well as attraction is a property common to
all electrified bodies.

CONDUCTORS AND NON-CONDUCTORS. — Pursuing our
investigation, new properties are developed. It is
found that while certain substances, as glass, ebonite,
and sealing-wax, show electric qualities, others, as brass,
iron, and copper, apparently do not show such qualities.
This led to the old division of all substances into
electrics^ a term applied to the former, and non-electrics,
applied to the latter.

But more thorough investigation has proved that
electricity may be generated by friction on the brass,
iron, and copper, as well as on the glass, ebonite, and
sealing-wax ; but that, when generated on bodies of the
former class, it is instantly distributed over the entire
body, and escapes to the earth unnoticed, if the body
be held in the hand, while, when generated on bodies
of the latter class, it is not so distributed, and does not
pass off in this way; bodies of the former class allowing
free electric movement, over the surface or through the
mass, while those of the latter class resist such move-
ment.

CONDUCTORS AND NON-CONDUCTORS. 5

To make this evident, let a short brass rod, of about
quarter inch diameter, terminating in a ball, be fitted to
an ebonite handle, as
represented by Fig. 3.
Let the brass be rubbed

briskly on woolen, silk, FiS- 3-The Insulated Metal Rod.

or india rubber, holding the instrument by the handle,
and it will attract and repel the pith balls in the same
way as the other electrified substances already used.
Copper, iron, or any other metal may be substituted
for the brass with the same result.

Kepeat the experiment, allowing the metal to touch
the hand, and the electric qualities disappear. This
shows that in the first experiment the electricity was
retained, because it could not pass through the ebonite
handle ; while, in the second, it passed off through the
hand.

The results obtained by experiments of this kind led
to the abandonment of the doctrine of electrics and
non-electrics, and the classification of all bodies as con-
ductors or non-conductors.

Experiment proves that all substances conduct elec-
tricity, and that they all offer a certain amount of
resistance to its passage. But it is found that the
relative proportions of conductivity and resistance vary
greatly in different substances. In some the conduct-
ivity is largely in excess, and they are called conductors ;
in others the resistance is- largely in excess, and they
are called non-conductors. Between these extremes
there are all degrees of variation ; so that in some sub-
stances the two properties are almost equally balanced.
Hence, since no exact rules can be given, we distinguish
the two classes by saying that a CONDUCTOR is any sub-

6 ELEMENTS OF STATIC ELECTRICITY.

stance of such low resistance that it can be used practi-
cally for the transfer of electricity; and a N OX-CON-
DUCTOR is any substance of such high resistance that it
can be used practically to prevent such transfer.

LIST OF CONDUCTORS AND NON-CONDUCTORS. — The
principal conductors are the metals, silver and copper
being the best. Among the partial conductors are the
different varieties of carbon, including coal, charcoal,
and graphite ; the acids, saline solutions, water, vegeta-
bles, and animals.

The principal non-conductors are caoutchouc, gutta-
percha, sulphur, and their compound, known as hard
rubber, vulcanite, or ebonite ; dry air, paraffin, shellac,
amber, resin, glass when free from metallic substances,
mica, silk, fur, wool, hair, feathers, bisulphide of carbon,
petroleum, and oil of turpentine.

Among the partial non-conductors are porcelain,
baked wood, paper, and leather.

INSULATOR DEFINED. — When a non-conductor is
used in connection with a conductor to confine elec-
tricity within certain limits, it is called an insulator;
and the conductor on which the electricity is confined,
or to be confined, is said to be insulated ; as a metal
placed on a glass or ebonite support, a copper wire
wrapped with silk or wool.

QUANTITY AND INTENSITY. — Electric quantity and
intensity are similar to the quantity and intensity found
in other more familiar forms of energy. The intensity
of any form of energy, other things being equal, is in-
versely proportional to the mass of the body in which it
is developed. A few strokes of a hammer on a small
piece of iron placed on an anvil will raise its temper-
ature to a burning heat; while the same number of

QUANTITY AND INTENSITY. 7

strokes on a large mass of iron will produce but very
slight change of temperature. The quantity of muscular
energy expended is the same in each case, but the inten-
sity of heat energy produced varies inversely as the
mass.

The intensity varies also as the resistance. A small
piece of wood held in the flame of a lamp is quickly
ignited at the end in the flame ; while the end held in
the hand shows no perceptible change of temperature.
But a brass rod of the same size, similarly held for the
same length of time, becomes too hot for the hand long
before the end in the flame is hot enough to ignite
wood.

In the wood, the intensity rises rapidly at the end
held in the flame, because the resistance prevents dis-
tribution of heat through the mass. But the low
resistance of the brass permits the rapid distribution
of the same quantity of heat through its mass ; so that the
intensity at the end in the flame is much less than that
of the wood.

In kindling a fire of anthracite coal, when the pro-
portion of coal is too great for the kindling-wood, the
heat generated by the consumption of the wood fails to
ignite the coal, because such coal being a comparatively
good conductor of heat, the amount is rapidly distrib-
uted through the mass, and hence the intensity at any
point is insufficient to produce ignition. But if the pro-
portion of coal be sufficiently reduced, the consumption
of the same amount of wood will produce ignition.
The quantity of heat imparted to the coal is the same in
each case, but its intensity is greater in the latter case.

In electric experiments there is a great difference
noticeable in the amount of work required to produce

8 ELEMENTS OF STATIC ELECTRICITY.

the same electric intensity on different bodies. Sealing-
wax and ebonite, for instance, are quickly electrified,
whil-e brass is electrified slowly. The reason is analo-
gous to that in the illustrations just given : the brass
being a good electric conductor, the electricity is in-
stantly distributed equally over every part of its sur-
face, and hence the quantity at any point being small,
the intensity is low. But the sealing-wax and ebonite
being good non-conductors, the same quantity of elec-
tricity is concentrated on those parts of the surface
brought into immediate contact with the rubber, instead
of being equally distributed over the entire surface ;
and hence the intensity at those points is proportion-
ately increased.

It will be shown hereafter that in static electricity
the electrification is on the surface. Hence, in this
case, electric intensity means quantity in proportion to
surface, whether it be the entire surface, as on a con-
ductor, or only those parts to which the electrification
is confined, as on a non-conductor.

It must also be understood, as will be shown more
fully hereafter, that the term intensity is as applicable
to a diminution of electric energy at a given point as
to an increase ; in the same sense as we speak of intense
cold, as well as of intense heat.

STATIC ELECTRICITY DEFINED.— The terms used to
distinguish different classes of electric phenomena, as
frictional, static, galvanic, chemical, magneto, tliermo,
take their origin from the different methods by which
electricity is generated, and the various conditions under
which its phenomena have been observed, and should
not be understood as referring to any difference in
the nature of the electricity produced.

STATIC ELECTRICITY DEFINED. 9

The term frictional has been used to designate that
class of phenomena now under consideration, since
friction is one of the principal agencies by which the
electricity is generated. But it seems more appropriate
to use a term embracing, not merely one agency by
which the electricity is generated, but also the various
phenomena produced, and distinguishing these phenom-
ena from those pertaining to electricity generated by
other methods. And since these phenomena refer
chiefly to electricity when stationary, the term static,
from the Latin sto, to stand, has been adopted, to dis-
tinguish electricity observed under these conditions
from electricity observed chiefly in a state of motion.

CHAPTER II.
ELECTRIC POTENTIAL.

POTENTIAL. — Potential, in the physical sense, is the
power to accomplish work. It derives its specific name
from the nature of the work, as gravity potential, heat
potential, electric potential.

A pound weight raised to the height of ten feet has
acquired ten foot-pounds of gravity potential, and has
the power, if allowed to descend to the same level, of
accomplishing ten foot-pounds of work, either in rais-
ing another weight, or setting machinery in motion by
which work may be accomplished.

A mass of metal whose temperature has been raised
from zero to one thousand degrees, has acquired one
thousand degrees of heat potential, and can accomplish
work to that amount in cooling to zero, either by heat-
ing another mass, or generating steam by which machin-
ery can be put in motion and work accomplished.

We have seen that bodies, when electrified, acquire
the power to attract or repel other bodies. This power
is called electric potential.

Suppose that the electric energy of the sealing-wax
in attracting the balanced rod, represented in Fig. 1,
Chapter I., were just sufficient, if expended without
loss, to move the rod one foot ; and, in doing so, to
overcome a resistance from inertia and friction repre-
sented by two ounces (one-eighth of a pound ) ; the

ELECTRIC POTENTIAL. 11

electric potential of the sealing-wax would equal one-
eighth of a foot-pound.

If only half this energy were required to overcome
inertia and friction, the other half might be expended in
lifting to a height of one foot an ounce weight attached
to a thread fastened to the end of the rod, and passing
over a pulley. In which case the work accomplished
by this half would be represented by one-sixteenth of
a foot-pound. Or the weight might be raised, or other
work to the same amount accomplished, by putting in
motion light machinery connected with the rod by gear-
ing at its center ; the added friction being included in
the ounce representing friction and inertia.

Repulsion would evidently produce the same results
in this case as attraction.

To distinguish between electricity and electric poten-
tial, we must bear in mind that electricity represents
the energy itself, while potential represents certain rela-
tions between this energy and matter. Hence we derive
the following definition :

Electric potential is the power which a body possesses to
accomplish work by virtue of its electricity.

DIFFERENCE OF POTENTIAL. — To accomplish work
in this way there must first be a difference of po-
tential.

The descending weight could not raise the other
weight unless there was a difference of level between
them. The heated metal could not heat a similar mass
unless there was a difference of temperature between
them. Neither could the electrified sealing-wax attract
the rod unless there was a difference of electric energy
between them. And these phrases, difference of level,
difference of temperature, difference of electric energy,

12 ELEMENTS OF STATIC ELECTRICITY.

are simply different forms of expression for difference of
potential.

To produce this difference work must first be ex-
pended, and this work is the measure of the potential
acquired.

The lifting of the pound weight ten feet against the
force of gravity gave it the ten foot-pounds of gravity
potential. The work of heating the metal, whether
represented by combustion, by friction, or by concus-
sion, gave it the one thousand degrees of heat potential.
And the rubbing of the sealing-wax gave it the one-
sixteenth of a foot-pound of electric potential.

As there is ordinarily no practical difference of elec-
tric potential between different points on the -earth,
within a limited area, its potential is considered zero,
and taken as the base of all measurements of electric
potential.

The qualification of this statement, as above, becomes
necessaiy, since there are often great differences of po-
tential over widely separated areas.

POSITIVE AND NEGATIVE. — Bodies whose potential
is higher than that of the earth are said to have positive
potential, while those whose potential is lower are said
to have negative.

The potential of bodies is also considered positive or
negative with reference to each other. If a body has
a higher potential than the earth, but lower than that
of another body, it is said to be positive with reference
to the earth, but negative with reference to the other
body. In like manner a body may have negative
potential with reference to the earth, but positive with
reference to another body of lower potential.

Hence, positive and negative are merely convenient rela-

ELECTRIC POTENTIAL. 13

tive terms to designate different degrees of potential and
not different kinds of electricity.

The sign ( + ) is used to denote positive potential, and
( — ) to denote negative potential.

The earth's potential, then, is the electric zero, just
as the freezing point is the zero of temperature in the
centigrade thermometer, and all uninsulated bodies are
said to be connected with the earth, and to have zero
potential when not under special influence from insu-
lated, electrified bodies in their vicinity.

When the electric potential of a body is changed
from zero by an increase of its electricity, it is said to
be positively electrified ; and when its potential is
changed from zero by a decrease, it is said to be nega-
tively electrified.

ELECTRIC MOVEMENT. — When a difference of electric
potential exists between different bodies, or different
parts of the same body, there is a constant tendency to
equalization.

A state of equilibrium seems to be the natural condi-
tion of bodies, and to produce difference of potential
requires, as we have seen, the exercise of force in the
performance of work, by which this equilibrium is dis-
turbed.

We find in other forms of energy, as gravity and heat,
the same tendency to equilibrium, requiring the exercise
of force to overcome it, as in the illustrations already
given.

The restoration of equilibrium is always effected by
a transfer of energy from the body having the greater
to the one having the less energy ; that is, from higher
to lower potential.

In the case of gravity this transfer of energy carries

14 ELEMENTS OF STATIC ELECTRICITY.

the body with it, as in the descent of a weight or the
movement of water from a higher to a lower level.
But in the case of heat and electricity, the energy may
move while the body remains stationary ; and it may be
transferred from one body to another, or from one part
to another of the same body. Thus the mass of metal,
in the illustration given, transfers its heat energy to
another mass ; and in like manner, when a metal rod is
heated at one end, the heat moves to the cold end.

Gravity apparently can move only by carrying the
body with it, while heat moves through the body with-
out producing change of position in its mass, like gravity.

A hot body transfers its heat to a cold one in its
vicinity, but does not attract it ; while gravity produces
mutual attraction between all bodies, but is not trans-
ferred like heat from one body to another.

But in electrified bodies we have both kinds of move-
ment. Like heat, electricity can move from one body
to another, or from one part to another of the same
body ; and, like gravity, it can carry the body with it.

Hence we must distinguish between the movement of
electricity and the movement of the electrified body.
Electric movement, like heat 'movement, is from higher
to lower potential. If one part of a conductor be elec-
trified, the electricity instantly distributes itself over
every part. If two insulated bodies, free to move, are
placed in each other's vicinity, like the pith balls of the
electroscope, the same tendency to equilibrium is shown
by their mutual attraction.

Though only one ball be electrified, yet it is evident
that their movement toward each other must be mutual,
and in proportion to their mass, since action and reac-
tion are equal : so that while the movement of the elec-

ELECTRIC POTENTIAL. 15

tricity is from the electrified to the non-electrified ball,
that is, from higher to lower potential, the movement
of the balls is mutual.

It will also be noticed that the movement of the non-
electrified ball is opposite to that of the electricity.
Hence, while electricity moves from higher to lower
potential, bodies under its influence may move in either
direction.

We have seen that when the two balls come into
contact there is a transfer of electricity from the elec-
trified to the non-electrified ball ; equilibrium is estab-
lished, and mutual repulsion follows, not only between
the balls, but also between them and the electrified
sealing-wax.

So long as a difference of potential exists there is
mutual attraction ; but when equilibrium is established
there is mutual repulsion. The same results may be
produced by numerous similar experiments, in which
different substances and different methods may be
employed. Hence we deduce the following important
principle :

Electrified bodies at different potentials attract, while
those at the same potential repel each other.

There can be no repulsion unless there is a difference
of potential between the electrified bodies and their
surroundings. For if the surrounding bodies \vere at
the same potential as the electrified bodies, the repul-
sion would be neutralized by their reaction. Hence
bodies at zero potential can show no repulsion. But in
all cases of electrification there is a difference of poten-
tial created in the body, either above or below the origi-
nal zero.

Indeed, attraction may account for the apparent

16

ELEMENTS OF STATIC ELECTRICITY.

mutual repulsion of bodies at the same potential, since
this difference of potential between the electrified bod-
ies and their surroundings must produce attraction and
tend to separate them.

But such outward attraction would not disprove the
existence of repulsion, though it might account for
some of its phenomena.

THE GOLD LEAF ELECTROSCOPE — As our investi-
gations now require a more sensitive instrument than
any which has yet been described, we here introduce
the gold leaf electroscope.

Fig. 4— Gold Leaf Electroscopes.

The style represented at A, Fig. 4, is convenient, and
easily constructed. It consists of a half-gallon tincture
bottle, fitted with an ebonite stopper, through the center
of which passes a small brass rod about five inches long,
which terminates about three-fourths of an inch above
the stopper in a brass disc about two inches in diame-
ter, having a round rim about three-sixteenths of an

ELECTRIC POTENTIAL. 17

inch ill diameter projecting from its lower surface, as
shown in the enlarged section at D.

To the lower end of the rod is attached a thin cross-
bar, about five-eighths of an inch long, which will pass
easily through the neck of the bottle. And from this
cross-bar are suspended two strips of imitation gold leaf,
each five-eighths of an inch wide by 2J inches long. A
small hole is drilled near the edge of the disc for con-
venience in attaching wires.

The leaves in this instrument lie close together, and,
consequently, must always be electrified at the same
potential; but in some experiments it is desirable to
electrify them separately, and for this purpose .a bottle
with a wide neck is used, which will admit an ebonite
stopper through which two rods can be inserted about
an inch apart, and from the cross-bar of each a single
leaf is suspended, the surfaces being parallel to each
other. This style is represented at #, Fig. 4. The rods
can terminate above in balls, or be bent outward and
terminate in discs.

Electroscopes may be constructed with thin metal
discs, attached to the glass opposite the leaves ; strips
of the same material extending down and connecting
with the earth. Brass rods surmounted with balls are
often used in the same way, as represented at (7, Fig. 4 ;
in which case a glass shade resting on a wooden base is
more convenient than the bottle form.

The object in either case is to have conductors at zero
potential near the leaves, which renders them niore
sensitive, and discharges them in case of too great
divergence ; thus preventing their adhesion to the glass,
which is often troublesome. Annoyance from the latter
cause is also obviated by using a bottle of globular form,

18 ELEMENTS OF STATIC ELECTRICITY.

the sides of which are too remote to be touched by the
leaves.

A brass cap, covering the glass above, as shown at (7,
is also used to screen the leaves from external electric
influence, and wire screens are likewise used for the
same purpose.

The use of the bottle, or glass shade, is to protect the
leaves from currents of air which would destroy them.
And the ebonite stopper is for better insulation, since
the glass generally used for bottles and shades is of
inferior insulating quality. The disc, or ball, and con-
necting-rod are for convenience in electrifying the
leaves, which are the efficient part of the instrument.

The following experiment will illustrate its use :—
Let the electrified sealing-wax touch the disc of electro-
scope A ; electricity is instantly transferred to the disc,
rod, and leaves, which are all good conductors, and the
leaves, being free to move, and at the same potential,
repel each other, and diverge.

If the disc now be touched with the finger, the elec-
tricity escapes to the earth, and the leaves, being reduced
to zero, converge.

The sensitiveness of this instrument is so great that a
chip of dry wood, less than a grain in weight, electrified
in cutting, and dropped on the disc, produces divergence
of the leaves. A wooden pen-holder, electrified by strik-
ing it lightly on the table, produces the same effect.

Hence, care must be observed to prevent the leaves
from being torn by sudden, spasmodic movements, Avhich
are liable to occur when experimenting with highly elec-
trified bodies in their vicinity.

MUTUAL EFFECTS OF FRICTION. — Thus far we have
considered only the effect produced on the sealing-wax,

ELECTRIC POTENTIAL. 19

glass, or other substance electrified by friction, without
reference to the effect on the substance by which it was
rubbed. But since action and reaction are equal, it is
evident that these two effects must, in some way, equal
each other ; that electricity, or its equivalent in some
other form of energy, must be produced on the rubber
as well as on the substance rubbed.

To test this, let a piece of flannel, after being used to
rub a stick of sealing-wax, touch the disc of electroscope
-4, Fig. 4, and the leaves will instantly diverge, showing
that the flannel has been electrified.

Substitute silk, fur, or any other substance used as a
rubber, and the same result will follow. Let the various
substances rubbed be also tested, and it will be found
that electrification has been produced on both rubber
and substance rubbed, at the same time, by the same
process.

Now let a rubber, about the same size as the sealing-
wax, be prepared, by wrapping a strip of wood in flannel
and insulating one end with a piece of india-rubber
tube.

Holding this rubber by the insulated end, let the
sealing-wax be rubbed with it ; and, keeping both still
in contact, lay them carefully on the disc of the electro-
scope, so that both shall touch it at the same instant,
and no divergence of the leaves will occur. Now lift
off the sealing-wax and they instantty diverge ; replace
it and they converge. Lift off the rubber and they
diverge, replace it and they converge again.

Let the experiment be made with any other two sub-
stances used to generate electricity by friction, as silk
and glass, ebonite and fur, and similar results will be
obtained.

20 ELEMENTS OF STATIC ELECTRICITY.

It will also be noticed that the approach of either
electrified body while the other lies on the disc causes
the leaves to converge, while its withdrawal produces
divergence.

There is often a slight divergence of the leaves when
both bodies are in contact on the disc, due to the diffi-
culty of producing perfect adjustment of contact, and
also to the fact that the electric condition of one body
may change more rapidly than that of the other, from
imperfect insulation or other cause.

The amount of divergence is also liable to vary, the
removal of one body producing greater divergence than
the removal of the other. This difference is also easily
accounted for by difference of mass, of conductivity, or
other cause.

Hence we deduce the following rule : When electricity
is generated on two bodies by their mutual friction, the elec-
tricity of each is neutralized by the presence of the other.

The effect of the mutual friction of the two bodies is
to create a difference of potential by the transfer of
electric energy from one to the other. As one gains
what the other loses, the amount of energy on the two
is not changed so long as they remain in contact, and
hence the potential of the electroscope is not disturbed.

But let one of the bodies be removed ; suppose it to
be the one to which energy has been transferred, the
potential of the remaining body being negative, there is
instantly a transfer of energy to it from the disc and
leaves, which thus become negative also.

The leaves, being both at the same potential, diverge
by mutual repulsion ; and that potential being less than
zero, the divergence is increased by attraction from the
higher potential of the glass and surrounding objects.

ELECTRIC POTENTIAL. 21

Replacing this body, let the one from which energy
has been transferred be removed ; the potential of the
remaining body being positive, there is instantly a
transfer of energy from it to the disc and leaves, making
them positive also. Hence the leaves diverge as before,
from mutual repulsion, and the divergence is increased
by attraction from the lower potential of the glass and
surrounding objects.

From this it will be seen that the effect on the electro-
scope is the same whether the potential of the electrified
body be positive or negative. In either case there is
mutual repulsion between- the leaves, from their being
at the same potential ; and mutual attraction between
them and surrounding objects, caused by difference of
potential.

The indications of the electroscope furnish no means
of distinguishing between positive and negative poten-
tial, being the same for both. And as this is true of
most of the phenomena pertaining to these two states,
it is difficult, in static electricity especially, to determine
which phenomena are positive and which negative.

There is no such well-marked distinction between
them as between the positive and negative states known
as heat and cold ; neither can we observe electric move-
ment as we can heat movement; since heat moves
slowly, while electricity moves with inconceivable ra-
pidity.

But if we can show cause for an accumulation of elec-
tric energy at one point and for its absence at another,
and show effects following such difference of energy, we
then have proof of the positive and negative potential
of the different points, which may be accepted as
reliable.

22 ELEMENTS OF STATIC ELECTRICITY.

Such proof will be furnished hereafter, and the further
consideration of this question must be deferred till the
examination of other phenomena shall enable the stu-
dent to comprehend such proof.

CHARGE DEFINED. — The term charge is used to ex-
press the condition of an electrified body when its poten-
tial is above or below zero. If its potential has been
raised above zero by receiving electricity, it is said to be
positively charged ; but if its potential has been reduced
below zero by the removal of electricity, it is said to be
negatively charged.

Hence we speak of a high negative charge in the same
sense as we speak of intense cold, meaning an intensity
of the negative condition caused by the absence of heat.

CHAPTER III.
THE NATURE OF ELECTRICITY.

THE CONSERVATION OF ENERGY. — A clear under-
standing of that great doctrine of modern science,
known as the conservation of energy, lies at the founda-
tion of a correct knowledge of electricity and electric
phenomena. Hence a brief examination of its prin-
ciples will not be out of place here.

Energy is a universal property of matter. It is the
principle of life and movement in matter in distinc-
tion from matter itself, inseparably connected with
matter and yet distinct from it : heat as distinct from
the heated body ; electricity as distinct from the elec-
trified body ; life as distinct from the living body.

Like matter, it manifests itself in various forms,
as gravity, cohesion, chemical affinity, light, heat, elec-
tricity. Like matter, its quantity in the universe is
fixed and definite, and cannot be increased or dimin-
ished. And hence, like matter, it is indestructible.

It may be transmuted from one form into another,
but in the transmutation there is no loss. One form
may re-appear in many forms, or the many be reduced
to the one.

In our experiments, muscular energy has been ex-
pended to produce electric energy ; but the energy pro-
duced must equal that which produced it, if the doc-
trine of the conservation of energy is true. And since

24 ELEMENTS OF STATIC ELECTRICITY.

it is evident that only a very small part of the mus-
cular energy expended would be required to move
the pith balls, the balanced rod, or the gold leaves, the
remainder must be accounted for.

This is easily done when we consider, first, that
the electric energy was equally divided between the
rubber and the substance rubbed; secondly, that only
a small part of the electric energy was used ; that the
electricity generated was sufficient for the performance
of the same work many times in succession, cither
with the rubber or substance rubbed; and that a
number of pith balls, placed on all sides of the
electrified body, might have been subjected to its
influence. Thirdly, we must consider the amount of
electricity lost from contact with the surrounding air;
and, lastly, that the amount of heat energy gener-
ated by the friction was probably equal to the electric
energy.

If the expended energy had been produced by a
descending weight, which should cause a glass or
ebonite cylinder to revolve in contact with a rubber,
and the sum total of the heat and electricity had
been recovered in the form of work which could be es-
timated, it would be found so nearly equal to the
number of foot-pounds expended by the descending
weight, that whatever difference existed could easily
be accounted for by the friction of the machinery and
other causes.

Experiments of this kind have been actually per-
formed, and the results verify the above conclusion.
Similar experiments have also been made with other
forms of energy, and like results obtained ; so that
the principles of the conservation of energy are now

THE NATURE OF ELECTRICITY. 25

well established, and universally recognized in all
practical work.

Another illustration may render the subject more
clear. A pound weight raised to a height of 20 feet
has acquired 20 foot-pounds of energy ; and, in de-
scending to its former level, can accomplish 20 foot-
pounds of work; as in raising to the same height
another weight of nearly equal mass. But if stopped
in its descent at a level of 10 feet, it has expended
only 10 foot-pounds of energy, and has still a reserve of
10 more.

It is evident, that to raise the weight in the first place
required the expenditure of 20 foot-pounds of energy ;
and that though this energy was consumed in the pro-
cess, and had disappeared, it was not lost, but merely
stored up, ready to be expended, either at once, by
the weight descending the entire distance, or in detail,
as when stopped half way or at any other point. If it
had descended but one foot, it would still have a re-
serve of 19 foot-pounds of energy.

It will be noticed that the second weight, raised by
the descent of the first, is required to be of less
magnitude, since part of the energy must be expended
in overcoming friction and inertia. For if it were of
equal magnitude, the force expended would exceed the
force stored up; since it must perform not only the
same work, but the added amount for friction and
inertia; in which case it would be possible to create
force, and the doctrine of the conservation of energy
would cease to be true.

It is immaterial whether the descent of the one pound
raises a small weight to the height of 20 feet, or a
laree weight to the height of one foot ; which it can be

26 ELEMENTS OF STATIC ELECTRICITY.

made to do by a system of ropes and pulleys. The
20 foot-pounds of energy expended must exactly
equal the 20 foot-pounds stored up; and the height
through which the large weight is raised is to that
through which the small weight descends in the in-
verse ratio of the mass of each.

In all electric work, of whatever nature, the same
principle will be found to hold true ; gravity potential
in this case representing electric potential in electric
work. Mechanical work may re-appear as electric work,
or electric as mechanical work; the energy produced
being always, in some form, equal to the energy
expended.

PIEAT, LIGHT, AND ELECTRICITY COMPARED. — The
weight of evidence goes to show that electricity, like
heat and light, belongs to that kind of energy known
as molecular ; and whatever is known as to one kind of
energy may, by analogy, be inferred as to other kinds
of the same class, with such modifications as distin-
guish different species of the same genus.

There is ample proof that heat is a mode of molec-
ular motion. Not that the heat produces the motion, or
the motion the heat, but that it is motion ; that the
molecules of matter being thrown into a certain kind
of motion, the result is the sensation known as heat.

According to the universally accepted theory of
light, it is another species of motion of the same
kind ; and there are indications that light and elec-
tricity are identical. But, if not identical, we may
at least assume that they are closely allied to each
other.

We find also that the same causes, acting at the
same time, on the same bodies, and under the same

TIIE NATURE OF ELECTRICITY. 27

circumstances, produce both heat and electricity, in
numerous instances; in others, equally numerous, both
heat and light, and in others, heat, light, and elec-
tricity.

The simultaneous production of heat and electric-
ity is seen in the examples already given of bodies
electrified by friction, of which heat is also a neces-
sary result.

Another prominent instance is the action of the
electric generators known as dynamos; in which the
evolution of heat is such that special provision for cool-
ing has to be made, to prevent injury. Here mechani-
cal action is the agent.

In the galvanic battery we have a well-known in-
stance of the production of heat and electricity by
chemical action ; as a certain amount of heat, more or
less perceptible, is always a result.

Instances of the simultaneous production of heat
and light are numerous and well known, as the heat-
ing of an iron rod, which becomes luminous when the
temperature rises to a certain degree ; whether it be
heated by friction, as of a shaft and journal, or by the
chemical action of a furnace.

The dynamo and galvanic battery have been re-
ferred to as producing both heat and electricity.
When a current of this electricity, of sufficient in-
tensity, is passed through a conductor of high resist-
ance, as a fine platinum wire, or a carbon filament, they
become luminous by incandescence ; and when passed
through two sticks of carbon slightly separated, we have
light of great intensity ; and there is, in both instances,
the evolution of intense heat. These are perhaps the
most striking examples which can be given of the sim-

28 ELEMENTS OF STATIC ELECTRICITY.

ultaneous evolution of heat, light, and electricity from
the same causes.

In the thermo-electric battery we have an example
of the direct production of electricity by heat.

POLARIZED LIGHT AND ELECTKICLTY. — Experiments
with polarized light, made by Faraday and others, fur-
nish strong evidence of the close alliance, if not actual
identity, of light and electricity.

It is known that light, from certain peculiarities of
reflection and transmission, undergoes a change, so
that when subsequently transmitted, its action is dif-
ferent from that of the original transmission, and this
change has been termed polarization.

Let a plate of tourmaline be so placed that a ray of
light falling on it shall be transmitted at right angles
to a particular direction within the crystal, known as
its optical axis. Then let another tourmaline plate be
so placed with reference to this one that their optical
axes are parallel, and that the ray shall pass through
both and form a bright spot on a screen beyond.

Now let either plate be turned, so that their optical'
axes are no longer parallel to each other, but still at
right angles to the ray ; the bright spot on the screen
will fade as the angle increases, till at 90 degrees it will
disappear. Continuing the rotation, it will re-appear,
increasing in brightness, till, at 180 degrees, it is en-
tirely restored ; then fading again till extinguished
at the end of the third quadrant, and again increasing
in brightness till restored at the end of the fourth
quadrant, or original position.

This alternation of brightness and extinction de-
pends on the relative angular position of the plates.
Let either of them be turned in either direction, and

THE NATURE OF ELECTRICITY. 29

the same result follows ; but when both are turned in
the same direction, there is no change in the brightness,
and no extinction of the light. Substitute one for the
other, and the same results are obtained.

It is evident, then, that the light in passing through the
first plate has undergone a change which affects its trans-
mission through the second, in any position except when
the optical axes of both are parallel; extinguishing it
entirely when they are at right angles to each other.

It is also found that this effect, termed polarization,
occurs to light transmitted through or reflected from any
transparent medium, as glass, selenite, Iceland spar,
and various liquids. Polished metals also produce the
same effect on reflected light. And this reflection or
transmission takes place at a certain angle, known as
the polarizing angle, which varies in each substance
by a certain definite amount.

Now let the ray be transmitted through, or reflected
from, a small piece of glass of suitable size or shape,
placed at the proper polarizing angle, and let the plates
be turned till their optical axes are at right angles, so
as to produce extinction ; then let the glass be sub-
jected to a powerful electric strain and the extinguished
light will re-appear, continue during the electric action,
and disappear when it ceases ; which shows that this
electric action has counteracted the effects of polar-
ization.

Similar experiments with various substances, too
numerous to detail here, show similar results.

Without anticipating another branch of our subject,
it may be stated here, that electricity and magnetism
are so closely allied that whatever affects one must
have some important relation to the other.

30 ELEMENTS OF STATIC ELECTRICITY.

Recent experiments have- demonstrated that unusual
disturbances in the variation of the magnetic needle
are coincident with unusual disturbances in the sun, in
connection with the phenomena known as sun spots ;
and that the telegraph and telephone are, at such
times, seriously disturbed by what are known, techni-
cally, as "electric storms"; that is, unusual disturb-
ances in the earth's electricity shown in the phenomena
known as earth currents.

It has also been found that these solar disturbances
are periodic; and that these periods, for the last hundred
and fifty years, correspond almost exactly with the
periods of unusual variation of the magnetic needle.

Since the sun is our chief source of light and heat,
since they are, in fact, the result of the constant dis-
turbance of the elements of that body; and since, when
this disturbance assumes an unusual phase, there is, co-
incident with it, an unusual disturbance in the earth's
electricity, it must be accepted as strong proof of a
common origin of heat, light, and electricity.

We have heat, light, and electricity derived from
friction, from chemical action, from magnetic action,
and from the sun. We have heat producing electric-
ity, and electricity producing heat and light ; and we
have electricity neutralizing the polarizing effect of
light. The evidence of identity then becomes cumu-
lative, while that of close alliance amounts to demon-
stration. Hence we may infer certain facts in regard
to the nature of electricity from what we know of
similar facts in regard to the nature of light and heat.
And we are also warranted in the conclusion, that a
well supported theory of light or heat requires but
little modification to adapt it to electricity.

THE NATURE OF ELECTRICITY. 31

l'*:
This much we certainly know, that they all are forms

of energy and that they radiate from the centers where
they are generated. Hence the term radiant energy
is equally applicable to each.

THE WAVE THEORY. — It has been assumed that a
subtle medium, termed ether, pervades all space ; that
it is so attenuated that it can insinuate itself between
the grosser molecules of material bodies ; so that solids
of the finest and closest texture, as well as liquids and
gases, are pervaded by it; and that light, and probably
electricity, are due to waves or undulations of this
ether. The evidence of the existence of such a medium
is almost wholly negative, and, like all negative evi-
dence, unsatisfactory. The assumption presupposes the
necessity of its existence.

It has been stated that energy is a universal property
of matter, and the converse may be accepted, that
energy cannot exist without matter. And since light,
coming from the sun, must traverse the in'terplane-
tary spaces, there must be matter there ; else we shall
be compelled to admit that energy can exist without
matter, which is contrary to all our experience.

The earth is surrounded by an atmosphere, to the
limits of which we cannot penetrate. In 1822, Dr.
Wollaston made a careful mathematical calculation, as
the result of which he claimed to have demonstrated
that the earth's atmosphere must have limits, beyond
which it cannot exist. And this apparent demonstra-
tion was accepted as authority, and remained unchal-
lenged for half a century. Meantime the wave theory
of light became prominent, and a medium being one of
its fundamental principles, the existence of the ether
was assumed, and is now generally accepted.

32 ELEMENTS OF STATIC ELECTRICITY.

But the researches of modern science have swept
away many of the errors of the past, and it is not im-
possible that Dr. Wollaston's demonstration may share
the same fate. Many eminent scientists, who have
made experimental investigations on the subject, hold
that the expansibility of the earth's atmosphere is
unlimited ; among whom may be cited Grove, Gassiot,
Geissler, and Dr. Andrews. And W. M. Williams,
in his work, " The Fuel of the Sun," claims to have
discovered a serious error in Dr. Wollaston's calcula-
tions, which vitiates his conclusion.

The assumption of these writers is that an atmos-
phere, the same as that of our earth, pervades all
space ; that in the interplanetary spaces it becomes ex-
ceedingly attenuated ; and that each of the heavenly
bodies attracts and surrounds itself with a portion of
it ; the extent and density of which is in proportion to
the mass of the body.

The high degree of vacuum which can now be attained
by improvements in the air-pump, seems to demonstrate,
that while electricity will pass more freely through rare-
fied air, on account of the reduced resistance, than
through air of ordinary density, it must still have a
medium in which to travel ; and that its passage through
an absolute vacuum, or space devoid of any known ma-
terial substance, is highly improbable. But as the best
attainable vacuum is still only an approximation to an
absolute vacuum, the full demonstration of this point
has not yet been reached.

The existence then of some elastic medium, by which
the two forms of radiant energy, known as light and
heat, can traverse the interplanetary spaces, is not
questioned. Nor does the theory of the unlimited ex-

THE NATURE OF ELECTRICITY. 33

pansion of our atmosphere conflict at all with the
theory of the universal existence of ether, since
the theory of ether is that it permeates all material
substances.

The wave theory assumes that radiant energy is
transmitted by the undulations of some medium ; that
an impulse originating at any center of energy, as the
sun, produces a wave which traverses this medium with
inconceivable velocity, till it reaches some distant
point, as the earth ; and that the constancy of such im-
pulses at every point on the sun gives rise to the phe-
nomena of solar light, heat, and electricity.

In like manner we may assume any other center of
energy, as a red-hot metal ball, radiating light and
heat; a stick of ebonite, excited by friction, radiating
electricity.

It is also assumed that the impulses radiate in straight
lines, while the undulations occur at right angles to
those lines.

To illustrate : — Drop a pebble on a smooth sheet of
water ; the impulse creates waves which radiate out-
ward in widening circles. The pebble has depressed
the water at the point where it struck, forcing the ad-
jacent water outward, and causing it to rise above the
general level ; then the downward impulse of this
wave, cinking under the force of gravity, raises the
original center, and also produces, by its outward im-
pulse, another wave beyond, as it descends by its inertia
below the general level.

As the water oscillates vertically above and below the
level, each successive impulse produces a new wave,
while the same process goes on in the outward waves,
creating new waves beyond, in ever widening circles,

84 ELEMENTS OF STATIC ELECTRICITY.

till the force of the original impulse has been exhausted,
and the water returns to its original level.

Now it will be perceived that there is no transfer of
the water from the center outward beyond the length of
the first wave. Part of the water forced outward by the
original impulse flows back again, while another part
flows outward, producing a new wave. The water is then
under the influence of two forces, one horizontal, the
other vertical, acting at right angles to each other; the
horizontal producing the wave length, that is, the
distance from crest to crest, or from hollow to hollow,
while the vertical produces the height, that is, the
vertical distance from hollow to crest, or from crest to
hollow.

In a similar way, it is supposed, occur the undula-
tions of the assumed elastic medium, with this excep-
tion, that the waves on the water occur in the same
horizontal plane, radiating outward in concentric cir-
cles, while those in the elastic medium occur in any
direction in which they are free to move ; radiating
outward in concentric spheres, if wholly unrestrained ^
or in sections of spheres or spheroids, if limited and
starting from impulses at various points on any surface,
either spherical, like that of the sun, or plane.

Having taken an illustration from a liquid, illus-
trations from solids will also be in point.

If a long rope, stretched lengthwise, with plenty
of slack, be held at one end, and jerked rapidly up and
dewn, it will be thrown into wraves, which will run
along its entire length.

Here it is evident that while the impulses given at
one end travel in waves to the other, the rope, as a
whole, remains stationary ; successive portions acting

->

TUK NATURE OF ELECTRICITY. 3o

as yielding levers to transmit the impulse" along its
length.

If one end of a lever be depressed below a horizon!
it receives a forward as well as downward movement,
in the arc of a circle, its opposite end receiving an up-
ward and backward movement. In this way each suc-
cessive portion of the rope oscillates horizontally as
well as vertically, modified by the difference between a
yielding and a rigid body.

Let a number of elastic balls be suspended in a
straight line in contact with each other. Draw back
the outer ball at one end of the line and let it swing
against the adjoining ball ; the impulse will be trans-
mitted along the line, and the outer ball, at the other
end, will swing off to nearly the same distance as that
through which the first ball swung, all the others re-
maining stationary.

Here the impulse is transferred from ball to ball by
virtue of their elasticity. When number one impinges
on number two the impact changes its shape slightly to
that of a spheroid; as it resumes its shape it imparts the
impulse to number two, by which it is imparted to num-
ber three, and so on through the line. But action and
reaction being equal and opposite, there is no perceptible
movement till the last ball is reached, which swings off,
since there is no ball to react against it. The impulse
travels, but the line remains stationary.

Here, also, it will be perceived that there is a radial
force acting at right angles to the horizontal force,
much the same as would result from a similar impact
if each ball were hollow, and its surface composed of an
infinite number of semicircles, joined at the points of
impact.

86 ELEMENTS OF STATIC ELECTRICITY.

Now the molecules of a metal rod may be com-
pared to an infinite number of these lines of balls ; and
it may be assumed that a heat impulse, or an electric
impulse, given at one end, moves along these lines
in some way analogous to that in which the impulse
moves along the lines of balls.

We are not obliged to confine ourselves to any spe-
cific method of movement; but may suppose a wave
movement, similar to that which takes place in the
slack rope, or on the water, if it shall seem to accord
best with known facts and phenomena.

In the polarization of light, it is supposed that the
waves assume a certain phase, in conformity with the
special arrangement of the molecules of the crystal.
Hence if the crystals are cut from the same block, and
placed in the same position, the phase will be the same
for each, and the light will pass through. But if the
second is turned at right angles to the first, the phase
produced by passing through the first will not be in con-
formity with the arrangement of the molecules in the
second, and hence the light cannot pass through.

Suppose the arrangement -of the molecules to be in
layers, or strata, like the sheets composing a ream of
note-paper, placed in a vertical position ; the waves
of ether would assume a vertical phase, and, meeting
the second crystal, placed in the same position, would
pass through. But if the second were turned, so as to
bring its strata to a horizontal position, the vertical
waves would be broken, and could not pass through.

Instead of the ether we may suppose the molecules
themselves thrown into waves, whose phase would con-
form to the structure of the crystal, and the same result
would evidently follow.

THE NATURE OF ELECTRICITY. 37

There is no reference, in this supposed case, to any
visible stratification of a crystal, as the experimental
ray is usually admitted at right angles to such stratifi-
cation ; the reference is to an invisible arrangement of
the molecules.

The same course of reasoning is applicable to heat,
or to electricity, but the phase of the heat wave, or
the electric wave, may be different from that of the
wave of light, so that a substance opaque to light, as
copper, might allow the free passage of heat or electric-
ity ; or a substance transparent to light, as glass,
might obstruct their passage.

And the medium in which the electric energy travels
may be the ether which is supposed to pervade the
different kinds of matter ; or the matter itself, in any
of its various forms, solid, liquid, or gaseous : as it
has been shown that undulations' may take place in
each of them.

CONDUCTIVITY FOR HEAT AND ELECTRICITY COM-
PARED.— It is very remarkable, and must be something
more than mere coincidence, that conductivity for heat
and electricity is nearly the same in the same sub-
stances. A good heat conductor is a good electric
conductor; a non-conductor of heat is a non-conductor
of electricity. So that if we know either the conduc-
tivity or resistance of any substance for heat, we
have, practically, its conductivity or its resistance for
electricity. This will appear from the table follow-
ing, showing the results obtained by Wiedmann and
Franz.

Hence if heat and light are modes of motion, travers-
ing various substances by undulations, we are warranted
in assuming the same with reference to electricity.

Substance.

Heat Conductivity.

Silver . . .

. . 100

Copper . . .
Gold ....

. . 74
53

Brass . . .

. . 24

Tin ....

15

Iron ....

12

Lead ....

9

Platinum . .

. . 8

German silver

. . 6

Bismuth

. . 2

38 ELEMENTS OF STATIC ELECTRICITY.

COMPARATIVE CONDUCTIVITY OF DIFFERENT SUB-
STANCES FOR HEAT AND ELECTRICITY, AS GIVEN BY
WlEDMANN AND FRANZ :—

Electric Conductivity.

100
73
59
22
23
13
11
10
6
2

Other observers place the electric conductivity of
some of these substances much higher, making the con-
ductivity of copper nearly equal to that of silver.

If the electric wave has its own peculiar structure, it
is evident that a substance whose structure is adapted
to it, or whose molecules easily adapt themselves to it,
would be a conductor; while a substance whose struct-
ure is not so adapted, or whose molecules resist such
adaptation, would be a non-conductor.

An attempt to insert a No. 36 screw into a No. 30
screw hole will fail, because the threads of the screws
are not adapted to each other. But let the same screw
be inserted into some yielding substance, as soft wood,
and the substance adapts itself to the structure of the
screw ; or, as we say, it cuts its own thread ; while a
rigid substance like iron resists such adaptation.

Something analogous to this may constitute the
difference between conductors and non-conductors,
and may also be the cause of other electric phenomena
of equal importance.

THE NATURE OF ELECTRICITY. 39

THE SPAEK AND SNAP. — As already stated, every
substance offers a certain degree of resistance to the
passage of electricity, and a result of this resistance
is the generation of heat, often accompanied with light,
and this effect varies as the resistance.

Platinum is a metal of high resistance, while that of
copper is very low; and a. fine platinum wire will be
brought to a white heat by an electric current which
would scarcely change the temperature of a copper wire
of the same size.

Air offers such high resistance that the passage of
electricity through it always produces both heat and
light, in the form of a bright spark. This occurs not
only when an electric charge passes through several
inches of it, but through the thinnest film ; the pres-
ence of heat, as well as light, being demonstrated by the
lighting of gas by a spark less than J inch in length.

A sudden condensation of the air, forced forward
and laterally by the charge, has been suggested as
the probable cause. If such condensation takes place,
heat would certainly be the result, as when air is com-
pressed by mechanical means. And perhaps it would
be accompanied by light, though this is not probable,
as combustion and incandescence are the only known
means of producing artificial light besides that now
under consideration, either of which would imply the
presence of some other substance besides air. But the
hypothesis seems to assume the passage of some material
substance through the air to produce the condensation,
of which there is no evidence.

But if, instead of condensation, we suppose undula-
tions to take place, giving great intensity of motion,
as the electric impulse, darting forward with incon-

40 ELEMENTS OF STATIC ELECTRICITY,

ceivable velocity, suddenly forces the resisting air into
the phases of the electric waves ; then the generation
of those other modes of motion, known as heat and
light, might easily be the result.

A sharp sound, varying from an insignificant snap to
a deafening report, always accompanies the spark. On
the condensation theory this is accounted for by the
sudden displacement and reflux of the air. But since
sound, like heat and light, is another mode of motion,
occurring chiefly in the air, it is evident that the wave
theory will best account for it ; the electric impulse
giving rise to these different modes of motion.

THE DUAL THEORY. — We have already seen, in ex-
periments with the pith ball electroscope, that the balls
may be attracted and repelled by electrified glass, seal-
ing- wax, and various other substances.

Let an electrified glass rod approach one of the balls ;
the ball is attracted, and, after contact, repelled. Now
let an electrified stick of sealing-wax be brought near,
and the electrified ball, which was repelled by the glass,
is attracted by the wax. Or let the ball be first elec-
trified and repelled by the wax, and it is attracted by
the glass.

Further experiments show that the same results can
be produced with other substances; glass representing
a certain class of substances, which show similar electri-
fication, and sealing-wax and resin another class, which
shows opposite electrification to that of glass.

Hence it has been assumed that there are two kinds of
electricity. One kind generated on the class of sub-
stances represented by glass, and the other on the class
represented by resin. The former was once designated
as vitreous, and the latter as resinous; but the term

THE NATURE OF ELECTRICITY, 41

' .

positive is now used instead of vitreous, and negative
instead of resinous. Used in this way, these terms
have no reference to a difference either in quantity or
intensity ; they express only a supposed difference in
kind, not in degree.

-This doctrine of the dual nature of electricity was
first proposed by Dufaye, and has since been strongly
maintained by many eminent scientists. Deschanel,
speaking of the phenomena under consideration, says :
" These phenomena clearly show that the electricity de-
veloped on the resin is not of the same kind as the elec-
tricity developed on the glass."

Now the only thing "clearly shown " is the difference
in the substances, not in the electricity. For we have
precisely the same electric phenomena of attraction
and repulsion with the glass as with the sealing-wax ;
but a third substance, the electrified pith ball, is at-
tracted by one and repelled by the other ; a result
which it would seem more reasonable to attribute to
the difference known to exist between the substances,
than to a difference supposed to exist in the electricity.
For it has already been shown that different causes, as
conductivity or resistance, influence the intensity of
electrification on different substances. Other causes
also, as a difference of temperature or mass, of hard-
ness or softness, density or porosity, doubtless contribute
to the same result.

But considering the quality of resistance alone, the
potential of any non-conductor, as glass, is liable to
vary greatly on different parts of its surface, when
electrified by friction ; and to differ from the potential
of sealing-wax, similarly produced on different parts
of its surface.

42 ELEMENTS OF STATIC ELECTRICITY.

The friction of the same rubber is also greater on
a substance like sealing-wax, whose surface is soon soft-
ened by the heat generated, than on a smooth, hard sub-
stance like glass, which is not affected in this way.

And, as already shown, difference of potential produces
attraction, while equality of potential produces repulsion,
between bodies. Hence the attraction by the sealing-
wax, of the pith ball electrified by the glass, is a neces-
sary result of difference of potential ; while its repul-
sion by the glass follows from equality of potential.
And the same will be true of the ball electrified and re-
pelled by the wax and attracted by the glass.

But if a difference exists in the kind of electricity
produced by the different classes of substances, we
should expect that difference always to manifest itself
whenever one of either class is employed as a generator.

But this is only true in a general way, to which the
exceptions are very numerous ; for it often happens that
glass and sealing-wax, or other substances belonging to
the different classes, when rubbed with the same rubber,
exhibit the same electric qualities. The same result
also will often follow \vhere different kinds of rubbers
are employed, as silk on one substance and woolen on
the other.

Such results are inconsistent with the theory of two
electricities ; but are easily accounted for by a differ-
ence, or an equality of potential, which we know is
liable to exist.

Hence, preference must be given to the doctrine of one
electricity, originally proposed by Franklin ; simple and
plain, like truth itself, and in strict accord with all elec-
tric phenomena ; whether pertaining to static electric-
ity, or to electricity under other forms.

CHAPTER IV.
INDUCTION.

IT is noticeable that in all our experiments thus far
the electrified body acts on the other bodies before there
is any actual contact. The knife-handle attracts the
spoon ; the sealing-wax, ebonite, or glass attracts the
balanced rod or the pith ball while separated from them.
And when either of these electrified bodies approaches
the gold-leaf electroscope, there is first a divergence of
the leaves before contact occurs.

It will also be noticed that this effect increases or
diminishes as the distance is increased or diminished.
And, further, that while the interposition of different
substances, as glass, paraffin, ebonite, air, wood, metal,
produce great variations in the effect, none of them
wholly prevent it.

There is evidently, then, an invisible influence ex-
tending to a certain distance from the electrified body
in every direction, and affecting everything within its
sphere, and this effect is called induction.

When an electrified body is brought near the disc of
the electroscope without touching it, the leaves diverge,
and on its removal converge again, showing no perma-
nent effect. But if it is allowed to touch the disc, the
leaves are electrified, and remain divergent after its
removal.

But if, instead of touching the disc, it be held near

44

ELEMENTS OF STATIC ELECTRICITY.

enough to produce divergence, as at J., Fig. 5, and, while
in that position, the disc be touched with the finger, as
at B, the leaves will converge, and remain so as long
as the electrified body is held near ; but on its removal
as at (7, they will diverge, and remain divergent, the
same as after contact of the electrified body with the
disc.

Fig. 5 — Induction Illustrated.

Here, then, is electrification by induction, without
any transfer of electricity by contact. How can this be
accounted for ?

When the electrified body is brought near, whether
its charge be positive or negative, the effect of induction
is to produce a temporary change. of the potential of
the electroscope, and the leaves diverge.

If the charge of the electrified body be positive, elec-
tricity is repelled from the disc to the leaves, and they
diverge, being positively electrified to the same poten-
tial, and hence mutually repellent, and also attracted
by the lower potential of surrounding bodies.

But if the electrified body be negatively charged,

INDUCTION. 45

electricity is attracted from the leaves to the disc, and
they diverge, being negatively electrified, and mutually
repellent, as before, and attracted by the higher poten-
tial of surrounding bodies.

Now, when the disc is touched with the finger, and
thus connected with the earth, if the charge is positive,
the potential of the electroscope is changed by the
escape of electricity to the earth under the influence of
the electrified body, and the leaves converge. But if
the charge is negative, the potential of the electroscope
is changed by the attraction of electricity from the earth,
and the leaves converge as before, equilibrium being re-
stored between the disc and leaves in each case.

The leaves remain convei gent so long as the electrified
body is held near ; the electroscope being still under the
influence of the force by which the change of potential
was produced ; which is evidently just equal to the re-
pelled, energy in the first instance, and to the attracted
energy in the second. But when the electrified body is
removed, this equilibrium is disturbed, and the leaves
diverge under the influence of mutual repulsion and
outward attraction, as already explained.

This experiment proves that a body connected with
the earth, and under the influence of induction, may
differ in potential from the earth, and is not necessarily
at zero potential from its earth connection. For it is
evident that such difference of potential existed during
the connection of the electroscope with the earth, else
it could not have become manifest when the connection
was severed and the inductive influence removed.. For
when the electrified body is removed before such con-
nection, the leaves converge, but when removed after it
has been made and severed, they remain divergent;

46 ELEMENTS OF STATIC ELECTRICITY.

showing that the difference of potential was created and
existed during the earth connection.

This point has an important bearing on phenomena to
be considered hereafter, in regard to which prominent
writers have been betrayed into serious mistakes from
having overlooked it.

INFLUENCE OF DISTANCE. — It is important to notice,
that when the electroscope has been charged by induc-
tion in this manner, and the electrified body is again
brought near, the leaves continue to converge as the
body approaches, and come together when it is in the
same position as when the disc was touched with the
finger. A nearer approach produces divergence, which
increases as the body is brought still nearer.

Let it now be gradually withdrawn, and the leaves
gradually converge and come together, when the body
reaches the same point, as before. Further withdrawal
produces divergence, which continues to increase, and
reaches its limit when the body is wholly removed.

If the electroscope be placed at various points, equally
distant from the electrified body, the effect of induction
will be the same, so long as the same distance from the
earth and surrounding objects is maintained. Hence
it is evident that electric energy, like other forms of
radiant energy, as light and heat, radiates equally
in all directions when not interfered with by other
influences.

Suppose the electrified body to be a small globe,
entirely removed from the earth, and surrounded with
a perfectly homogeneous medium, it would be the
center of a sphere of inductive influence. And suppose
the lines of force radiating from it to be cut by the
surfaces of two imaginary concentric spheres of differ-

INDUCTION. 47

ent sizes, one placed outside of the other, and having
the electrified globe for their common center.

Since the surfaces of spheres are to each other
as the squares of their radii, and since the radii meas-
ure the distances from the center, the surfaces are to
each other as the squares of their distances from the
center.

But as each surface embraces all the lines of force,
the intensity of force on equal surface areas of the two
spheres would be in the inverse ratio of their entire sur-
faces ; and hence would vary inversely as the squares
of their distances from the center.

Hence, electric induction varies inversely as the square
of the distance.

Practically the conditions of the supposed case are
never exactly fulfilled ; but that does not affect the
correctness of the principle, which is the same in elec-
tricity as in light and radiant heat.

CYLINDER ELECTRIFIED BY INDUCTION. — The effect
of induction may be further illustrated by an insulated
cylinder of conducting material, placed between two
spheres of similar material, one insulated, and the other
connected with the earth by a chain, as shown in Fig.
6 ; the cylinder having mounted on it three pith-ball
electroscopes, connected with it by conductors.

If the insulated sphere, A, be positively electrified,
electricity will be repelled by induction from the end of
the cylinder next A to the end next B. And since B
is connected with the earth, the electricity accumulated
on the end of (7, next to it, will repel to the earth from
B an amount equal to the positive charge on A.

Hence the pith ball next A, being negative and A
positive, is attracted by A, while the one next B, being

48 ELEMENTS OF STATIC ELECTRICITY.

positive and B negative, is attracted by B ; but the
central ball, being neutral, remains unmoved.

If the sphere, A, be negatively electrified, these condi-
tions will all be reversed. Electricity will be attracted
to the end of C next A, and a positive charge, equal
to the negative on J., attracted from the earth to B.
Hence the balls will assume the same positions as
before.

Fig. G — Cylinder Electrified by Induction.

Similar inductive effects can be produced on the cyl-
inder by the sphere A alone, but less marked than when
two spheres are used; and, for such an experiment, tin-
foil electroscopes are better than those made with pith
balls, being more sensitive.

THEORY OF INDUCTION. — It Is noc known how in-
ductive force is transmitted. The hypothesis has been
advanced that it is by a certain strain of the medium ;
as when a weight is lifted by a rope or pushed by a pole,
the energy is transmitted in one case by the tension of
successive portions of the rope, and in the other by a
compression of successive portions of the pole. In
either case the energy or stress produces a strain, which

INDUCTION. 49

runs through the substance of the medium till it reaches
the object, and the continued stress produces continued
strain. Something analogous to this, it is assumed,
takes place in the transmission of electric energy by
induction.

This hypothesis has the sanction of eminent author-
ity, and may assist us in arriving at a solution of the
problem. Gordon says: "If electric induction were a
4 direct action at a distance,' we should expect that it
would be transmitted equally through all insulators.
One of the strongest arguments for supposing it to be
a strain of the particles of the insulator is found in the
fact that different insulators transmit it with very differ-
ent strengths."

"Induction, so far from being a 'direct action at a
distance,' is most certainly transmitted by the particles
of the dielectrics, and is affected by almost every molec-
ular change which may occur in them."

And he defines strain, as here used, to mean " an
alteration of size or shape," including "all alterations
of volume," " all twistings and bendings, and all vibra-
tory motions other than those of a rigid body as a
whole."

The wave theory agrees with the views here expressed ;
for we have only to conceive that this " strain " consists
in a " vibratory motion," that is, in undulations of the
medium.

It is also in accordance with the analogy of similar
transmission of other forms of radiant energy. And, if
all energy has a common origin, it is reasonable to sup-
pose that the transmission of its different forms would
present striking analogies.

INFLUENCE OF DIELECTRIC. — In order to observe

50 ELEMENTS OF STATIC ELECTRICITY.

induction there must be two or more bodies at differed
potentials placed in each other's vicinity, and these must
be separated by an insulator; for, if separated by a con-
ductor, equilibrium would at once be restored, and in-
duction could not take place.

Insulators through which induction takes place are
called dielectrics, from the Greek &«, through. Air was
the dielectric between the electroscope and electrified
body, and between the spheres and cylinder, in the ex-
periments already given.

Now, since conductors permit electricity to pass
through them easily, while insulators resist its passage,
there must be some peculiarity in the nature or arrange-
ment of the molecules which makes two bodies of the
same class similar in this respect, while two of opposite
classes are dissimilar.

Hence we can easily conceive that when two insulated
conductors, at different potentials, are brought into con-
tact, the undulations of their molecules would assume
the same phase, and equilibrium take place; but that
when those undulations are transmitted through a die-
lectric, they undergo such a change that, the phases of
the undulations not being the same, there is a repulsion
instead of an intermingling, which results in creating
opposite potentials in adjacent parts, on either side of
the dielectric, the negative of one being equal to the
positive of the other.

And since in the transmission, part of the energy is
consumed in overcoming the resistance, difference of
potential, on opposite sides, must result from this cause
also.

If either conductor be removed, still remaining insu-
lated, the equilibrium of each will be restored, and its

INDUCTION. 51

potential be found the same as before it was brought
within the sphere of inductive influence, showing that
no permanent effect has resulted.

Hence it will be seen that the effect of induction is
opposite to that of contact; the latter producing perma-
nent equilibrium between conductors, while the former
produces temporary disturbance of equilibrium.

SPECIFIC INDUCTIVE CAPACITY. — It has already been
stated that electric induction takes place through all
substances, but in different degrees ; and, since it
is found that each has an inductive power peculiar
to itself, this property is called its specific inductive
capacity.

The importance of this subject will be understood
when it is considered that it affects enterprises in-
volving large capital, public convenience, and public
safety ; as in the transmission of electric energy by
insulated conductors, as telegraph and telephone wires,
ocean cables, and electric light wires ; including the
important question of underground transmission in
cities.

Hence, for the last forty years, it has engaged the atten-
tion of such men as Faraday, Boltzmann, and many
others, including the earlier researches of Cavendish,
who seems to have been the first to investigate it, but
whose experiments on this subject have only recently
been published.

The general method of investigation is as follows: —
The inductive capacity of dry air at the barometric
pressure of 760 millimeters (29.92 inches) and at the
temperature of 0° C. (32° Fahrenheit) is made the
standard unit by which the inductive capacities of all
other substances are estimated.

52 ELEMENTS OF STATIC ELECTRICITY.

To illustrate : — Suppose we have two insulated metal
plates, A and B (Fig. 7j, separated by an air space C ' ;
let A be electrified and J9. connected with the disc of
an electroscope. First note the amount of divergence
of the leaves ; then let a plate of glass, cake of paraffin,
or any other insulator which will exactly fill the space

Fig. 7 — Specific Inductive Capacity Illustrated.

(7, be introduced between the plates, and note the diver-
gence of the leaves now, as compared with the former
divergence.

As this insulator has displaced the air, it is evident
that its inductive capacity, as compared with air, is
shown by the difference in the divergence of the leaves.

INDUCTION. 53

If that divergence has increased, then the power of this
insulator to transmit electric influence — that is, its spe-
cific inductive capacity — is greater than that of air;
otherwise, it is equal to, or less than that of air.

From this we see that specific inductive capacity varies
inversely as insulation. Hence this property is almost
infinite in the best conductors ; while in the best insu-
lators it is the reverse.

By methods similar to the above, with the aid of
improved instruments, to be described hereafter, the
specific inductive capacities of a number of substances,
including the principal insulators, have been carefully
estimated by Boltzmann, Gordon, and others: and from
the results obtained by them the table on the next page
has been prepared, in which the general averages are
given.

The results obtained by different observers differ so
widely that they can only be regarded as approximate,
and will undoubtedly require future correction, when
improved methods shall give greater accuracy.

The table shows the electric resistance of glass to be
much less than that of ebonite ; the inverse ratio being
5.87 to 2 89 : and this is doubtless true of glass, in the
average. But, if the best insulating glass were com-
pared with the best insulating ebonite, the ratio might
require to be reversed. Ebonite, when subjected to a
powerful electric strain, seems to yield gradually, and
allow the electricity to creep through it ; and, by con-
tinued strain, its electric resistance soon becomes
permanently impaired : while the best insulating glass
rigidly resists, and suffers fracture before yielding.

But, according to Gordon, the electric resistance of
glass also becomes somewhat impaired by long use ; or,

54 ELEMENTS OF STATIC ELECTRICITY.

which is the same thing, its specific inductive capacity
is increased. All of which goes to prove that electric
transmission depends on molecular structure.

SPECIFIC INDUCTIVE CAPACITIES OF VARIOUS
SUBSTANCES.

Standard.

Air at 0° C. temperature and 760 mm. pressure, 1.0

Solids.

Paraffin, 2.09

Caoutchouc, 2.23

Gutta-percha, 2.46

Shellac, 2.85

Ebonite, 289

Sulphur, . .• 2.95

Resin, . . . 3.6

Glass, average of various kinds, 5.87

Liquids.

Bisulphide of carbon, 1.81

Petroleum, 2.05

Oil of turpentine, 2.19

Gases.

Hydrogen, H, at 0° C. and 760 mm., .99941

Carbonic oxide, CO, " " 1.00001

Marsh gas, CH4, « 1.00035

Carbonic dioxide, CO2, " 1 00036

Nitrous oxide, NO, " 1.00039

Olefiant gas, C2H4, " 1.00072

CHAPTER V.
ELECTKIC DISTRIBUTION AND CONDENSATION.

EQUIPOTENTIAL. — A charge of electricity given to
any part of a good conducting surface is instantly dis-
tributed equally over every part, and such a surface is
called equipotential. For the momentary increase of
electric energy at any point creates electric movement
from higher to lower potential, which instantly results
in the establishment of equilibrium at every point.

Separate points on such a surface are called equipo-
tential points, and a line of such points an equipotential
line.

LINES OF FORCE. — The direction along which elec-
tricity tends to move, from a point of higher to one of
lower potential, is called a line of force. Such lines
are perpendicular to the equipotential surfaces at the
points ; for, as the tendency is to move from one point
to the other, it would be from one such surface to the
other ; and if the line differed from a perpendicular, it
would imply, by the resolution of forces, that there
could be two lines of force at right angles to each
other, one of which would lie in an equipotential sur-.
face ; implying two points at different potentials in such
surface, which would be an impossibility.

SURFACE CONDENSATION. — Since the surface of a
solid sphere of any good conducting material is evi-
dently equipotential, we may regard its interior as

56 ELEMENTS OF STATIC ELECTRICITY.

composed of an infinite number of such surfaces, or
spherical shells, having a common center ; and their
radii as equipotential lines cut by such surfaces. From
which it is evident that no difference of potential could
exist in the interior of such a sphere.

If it were insulated, a positive charge communicated
to it would evidently be distributed equally through
every part, if there were no influence tending to pro-
dace a different effect. But, since the sphere would
be at a higher potential than its surroundings, induc-
tion would create lines of force in the direction of the
radii, which must result in the condensation of the en-
tire charge on the surface.

Also, since every portion of the sphere is at the
same potential, and since electrified bodies at the same
potential repel each other, it is evident that the mole-
cules would be self-repellent. But since they are rigid,
the electricity of each molecule would repel that of
every other, and move in the direction of least resist-
ance. Let a row of molecules composing a diameter
be selected, the direction of least resistance would be
from the center each way. For, if surface condensa-
tion takes place (and experiment shows that it does),
as the electricity of the molecules near each end of the
diameter became condensed at the extreme points, its
reaction being thus neutralized, more would be repelled
from the center, and. this would continue till all the
electricity of the diameter was condensed at the ends.

But since the ends are points on the surface, and the
surface is made up of an infinite number of such points,
it is evident that the entire charge would be condensed
on the surface.

Hence surface condensation takes place under the

ELECTRIC DISTRIBUTION AND CONDENSATION. 57

influence of attraction from without and repulsion from
within, in the direction of the radii.

If the charge be negative, the potential of surround-
ing bodies being higher than that of the sphere, elec-
tricity is, in like manner, repelled from the surface
toward the center ; and the negative charge takes place
on the surface, as the positive charge did in the first
instance. Hence the condensation is now in the inte-
rior, leaving the surface negative.

Hence surface charge, if positive, takes place under
the influence of attraction from without and repulsion
from within ; but, if negative, under the influence of
repulsion from without.

In either case the air is the dielectric between the
electrified sphere and surrounding bodies : and when
the charge on the sphere is positive, a negative charge
of corresponding amount is induced on adjacent parts
of surrounding bodies ; electricity being repelled from
them by the higher potential of the sphere. But when
the charge on the sphere is negative, the charge on
adjacent parts of surrounding bodies is positive ; elec-
tricity being attracted to them by the lower potential
of the sphere.

Now since surrounding bodies, as a whole, are at
zero, and this positive charge, in their adjacent parts,
results from the negative attraction of the sphere, it
is evident that the interior potential of the sphere, as a
whole, cannot rise above zero; the negative potential
of its surface being exactly equal to the positive of
adjacent parts of surrounding bodies, just as their
negative potential was equal to the sphere's positive
surface potential in the first instance. Now, since a
solid of any conceivable shape could be cut from such a

58 ELEMENTS OF STATIC ELECTRICITY.

sphere without altering the electrical conditions named,
it is evident that, A charge of electricity communicated
to any solid conductor will be condensed on its surface.

SURFACE TRANSMISSION. — It is also evident, that
although a static charge will be thus condensed on the
surface, electric transmission is not confined to the sur-
face ; since surface condensation is due to induction
and repulsion, which implies the possibility of trans-
mission through the substance to reach the surface.

Hence, although induction operates during transmis-
sion, it cannot prevent transmission through the sub-
stance : so that it must not be inferred that the con-
ducting power is in proportion to the surface, but to the
mass of the conductor.

Hence a charge of electricity which could be easily
transmitted by a solid rod might be sufficient to melt
a thin tube of the same diameter.

HOLLOW CONDUCTORS. — The same reasoning which
applies to an electric charge on a solid sphere will also
apply to one on a hollow sphere. For if any number of
the spherical shells composing the interior be removed,
it does not alter the equipotential of the remaining
ones, nor of their radii ; neither can it change the induc-
tion of the outside surroundings.

And as the form may be altered without changing
these electric conditions, the same reasoning will apply
to any hollow conductor.

Hence, A static electric charge, communicated to a hol-
low conductor, will be condensed on its external surface.

PROOF PLANE — But all our conclusions should be
the result of experiment ; to aid us in which we now
require the little instrument called the proof plane,
represented in Fig. 8 ; which consists of a small brass

ELECTRIC DISTRIBUTION AND CONDENSATION. 59

disc, two inches in diameter, to which is attached a
light ebonite handle, 12 inches long. A light, flat
spring, which lies close to the disc, its lower end free,
and its upper end attached to the handle, will be found
convenient for attaching tin-foil in some experiments.

Fig. 8— Proof Plane.

The proof plane is used for examining the electric
condition of bodies, and for transferring a small charge
of definite amount. Care should be used to prevent
the handle from becoming charged, which may happen
from friction against the clothing or otherwise.

EXPERIMENTS WITH HOLLOW CONDUCTORS. — Let a
charge of electricity be given to the insulated sphere A,
Fig. 9, which has an
opening in the top. In-
troduce the proof plane
through this opening,
taking care to prevent
contact with the edges ;
and touch the inside sur-
face and then the disc
of the electroscope, with
it. As the leaves show
no divergence, it proves
that the inside is not
electrified.

Now touch the outside, and then the disc, and the
leaves diverge ; proving that the charge is on the out-
side surface.

Apply the same tests to the insulated cylinder B, and

GO ELEMENTS OF STATIC ELECTRICITY.

the same results will follow. And this cylinder may
be composed either of sheet metal or wire gauze without
affecting the results.

Cylinders of the latter kind are often used to protect
electroscopes from the induction of electrified bodies
in their vicinity.

Repeat these experiments, communicating the charge
to the inside surfaces of the globe and cylinder, and the
results will be the same ; showing that no charge can
remain on the inside.

Fig. 10— Faraday's Bag.

BAG EXPERIMENT. — To test this more thoroughly,
Faraday constructed a cone-shaped linen bag, shown in
Fig. 10 ; attached to its mouth a ring insulated on a
stand, and to its apex two silk cords, by which either
surface could be turned outward.

An electric charge was communicated to it, and, on
testing with the proof plane and electroscope, was found
to be entirely on the outer surface. The surfaces were
now reversed, and the charge was found to have been
reversed also, going to the outside, as before.

PAIL EXPERIMENT. — The following experiment by
Faraday shows the effect of induction on a hollow con-
ductor :

Let a tin pail J., Fig. 11, or any similar hollow con-

ELECTRIC DISTRIBUTION AND CONDENSATION.

61

ductor, be insulated and connected by a wire with an
electroscope U, and let an electrified metal ball B be
lowered into it by a silk cord. The leaves will diverge
as the ball enters, and the divergence increase till the
ball has passed some distance below ' the edges : after
which the divergence is not increased by its further
descent.

Fig. 11— Pail Experiment.

If it be lifted out without having touched the pail,
the leaves will converge, and the ball show no loss of
charge : but, if allowed to touch while below the edge,

62 ELEMENTS OF STATIC ELECTRICITY.

the leaves will remain divergent after its removal, but
show no increase of divergence by the contact; and the
ball, after removal, will be found entirely discharged.
This experiment proves : —

1. That the induction of the electrified ball has re-
pelled electricity from the inner to the outer surface of
the pail if the charge was positive, or attracted elec-
tricity from the outer to the inner surface if the charge
was negative ; in either case producing a divergence of
the leaves.

2. It proves that induction increases as the ball de-
scends, shown by the increasing divergence of the leaves,
till all the lines of force, which can be included within
the pail, are cut by its surface, after which there is no
further increase of divergence.

3. It proves that there is no permanent effect if
there is no contact ; since the leaves converge when the
ball is removed.

4. It proves that the induced charge on the pail is
exactly equal to the charge on the ball, since no increase
of divergence occurs from contact, although the entire
charge has been communicated to the pail, as shown by
the ball having lost its charge. But this can be strictly
true only when all the lines of force are cut by the pail ;
but since some of the nearly vertical lines must escape,
no matter how deep the ball descends, there must be a
slight increase of divergence by contact, though it may
not be perceptible.

If a charge be given to the pail and the ball be low-
ered into it by a wire held in the hand, the divergence
of the leaves, caused by the charge on the pail, will be
perceptibly reduced as the ball descends.

This proves that the inner surface of a hollow con-

ELECTRIC DISTRIBUTION AND CONDENSATION. 63

ductor can be charged by induction. The charge on
the pail, if positive, repels electricity from the ball,
through the wire and hand, to the earth ; or, if nega-
tive, attracts electricity from the earth; and in either
case, a certain degree of equilibrium follows, causing a
corresponding convergence of the leaves.

Entire convergence cannot be produced, since only
a small portion of the lines of force from the pail are
cut by the ball ; while, in the former experiment, nearly
all those from the ball were cut by the pail. For this
reason a large ball is best for the second experiment
and a small one for the first.

If the ball, in the second experiment, is lowered by a
silk cord instead of a wire, there is no perceptible effect
on the leaves, since induction cannot increase nor dimin-
ish the electricity of the ball when there is no earth
connection.

COMBINATION OF PAILS. — The following experiment
was made by Faraday with a combination of hollow
conductors : —

Let four pails of different sizes be placed on an insu-
lated support, and arranged one within the other as
shown in Fig. 12 : and let them be insulated from each
other at bottom by cakes of paraffin, or any other good
insulator, placed between them. Let silk cords be
attached to the three inner ones, and the outer one be
connected with an electroscope.

On lowering the charged ball into the innermost one,
the leaves diverge as in the first experiment ; contact
between the ball and pail producing no increase of
divergence, and the ball is then found to be discharged,
as before ; which proves that the interposition of the
insulated pails, 2 and 3, has not affected the induction.

64

ELEMENTS OF STATIC ELECTRICITY.

Now let pail No. 4 be lifted out by the silk cord, and
the leaves will converge, and diverge again when it is
replaced, showing that the charge on the ball was trans-
ferred to it.

Fig. 12— Combination of Pails.

Let a connection be now made by pieces of copper
wire, let down by silk threads, between each of the
pails successively, beginning with 4 and 3, till all four
are in electric connection, and let the effect on the
leaves be observed as each connection is made. The
results will be found the same as in the first experiment,

ELECTRIC DISTRIBUTION AND CONDENSATION. 65

when but one pail was used: which proves that the
interposition of interior surfaces has no effect on induc-
tion ; nor can it prevent the entire charge from going
to the outside surface when the four pails are in electric
connection; for if the three inner pails be now removed,
they will be found to have lost their charge; but there
will be no change in the divergence of the leaves.

This experiment is an actual demonstration of what
has already been stated, that the interior of a solid con-
ductor, or the shell of a hollow conductor, may be
regarded as composed of an infinite number of equipo-
tential shells or surfaces, from which a charge of elec-
tricity must always pass to the outside surface.

FARADAY'S HOLLOW CUBE. — A most remarkable ex-
periment in this connection was made by Faraday with
a hollow cube of wood, measuring twelve feet each way,
covered with tin-foil, insulated and charged by a power-
ful electric machine.

He says : " I went into this cube and lived in it,
using lighted candles, electrometers, and all other tests
of electrical states. I could not find the least influence
upon them, or indication of anything particular given
by them, though all the time the outside of the cube
was powerfully charged, and large sparks and brushes
were darting off from every part of its outer surface."

This experiment verifies the statement made on page
12 in regard to zero potential ; showing that however
strong the electrification, no indications of electric action
are perceptible within a space where there is perfect
equilibrium. So that even if the whole earth were as
powerfully charged, in proportion to its size, as Fara-
day's cube, we, who live on it, could perceive no electric
action, if the charge were as uniform as on the cube.

66 ELEMENTS OF STATIC ELECTRICITY.

But if it be objected that the case is not parallel, see-
ing that we live on the surface, it must be remembered
that we have an atmosphere above us which is a part of
the earth's matter; so that, although we live on the
solid surface, we do not live on the outer surface : and
the surface on which we live is practically equipoten-
tial over limited areas.

Faraday, evidently, might have generated electricity
with insulated instruments, inside the cube, and con-
densed it on insulated conductors, without either dis-
turbing the electric conditions by which he was sur-
rounded, or being prevented by them : just as we do
without disturbing the earth's electricity, or being pre-
vented by it. But any connection by a conductor,
between his instruments and the cube, would have
caused the charge to disappear; just as a similar con-
nection with the earth produces the same result.

THICKNESS OF ELECTRIFIED SURFACE. — The idea
of surface condensation implies that an electrified sur-
face must be something more than a mere superficies.
It must have a certain degree of thickness, the elec-
tricity penetrating the conductor and surrounding air
to a certain depth, in proportion to the resistance of the
air, and the attraction or repulsion of the charge on
the conductor. Hence the amount of static charge
which may be condensed on a conductor, per unit of
surface, depends on the resistance of the air.

CONVECTION. — It has already been shown that dry
air is one of the best insulators ; but, since it is a fluid,
its resistance cannot be so great as that of a solid of the
same insulating power; for the air molecules, in contact
with an electrified surface, becoming charged, fly off
under the influence of repulsion and induction, while

ELECTRIC DISTRIBUTION AND CONDENSATION. 67

those farther out rush in to take their place ; creating
air currents around the conductor, by which its elec-
tricity is gradually dissipated. The removal of electric-
ity by the air in this way is called convection.

VARIATION OF CHARGE. — Since the insulating power
of the air varies greatly with its humidity and tempera-
ture, and since its electric potential is also variable, the
charge which may be condensed on a conductor will
vary in like proportion ; dry, cold air being much more
favorable to the condensation of a high charge than
damp, warm air; and air at a high electric potential
than air at a low potential.

Analogous to this is the influence of atmospheric
pressure on steam ; the temperature varying with the
pressure under which it is generated. Here pressure
constitutes resistance, while in the case under consider-
ation the resistance is due to the causes mentioned.

Equal electric condensation on every part of the sur-
face is never practically true ; as the induction of sur-
roundings varies, and form, as will be shown hereafter,
has an important influence. It could only be true of
an insulated sphere, surrounded by a homogeneous
medium, and removed from all other influences.

INFLUENCE OF FORM. — It has already been stated
that form exercises an important influence on the
amount of static charge which may be condensed on a
conductor ; and that a charge on an insulated sphere
is equally distributed over its surface, when the sur-
rounding induction is equal : also that the air, by its
insulation, retains this charge on the surface, and
by its convection gradually removes it. It is evident
also that these forces act at equal distances from the
center.

68

ELEMENTS OF STATIC ELECTRICITY.

Fig. 13— Spheres in Contact.

ELECTRIFIED SPHERES. — Let two insulated metal
spheres, of equal size and similarly charged, be placed
in contact, as represented in Fig. 13. It is evident that
either .of them, separately, would fulfill the conditions

just named; but
when placed in con-
tact, they must be re-
garded as one mass,
having its center at
the point of contact ;
the electric distribu-
tion being the same
on each.

Hence the forces
of induction and re-
pulsion which before acted to remove electricity from
the center of the single sphere to the parts most remote
from it — that is, to the surface — now act in the same
manner, to remove electricity from this new center to
those parts of the mass most remote, that is, to the
points A and B, and the surfaces surrounding them.

There must also be a certain amount of electricity
distributed over the entire surface of each sphere ; and
there must be repulsion between the surfaces adjacent
to the point of contact : so that the charge will be zero
at this point, and increase each way toward A and B.

This may be demonstrated by touching the points
A, B, and C with the proof plane, and, after each con-
tact, bringing it near the disc of the electroscope ;
taking care to discharge it with the finger before mak-
ing the next test.

It will be found that the central point shows scarcely
a trace of electricity, while the points A and B are

ELECTRIC DISTRIBUTION AND CONDENSATION. 69

strongly electrified. The same test, applied to inter-
mediate points, shows the charge on them to be in pro-
portion to their distance from the central point.

ELECTRIFIED CYLINDER.— Instead of the two spheres,
we may substitute an insulated metal cylinder, with
hemispherical ends, provided with pith - ball • electro-
scopes at the ends and center, as represented in Fig. 14.

A light charge of electricity G^^
on the cylinder will cause the
balls at the ends to diverge in
opposite directions, while the
central ball will remain un-
moved, or but slightly affected ;
showing that the principal part
of the charge is condensed on
the ends, and that induction
and repulsion are operating to
remove electricity to the points Fig' ^-
farthest from the center, as shown by the position of
the balls at the ends.

If a sphere be made to oscillate near one of the balls,
at right angles to the length of the cylinder, the effect
of induction will be shown by the ball following the
movement of the sphere.

INFLUENCE OF POINTS. — If a cylinder having cone-
shaped ends be substituted for the one with hemispher-
ical ends, dissipation of the charge, instead of condensa-
tion, will occur. For, on the hemispherical ends, the
charge is retained by the resistance of the air on the
surface ; but the cone-shaped ends terminate in points
which have no surface, hence there can be no resistance.

But if resistance is removed, even from a single point,
it is evident that the entire charge must pass off through

70 ELEMENTS OF STATIC ELECTRICITY.

that point ; since the removal of electricity from any
point on a surface creates a difference of potential be-
tween that and surrounding points, producing an elec-
tric movement in the direction of the point of no
.resistance, which must extend to every part of the sur-
face, and continue till equilibrium is restored.

Instead of the cylinder with cone-shaped ends, we
may use one with needles attached to the ends, as repre-
sented in Fig. 15. A wooden cylinder covered with
tin-foil can easily be changed in this way.

It will be impossible to
charge such a cylinder, even if
only a single needle be at-
tached to any part of the sur-
face. A projecting angle on
any part of a conductor will
tend to produce the same re-
sult.

Effects somewhat analogous
to these may be obtained by
Wi7h Points Dipping into water a spherical
Attached. body, and also a sharp-pointed

spike having the same amount of surface. On lifting
out the spherical body, water will adhere to it, and col-
lect in a large drop at the lowest part ; being held there
by adhesion and atmospheric pressure. But if the
spike be lifted out, point downwards, the water will
drop off when it reaches the lowest point, there being
no surface there on which it can be retained by those
forces.

ELECTRIFIED SPHEROID. — If a metal sphere be flat-
tened at the poles till it assumes the form of an oblate
spheroid, as shown at A, Fig. 16, the face of a cross-

ELECTRIC DISTRIBUTION AND CONDENSATION.

71

j
V

j

section through the poles, as shown at j5, will have the

same form as a cylinder with hemispherical ends. And

since it has been shown that a charge of electricity on

such a cylinder is condensed

on the ends, it is evident that

a charge on such a spheroid

will, in like manner, be con-

densed on its outer edge.
ELECTRIFIED Disc. — If a

flat metal disc, with a thin

edge, be electrified, the charge

will go to the outer edge, as in

the last case. But resistance,

being in proportion to surface,

is very small on such an edge, Fig' 16-Electrified Spheroid.

and the charge is rapidly dissipated. Hence such a

disc, when constructed for the purpose of condensing

electricity on it, should be pro-
vided with a round rim, which
may be called a resistance rim.

If it be insulated, and there
be placed on its opposite sides,
near the edge, two little metal
stands with pointed stems, on
which are balanced light metal
pointers, having arms of tin-

Fig. 17-ElectrifledDisc. equal ^^ &g shown jn Fig>

17, a charge of electricity given to it will cause the
pointers to arrange themselves in the direction of the
radii, showing that the electric force is from the center
outward.

CHAPTER VI,

ACCUMULATORS.

THE CHARGED PANE. — The electric charge which
be condensed on the surface of an insulated con-
ductor is comparatively small, when such a conductor
is remote from inductive influence.

But when another conductor, having a connection
with the earth, is placed in its immediate vicinity,
the charge may be greatly increased.

To prove this, let a sheet of good insulating glass,

varnished with shellac, be
coated on opposite sides with
tin-foil, to within about two
inches of its, edge, and placed
on an insulating support, as
shown in Fig. 18. A small
charge can be given to the
Fig. 18-The Charged Pane, tin-foil, on the upper surface,
which will be indicated by sparks passing between it
and the body from which the charge is given. But the
limit is soon reached, and no more sparks will pass.

Now let the lower surface be connected with the earth
by a strip of tin-foil, and sparks will again pass freely
between the charging body and the upper surface, till
a charge greatly in excess of the former is given.

If the tin-foil strip be suspended with its lower end
near a conductor, as shown, sparks will pass between

AC CUM ULA TORS. 1 3

it and the conductor, simultaneously with the sparks
on the upper surface ; indicating that each surface re-
ceives the same amount of charge.

But the potential on opposite surfaces will be oppo-
site. If the upper surface acquires positive potential,
by an increase of electricity, the same amount will be
repelled from the lower surface, making it negative.
But if the upper becomes negative by a decrease,
electricity, to the same amount, will be attracted to the
lower surface, making it positive.

To prove that these charges are equal, let the tin-
foil strip be removed
after the plate has been
charged ; and a wire,
held by a piece of
india - rubber tube, to
insulate it, be bent so
that its ends come into
contact with the oppo-
site surfaces, as shown

Fig. 19— The. Pane Discharged.

in Fig. 19 : a flash and

report will follow, and both surfaces, after the wire
has remained in contact for a few moments, will be
found completely discharged.

Now, since the removal of the strip produced com-
plete insulation, perfect equilibrium could occur only
by the positive of one surface being exactly equal to
the negative of the other.

Since induction varies inversely as the square of
the distance (page 47), it is evident that, if this factor
alone is considered, the amount of charge which can be
given will be in the inverse ratio of the thickness of
the glass, and hence greater on thin than on thick

74 ELEMENTS OF STATIC ELECTRICITY.

glass. But since the resistance of glass is in the
direct ratio of its thickness, when the specific induct-
ive capacity is the same, this factor also must be
considered.

Hence, in the construction of instruments involving
these principles, if great sensitiveness and a low poten-
tial is desired, the glass, or other dielectric, should be
thin : but if the highest attainable potential is desired,
there should be sufficient thickness to resist fracture
or puncture.

The uncoated margin must also be wide enough to
m^ike the resistance there equal to that of the thickness ;
a small fraction of an inch in thickness having a re-
sistance equal to that of several inches of surface.

No definite rules can be given, as the resistance of va-
rious kinds of glass, and other dielectrics, varies greatly,
as well as the cases in which they may be required.

As the positive and negative on opposite surfaces are
equal, it is impossible for a change of potential to occur
on either surface without a corresponding change on
the opposite surface. Hence a conductor brought into
contact with either surface alone will not change its
potential, unless directly or indirectly connected with
the opposite surface. Hence the charge on each surface
is said to be bound by the opposite charge.

The convection and conduction of the air, so far
as it can act equally on both surfaces, will in time re-
store equilibrium. It may also be restored by the oscil-
lation of a solid body, as a pith ball, suspended between
conductors connected with both surfaces ; or, by direct
connection through a conductor, as already explained.

Instruments constructed for accumulating electricity
in this way are called accumulators, or condensers.

ACCUMULATORS. 75

THE LEYDEN JAR. — The first discovery of an accu-
mulator was made by Kleist, a clergyman of Cammin,
in Pommerania, who stated in a letter to Dr. Lieber-
kiihn, of Berlin, Nov. 4, 1745, that by pouring a little
mercury, " spirits," or water, into a phial and con-
necting it with a nail through the cork, he could
electrify it through the nail, ignite "spirits of wine"
with it, and receive a shock by touching the nail with
his finger.

The same discovery was made in the following year
in Leyden, by Cuneus, a pupil of Musschenbroek, who
electrified some water in a flask, which he held in his
hand, by bringing it into contact with a chain from the
conductor of an electric machine. On attempting to
remove the chain with his other hand, he received an
electric shock which so frightened him that he dropped
the flask. Musschenbroek, having tried the experiment,
said he would not take a second shock for the crown
of France.

The discovery created great excitement, and led to
the construction of improved instruments, to which
the name "Leyden jar" was given.

The water in this instance constituted the inside
coating, the hand the outside coating ; and, when the
other hand touched the chain, both surfaces were con-
nected by a conductor, and a discharge followed, which
produced the shock.

Fig. 20 represents the Leyden jar as it is usually
constructed. The essential elements are two conduct-
ing surfaces separated by a dielectric ; but, for conven-
ience in charging and discharging, a wooden cap is
fitted to it, through which passes a brass rod, terminat-
ing in a ball above, and to its lower end is attached a

76

ELEMENTS OF STATIC ELECTRICITY.

light Spring, or a chain, which comes into contact with
the inside coating.

Tin-foil is the usual coating, and is put on with paste,
covering both surfaces equally to within about three
inches of the top. Light sheet brass makes a more
substantial outside coating, and does not require at-
tachment to the surface.
It can also be used for
the inside coating, when
the mouth is the full
width of the jar and the
sides are straight. Sul-
phuric acid is also some-
times used for the inside
coating of jars designed
fur special purposes.

An instrument called
a discharger is also repre-
sented at A, in Fig. 20.
It consists of a curved
brass rod, terminating
in balls, and having an
insulating handle, of
ebonite or glass, at-

Fig. 20-Leyden Jar and Discharger. tached to its Center. It

is sometimes jointed at the center, and furnished with
two handles, as represented at B, Fig. 20. Its use is the
same as that of the bent wire already described.

The Leyden jar can be made of any insulating
material capable of being molded into the proper form;
but glass seems to be the only substance capable of
resisting the enormous strain to which the dielectric is
subjected under a full charge.

A CCVMULA TORS. 1 1

Glass suitable for the purpose must be free from any
substance which makes it a partial conductor. Hence
such glass as is commonly used for fruit jars, candy
jars, and druggist's bottles cannot be used, since it con-
tains metallic substances.

Glass of a bright green color, free from bluish tint,
also the kind known as "hard flint," makes the best
Ley den jars

The Leyden jar is charged by an electric machine ;
its inner coating being connected with the machine,
and its outer coating with the earth, or with the op-
posite electrode of the machine ; though it is not
material which coating is connected with the machine,
except as a matter of convenience. The jar may
be insulated, and the charge given to the outer
coating, if the inner coating is connected with the
earth.

It is also immaterial whether the charge given is
positive or negative, as the opposite charge will always
be induced on the opposite surface ; electricity being
repelled to the earth when a positive charge is given,
or attracted from the earth when negative is given.

The electromotive force (E. M. F.) of a jar is equal
to the difference of potential between its inner and
outer coatings.

CHARGE BY CASCADE. — The method of charge by
cascade, first proposed by Franklin, is as follows : Let
a number of jars of equal size, as A, B, C, Z>, be
arranged as represented in Fig. 21 ; the outer coating
of each, commencing with A^ being connected with the
inner coating of the one next to it ; D having its outer
coating connected with the earth, and A having its
inner coating connected with the machine. And let A,

78 ELEMENTS OF STATIC ELECTRICITY.

B, and C be well insulated on cakes of paraffin or some
equally good insulator.

A positive charge given to the inner coating of A
will induce negative on its outer coating, by repelling
the same amount of electricity; and this repelled
charge must go to the inside of B, since it has no other
outlet. Hence the inner coating of B will be positively
charged, and electricit}^ will, in like manner, be repelled
from its outer coating to the inner of C. Hence the
charge of each jar in the series will be similar to that
of A ; electricity from the outer coating of D being
repelled to the earth.

Fig. 21— Jars in Cascade.

As the energy expended is distributed among four jars,
it is evident that the charge of each must be much less
than if the same amount had been expended in charging
one jar : since the energy accumulated cannot exceed
the energy expended. But, as the charge is in the
inverse ratio of the thickness of the glass, the resist-
ance from this source must increase from A to 7>, in
proportion to the number of thicknesses interposed :
and the charge must vary in the same ratio ; the neg-
ative being greatest on the outer coating of A, where
only one thickness is interposed, and least on the outer
coating of D, where four thicknesses are interposed ;

ACCUMULATORS. 79

C

the positive on the inner coatings varying in the same

ratio. The same variation must also occur in the
resistance of the connectors, and produce a similar
effect, in a limited degree ; the resistance of a conductor
being directly as its length.

If the charge given to the inner coating of A be
negative, the electric movement is reversed; all the
inner coatings becoming negative, and the outer pos-
itive ; electricity being attracted from the earth to the
outer coating of D.

The insulations and connections should receive care-
ful attention, so as to prevent loss by leakage ; which
will inevitably occur if the insulation is imperfect, or
if the connectors have points, sharp edges, or projecting
corners.

After the charge is given, the jars should be sep-
arated, placed in connection with the earth, and each
discharged separately. A single jar, charged to the
same amount, should then be discharged, and the
results -compared.

This method will indicate, roughly, the amount of
charge of each jar ; but the electrometer, to be de-
scribed hereafter, will give more accurate results.

THE LEYDEN BATTERY. — When a number of jars
have their inner coatings joined by conductors, and
also their outer coatings in like manner, the combi-
nation is called a Leyden battery.

A convenient form of such a battery is represented
by Fig. 22, in which connectors between the inner
coating radiate from a central jar. The outer coatings
are made of sheet brass, nickel-plated, and screwed to a
wooden base, their connections being made with copper
wires attached to the points of the screws underneath.

80 ELEMENTS OF STATIC ELECTRICITY.

This construction for the outer coatings makes them
durable, gives the jars a firm attachment, and adds
greatly to the neatness and beauty of the instrument.

The E. M. F. of a Leyden battery is the same as that
of a single jar having the same amount of coated sur-
face. There can be no increase of intensity from any
special arrangement of the jars, as such a battery is
merely an accumulator, and not a generator "of electric-

Fig. 22— Leyden Battery.

ity. But when great E. M. F. is required it is generally
more convenient to use a battery than a single jar of
equal energy. And, in case of fracture from an over-
charge, a small jar can be replaced at less expense than
a larger one.

In charging or discharging a battery, it is immaterial
which jar is selected : for all the inner coatings being
connected together, as well as all the outer coatings,
each is practically the same as a single coating of equal

ACCUMULATORS. 81

size ; and connection with any part of either coating
affects the whole of that coating.

DISCHARGE THROUGH A BOOK. — The discharge
from a Leyden jar or battery, passed through a card or
a thin book, leaves a puncture, with a burr projecting
from each surface.

Fig. 23— Discharge Through a Book.

To perform this experiment successfully, let one
knob of the discharger be placed in contact with the
outer coating, and the other in contact with the book ;
and let the book, held by its edge, with the knob
against it, be brought quickly into contact with the
knob of the jar, and the discharge will take place as
shown in Fig. 23.

82 ELEMENTS OF STATIC ELECTRICITY.

In this way a book of one hundred or more pages may
be perforated.

If the book is first placed in contact with the knob
of the jar, part of the charge will escape from the edges
and corners of the leaves, and the experiment is liable
to fail.

The burr projecting from each surface, after the dis-
charge through a book or card, has been relied on as a
proof of the dual nature of electricity, and ascribed to
the rush of positive and negative in opposite directions.
It is also attributed to the expansive force of heat, or of
gas, generated by the discharge.

The first theory cannot be accepted, unless we have
stronger proof of the dual nature of electricity than is
afforded by this experiment. And the second also fails ;
since in the case of a discharge through a book, the
leaves may be held so loosely as to allow a free outlet
for expansion from heat or gas, and yet the burr turns
in opposite directions from a point near the center of
the book, and becomes more prominent when the leaves
are thus held than when they are compressed ; whereas,
if the burr were due to the expansive force of confined
heat or gas, the reverse would be true.

Since these theories are unsatisfactory, let us en-
deavor to explain this phenomenon in accordance with
the principles which we have been considering.

Let a jar be charged on its inner coating, and
discharged through a book, as represented in Fig. 23.
Suppose the charge to be positive, electric movement
being from higher to lower potential, it would be from
the knob of the jar to the nearest knob of the dis-
charger. The entire charge of the inner coating,
passing out through the knob, would induce a high

ACCUMULATORS. 83

negative potential on that point, on the nearest surface
of the book, in a line between the knobs ; repelling the
electricity of the book along that line to the opposite
surface, which would thus become highly positive.

The paper being a very imperfect conductor, the
charges thus induced do not spread rapidly, but remain
concentrated for a moment on small circular spaces
around each of these points ; the greatest intensity
being at the centers. Hence there is a powerful at-
traction between the knob of the jar and this negative
point on the surface of the book ; and also between the
knob of the discharger and the positive point on the
other surface ; under the influence of which the paper
on each surface gives way and bursts outward toward
the knobs ; that surface next the knob of the jar being
attracted, and that next the knob of the discharger
repelled.

As each outward leaf bursts, the next, becoming
then the outer one, bursts also, till the perforation is
complete from the center each way. All of which
occurs instantaneously.

Meantime the electricity from the knob of the jar
follows up this inductive effect on the electricity of the
book; but meeting great resistance from the imper-
fectly conducting paper, and the air between the leaves,
it is concentrated on each leaf successively ; so that the
inductive force is constantly in advance of the charge,
the leaves and layers of air between them constituting
the dielectric.

It will be noticed, then, that this is not a case of
energy going through a passive medium, but of energy
acting on the energy of that medium, causing it to become
active and perform work.

84 ELEMENTS OF STATIC ELECTRICITY.

It should also be noticed that when the leaves are
held loosely, the thickness of the air dielectric is in-
creased ; each layer of air having a charged surface of
partly conducting paper on each side of it, is in the
position of the coated pane, a powerful attraction
between the surfaces acting across it. And when the
paper bursts there is more room for the formation of a
burr, and less resistance to the tearing of the paper,
which accounts for the increased prominence of the
burr.

If the charge of the jar is negative, the same results
occur in reverse order.

THE RESIDUAL CHARGE. — When a Leyden jar is
discharged, there still remains a slight difference of
potential between the coatings, which is known as the
residual charge. Hence, a small discharge can be
obtained a moment after the first ; and this also leaves
a residual, bearing about the same proportion to the
second discharge as the second to the first, when the
same length of time elapses between them. A number
of successive discharges may thus be obtained, which
constantly decrease in amount till no further discharge
is perceptible. But, even then, it is not probable that
perfect equilibrium is restored.

To understand this, we must remember that even
the best dielectric is a partial conductor: and that
while electric movement is instantaneous in a good
conductor, it is very slow in a non-conductor. In the
Leyden jar we have a combination of both — two con-
ductors separated by a non-conductor. And, when the
charge is given, every part of each coating instantly
becomes electrified, one coating positively and the other
negatively, on the surfaces next the glass.

ACCUMULATORS. ' 85

The electricity, on the positively electrified coating,
slowly penetrates into the glass, acting inductively on
its electricity, which it repels from the opposite sur-
face ; and producing, probably, a temporary strain or
distortion of its structure.

When the first discharge takes place, there
is a relief from this strain; and, as the
electrified glass slowly returns to its former
state, the electricity which had penetrated it
returns to the conducting surface.

This view receives confirmation from the
fact that delay increases the residual charge,
giving time for the electricity to come out of
the glass and accumulate : while it has the
opposite effect on the primary charge, reduc-
ing it by giving time for dissipation.

Tapping the jar lightly hastens the in-
crease of the residual charge, the vibratory
motion thus given to the glass tending, prob-
ably, to relieve the electric strain.

JAB WITH MOVABLE COATINGS. — If a with 2

Leyden jar be constructed Avith any rigid Coatluss-
metal, as sheet brass, for both coatings, as suggested on
page 76, and the conducting rod be attached to the
inner coating, the coatings may be removed and re-
placed at pleasure, as represented in Fig. 24 : and we
have the means of investigating certain phenomena in
regard to the electrification of the different parts.

Let a charge be given to such a jar, and the coatings
removed carefully, so that they shall not be connected
by a conductor during removal: they may now be
brought into contact without producing any electric
effect ; and the jar also may be handled with a like

86 ELEMENTS OF STATIC ELECTRICITY.

result : but, on replacing the coatings, a full discharge can
be obtained, the same as if they had not been removed.

But if, while the coatings are removed, the jar be
examined by touching both surfaces with the finger and
thumb, or a small discharger, made with a bent wire,
at any point below a line marking the position of the
upper edges of the coatings, a discharge can be obtained
from that point. In this way a number of small dis-
charges can be had from various points, but no general
discharge.

This proves that the charge remains on the glass, while
the coatings are removed ; but that the resistance of the
glass prevents a general discharge. But it cannot be
accepted as proof that the charge is confined to the
glass, when the coatings are in contact with it ; unless
it can be shown that the charge remains on the glass
after the removal of both coatings at precisely the same
instant ; which could not be done with the care neces-
sary for so delicate an experiment. But when the
coatings are removed separately, the charge must be
transferred to the glass during the removal of each :
since it is impossible to produce any change of poten-
tial on either surface, unless a corresponding change
is produced, at the same instant, on the opposite surface;
each being bound by the opposite.

VARIOUS EFFECTS OF THE DISCHARGE. — The dis-
charge of a Ley den jar of moderate size is sufficient to
explode gunpowder, and to ignite various substances;
as phosphorus, powdered resin, sulphuric ether, and
alcohol; while that of a large Leyden battery fuses
wires, magnetizes steel, 'and destroys animal life.

With a battery of 550 square feet of coated surface,
large steel bars have been magnetized, iron wires, T£o

ACCUMULATORS. 87

of an inch in diameter, and 25 feet long, melted into
globules; and tin wires, -fa of an inch in diameter, and
8 inches long, dissipated in smoke.

Tyndall accidentally received a charge from a Ley-
den battery of " fifteen large jars " during a lecture,
and describes his experience as follows : " For a sensi-
ble interval life was absolutely blotted out, but there
was no trace of pain. After a little time consciousness
returned; I saw confusedly both the audience and the ap-
paratus. But though the intellectual consciousness of my
position returned with exceeding rapidity, it was not so
with the optical consciousness. For my body presented to
my eyes the appearance of a number of separate pieces.
The arms, for example, were detached from the trunk
and suspended in the air. In fact, memory and the
power of reasoning appeared to be complete long before
the restoration of the optic nerve to healthy action."

Gunpowder cannot be exploded by the ordinary
discharge; the only effect of which is to scatter it. But
when the discharge is retarded, by introducing into the
circuit an imperfect conductor, as a wet string, it
explodes readily. By this method also gun-cotton,
phosphorus, and other highly inflammable substances
may be ignited.

For such experiments the universal discharger, rep-
resented by Fig. 25, is convenient. It is constructed
with a base, in the center of which, mounted on a stem,
is a small circular tablet of some insulating material, as
ebonite ; and at each end, mounted on insulating stems,
are brass sliding rods, each terminating in balls, and
passing through a socket hinged on the top of its stem.
A plaster of paris receptacle, to hold inflammable
substances, should also be provided.

88

ELEMENTS OF STATIC ELECTRICITY.

The substance to be operated on is placed in the
receptacle on the tablet, the inner terminals of the
sliding rods adjusted on opposite sides of it, and the
outer terminal of one rod connected with the outer
coating of the jar or battery ; and the circuit completed
by connecting the outer terminal of the other rod with the
knob of the jar or battery, by the discharger, as shown.
The wet string, or other imperfect conductor, when

used, can be intro-
duced into any con-
venient part of the
circuit, as at S.

SPONTANEOUS DIS-
CHARGE.— A sponta-
neous discharge is
liable to occur in
attempting to charge
a jar beyond its ca-

Fig. 25— Universal Discharger. pacity : and, if the

glass is thin at any point, it may be fractured in this
way ; but if the resistance of the insulating margin is
less than that of the glass, the discharge will take place
over that surface, without injury to the jar, electricity
always following the path of least resistance, whether
longer or shorter.

DISRUPTIVE DISCHARGE. — When a discharge takes
place through the air or any other dielectric, it is
termed disruptive; since the electricity must force a
passage and break down opposing barriers. Such a
discharge is always accompanied with light, heat, and
sound ; as expressed by the terms spark and snap, flash
and report — effects due to the resistance encountered,
and not qualities inherent in electricity.

A CCUMULA TORS. 89

SILENT DISCHARGE. — But when the discharge takes
place through a good conductor of sufficient size, it is
termed silent; since light and sound are absent; the
resistance encountered being only sufficient to produce
a slight amount of heat.

The discharge through a point is also termed silent ;
since a point, as already shown, offers no resistance ;
and hence there is little or no sound, even when the
discharge passes through intervening air. A battery
discharge, sufficient to destroy life, may be received
with impunity through the point of a cambric needle,
held in the hand, without producing any unpleasant
sensation.

LICHTENBERG'S FIGURES. — If, on a plate of ebonite,
or of glass varnished with shellac, figures be traced with
the knob of a positively charged Leyden jar, and sulphur
dusted over the surface, inclining the plate and tapping it
to remove the surplus ; the sulphur will adhere to the
lines traced, spreading out in a beautiful fringe, as shown
in Fig. 26, which is from a photograph of a figure made
in this way.

A similar result can be obtained by tracing lines
with the outside of this jar, or with the knob of a
negatively charged jar, and dusting the surface with
red lead.

Or a mixture of sulphur and red lead may be used, and
separate figures traced ; the jar being charged positively
for one figure, and negatively for the other. The sulphur,
it is claimed, adheres to the positive, and the lead to the
negative lines. Any non-conducting surface may be
used, also various other powdered substances.

It should be noticed that the loss of charge, whether
positive or negative, from the inner coating, while tracing

90 ELEMENTS OF STATIC ELECTRICITY.

the figures with the knob, is balanced by an equal loss
from the outer coating through the hand in which the jar
is held. Hence, when the tracing is made with the outer
coating, the knob must be held in the hand, to produce
the same effect on the inner coating : the jar being first
placed on an insulator to prevent a discharge and conse-
quent shock, by indirect connection through the earth.

Fig. 26 — Lichtenberg's Figures.

An inspection of the figure shows, that at the point
where it begins above, the fringe lines radiate from a com-
mon center ; but that, as the curve is produced from right
to left downward and from left to right upward, they
point diagonally in the direction in which the knob of
the jar moves. The explanation is as follows : — The

A CCUMULA TORS. 91

surface being a non-conductor, the electricity has to force
its way against strong resistance, bursting through at the
points where resistance is least, and forming the fringe.
The strongest effect is produced where the knob first
approaches the surface ; as the jar has then a full charge :
and the first action is a disruptive discharge through
the air, producing the circular, star-like figure, at that
point. But as the knob moves along the surface, after
contact, new lines start out at right angles to the line
of movement. And as the knob leaves a point where
such a line has started, it exerts an inductive action on
the original impulse, which tends to turn this line for-
ward; the diagonal direction being the resultant of
these two forces acting at right angles to each other.
And the forked branches are the result of similar
inductive action of the main fringe lines on the branch
lines. •

We have, in this experiment, a graphic demonstration
of the effect of an insulating surface in resisting electric
movement : since the figures show the exact location of
the electric force ; which, we see, is confined chiefly to
the tracings, spreading only to the limited extent
represented by the fringes.

It also shows that the effects produced in different
substances, by opposite electric influences, are depend-
ent on the electric condition of the substances them-
selves: so that a mixture or a compound may, in this
way, be separated into its elements. The sulphur in
this experiment becoming negative, as claimed, by
friction, is attracted to the positively charged lines,
while the red lead, becoming positive, is attracted to
those negatively charged. This principle has numer-
ous useful applications in the arts.

CHAPTER VII.
ELECTEIC GENERATORS.

THE ELECTROPHORUS AND FRICTIONAL MACHINE.

THE only electric generators noticed thus far are the
rods of glass, ebonite, and sealing-wax ; rubbed with
silk, woolen, or fur: but it is evident, that for such
work as the charging of Leyden jars and batteries, and
similar experiments, we require generators of far greater
capacity. But it was thought best to anticipate their
existence, and defer their introduction till there had
been a full consideration of the principles on which the

various kinds de-
pend: so that they
might all be in-
cluded in one com-
prehensive view ;
from which the
merits of each, and
Fig. 27-Eiectrophorus. the principles of

its construction could be more fully ascertained.

THE ELECTROPHORUS. — This instrument, invented
by Volta, is one of the simplest forms -of a static gen-
erator ; but it is of great utility in furnishing an
unfailing, though limited supply of electricity, for
numerous delicate experiments.

The following style, designed by the author, and
represented by Fig. 27, makes a handsome, convenient,
and very efficient instrument.

ELECTRIC GENERATORS. 93

On a wooden base thirteen inches square, constructed
of layers glued together to prevent warping, is placed
a thin sheet of brass of the same size ; over which is
placed a sheet of ebonite of equal size, iV of an inch
thick ; and both attached to the base by screws near
the corners.

On the ebonite is placed a circular plate or cover,
made of No. 20 sheet brass, twelve inches in diameter,
perfectly flat, and having a round resistance rim joined
to the upper surface. In its center is an ebonite handle,
seven inches high ; and from its rim projects a goose-
neck, made of No. 8 brass rod, terminating in a half-
inch brass ball ; near which, on the edge of the base, is
a brass strip, f of an inch wide, connected with the
lower plate.

The base may be made of metal, if preferred, in which
case the lower plate and strip are unnecessary, the base
itself taking the place of the plate.

In this instrument we have two conductors separated
by a dielectric ; the upper one insulated, and the lower
connected with the earth.

The cover being removed, the dielectric is beaten
briskly with a piece of catskin, or other fur, by which
its upper surface is electrified ; and the cover is then
replaced.

Suppose the charge to be negative ; electricity having
been removed by the fur, the same amount is attracted
from the earth to the under surface of the dielectric,
and to the upper surface of the brass plate in connection
with it ; which thus become positive by induction. The
under surface of the cover also becomes positive and its
upper surface negative.

Let a connection now be made between the lower

94

ELEMENTS OF STATIC ELECTRICITY.

plate and cover, by touching the strip and knob with
the finger and thumb, or a small discharger ; the elec-
tricity accumulated on the lower plate will pass to the
cover, producing a shock if passed through the hand.

The cover thus becomes positive ; but its charge is
neutralized, or bound, by the negative of the dielectric.
Let it be lifted off by the insulating handle ; its charge
being no 'longer 'bound, a discharge, producing a spark,

an inch or more in
length, takes place,
when the knuckle
or any conductor
is presented to the
knob, as shown in
Fig. 28.

The removal of
the cover with its
positive charge,
having left the up-
per surface of the
dielectric negative,
a positive charge
is again attracted
to the under sur-
face and plate, as before ; and the cover, having been
discharged and replaced, the process may be repeated
with the same results an indefinite number of times,
and Leyden jars charged, or other electric work
performed.

Suppose the original charge to be positive, the same
results occur in reverse order. Electricity having been
imparted by the fur to the upper surface of the dielec-
tric, the same amount is repelled from the under

Fig. 28— Discharge of Electrophorus.

ELECTRIC GENERATORS. 95

surface and plate, making them negative. The under
surface of the cover also becomes negative and its
upper surface positive. Connection being made as
before, electricity passes from the cover to the lower
plate ; leaving the cover negative, and its charge bound
by the positive on the upper surface of the dielectric.

The cover being removed, and a conductor presented
to the knob, a discharge takes place ; electricity now
passing from the conductor to the cover, instead of from
the cover to the conductor as before.

The removal of the cover, with its negative charge,
having left the upper surface of the dielectric positive,
electricity is again repelled from the under surface and
plate by induction : and the cover having been restored
to zero and replaced, the process may be repeated as
before.

We see, then, that when the charge is negative,
electricity is attracted from the earth to the lower
plate, then passes to the cover, and then from the cover
to the presented conductor; but when the charge is
positive, electricity is repelled to the earth from the
lower plate ; then an equal amount passes from the
cover to the lower plate, and the same amount passes
to the cover from the presented conductor.

Hence, when the instrument receives a positive charge,
it gives a negative charge ; and when it receives a neg-
ative charge, it gives a positive charge.

It will also be noticed that the initial charge is given
by friction, but all subsequent charges are obtained by
induction.

If the cover be removed, without first making con-
nection between it and the lower plate, no charge will
be found on it; since it has neither gained nor lost

96 ELEMENTS OF STATIC ELECTRICITY.

electricity through any external source ; and its own
electricity, being merely changed to the upper or lower
surface, by the positive or negative of the dielectric, is
restored to zero when removed from that influence.

This connection may be made automatically fby plac-
ing a short brass pin in a hole made through the dielec-
tric, its upper end even with the upper surface, so that it
shall touch the cover and also the lower plate. This
makes the instrument more convenient for obtaining
charges in rapid succession : but, when used to demon-
strate the principles involved in its construction, as
above, the pin should be removed.

The top of the handle should be grasped, when
removing the cover, to prevent a partial discharge
through the hand.

The electrophorus will retain its charge for months ;
and, like the Leyden jar with movable coatings, can be
taken apart and put together again without perceptible
loss of charge ; but, when not in use, the charge is
gradually dissipated, so that only a residual remains.
Hence it should be charged again before immediate
use, if great efficiency is desired. This property of
constancy probably suggests the name, electrophorus,
electricity-bearer, from qisaco to bear, qfaxTQov electricity.

THE FKLCTIONAL MACHINE. — The principle of this
machine is the same as that of the rod and rubber. It
was invented by Otto Guericke, and consisted, at first,
of a globe of sulphur, revolved on an axis by a crank,
the hand being used as a rubber. Subsequently a globe
of glass was substituted for the sulphur ; but as insu-
lation was disregarded in both styles, only feeble results
were obtained, and the machines fell into disuse.

Boze, of Wittemberg, revived and improved them,

ELECTRIC GENERATORS.

97

using the glass globe, and a band wheel and belt to
increase their speed ; and collecting the electricity on
an iron tube, suspended by silk cords, from which hung
a chain in contact with the globe.

Furt' er improvement was made by the use of a
leather rubber stuffed with hair : and subsequently the
globe was replaced by a glass cylinder, on one side of

Fig. 29— Plate Electrical Machine.

which the rubber was mounted on a glass pillar ; and,
on the other side, similarly mounted, was a brass cylin-
der, called the prime conductor, from which a row of
points projected toward the glass. An oil silk flap
enveloped the upper part of the glass cylinder ; and a
chain was used to connect either the rubber or the
prime conductor with the earth, as desired.

The plate machine, invented about 1787, was con-

98 ,: ELEMENTS OF STATIC ELECTRICITY.

structed 'on the same principles, a glass plate being
substituted for. the glass cylinder, and has now come
into general use. Fig. 29 represents one of the prevail-
ing styles.

It consists of a disc of plate-glass -A, mounted on a
wooden base with wooden or glass pillars, and revolved
by a crank with an insulated handle. A pair of rub-
bers B, made of soft leather or felt, are pressed against
the glass on opposite sides by a pair of brass springs (7,
the pressure being adjusted by a screw. These are
mounted on a glass pillar, and connected above with a
brass ball ; and a brass chain, which may be removed,
connects them with the earth.

Mounted on a glass pillar is the prime conductor _Z),
made of brass, and consisting of a pair of balls, from the
lower one of which projects a pair of combs, which
extend on opposite sides of the glass, and whose teeth
come within a quarter of an inch of it. And, from the
opposite side of the same ball extends a rod, terminat-
ing in a small ball.

A silk cover envelops the lower part of the glass plate,
and the rubbers, on the surfaces in contact vvith the glass,
are coated with an amalgam, composed of five parts
zinc, three parts tin, and nine parts mercury, melted
together, pulverized, and made into a paste with lard.

The machine should be dry and warm before use, as
moisture condenses on the surface of the glass when it
is colder than the atmosphere, and suspends insulation.
For this reason ebonite pillars have an advantage over
glass, being less liable to condense moisture.

Ebonite has also been used for the plate, but is not
so reliable as glass ; and its liability to warp with heat,
when in thin plates, makes it very objectionable.

ELECTRIC GENERATORS.

ITS MODE OF ACTION.— The plate being re^piy/ed'iil? -y
the direction of the arrow, electricity is gener5tf«5^%
the friction of the rubbers; the charged surface of the
glass passing directly into the silk cover, which prevents
loss of charge from contact with the air.

O

If the charge on the glass is positive, when the
charged surface comes opposite the combs, electricity
passes through them from the plate to the prime con-
ductor, where it accumulates. The glass, being thus
discharged, passes round again to the rubbers, which,
having become negative from parting with electricity
to the glass, have received electricity from the earth
through the chain.

Each portion of the plate is thus alternately charged
and discharged, as it passes first to the rubbers, and
then to the combs; the lower half being constantly
positive, and the upper half at zero, except the resid-
ual ; electricity passing to the rubbers from the earth,
and being carried round by the plate to the prime
conductor.

If the charge on the plate is negative, the transfer
takes place in reverse order ; electricity passing from
the prime conductor to the plate, from the plate to the
rubbers, and from the rubbers to the earth; the prime
conductor becoming negative and the rubbers positive.

If the prime conductor be placed in connection with
the earth, by having the chain transferred to it, the
charge, whether positive or negative, will take place on
the ball and other parts connected with the rubbers.

If the prime conductor and rubber be connected by
the chain, no charge can occur on either; since elec-
tricity constantly passes from one to the other through
the chain, as it is generated.

100 ELEMENTS OF STATIC ELECTRICITY.

If the chain be removed entirely, only a very limited
charge can occur, derived from the material of the
machine itself.

The limit of the charge is reached when its potential
energy, whether positive or negative, so far exceeds the
resistance of the air, that the loss of charge by convec-
tion, as explained on page 66, shall equal the energy
generated. When the atmosphere is damp, or its
electric potential low, this limit is soon reached; but
when dry, and at a high electric potential, a much
greater charge can take place.

MACHINE DESCRIBED BY NOAD. — The largest ma-
chine of this kind of which we have any record was
made some years ago for the Panopticon of Science in
London. According to Noad, it had a plate ten feet in
diameter, three pairs of rubbers, each three feet in
length, and a pear-shaped prime conductor, six feet
in length, and four feet in diameter at its widest
part.

It was operated by steam power, and gave sparks
fifteen to eighteen inches in length; and charged to its
full capacity, in less than a minute, a Leyden battery
of thirty-six jars, having one hundred and eight square
feet of coated surface.

MEASUREMENT or ENERGY. — The amount of elec-
tricity which a well-constructed machine can generate
is in proportion to the surface area of the plate, which
may be increased to any practicable limit, the other
parts being increased in like proportion. It is roughly
estimated by the number of sparks of a given length
and energy which can be obtained in a given time,
when an uninsulated conductor is brought near the
prime conductor; or by the length of time required to

ELECTRIC GENERATORS.

101

charge a Leyden jar or battery having a given amount
of coated surface.

The results are only approximate, especially those
by the first method, for the following reasons. Length
of spark is not a true index of energy ; since a short,
thick spark may have greater energy than a long, thin
one: and our estimate of the comparative energy of
each from its appearance, and the accompanying snap,
is liable to be very inaccurate. The spark accom-

Fig. 30— Lane's Unit Jar.

panying the discharge of a Leyden jar or battery is
generally quite short, though its energy often greatly
exceeds that of any single spark of much greater length,
given by the machine in charging it.

The humidity of the air and its electric potential,
being liable to great variation, produce a corresponding
variation in the results obtained at different times.

The charge and discharge of a Leyden jar of a given
capacity, in a given time, is a more reliable method.

102

ELEMENTS OF STATIC ELECTRICITY.

The jar should be made self-discharging, by bringing
the knob of a conductor, connected with its outer
coating, within sparking distance of the knob of the
jar.

Lane's unit jar, shown in Fig. 30, is constructed on
this principle.

A bent brass rod is connected by a band to the outer
coating; its upper end terminating in a ball through
which passes a horizontal sliding rod, terminating in a
ball at its inner extremity; and having an ebonite
handle at its outer extremity, by which the ball can

be adjusted to any required
distance from the knob of the
jar.

To estimate the comparative
energy of different machines,
a uniform rotation of the plates
must be maintained by a given
number of revolutions per min-
ute ; and the number of dis-
charges in a given time of the
unit jar, connected with the
prime conductor, will then be approximately correct
for the energy of each.

THE ELECTRIC CHIME. — This instrument is used in
connection with the machine, to illustrate electric
attraction and repulsion.

It may be mounted on a separate stand, or hung from
the projecting rod of the prime conductor. Fig. 31
represents a common style used in this way.

It consists of three bells suspended from a brass rod;
the two outer ones by brass wires or chains, and the
central one by a silk cord ; a brass chain connecting it

Fig. 31— Electric Chime.

ELECTRIC GENERATORS.

103

with the earth. Between the central and outer bells
are two small brass balls, suspended by silk cords.

When the machine is put in operation, the outer bells
receive a charge from the prime conductor; this acts
inductively on the insulated balls, which are at zero,
attracts, and, after contact, repels them. Being now
charged the same as the outer bells, they act inductively
on the central bell, repelling or attracting electricity
through its chain, according as their charge is positive
or negative ; and pro-
ducing on it a charge
of the opposite kind,
they are attracted to it
and discharged. Be-
ing now at zero, they
are attracted to the
outer bells, as before ;
and in this way the
three bells are made
to ring.

IMAGE PLATES. —
These are used to

show the effect of ill- FiS- 32-Image Plates.

duction between two conducting surfaces, as repre-
sented by Fig. 32.

From the projecting rod of the prime conductor, a
brass plate, having a resistance rim, is suspended by a
wire or chain: and under it. on an insulating stand, is
placed another similar plate, made a little larger, and
joined to the insulating support by a sliding rod, by
which the distance between the plates may be adjusted,
a chain connecting it with the earth.

When the machine is put in operation, the upper

104 ELEMENTS OF STATIC ELECTRICITY.

plate will have the same charge as the prime conductor.
If the charge be positive, electricity is repelled by in-
duction from the lower plate to the earth, through the
chain ; if negative, it is attracted through the same
medium ; and, in either case, the plates are oppositely
charged to the same potential, the air being the dielec-
tric.

When the space is properly adjusted, pith balls or
images, placed on the lower plate, are alternately at-
tracted and repelled, dancing up and down between
the plates in a manner which is often quite amusing.

If electric connection
with the earth be sev-
ered by removing the
chain, this effect will
cease : which proves
that the opposite poten-
tials of the plates was
caused by the transfer
Fig. 33-The Eteoric Whirl. of electricity to or from

the earth, as stated.

THE ELECTRIC WHIRL. — This little instrument,
shown in Fig. 33, consists of a set of pointed brass arms
attached to a common center, which is pivoted on the
point of a vertical rod connected with the prime con-
ductor; the arms being bent, so that when passing a
given point each shall turn in the same direction.

When the machine is put in operation, the air in
front of each point becomes electrified, either positively
or negatively, by the passage of electricity either from
or to the point; while that back of it is oppositely
electrified by induction. This causes repulsion from
the air in front, and attraction toward that at the back,

ELECTRIC GENERATORS. 105

producing rotation of the instrument in the opposite
direction to that in which the points turn.

The effect of a stationary point in producing a cur-
rent of air is shown in Fig. 34; where the flame of a
candle is represented as blown from a point attached
to the prime conductor.

The direction of the air current will be the same
whether the charge is positive or negative : since, in
either case, the air embraced within a sphere of which
the point is the center will have the same potential as
the prime conductor; while that outside of this sphere
will assume the opposite potential by induction. Hence
the air near the
point becomes
self-repellent,
and is also at-
tracted by the
air outside ; that
directly in front

Of the point Fig 34_Air Current from "a'print.

being repelled

with the greatest force, produces a current in that di-
rection, while the air on either side is attracted, and, in
its turn, again repelled.

ARMSTRONG'S HYDRO - ELECTRIC MACHINE. — Fig.
35 represents a machine invented by Sir William Arm-
strong, about 1840, which generates electricity by the
discharge of partially condensed steam.

It consists of a boiler and furnace mounted on glass
pillars; the boiler being provided with steam and water
gauges, a safety valve, and a condenser inclosing sev-
eral small pipes, through which the steam escapes.

These pipes are surrounded with filaments of cotton,

106

ELEMENTS OF STATIC ELECTRICITY.

the lower ends of which are immersed in cold water at
the bottom of the condenser : and the water being thus
raised by capillary attraction, cools the pipes, producing
partial condensation of the steam ; thus charging it
with water in fine drops, by the friction of which
against the pipes electricity is generated; the steam

Fig. 35 — Armstrong's Hydro-Electric Machine,

being discharged against a row of points connected
with the prime conductor.

Each pipe is furnished with a wooden tip : and the
friction is increased by a tongue of metal, around which
the steam must pass before entering the tip, as shown
by the enlarged section at letter A.

A machine of this kind, constructed for the Royal

ELECTRIC GENERATORS. 107

Polytechnic Institution in London, had a boiler seventy-
eight inches long, and forty-two inches in diameter, with
forty-six steam jets. It gave sparks twenty-two inches
in length, and charged to its full capacity, in six to eight
seconds, a Leyden battery, having eighty square feet of
coated surface.

Another one, described by Noad, had one hundred
and forty steam jets, gave sparks of the same length,
with thrge or four times the rapidity; and charged, to
its full capacity, a Leyden battery having 1,188 square
feet of coated surface, sixty times in a minute.

But though capable of such powerful effects, this
machine is not practical. It is inconvenient to manage,
requires distilled water, careful cleansing of the boiler
after use, and great steam pressure. Its operation is
accompanied with a deafening noise, and the escape of
a great volume of steam, producing dampness and other
unpleasant results, when used in a room. Hence its
chief value is in the demonstration of the important
fact, that electricity may be generated in this way.

CHAPTER VIII,
ELECTRIC GENERATORS.

THE HOLTZ AND TOPLER MACHINES.

INFLUENCE MACHINES. — Previous to 1865, frictional
machines were the principal electro - static generators
in use. But that year marked an era in electric prog-
ress by the invention of two new machines of remark-
able energy, by the German electricians, Holtz and
Topler; to which the name influence machines was
given, from their being constructed with two or more
glass plates, arranged to generate electricity by their
mutual inductive influence.

Both machines are very similar in construction ; the
principal difference being, that the Holtz requires to be
incited by an initial charge from an external source,
while the Topler is self-inciting.

THE HOLTZ MACHINE. — This machine, of which
there are several different styles, is represented by Fig.
36. On a wooden base are mounted two glass plates ;
the rear plate B stationary, and supported by three
ebonite insulators, two below and one above ; while the
front plate A revolves in the direction of the arrow,
on a steel shaft, which passes through an opening in
the center of the plate B, and is attached to the post
at M. A is mounted on an ebonite hub, attached to a
hollow shaft of brass, which revolves on the fixed shaft,
and carries, at the end next the post, a small pulley,
from which a belt extends to the driving wheel, which

ELECTRIC GENERATORS. 109

is revolved by a crank with an ebonite handle. The
relative sizes of the wheel and pulley are such as to
give the plate four to six revolutions for each revolu-
tion of the driving wheel, the plates of small ma-
chines requiring a more rapid revolution than those of
larger ones. In front of the plate A, J of an inch from
the glass, are the combs T^and H, attached to a brass
core at the center of the ebonite disc M ; and the
combs IT and L, insulated by their attachment to ebon-

Fig. 36-The Holtz Electric Machine.

ite rods projecting from the disc Jf, and connected by
brass rods with the Leyden jars C and D, and with the
sliding-rods P and R. These sliding-rods have ebonite
handles, and terminate in brass balls at their inner ex-
tremities.

The plates are of sheet glass, about \ of an inch thick ;
of good insulating quality, and well coated with shellac.
The stationary plate B has two circular openings called

110 ELEMENTS OF STATIC ELECTRICITY.

windows, directly opposite the combs IT and L ; and, on
its rear surface, are cemented two paper inductors T
and X; T extending from H to L, and X from V to K ;
and each armed with a row of points, projecting into
each window.

Machines of this kind are often constructed with
more than two plates ; sometimes with a large number.
The plates are also sometimes placed in a horizontal
position. Ebonite plates are also used ; but are objec-
tionable, for reasons already given.

THE TOPLER MACHINE. — The Topler machine has
the same general construction as the Holtz ; but, on
the front surface of the revolving plate, are cemented
a number of small metal discs, called carriers; usually
made of tin-foil with raised brass centers, which, as the
plate revolves, are brought into contact with four wire
brushes; two attached to the stationary plate, and two
to the uninsulated combs. In this way the machine is
made self-inciting, as already mentioned.

The windows, and the rows of points projecting into
them, used in the Holtz stationary plate, are omitted
from the stationary plate of the Topler: and the paper
inductors are made longer, and have small tin-foil in-
ductors under them, connected, by tin-foil strips, with
each other and also with the two brushes attached to
this plate.

Fig. 37 represents a Topler machine constructed by
the author, and patented April 10, 1883, and December
8, 1885. The principal points covered by the patents
are as follows : —

1. The outside coatings of the Leyden jars C and
D are of sheet brass, f nickel plated ; and are screwed
firmly to the base ; forming cups into which the jars

ELECTRIC GENERATORS.

Ill

fit closely, and are thus held in a fixed position ; afford-
ing a firm support to the parts connected with them,
and preventing liability to accident or injury to the jars
or plates.

2. The induced current from these outside coatings
is conveyed down by the brass screws which attach

Fig. 37— Atkinson's Topler Electric Machine.

them, and along copper wires underneath, to the termi-
nals of the switch S ; through which, when closed, it
passes from one jar to the other ; but when open, as in
the cut, it passes by the brass sockets, seen on the edge,
which are also connected with the terminals, out
through the conducting cords, and a person, or other

112 ELEMENTS OF STATIC ELECTRICITY.

object, connected with their outer extremities. As this
induced current flows simultaneously with the direct
current from the inside coatings, the switch and sliding-
rods place it completely under control of the operator.

3. The brush holders, E and F, are attached to the
plate B, through holes near its edge ; thus giving a di-
rect passage to the electricity from the carriers on the
plate A, where it is generated, through the glass, to the
tin-foil inductors, represented by the dark shade, and
the paper inductors T and X, represented by the light
shade. By passing the electric charge through the glass,
inside its edge, an insulating margin is interposed be-
tween the conductors and the edge, thus preventing
loss from leakage, which is unavoidable when the brush
holders are attached by clamps or ears on the edge.

4. The carriers on the plate A are of sheet brass,
with raised centers, and are nickel plated, making them
both durable and ornamental. The hard nickel surface
is not affected by the action of the brushes, or the elec-
tricity, while tin-foil soon becomes defaced: and the
carrier, being practically one piece, and its entire sur-
face cemented to the glass, its raised center cannot be-
come detached, as may happen when the center is put
on separately over a tin-foil base.

5. The combs V and K, also H and L, radiate at an
angle of 45 degrees to each other, from the central disc
M, to which they are attached ; so that any possibility
of error in regard to their position, or of displacement, is
practically impossible.

The following improvements may also be noticed: —
The base is made of two-inch strips, glued together

lengthways, and heavy cleats screwed on underneath ;

giving all the advantages of iron as to freedom from

ELECTRIC GENERATORS.

113

warping, with the insulation and elegant finish of the
wood. The driving wheel is of ebonite and the iron
casting, on which it is mounted, slides in grooves on an
iron plate, and is moved by the adjusting screw 0, to
regulate the tension of the belt.

Fig. 38— Atkinson's Four-Plate Topler Machine— Front View.

The ebonite insulators, which support the plate J9,have
soft rubber packing, to ease the pressure on the glass.

The conducting rods of the Leyden jars pass through
ebonite caps with cork attached underneath, which
gives them a fixed vertical position, and affords firm
support to the sliding-rods and the combs connected
with them above.

114

ELEMENTS OF STATIC ELECTRICITY.

THE FOUR-PLATE TOPLER MACHINE. — This machine
has the same construction in front as the two-plate ma-
chine as shown by Fig. 38, but a special construction
for the two rear plates which will be understood by
reference to Figs. 39 and 40.

The end view, Fig. 39, shows two pairs of plates, the
position of the rear pair being reversed, which brings
the stationary plates into the center, back to back, be-
tween the revolving plates ;
so that the inductors are on
the inner surfaces of the
stationary plates, and the
carriers on the outer sur-
faces of the revolving plates,
which being mounted on
the same shaft, with a col-
lar between them, revolve
in unison.

The combs L and K, and
F'and H, have curved rods
L which pass round the
plates and support dupli-
cate combs in the rear as

Figi 30_Atkinson's Four-Plate shown ill the Cuts. The
Topler Machine-End View. brusnes are algo duplicated

as shown : so that with the exception of the Leyden
jars and switch, and parts connected with them, this
is practically a double machine.

In like manner an eight-plate machine may be made
by doubling these parts of the four-plate.

When the large Topler or Holtz machines are
wanted for constant use, the motive power is usually
supplied by a steam or gas engine, or a water motor.

ELECTRIC GENERATORS.

115

In which case the driving wheel is not used ; the belt
passing directly from the small pulley connected with
the plates, to a pulley attached to the engine or motor.

MODE OF ACTION OF THE TOPLER. — To compre-
hend the action of any electric generator, the following
essential principles in their construction should be kept
distinctly in view.

To generate electricity, is to create a difference of
electric potential ; the efficiency of all generators,

Fig. 40— Atkinson's Four-Plate Topler Machine— Rear View.

whether batteries, dynamos, or glass plate machines,
depending on the difference of potential which each is
able to create and maintain within the apparatus itself.
And the work to be done by such an apparatus is the
restoration of equilibrium, through an exterior circuit ;
and may consist in producing heat or light, chemical,
mechanical, or physiological action.

Let us consider how these principles apply to this
machine.

116 ELEMENTS OF STATIC ELECTRICITY.

Fig. 37, page 111, represents the machine with the
sliding electrodes P and R separated. Suppose the
switch AS' to be closed and the machine put in operation.
It will be seen that as the plate A revolves, the raised
centers of the six carriers are brought into contact with
the wire brushes attached to the holders E and F; each
opposite pair touching opposite brushes, successively,
at the same instant. The friction generates electricity,
which diffuses itself over the carriers on A, and the in-
ductors on J5, with which they are, at the instant of
contact, in electric connection. The potential of car-
rier and inductor, during contact, will be the same ; at
the next instant the carrier passes on, and is insulated
from the inductor, and carrier and inductor now act in-
ductively on each other, and multiply the initial charge
given by the friction of contact. As it accumulates, it
spreads over the paper inductors ; these act on the
opposite surfaces of the glass, till both surfaces of both
plates become charged ; the initial charge being still
continued by the constant friction of the carriers and
brushes.

But, since both sides of the machine are of similar
construction, and since the mode of action on both
sides is apparently the same, the question arises, how
any difference of potential, or electric charge can be
accounted for.

And first, it will be noticed, that the position of the
plates being vertical, their lower halves are nearer to
the earth, by their semi diameter, than the upper halves,
and consequently, more under the influence of its in-
ductive action, by the square of that distance. The
lower halves are also in close proximity to the Leyden
jars, the driving wheel, and the belt, and subject to their

ELECTRIC GENERATORS. 117

inductive influence; and the plate B is supported on two
insulators, while the upper half has but one, and hence
has the advantage of the better insulation of the air.

To this lower half of B, and subject to these influ-
ences, is attached the brush holder F, while E is
attached to the upper half, and remote from them.
Hence, the carriers brushed by E, and descending to-
wards L, must acquire a higher potential than those
brushed by F, and ascending towards K.

An accumulation of electricity must also occur at the
lower ends of the inductors ^and X, from the induct-
ive influence of the earth ; and as the brush holder F
is placed at the lower end of X, it furnishes an outlet
to a portion of this charge, as seen at night by the
brushes of light from this holder to the outside of the
jar (7, and other parts in close proximity.

The lower end of T, on the contrary, is well insu-
lated ; hence the potential of T, from the heavier charge
at its upper end, and the better insulation at its lower
end, must be much higher than that of X, where the
influences are just the reverse.

This accumulation, or high positive potential at the
lower end of T, produces a high negative potential at
that point on the plate A, and its carriers, as it revolves ;
as shown by the brush of light, seen in the dark, from
the uninsulated comb F, marking the flow of electricity
to the upper part of the plate, as it passes under that
comb; the outflow of the current received through the
comb H. This brush of light extends downward, as
the charge increases, almost to the comb K: and a sim-
ilar brush extends downward from K, marking the
outflow of electricity from the interior of the jar (7, as
explained hereafter: while the points of the combs, L

118 ELEMENTS OF STATIC ELECTRICITY.

and H, where the charge is received, show only a glow
of light.

These brushes of light always turn in the opjtosite
direction to that in which the plate A revolves ; differ-
ence of potential between the comb and that portion of
the plate approaching it producing attraction ; while
equality of potential between the comb and that portion
of the plate receding from it produces repulsion. (seep. 224.)

Following any opposite pair of carriers, as TFand Z, we
find that as Z passes under the wire brush _F, W passes
under E ; and as Z moves on to the insulated comb 7T,
TFat the same instant arrives at L ; but Tf, as already
shown, has a higher potential than Z, and, at this point,
a peculiar adjustment takes place. W gives up its
charge through the comb L, to the inside of the Ley-
den jar D. This creates a positive charge on the inside
of .Z>, which induces a negative charge on its outside.
The electricity thus repelled, passes to the outside of
(7, making it positive, and inducing negative on its
inside ; and this repelled electricity flows through the
comb K to the plate A, as already shown. W then
moves down to the uninsulated comb ZT, while Z moves
up to V. Each now passes under the wire brush at-
tached to its respective comb, and the combs being
attached to the brass core at the center of M, the carriers
are put in electric connection with each other, and their
potential equalized by the flow of the residual charge
from If to F", as already described; so that each arrives
at the original position of the other at the same poten-
tial, ready to repeat the same process.

It should be noticed, that the residual is slightly in-
creased by induction from Tand X, as the carriers move
from the combs L and .Zf to the combs JTand F".

ELECTRIC GENERATORS. 119

The surfaces of the plates, on which the carriers and
inductors are mounted, assume the same potential as
the carriers and inductors attached to them, while their
opposite surfaces have the reverse. Opposite parts of
the same surface are also in opposite electric states : the
section L M H, for instance, having a potential oppo-
site to that of V M K; change of potential on these
surfaces following that of the carriers and inductors,
already described.

It will be noticed that the office of the brushes, E and
F, is the reverse of that of H and F". E and F generate
by friction, while H and V discharge by contact. And,
while the combs, K and L, aid in creating a difference
of potential, the combs, H and F", aid in restoring equi-
librium.

When the difference of potential between the inner
coatings of the jars becomes sufficient to overcome the
resistance of the air, a discharge from the inner coating
of D to that of C takes place between the terminals of
the sliding electrodes R and P ; and, at the same
instant, a discharge from the outer coatings takes place
through the switch and connections, from 0 to D, to
restore equilibrium between them, and thus complete
the circuit.

A spark and snap, from the resistance of the air,
accompanies the discharge between the inner coatings ;
and the same will occur between the outer coatings if
the switch is open ; but, if closed, the discharge takes
place silently. The plates and other parts being, at the
same instant, relieved of strain, there is a restoration of
equilibrium in the whole machine.

The above explanation applies to the machine when
it is put in operation from a state of absolute rest ; but

120 ELEMENTS OF STATIC ELECTRICITY.

when it is in a high state of activity, there frequently
occurs a reversal of potential after a discharge, as shown
by the reversal of the brushes of light from the combs.
To account for this it must be considered, that the
residual which remains after the primary discharge may,
from unequal resistance, be greater on one side than on
the other; and after being relieved from strain by the
primary discharge, it will operate to give a slight pre-
ponderance of potential to that side, which is rapidly
multiplied by induction, as the rotation of the plate
continues.

A reversal can also be produced by a temporary
reversal of rotation, as explained on page 140 ; or by
touching the inductors, or parts connected with them,
while in action, which would reduce the potential at that
point. Special conditions may also exist in certain
machines, which will reverse the ordinary mode of
action ; as, for instance, a difference of thickness on
opposite parts of a glass plate ; or in opposite jars.

It should be noticed that the electric charge is
instantly diffused over the metal carriers and inductors,
more slowly over the paper inductors, and still more
slowly over the shellacked surfaces of the glass plates.
So that when the machine is put in action, after a con-
siderable interval of rest, three or four seconds elapse
before it becomes fully charged, and a crackling sound
is heard from the electricity forcing itself over the
resisting surfaces of the paper and glass.

The condition of the air, as to its insulation, influ-
ences the whole operation of this machine. An air
space insulates the plates, and also the jars, with their
rods and balls, from each other ; and as a damp atmos-
phere lessens this insulation, it will decrease the energy

ELECTRIC GENERATORS. 121

of the machine in like proportion. A film of moisture,
settling on the plates, will often so reduce the insula-
tion, that the slight initial charge by the action of the
brushes is conducted over the damp surface as fast as
it is generated ; so that no difference of potential, and
consequently no permanent charge, can occur. And as
the machine is much more sensitive to such influences
than the operator, the latter is often puzzled to know
why it will not generate. The simple and effectual
remedy, in all such cases, is to dry it. This may be
done by a fire, a kerosene lamp, a hot iron, or by
the sun's heat, though artificial heat is generally more
effectual.

Warm days, before or after rain, when the atmos-
phere is loaded with moisture, are the most unfavorable.
At such times the plates should not only be dried, but
warmed, as moisture will continue to be deposited so
long as they are colder than the air.

The electric conditions in upper rooms, other things
being equal, are more favorable to the operation of the
machine than in those on the ground floor.

MULTIPLICATION or THE CHARGE. — The multipli-
cation of the initial charge proceeds with great rapidity.
During the first revolution of the plate A, each tin-foil
inductor receives six direct charges from the contact of
its connecting brush with each of the six carriers: and
also six inductive charges of equal amount, as each
charged carrier passes it. So that at the end of the
first revolution, it has accumulated twelve charges ; and,
during that revolution, it has reacted inductively on each
passing carrier with this constantly increasing energy,
increasing the energy of the carrier in like proportion.

At the beginning of the second revolution, it has

122 ELEMENTS OF STATIC ELECTRICITY.

twelve times the inductive energy which it had at the
beginning of the first; and this energy continues to
increase, and react on the carriers, at the same rate as
before. And as the plate makes about five revolutions
per second, the rate of increase on any tin-foil inductor
is about sixty increments per second.

But as the charge spreads from the tin-foil inductors
over the paper inductors and adjacent parts of the sta-
tionary plate; and from the carriers over adjacent parts
of the revolving plate, each point on each plate, within
the charged areas, becomes a center of direct and induct-
ive action in the same manner as the metal inductors and
carriers. So that even an infinitesimal charge is increased
in a few seconds to the full capacity of the machine.

HOLTZ AND TOPLER MACHINES COMPARED. — Since

the chief difference between the Holtz and Topler con-
sists in the latter being self-inciting, the mode of action
is essentially the same in each.

The Holtz may receive its initial charge from a fric-
tional machine, an electrophorus, or any similar, exter-
nal source : but the usual method of charging is by
means of a piece of ebonite, electrified by the fur of a
cat-skin.

The electrified ebonite is held in contact with one of
the paper inductors on the stationary plate, which is
thus charged ; a portion of the charge being commu-
nicated to the revolving plate through the points which
project into the windows; and this plate is made to
rotate rapidly, so that the charge is soon multiplied to
the full capacity of the machine, if the atmospheric con-
ditions are favorable; and the ebonite is then removed.

It will thus be seen that the initial charge in both
machines is produced by friction and multiplied by

ELECTRIC GENERATORS. 123

induction. In the Holtz it is derived from an external
source, begins 011 the stationary plate, and is then com-
municated to the revolving plate. In the Topler it is
produced by the machine itself, begins on the revolving
plate and is then communicated to the stationary plate.
In the Holtz it occurs on one side only. In the Topler it
is simultaneous on both sides. In the Holtz it ceases when
the plates are charged. In the Topler it is continuous.

The absence of the brushes, carriers, and metal in-
ductors from the Holtz increases the internal resist-
ance, making it more difficult to charge, but giving
better insulation, and consequently greater energy than
a Topler of the same size.

But the action of a Holtz is much more liable to
interruption from dampness, and a low electric poten-
tial in the atmosphere : since it receives only a small
initial charge, which is soon discontinued ; while that
of the Topler is constant, from the continuous action
of the carriers and brushes. So that a well constructed
Topler, with ordinary care, is reliable in any state of
the atmosphere, while a Holtz is very unreliable.

COMPARISON BY DR. HOLTZ. — In reply to an
inquiry as to whether the Topler machine was an
original, independent invention, or only a modification
of the Holtz, the author received a letter from Dr.
Holtz, written from Greifswald, Germany, March 20,
1883; in which he says, that his machine, as first de-
scribed in PoggendorfTs Annalen, in 1865 (volume
125, page 469, and volume 126, page 157), had "two
discs rotating in opposite directions, without stationary
discs " ; and that " The Topler machine, invented at the
same time, was a combination of two pairs of discs";
two movable and two stationary.

124 ELEMENTS OF STATIC ELECTRICITY.

He then says : —

" Topler has recently rejected his system and adopted mine,
because it is simpler, and, at the same time, more effective. The
application of the pointed combs and the non-covered movable
discs is also my invention, since the Topler machine had only the
tin-foil coverings and sliding springs. (Schleifende Federn.)

11 1 had been accustomed to the same, indeed, already ; although
not with independent acting, influence machines, but rejected
them on account of the smaller spark-length.

" Topler has also lately adopted my principle of the pointed
combs, and the non-covered discs ; but so far modified, that besides
the pointed combs and non-covered discs, he yet allows to act, at
the same time, small pieces of tin-foil (or pieces of metal), and the
sliding springs. This has the advantage that the machine excites
itself, and is less sensitive to moisture ; but also the great disad-
vantage, that the sparks become shorter, and a constant reversal
of current follows. Besides, a certain mechanic, Voss, also claims
this machine, so modified, as his merit ; but unquestionably Topler
was the first who showed that influence machines, with metallic
covering and sliding springs, excite themselves.

" The entire form of the machine, its symmetrical construction,
the one-sided support of the axis, the application of a sheath
running upon a pin fastened on one side, the application of the
so-called rotary diametrical (double) pointed combs, the applica-
tion of the so-called condensers (small Leyden jars) for increase of
spark-length, is all mine, as published in the year 1869, by Professor
Poggendorff (PoggendorfFs Annalen, vol. 136, page 171).

" Yours truly, DR. W. IIOLTZ."

The " sliding springs " mentioned above, doubtless
refers to a style of construction in which the springs
glide continuously over the surface of the glass ; essen-
tially different, and differing in its effect, from that of
the brushes, which touch only the raised centers of the
carriers, and are wholly insulated from the glass;
giving alternate contact and insulation, making induc-
tion much more effective. The latter construction is
attributed to Voss.

CHAPTER
EXPERIMENTS WITH THE TOPLER MACHINE.

IN experiments with the frictional machine, such as
the charging of Leyden jars, and the ringing of bells,
as already described, induction is produced by connect-
ing one part of the apparatus with the earth, and
another part with the prime conductor. But in the
Holtz and Topler, the charge is accumulated in the
Leyden jars instead of on a prime conductor ; and any
change of potential in one jar must be compensated by
a corresponding inductive change in the opposite jar.
Hence to obtain the full inductive effect, connection
must be made with the opposite jars.

For convenience in making this connection, holes are
drilled in the knobs surmounting the jars, and the
charge is conveyed by insulated conducting cords, hav-
ing brass tips which fit these holes.

Thus, by connecting the inner and outer coatings of
a Leyden jar or battery with the opposite jars in this
way, a full charge can be given very rapidly.

In a similar manner, image plates, bell chimes, and
other apparatus, mounted on separate stands, can be
connected and used.

ELECTRIC CHTME FOR TOPLER MACHINE. — Fig. 41
represents a chime designed by the author, which is
mounted on the machine itself. It consists of two
brass arms A and J5, insulated by an ebonite connector

126

ELEMENTS OF STATIC ELECTRICITY.

C ; the tips of the arms being fitted to the holes in the
knobs of the jars.

A bell is suspended from each arm by a brass rod ;
and a brass ball suspended by a silk cord from the
ebonite connector hangs between them.

As each bell is at the same potential as the jar with
which it is connected, the ball is alternately attracted
and repelled, causing the bells to ring.

Instruments of this kind have no practical use, except
to illustrate the principles of the science.

c APPARENT TIME OF

THE ELECTRIC DIS-
CHARGE AN OPTICAL
ILLUSION. — The car-
riers on the revolving
B plate of a Topler afford
special facilities for
this experiment. They
are usually six discs,
arranged in a circle,
and present the ap-
pearance of a con tin-
Fig. 41-Chime for Topler Machine. ^^ ^^ ^ ^^

the machine is operated in the light ; but when oper-
ated in the dark, they are seen only when the spark
renders them visible ; and, instead of the bright ring,
each appears by itself, apparently motionless, and as
perfect in form as if really so, just as if the movement
of the plate were momentarily arrested during the
passage of the spark.

This apparent time of the spark may be estimated
at i second ; but if the carriers were really visible
during that time, the ring-like appearance would be

EXPERIMENTS WITH THE TOPLER MACHINE. 127

unavoidable, as will appear from the following calcu-
lation.

Suppose the revolving plate to have an average speed
of 4 1 revolutions per second, it is evident that each
carrier would make a complete revolution in less than
i second ; consequently if that were the actual duration
of the spark, each would be continuously visible round
the entire circle, and hence even a single carrier would
produce the bright ring. But it is only necessary to
this result that each should be visible until it takes the
place of its predecessor — that is during its passage of J
of the circle, which reduces the time to aV of & second.

But if they were visible even half that time, -^ of a
second, and each were 1J inches in diameter, and their
distance, from center to center, 6 inches, we would
have 6 ellipses, each having a length equal to twice its
breadth.

From this it is evident that the smallest conceivable
duration of spark must produce an ellipse : but as each
presents the appearance of a circle, with no tendency
to elliptical form, the conclusion is inevitable that the
apparent duration of the spark is an optical illusion,
and that its time is so nearly zero, that it cannot be
estimated.

We must conclude, then, that at the instant of dis-
charge the image of the carrier is photographed on the
retina of the eye, and at the next instant darkness
supervenes : but the sensation on the retina has a mo-
mentary duration, during which the carrier appears
stationary, while in reality it may have passed entirely
round the circle.

It is important to notice, in this connection, that the
appearance and disappearance of the carriers depend

128 ELEMENTS OF STATIC ELECTRICITY.

on the rapidity of the discharge ; and when the spark
is made so short and rapid as to be apparently contin-
uous, the carriers appear and disappear with each snap,
like a succession of views in a rapidly moving panora-
ma, proving that the apparently continuous spark is a,
succession of sparks so rapid as to give the impression
of continuity.

As a flash of lightning is only the same thing on a
grander scale in nature's own laboratory, we must con-
clude that the passage of electricity from cloud to cloud,
a distance often of many miles, is so rapid as to defy
human calculation. We notice this in chain lightning,
when the flash, sometimes three to five miles long, is
seen throughout its entire length at the same instant,
as if suddenly photographed on the cloud.

TRANSMISSION OF POWER BY STATIC ELECTRICITY.
— Two machines are necessary for this experiment —
one called the primary, and the other secondary. The
secondary should be a very light running machine ;
hence it is better to make it smaller than the primary,
and the driving wheel and switch may be dispensed with.

Let the machines be placed near each other, in the
same relative position, the secondary in front ; and
connected together by conducting cords or wires, joining
similar pairs of Leyden jars: and let the sliding elec-
trodes be separated beyond sparking distance. Now
let the primary machine be put in operation, and the
movable plate of the secondary will rotate in a direction
opposite to that of the primary. If the electric energy
should not be sufficient to overcome the friction and
inertia, in starting, the plate of the secondary may be
put in rotation by hand, and its motion will then be
sustained by the electric action.

EXPERIMENTS WITH THE TOPLER MACHINE. 129

The explanation is as follows. When a Topler ma-
chine is in operation, there is a strong attraction be-
tween the plates, the result of induction from the
opposite electric states of the parts in proximity. This
attraction which constantly increases up to the instant
of discharge, acts as a resisting force which must be
overcome by the force used to rotate the plate. Now,
when the two machines are connected, this electric
force is transmitted to the secondary, where, having no
mechanical force to oppose it, as in the primary,
it causes the rotation of the plate in the opposite
direction.

Thus the mechanical force in the primary is trans-
muted into electric force, passes <fver to the secondary
and reproduces mechanical force ; the force applied to
the primary being expended in the secondary.

The apparatus thus becomes a scientific bank, with
its receiving and paying tellers. But nature is a
shrewd banker, and always exacts full discount ; hence
the mechanical energy, paid in to the primary, is dis-
counted by friction, leakage, and heat; so that the
remaining energy may not be sufficient to start the
plate of the secondary into rotation without an ad-
ditional payment.

The sliding electrodes in the secondary machine may
l)e adjusted to produce the electric discharge with spark
and snap, instead of the mechanical rotation of the
plate ; thus illustrating the transmutation of force, at
will, from mechanjcal to electric, and from electric
either back again to mechanical, or to the heat, light,
and sound of the electric discharge.

SOUECE OF ELECTRIC SUPPLY OF THE TOPLEK MA-
CHINE.— The earth, the machine itself, and the air are

130 ELEMENTS OF STATIC ELECTRICITY.

the only sources from which an electric machine can
derive electricity.

With the common friction machine a connection
with the earth is indispensable, and only a very limited
charge can be obtained without it ; the transfer being
either from the earth to the machine, or from the ma-
chine to the earth, as explained on page 99. Hence it
is often compared to a pump, drawing electricity from
the earth through a chain. Remove the chain and the
supply ceases.

But with the Topler a similar earth connection
diminishes the charge ; showing a loss instead of an
increase of charge. Indeed, perfect insulation of the
generating parts is an essential feature of the machine.

To demonstrate this more perfectly, let the machine
be put in operation on an insulated platform, when it
will be found that there is not the slightest perceptible
diminution of electric energy. It is evident, then, that
the earth is not its source of supply.

A certain amount is, no doubt, obtained from the
material of the machine itself; but this source would
soon be exhausted by such experiments as the charg-
ing of a large Leyden battery ; whereas such a battery
may be charged without diminishing the energy of the
machine.

The air, then, is the only remaining source, and the
large amount of ozone generated by this machine is
conclusive evidence of its electro-chemical action on
the air, and strong, presumptive evidence that the air,
thus acted upon, has furnished the electricity whose
action has changed the oxygen to ozone.

This would imply that ozone is the result of depriv-
ing air of a portion of its electricity ; whereas if the

EXPERIMENTS WITH THE TOP LEE MACHINE. 131

electricity were derived from the earth, we must infer
that its generation precedes the generation of ozone,
instead of being coincident with it. But the insulation
proves that the earth does not supply the electricity ;
so that the weight of evidence is in favor of ozone being
the direct result of electric generation, rather than a
result of subsequent electric action. And, if such is
the case, it is strong proof that the air is the chief
source of electric supply.

The generation of ozone by atmospheric electricity
during thunder storms is a well-known fact; and clouds,
floating miles above the earth, must obtain their elec-
tricity either from their own vapor, or the air, or both.
Such clouds, at different electric potentials, insulated
from the earth, acting inductively on each other, and
finally producing a discharge, fulfill the same conditions
as exist in the Topler machine ; and the generation of
ozone is doubtless due to the same cause in both. And
since the vapor of the cloud corresponds to the material
of the machine, and it has been shown that the electric
supply of the machine from its own material must be
very limited ; and since the machine operates most
effectively in a dry atmosphere, and hence does not
derive its electricity from vapor ; we may infer that the
electric action is the same in both cases, and that the
air is the chief source of electric supply.

It is evident from the movement of particles of dust
and other light bodies towards the machine, that the
air in which these atoms float must have a similar
movement ; that currents of air are constantly flowing
to the machine and that this air, after being changed
to the same electric potential, is repelled, and air at a
different potential flows in to take its place ;. a move-

132 ELEMENTS OF STATIC ELECTRICITY.

merit similar to that which takes place in the hot and
cold currents round a heated stove.

But the initial charge is undoubtedly from the ma-
terial of the machine itself, and results from the friction
of the brushes on the carriers ; after which follows the
increase by induction and the action on the air.

ELECTRICITY GENERATED BY THE FRICTION OF
METALS. — The old division of all substances into elec-
trics and non-electrics was the exponent of the idea
then prevalent, that only certain substances, as glass,
sealing-wax, and other non-conductors, comprised in a
very brief list, were capable of electric excitation.
While this view is no longer maintained, yet, since in
nearly all experiments illustrating the elements of static
electricity, glass, sealing-wax, ebonite, silk, wool, fur,
and other non-conductors, are almost exclusively em-
ployed as generators, we are apt to lose sight of the
fact that metals and other conductors are capable of
generating electricity by their mutual friction. And
yet this is one of the most important principles of static
electricity. It is that which liberates our ideas of elec-
tricity from the narrow bounds to which they were
once confined, proving that it is not a special property
of certain substances, but a universal property of mat-
ter, one form of that energy which pervades and con-
trols the universe.

This point has been already illustrated, but the
Topler machine affords special facilities for illustrating
it more fully. In it the initial charge is produced by
the friction of metal brushes on metal carriers. True,
both carriers and brushes are attached to glass, and the
glass subsequently acts by induction as a generator;
but the friction is confined to the carriers and brushes

EXPERIMENTS WITH THE TOPLER MACHINE. 133

alone; and, so far as the electricity is obtained from
this source, the glass acts only as an insulator to pre-
vent the escape of the electricity generated by the
friction, from which the initial charge is derived.

It is not even necessary that the metals should be
different. The machines here described are constructed
with brass carriers, and brushes of brass wire; and,
though the carriers are nickel-plated, so that the friction
is that of brass brushes on a nickel surface, yet carriers
left.unplated give equally as good results.

THE SPARK; ITS DIRECTION, SUBDIVISION, AND
COLOR. — The spark from a Topler machine presents
phenomena which demand careful investigation.

As already shown, the apparent time of the discharge
is an optical illusion, time being practically annihilated;
so that it is impossible, from observation, to tell in what
direction the discharge takes place. A brilliant streak
of white light, extending from one electrode to the other,
suddenly appears and disappears, leaving us in igno-
rance as to the direction in which it moves. But the
following experiment affords better opportunity for
observation.

As already shown, the electric connection between the
inside coatings of the Leyden jars may be interrupted
by separating the sliding electrodes, and that between
their outside coatings by opening the switch. Put the
machine in operation in a darkened room at night, with
the switch open, arid the sliding electrodes separated
three or four inches. From the electrode P, Fig. 42,
a brush of violet -colored light, diverging from a small,
circular space, extends about f of an inch towards the
opposite electrode, accompanied by a hissing sound.
The opposite electrode, R, remains comparatively qui-

134 ELEMENTS OF STATIC ELECTRICITY.

escent at first, showing only a glow of light ; but, as
the electricity accumulates, there is a sudden outburst
from it, accompanied by phenomena of the most inter-
esting and varied character.

A brush of light, of a faint white, or violet color,
darts across the intervening space, diverging towards
the center, and converging as it meets the brush from
the opposite electrode ; forming an elliptical figure, two
or three inches in diameter, extending from one elec-
trode to the other. Through the center of this brush
shoot out long tongues of red and violet light, curving
and branching in a variety of fantastic forms. Some-
times five or six of these appear at once, like fiery
serpents, hissing, spitting, and darting out their red
forked tongues. Sometimes the appearance is that of
a miniature tree, its main trunk branching off at various
angles and curves. Then, again, the brush disappears,
and we have a single, straight, violet colored stem,
about | of an inch long, which divides into a great
number of bright rays, radiating in straight lines from
the end of the stem, and forming a globe of white light,
about three inches in diameter : the whole resembling
a little bush of remarkably regular appearance, in
marked contrast with the curved and contorted phe-
nomena just described.

Between this, and the short brush on the opposite
electrode, a dark space intervenes, into which the rays
pass and intermingle ; the brush from the electrode R
being largely in excess of the other, and showing far
greater energy ; but more fitful, coming at first in jets,
with a spitting sound, while the other is more constant,
with a steady, hissing sound.

As explained on page 118, electric movement is from

EXPERIMENTS WITH THE TOPLER MACHINE. 135

the comb L downwards into the jar D, then along the
switch, when closed, and its connections to (7, and up-
ward out of C to the plate and carriers; and, in part,
along the electrode P towards 72, which connects with
the inside of D: the glass of the jars, and the air space
between P and R forming barriers at which electricity

42— Experiments with the Topler Electric Machine.

accumulates on the side towards which the movement
takes place, induction producing a corresponding neg-
ative on the opposite side.

D has the higher potential, as already shown, but its
inside charge is bound by an equal negative on its
outer coating; while electricity is repelled from the

130 ELEMENTS OF STATIC ELECTRICITY.

inside of C. Hence, when the switch is open, we have
the difference in the brush discharge already described.

But as the higher charge of D continues to accumu-
late on its inside coating, the tension increases on the
electrode R, till the electricity finally bursts through
the resisting air from R to P ; producing the spark and
snap when the switch is closed, as already explained.

The effect of opening the switch is to substitute for
this metal conductor, which has comparatively no
resistance, a portion of the base, which is of kiln-
dried wood, and offers high resistance. This retards
the current, producing the difference of phenomena
between the bright, instantaneous spark of white light,
with its sharp report, and the slow moving brushes of
violet light, with their hissing, spitting sounds; and
from this slow movement we are able to determine the
direction of the discharge, as already shown.

The cause of the subdivision of the spark when the
switch is open next claims attention. It has been
shown that the discharge between the inside coatings
through the electrodes P and 72, and the intervening
air space, is dependent on the counter discharge
between the outside coatings, through the switch, when
closed, or, through the kiln-dried wood of the base,
when the switch is open. This discharge may be seen
by opening the switch, half an inch or more, so that
the resistance of the air is less than that of the wood.
We then have a bright spark below, simultaneous with
the spark above. But when the switch is opened so
that the resistance of the air is greater than that of the
wood, the discharge below takes place silently through
the wood, and we have above, the subdivided, colored
discharge already described.

EXPERIMENTS WITH THE TOPLER MACHIN ^1 '

•

Witli the switch closed, reducing the resistance Below
to zero, the discharge through the air is instantaneoufc^V/
and there is seldom any subdivision, except that a long
spark from a heavy charge sometimes divides into two,
slightly separated during a part of their course. But,
with the switch open, the high resistance retards the
lower discharge, which is compelled to force its way
slowly through the kiln-dried wood; making the
change of potential between the outside coatings slow
and gradual, and producing a similar effect on the
inside coatings. Now, as the spark is caused by the
electricity forcing its way through the air, whose elec-
trified molecules are at the same potential near each
electrode, and hence self-repellent, while the surround-
ing air is at a lower potential and attractive, these
forces, acting in part at right angles to the original
impulse, during the comparatively slow progress of
the discharge, produce the brushes of diverging rays
already described. Various influences, such as currents
of air, particles of dust, and the induction of electricity
generated on adjacent parts of the machine, curve and
contort the spark, producing the peculiar phenomena
already described in connection with the brushes, and
also affecting the long blight sparks in a similar manner.

We next notice the color of the spark. Light is a
mode of motion, and its color is influenced by the
intensity of the motion. A bar of iron, dra.wn from the
furnace, ready for rolling or welding, is said to be at a
white heat; as it cools it changes to a red heat. Here
the color of the light depends on heat, which is also a
mode of motion ; and as the intensity of the heat mo-
tion decreases, the light changes from white to red of
various shades, till the bar resumes its original color.

138 ELEMENTS OF STATIC ELECTRICITY.

The brilliancy of the arc in the electric lamp is due
to the intensity of the motion, while the softer light of
the incandescent lamp results from a motion less intense.
When an electric lamp is being lighted or extinguished,
the change of color from white to the various shades of
red is evidently dependent on decrease of motion.
Must we not conclude then that the white light of the
electric spark, when the switch is closed, is due to
intensity of motion, and the colored light with the open
switch, to decrease of intensity, as in the iron bar or
the carbon of the electric lamp ? Or, if light and heat
are modes of motion, is not the evidence equally strong
that electricity is a mode of motion? Or may we not
go still farther, and say that light, heat, and electricity
are only different manifestations of that energy which
is a universal property of all matter, of which the ex-
periments here given are an additional proof? For in
the electric spark, we have light, heat, and electricity
combined.

Having stated that D is usually the jar of higher
potential, it should be explained, that there is fre-
quently a temporary reversal of potential; and, when
this occurs, all the phenomena here described are
reversed also. The cause of this reversal will be ex-
plained in connection with the following experiment.

DIRECT AND REVERSED ROTATION. — A Topler ma-
chine can be charged only by revolving the smaller
plate in a given direction; which, in the machine
represented, is shown by the arrow.

The reason is this: In order to store up electricity
in the Leyden jars, each carrier must pass from an
insulated brush, where it is charged, directly to a comb
connecting with a Leyden jar, before it passes to an

EXPERIMENTS WITH THE TOPLER MACHINE. 139

uninsulated brush, where it is discharged. Thus the
carrier TF, charged by the friction of the insulated
brush E, must pass to the comb Z, connecting with the
jar D, and give up its principal charge, before passing-
to the uninsulated brush and comb H, where its resid-
ual is discharged through the brass rod H F", which
puts it in electric connection with the carrier Z, of
opposite potential. Reverse the rotation, and the
carrier TF, starting from E^ would give up its princi-
pal charge to the uninsulated brush and comb at F~,
before reaching the comb K, connecting with the
Ley den jar (7, where only the residual would remain.
It must also be noticed that the charge is greatly
increased, both on the carrier and adjacent portion of
the plate, by passing the inductor T, attached to the
stationary plate B ; whereas, when the rotation is re-
versed, the carrier leaves the inductor and passes the
space between T and X, where the induction is almost
zero. Thus it is evident that no storage of electricity
in the Leyden jars, and hence no permanent charge can
be obtained from a reversed rotation.

HIGHER POTENTIAL OF JAR D. — It has been shown,
that from the higher position, and hence better insula-
tion of the brush E, and upper half of the revolving
plate .A, as compared with the lower position, and con-
sequent inferior insulation of the brush F, and lower
half of A, the potential of the jar .Z), receiving its
charge from the former, must be higher, as a rule, than
that of C, which receives its charge from the latter.

Repeated experiments, made by the author with a
number of different machines of this kind, fully confirm
this view. The higher potential is shown by the fre-
quent partial discharges between the inside and out-

140 ELEMENTS OF STATIC ELECTRTCITY.

side coatings of this jar ; and, in case of fracture, which
sometimes occurs from an overcharge, it is always this
jar which is broken : and the fracture always occurs on
the side nearest the opposite jar, showing that the
charge is attracted to that side, and electricity repelled
from the outside coating, creating a sufficient difference
of potential between the two coatings to overcome the
resistance of the glass and perforate it.

REVEKSAL OF POTENTIAL. — It has been already
stated that there is frequently a temporary reversal of
potential. Such a reversal can be produced, if desired,
by joining the electrodes P and 72, and reversing the
rotation of the plate till the machine is fully discharged;
then separating P and R while the reversed rotation is
continued, and then resuming the direct rotation, when
a complete reversal of potential will be found to have
occurred, which will continue till again reversed by a
similar experiment, or till the machine has had a period
of rest. The explanation is as follows: —

When P and R are separated, and the rotation re-
versed, the same causes which before operated to raise
the potential of D above that of 67, now operate to raise
the potential of O above that of 7), but in a very lim-
ited degree. For, as already shown, any carrier, as W,
charged by the brush E, would now give up its princi-
pal charge to the brush and comb at V; but the resid-
ual, slightly increased by the inductor X, would be
given up, through the comb TTto the jar O; while the
opposite carrier Z, would give up its principal charge
at H, and carry its residual to the comb L, and the jar
Z>, after a slight increase by the inductor T. But the
difference of insulation between the upper and lower
parts affect these residual discharges in the same man-

EXPERIMENTS WITH THE TOPLER MACHINE. 141

ner as the principal discharges, and hence operate to
make the potential of 6Y, receiving its charge from
above, higher than that of Z>, receiving its charge from
below. This residual is not sufficient of itself to bring
the machine into action, but it creates a slight differ-
ence in favor of (7, sufficient to sustain a reversal of
potential when the direct rotation is resumed.

THE F ARABIC CURRENT. — The faradic current con-
sists of a series of electric impulses following each other
with great rapidity. It is obtained from the battery
and coil by a spring vibrator, which opens and closes
the circuit; and from the magneto-electric machine by
a revglving electro-magnet and commutator.

Both these instruments have, for many years, been
extensively used in medical practice ; but the use of a
static machine for this purpose is quite recent, and the
switch, on the machine here represented, affords special
facilities for producing and utilizing this current. In
Fig. 42 are shown two sockets, on the front edge of
the base, connecting with the terminals of the switch,
into which are inserted the tips of conducting cords, to
the outer extremities of which may be attached metal
handles, as shown, or other electrodes suitable for the
use of this current, for medical or scientific purposes.

As already explained, when the machine is in oper-
ation there is a constant movement of electricity
through the switch and its connections, from 1) to (7,
while the charge is accumulating; and the counter dis-
charge through them, from 0 to D, is simultaneous with
the discharge above, from R to P. When the switch is
open and the cords attached, as shown, this discharge
must either force its way through the kiln-dried wood,
or pass out through the cords and any object connected

142 ELEMENTS OF STATIC ELECTRICITY.

with their outer terminals, according to the degree of
resistance offered by each path respectively. If a per-
son, or a number of persons with hands joined, grasp
the handles, the resistance will be less than through the
wood, and they will feel the effects of the discharge.
This discharge is regulated by the distance to which R
and P are separated. With a separation of TV of an
inch, on a large machine, the discharge is so rapid that the
distinction between the impulses can scarcely be per-
ceived ; producing a faradic current smoother than can
be obtained from the best batteries, while a separation
of i an inch produces effects which the strongest nerves
cannot endure.

This current, in its milder form, cannot be distin-
guished from that obtained from the battery, or mag-
neto-electric machine : but, in its more powerful effects,
it is more impulsive ; coming in jets, with cumulative
force, like the rapid blows of a planishing hammer. In
the battery current, the stronger effects show increased
intensity, and a greater tendency to muscular contrac-
tion ; while increase of strength in this current is
due to the slower impulses giving more time for the
accumulation of electric energy.

THE ELECTRIC BATH AND ELECTRIC WIND. — Charg-
ing a person on an insulated stool is one of the most
common experiments in static electricity, but it has
only recently come into use in medical practice; and,
instead of the stool, an insulated platform, on which
one or more persons can be comfortably seated, has
been substituted; the treatment being known as the
"Electric Bath."

When the patient is seated, as above, the electrodes
P and R, drawn out beyond sparking distance, and the

EXPERIMENTS WITH THE TOPLER MACHINE. 143

switch closed, a connection is made between the pa-
tient and the machine by a conducting cord; one end
being attached to the ball surmounting one of the
Leyden jars, and the other end to the chair. A similar
connection is made between the opposite jar and the
floor near thg platform, to create a certain degree of
induction, and so facilitate the process of charging,
which is now done by putting the machine in oper-
ation. Very little sensation is experienced from this
charge, but its effect in certain nervous diseases, which
cannot be treated with the battery, such as St. Vitus
dance, is said by medical men to be very soothing. In
other cases, sparks are drawn from the patient with the
hand or a suitable electrode, as a ball, roller, or sponge,
attached to the cord from the opposite jar, and held by
an insulating handle.

The electric wind is given by a point electrode,
attached as above, either with or without the insulated
platform. A gentle current of electrified air from the
point fans the patient, producing a delightfully sooth-
ing sensation.

Electric treatment of this kind can be given only by
static electricity, and its value must be determined by
the medical profession, among whom it is coming into
favor; being used and recommended by physicians of
eminence.

GAS LIGHTING. — Lighting the gas in churches and
public halls by electricity is commonly done by a bat-
teiy and coil, but the Topler machine can also be used
for this purpose. With either method there must be
wires connecting the generator with the chandeliers,
wires connecting the chandeliers together, and also the
separate burners; all arranged in one circuit and prop-

144 ELEMENTS OF STATIC ELECTRICITY.

erly insulated. At each burner there is a break in the
circuit, so arranged that a short spark will pass through
the gas; the ends of the wire being attached to an
insulator fitted to the burner.

With the battery there is a ground wire, and con-
nection with the gas pipe to complete the circuit; but,
with the machine, the circuit is made by two separate
wires, connecting the chandeliers with the balls sur-
mounting the Leyden jars. On account of the greater
intensity of static electricity, these wires must be thor-
oughly insulated with thick rubber tubing, wherever
they are liable to come in contact with the walls or gas
fixtures. With these arrangements properly made, it
is only necessary to close the switch, separate P and R,
to the full extent, turn on the gas, and put the machine
in operation. The resistance of the air between P and
R, being greater than the resistance of the wires and
the short breaks between their terminals at the burners,
the sparks take place at the burners, and the gas is lit.

As to the expense, convenience, and efficiency of this
system, as compared with the battery system, only gen-
eral statements can as yet be made. The first cost
would probably be about the same; after which there
would be no further expense with the machine, which,
with proper care, should remain in good working order,
for this purpose, for an indefinite term of years; while
the battery requires frequent renewal of the fluid, and
occasional renewal of the zinc, besides cleansing and
amalgamating.

As to efficiency, the greater intensity of the spark
from the machine will be evident, when we consider
that a machine of very moderate size will easily pro-
duce sparks three to five inches in length, while a very

EXPERIMENTS WITH THE TOPLER MACHINE. 145

large battery and coil would be required to produce the
same result. But this should be taken merely as an
indication of comparative intensity ; as, practically, only
very short sparks are required : so that a battery and
coil of medium size is generally sufficient.

A damp atmosphere does not affect the battery,
while it lessens the energy of the machine; and, in
unskillful hands, may interfere with its practical
efficiency. But, with either system, the person in
charge should have a thorough knowledge of its care
and management: in which case the machine can
always be kept in practical working order.

CHAPTER X.
ELECTEIC TRANSMISSION IN VACUA.

ELECTRIC TRANSMISSION IN Low VACUA. — Let a
glass tube, about thirty inches long, be provided
with brass caps at each end, fitting air tight; from
each of which a pointed brass rod projects inwards.
And let a stop-cock be attached to one of the caps, by
which the tube can be connected with an air pump, as
shown in Fig. 43.

Let the tube be insulated, and the caps connected by
conducting cords with the balls surmounting the Ley-
den jars of the Topler machine; the sliding electrodes
being separated to their full extent. When filled with
air, at the ordinary atmospheric density, it will be found
impossible to pass an electric charge through a tube of
this length: but let it be connected with an air pump,
and the air well exhausted, and a charge Avill easily pass
through. This proves that air at the ordinary density
has a much higher electric resistance than rarefied air.

But if a high degree of vacuum is produced, it will be
found much more difficult to pass the charge through ;
which indicates that a medium, consisting of some
material substance, is essential to electric existence
and movement; and that if it were possible to produce
an absolute vacuum, electricity could not pass through.

If the above experiment be performed in a dark room,
flashes of red and violet colored light will be seen to

ELECTRIC TRANSMISSION IN VACUA. 147

accompany the discharge, strongly resembling the cor-
uscations of the aurora polaris. Hence tubes, used for
this purpose, have been called aurora tubes.

GEISSLER TUBES — Improved tubes of this
kind, called from their inventor Geissler tubes,
are constructed with fine platinum wire sealed
into their extremities; the points projecting
inwards, and loops formed outside for the
attachment of conducting cords or wires. The
glass is bent into a variety of graceful curves
and folds: small tubes, bent in this manner,
being inclosed, for protection, in large straight
ones; and thus long, frail tubes are reduced to
compact, convenient forms, in which they can
be safely handled, as shown in Fig. 44.

The air is exhausted from them by a mercury
pump, after which they are hermetically sealed.
The expansion of the fine platinum wires being
very slight and nearly the same as that of the
glass, is not sufficient to cause fracture, hence
the vacuum produced in well-constructed tubes
remains permanent for years.

Beautiful fluorescent effects are obtained by
constructing such tubes of uranium glass. Sim-
ilar effects are also obtained by introducing into
them various solids and gases; as sulphate of
quinine, fluoride of boron, fluoride of silicon,
iodine, hydrogen, and nitrogen ; which give
certain characteristic colors, when subjected to
electric action. ^g. 43-

The effect of the discharge is greatly increased Tube-
if a break be made in the connection between one end
of the tube and the machine, so as to introduce a short

148 ELEMENTS OF STATIC ELECTRICITY.

Fig. 44— Geissler Tubes.

ELECTRIC TRANSMISSION -IN

149

ix</A0
spark into the circuit. The electricity then accumulate

and a rapid succession of brilliant discharges is the resul

The same effect can be produced by opening the
switch, connecting the tube with its terminals, arid
slightly separating the sliding electrodes P and R.

When the spark between P and R is apparently
continuous, the pulsations in the induced discharge
through the tube are distinctly visible in the alterna-
tions of light and shade; proving that the discharge
consists of a series of distinct impulses.

Fig. 45— Rotary Movement in High Vacua.

ELECTRIC TRANSMISSION IN HIGH VACUA. — The re-
sidual air in the ordinary Geissler tube is about jo^An
of an atmosphere, but Crookes has produced tubes in
which the residual is less than Tutr&injtf °f an atmos-
phere ; and the electric discharge in such tubes presents
certain peculiarities not observed in ordinary vacua.

Electric action on substances inclosed in such tubes,
and on the glass itself, is increased in the ratio of the in-
creased vacuum; since those substances receive the force
of energy which, in lower vacua, is expended on the air.

This increased action is shown by an increase in the
light and heat developed in them, and in the attractive

150 ELEMENTS. OF STATIC ELECTRICITY.

force exerted on them, as shown when they are free to
move.

Fig. 45 shows such a tube, having a glass railway on
which is placed a roller with mica vanes, and the
electrodes so placed that the upper vanes are, in line
between them. When an electric current passes
through the tube, these vanes, being at zero potential,
are attracted by the higher potential of the positive
electrode, producing a rotary movement of the roller
from negative to positive ; the force being sufficient to
move it up an incline.

Fig. 46 — Rotary Movement Reversed.

Fig. 46 shows a tube in which a wheel with mica
vanes is so mounted that its center is in line between
the positive electrode, and the center of the negative.
The negative electrode a b is cup-shaped, and its con-
cave surface turned towards the positive : so that the
lines of force may be brought to a focus, and concen-
trated on the vanes. And between its center and the
wheel is placed the mica screen c d.

A magnet, #, is suspended above the tube, between

ELECTRIC TRANSMISSION IN VACUA. 151

the screen and negative electrode, in such a manner that
it can be turned so as to reverse the position of its poles.

By this means the electric current may be attracted
or repelled, so as to pass over or under the screen.
When it passes over the screen, the upper vanes are
attracted towards ihe positive electrode, producing
rotation of the wheel in accord-
ance with such movement : but
when the position of the magnet
is reversed,the current is repelled
and passes under the screen, and
the lower vanes are attracted,
reversing the rotation.

The glass in these tubes is
usually of very low insulating
power, much lower than that
of air at the ordinary density.
Hence the electric resistance in
high vacua, being much greater
than in the glass, electric move-
ment takes place through the
vacua and glass respectively, in
the inverse ratio of the resist-
ance of each.

Fig. 47 represents a tube in
which the negative electrode

. , /. IIP i . i /. Fig. 47 — Glass Illuminated

consists of a half cylinder of

aluminium, supported, near the center of the tube, on a
small glass tube, b; through which a copper wire ex-
tends, connecting the aluminium with the platinum
terminal below.

The ends of both electrodes come near the walls of
the tube; and when the electric charge passes, its prin-

152 ELEMENTS OF STATIC ELECTRICITY.

cipal effect is produced on the glass, which gives a brill-
iant green light ; the illuminated surface terminating in
points near the extremities of the negative electrode.

The influence of induction on the walls of the tube,
as well as the conductivity of the glass, is illustrated in
Fig. 48 ; which represents a pear-shaped tube, having
for its positive electrode an aluminium cross, £>, placed
near its broad end; the negative electrode a being
cup-shaped as in Fig. 46. This cross is hinged at bot-
tom to the platinum terminal; so that, by a movement
of the tube, it can easily be brought to a horizontal or
a vertical position.

Fig. 48— Inductive Action of Metal Screen.

When the charge is passed through the tube, the
cross, when vertical, as shown in the cut, exerts a strong
inductive influence on the broad end of the tube, to the
left; over a space inclosed by lines extending over its
edges, from the negative electrode : repelling electricity
from this space, and screening it from the action of the
negative electrode, which attracts electricity from the
other parts of the tube, and from the surrounding air.

ELECTRIC TRANSMISSION IN VACUA. 153

Hence electric action within this space is neutralized ;
producing the dark shadow c d shown on the broad
end ; while the rest of the tube is illuminated.

When the screen is thrown down a luminous cross
takes the place of the dark shadow: but this higher
illumination soon fades, since electric action on this
space is now the same as on the rest of the tube.

If the tube be used again, after a period of rest, the
shadow can be reproduced ; but is never so strong as at
first. This proves that the glass has been subjected to
an electric strain, which has permanently lessened its
insulating power.

The illumination of the glass is due to its resistance ;
just as the bright spark is due to the resistance of air
at the ordinary density, and the faint glow, to the
reduced resistance in vacuum. Hence, when electric
action begins, after the screen is thrown down, the
resistance being greater on the spot which was pro-
tected by the screen, we have the bright cross where
the dark one was : but when the electric strain has so
affected the relations of the molecules to each other, as
to lessen the resistance, this first bright glow ceases, and
the illumination is the same as in the rest of the tube.

This action on the glass, as shown in Figs. 47 and 48,
is accompanied with heat as well as light ; the- tube
shown in Fig. 47 becoming intensely hot, at those
points where the greatest electric energy is concen-
trated.

Fig. 49 represents a tube constructed to show this
heating effect in a very striking manner. Its upper
part is enlarged into a globular form : and, at the bot-
tom, is the concave negative electrode, of aluminium,
already described ; which is so placed that it brings

154

ELEMENTS OF STATIC ELECTRICITY.

the lines of force to a focus on a piece of iridio-platinum,
£>, placed in the center of the globe. This, being a
metal of high resistance, becomes white hot under the
electric action ; glowing with intense brilliancy, and
finally melting.

The walls of the globe, being remote from the line

between the electrodes,
which is comparatively
short, the glass is less af-
fected than in the long
narrow tubes: so that elec-
tric action is chiefly con-
centrated on the object at
the center.

Crookes attributes all
these phenomena to the
impact of the residual air
molecules, which he desig-
nates as "radiant matter";
and claims that the mole-
cules move independently
of each other, and are
driven with such force
against the glass and other
objects, as to produce the
various phenomena de-
scribed.

Gordon considers this theory reasonable, and elabo-
rates it at considerable length : but it is not generally
accepted ; and it is believed that the explanations here
given will be found more in accordance with well estab-
lished electric principles.

Fig. 49-Heat Produced in High
vacua.

CHAPTER XL
ELECTROMETERS.

PROGRESS in every department of science is largely
dependent on exact measurement, since it is only by
this means that we get an accurate knowledge of
relative values. The thermometer enables us to in-
vestigate the laws of heat; the barometer gives us a
knowledge of atmospheric pressure, and the various
matters relating to it. And in chemistry and astron-
omy almost every step depends on such measurement.
Even our ordinary business transactions, and the value
of our currency, are regulated by the common scales,
by which we measure the force of gravity.

Electric science is no exception to this rule. We
require to know, accurately, relative differences of
potential; the conductivity and resistance of various
substances ; the force of electric attraction and repul-
sion, the comparative energy of the various instruments
used for generating and accumulating electricity ; and
other matters of similar importance.

But electric measurement presents peculiar difficul-
ties not met with in the measurement of other forms
of energy. In the measurement of gravity, we deal
with a force easily controlled, the direction of whose
movement is always known, and which, on the various
parts of the earth's surface, is subject to but slight
variation.

156

ELEMENTS OF STATIC ELECTRICITY.

In heat we have a force, susceptible of easy control ;
its movement slow, and its direction easily ascertained.
But electricity moves with the rapidity of thought;
its direction is difficult to ascertain ; and it defies our
utmost efforts at absolute control; so that the results
of measurement, by our best constructed instruments,
fall short of perfect accuracy.

In static electricity less progress has been made in
measurement than in other forms of electric energy,
whose practical applications are more numerous.

The electroscope, sometimes
classed with electrometers, in-
dicates the presence of an elec-
tric charge, but cannot be said
to measure it, except as such
indication may show an in-
crease or diminution of a light
charge. Lane's unit jar may
be considered an electrometer,
and the methods of measure-
ment by it, and by sparks
from the Holtz and Topler
machines, belong to the same
class : but both methods are
very inaccurate, and can be
used only in special cases.

COULOMB'S TORSION BALANCE. — To Coulomb is
due the credit of the first efforts at accuracy in elec-
tric science ; and the torsion balance, which is still
extensively used, was his invention and may properly
be regarded as the first electrometer.

It is represented by Fig. 50; and consists of a glass
cylinder A J., to the top of which is attached, at the

Fig. 50— Coulomb's Torsion
Balance.

ELECTROMETERS. 157

center, a glass tube D D, to each end of which is fitted
a brass collar. An enlarged section of the upper end
of this tube and its attachments, representing what is
known as the torsion head, is shown separately ; in
which it will be noticed, that the brass collar a has
fitted to it a cap b with a projecting rim ; on the
upper surface of which is a graduated scale, of 360
equal divisions. This cap is capable of being turned
horizontally, so as to bring the several divisions of the
scale under a pointer <?, attached to a.

In the center of b is a close fitting brass rod c?,
with a broad head by which it can be turned, when b
is held firmly ; or the rod may be allowed to turn with
b. Attached to this rod is a fine wire, which sustains,
at its lower extremity, a horizontal shellac rod /,
carrying at one end a small gilt ball g. Opposite this
ball, on the cylinder A A, is & graduated scale a a,
having 360 divisions, to correspond to those of the
upper scale. Opposite the zero of this scale is a gilt
ball gf, of the same size as the other gilt ball, and sup-
ported by a shellac rod /", by which the ball can be
introduced through an opening in the top of A A.

The instrument is supported on a base, having level-
ing screws ; and the air, in the interior, kept dry with
chloride of calcium.

To use this instrument, the cap b is turned till the
zero of the upper scale is brought under the pointer c.
The rod d is then turned till the movable ball g just
touches the fixed ball </, without torsion of the wire.
The zeros of the two scales will then be practically in
the same vertical plane.

The fixed ball cf is then taken out, electrified, and
replaced as before, in contact with the movable ball g.

158 ELEMENTS OF STATIC ELECTRICITY.

Both being the same size, the charge is equally divided
between them ; and, being at the same potential, the
movable ball g is repelled to a distance indicated by
the number on the lower scale : at which point the
force of repulsion is balanced by the torsion of the wire.

The cap b is then turned in opposition to the repul-
sion, so as to bring the ball g nearer to g'\ the distance
being indicated on the upper scale. The torsion of the
wire is thus increased, and repulsion again balanced by
torsion in the new position.

It is known that the force of torsion is proportional
to the angle of torsion : and since this force has to be
increased to oppose the increase of repulsion, as the
balls are brought closer, the point to be determined is
the ratio of increase of force, as compared with the
reduction of distance between the balls; which is done
by comparing the distances from zero, indicated on the
upper and lower scales.

The following is one of Coulomb's experiments for
this purpose : The first distance to which the movable
ball g was repelled being 36°, it was found necessary,
in order to reduce this distance to 18°, to turn the cap
b through 126°; and to reduce the distance to 8£° re-
quired an additional rotation of the cap through 441°.

The distances 36°, 18°, and 8i° are to each other,
practically, in the ratio of 1, i, and I ; and the forces
of repulsion at these points were balanced by torsions
of 36°, of 126°+ 18°= 144°, and of 441° 4 126° -f 8£°=
5751°, respectively.

Now since 144 = 4x36, and 575| (practically 576)
=16x36, it will be seen that as the distance between
the balls is divided by 2 or by 4, the force of repulsion
is multiplied by 4 or by 16 ; and thus Coulomb proved

ELECTROMETERS

159

that electric repulsion varies inversely as the square of the
distance.

INACCURACY OF THE TORSION BALANCE. — In the
use of this instrument, as above, the arc is assumed as
the distance between the balls, while the actual dis-
tance is the chord of the arc ; but since these distances
are in the same proportion, the accuracy of the results
is not affected.

It is also assumed that the arm of the lever, by which
repulsion produces torsion, is the distance from the
center of motion to the cen-
ter of the ball g. But this
is true only when the balls
are in contact. In every
other position, this arm is
represented by a perpendic-
ular from the center, on the
chord which cuts the centers
of the two balls : and as the
ball g moves round, and the
chord increases in length,
this perpendicular decreases ;
and vanishes when the chord
equals the diameter.

This is shown in Fig. 51, where b represents the first
position of the balls, when the arm equals a b : but
when g moves round to 6*, the line a f represents the
arm ; and when it moves to d, the short line a m rep-
resents the arm ; and at n the arm vanishes.

This may be made more plain, by considering that
the ball g is moving under the influence of two forces,
electric repulsion, and the rigidity of the shellac rod,
by which it is held at a fixed distance from the center.

Fig. 51 — Arm and Angle of
Repulsion Illustrated.

160 ELEMENTS OF STATIC ELECTRICITY.

When motion begins, at 5, these forces act at right
angles to . each other but as the ball moves round, the
angle of repulsion constantly decreases; being repre-
sented at £, by the angle a b c; and at d, by the
angle a b d; and vanishing at w, where the two forces
are in direct opposition.

In this position the force of repulsion opposes further
movement : for, as it radiates equally in every direction
from the two balls, its force on opposite sides of n is
equal. But since, in the experiment given, the greatest
angle was 36°, which is only one-fifth of the semi-circle,
the error is not sufficient to affect the result seriously.

Another inaccuracy results from lack of rigidity in
the fine wire, which causes it to deviate slightly from a
true vertical, under the influence of repulsion ; moving
its lower extremity out of the center.

There is also a slight inaccuracy resulting from the
force of repulsion being estimated from the centers of
the balls, instead of from their nearest points.

It is also assumed that electric repulsion remains
constant during the experiment: which would not be
strictly true; since there is a continual reduction of
energy, from causes already explained, which would
produce serious error, if the experiment were of long
duration.

Since each ball becomes a center of electric radiation,
it is evident that the lines of force cut by each repre-
sent but a very small part of the entire repulsive energy.
But since the balls are of equal size and equal poten-
tial, it may be assumed that the proportion between
the energy actually measured, and the entire energy, is
the same in each ball. But an instrument embracing
all the lines of force would evidently be more reliable.

ELECTROMETERS. 161

These inaccuracies doubtless account fWcJhe slight
error observed in Coulomb's experiment, am^ifrnd to
confirm the correctness of his results by showing suf-
ficient cause for the error.

ATTRACTED-DlSC ELECTROMETERS. — Sir W. SnOW

Harris was the first to construct an electrometer on the
attracted-disc principle. His instrument consisted of a
scale beam, carrying at one end a pan for the weights,
balanced at the other end by an insulated metal disc, sus-
pended horizontally over a similar fixed, insulated disc.

An electric charge being given to the lower disc, the
force of attraction between it and the upper disc was
measured by weights placed in the scale pan.

The rapid loss of charge, from the edge of the elec-
trified disc, was the chief objection to this instrument.
But the principle has been adopted, and the construc-
tion improved by Sir William Thomson, whose instru-
ment, shown by Fig. 52, is described as follows: —

THOMSON'S ABSOLUTE ELECTROMETER. — This in-
strument consists of two distinct parts; one for testing
and maintaining a certain constant auxiliary potential
V, and the other for determining, in absolute measure,
the difference between the potentials of any two given
conductors. The first of these parts embraced a Ley-
den jar, forming the case of the instrument, an idio-
static gauge, -and a replenisher E.

The Leyden jar is a glass cylinder, closed at top and
bottom by metal plates; and coated, inside and out,
with tin-foil, in which openings are left for viewing the
internal parts; and an uncoated surface, for insulation,
left at the top and bottom, between the inner coating
and the metal plates.

The idiostatic gauge will be understood from Fig.

162 ELEMENTS OF STATIC ELECTRICITY.

Fig. 52— Thomson's Absolute Electrometer.

ELECTROMETERS. 163

53. A small aluminium plate P is fitted to a square
hole in the metal plate 6r, like a trap-door, without
touching the edges. To one side of P is attached an
arm A, of the same material, enlarged at its junction
with P, and bent, so that when the surfaces of P and
Gr are in the same plane, the arm is elevated a little
above 6r, and is parallel with it.

A platinum wire / stretched between two supports,
attached to (7, passes through the enlarged part of the
arm ^, over a slight projection ; supporting P, and, by
its torsion, regulating its movements. At the outer
end of the arm is a fork
F ; and between its
prongs is a little white
enameled standard, at-
tached to G- ; having,
on its outer face, two
black dots, close to-

gether, and in the same Fig 53_The Idiostatic^T
vertical line. A black
hair, stretched across the fork, and viewed through the
lens Z, moves up and down in front of the dots ; and
conies exactly between them, when the surfaces of P
and G- are in the same plane. This is called the sighted
position.

Under the plate G- is seen, in Fig. 52, a circular
metal plate F, supported on a metal rod, attached to
the metal plate A, which is in contact with the inner
coating of the Ley den jar; so that A and Pare always
at the same potential V, as this coating. The distance
between F and 6r is so regulated, that when the poten-
tial of F is V, its attraction for the plate P overcomes
the torsion of the platinum wire, and keeps P in the

164

ELEMENTS OF STATIC ELECTRICITY.

sighted position : and, in this way, the constancy of the
potential J^is tested.

This constancy of potential is maintained by the
replenisher seen at JS, which is practically a small To-
pler machine ; and is shown separately in Fig. 54. A
and B are two insulated metal inductors, to which are
attached two receiver springs a and b. C and D are
two contact springs, in electric connection with each
other, but insulated from the other parts.

P and Q are two metal
carriers, attached to an eb-
onite cross-piece, through
which passes the ebonite
axis T, which can be ro-
tated by the milled head
E: so that the carriers
P and $, revolving inside
the inductors A and B,
shall successively touch
the springs a, J>, b, 0.

When the replenisher
is in its place, as shown

in Fig. 52, the inductor A is put in electric connection
with the disc A ; which is supported in connection with
the inner coating of the Leyden jar : while the inductor
B, being in contact with the cover, is in electric con-
nection with the outer coating. And since the replen-
isher operates on the principle of the Topler machine,
already described, its rotation, either direct or reversed,
will raise or lower the potential of the jar : and so keep
the potential of the plate A, and of the idiostatic gauge,
connected with it, at the constant potential F", as
shown by the gauge.

Fig. 54— The Replenisher.

ELECTROMETERS. m 165

The second part of the electrometer consists of the
apparatus for expressing differences of potential, be-
tween conductors, in absolute measure. The metal
plate A, called the guard plate, has, at its center, a
circular opening about If inches in diameter, to which
is fitted the disc (7; which just fills it without touching
the edges; and is made of thin aluminium, flat and
smooth on its under side, but strengthened by a rim,
and radial arms, on its upper side. It is supported by
three light steel springs, shaped somewhat like tuning-
forks, and placed horizontally, at equal distances apart ;
one of which is shown at S. The lower end of each is
attached to the center of (7, and the upper end to a
brass socket, which is cemented to the lower end of a
glass rod, shown at /; which insulates it from the
metal rod above ; to the lower end of which the glass
rod is attached. And the metal rod is moved vertically
in guides by the micrometer screw M ; the movements
being registered by the scale 6r, and the graduated disc
D.

To the center of the disc 0 is attached a fine hair:
in front of which a lens, II, is so placed as to form, at
its conjugate focus, near the surface of the jar, an
image of the hair ; which may be viewed through the
eye-piece at L. This image is seen exactly between
the points of two screws K, when the lower surfaces
of the disc (7, and guard plate A, are in the same plane :
which is called the sighted position.

On a support below A, is the metal disc B, known
as the attracting disc ; insulated from the jar, and mov-
able vertically by the micrometer screw M' ; the move-
ments being registered by the scale R, arid the grad-
uated disc T. It is connected with the electrode -ZV", by

166 ELEMENTS OF STATIC ELECTRICITY.

which it can be put in electric connection with bodies
whose potential is to be tested.

The attracted plates P and C are really movable
centers of the guard plates Cr and A; and since loss of
charge, from radiation and otherwise, affects chiefly the
outer edges, the small centers are practically unaffected
by such loss. Hence the large discs Cr and A are
appropriately called guard plates.

MODE OF USING THE ABSOLUTE ELECTEOMETEE. —
The plates are first brought to zero potential, by put-
ting them, for an instant, in electric connection, by the
electrode -ZV, connecting with Z?, and a wire connecting
with A through the cover. The disc O is then brought to
its sighted position by the micrometer Jf, and the read-
ing noted. A known weight, w, is then placed upon it
so as to depress it below the level of the guard plate
A; and M is turned till C is again raised to its sighted
position : the reading is noted, and the weight removed.

The Leyden jar is then charged to potential V, as
determined by the idiostatic gauge, and kept constant
by the replenisher, during the experiment. The disc
B is now put into connection with the outside coating
by the electrode N; and the micrometer Mr turned till
the attraction of B on the disc C brings it again to its
sighted position. Hence the attraction of B is known to
be equal to the weight w. This reading being noted, B
is insulated, and the bodies, the difference of whose po-
tentials x and z is required, are successively put into
contact with B through N. The distances d and h
through which B has to be moved to bring the disc (7,
in each case, to its sighted position, are noted, and the
difference of potential can then be calculated.

FOE CHAEGED SUEFACES. — With a

ELECTROMETERS. 167

given charge, the electric energy at any point on a con-
ductor, called its surface density, is in proportion to its
surface area. Let Q represent the surface density, then
the electric force, exerted by a charged conductor on a
point near it, equals Q multiplied by the surface area.

On a sphere the surface equals the square of its ra-
dius multiplied by 4x3.14159. If 3.14159 = ^, and
radius = 1, we have I2 x 4 a = 4 n. Hence the force
exerted by a charged sphere on a point near it equals
4 7t Q ; and the force exerted by a charged hemispher-
ical surface equals 2?tQ.

The hemispherical surface may be considered as
made up of the bases of an infinite number of small
cones, having their apexes at the center. Hence each
base subtends a solid angle : and lines of force, extend-
ing from surface to center, are everywhere normal to
the surface.

Now if we conceive a plane surface applied to the
hemispherical surface, and these cones extended to
meet it ; we find that the lines of force, extending from
these bases, are oblique to the plane surface. Hence
each one can be resolved into two components, one
normal to the plane, and the other acting along it.
But since there are an infinite number of these cones,
the lines of force from whose bases may all be resolved
in this way; the components along the plane, all
around, neutralize each other, leaving only the normal
components; whose force equals the sum of all the
solid angles multiplied by the surface density, which, as
we have seen, equals 2 TIQ. Hence the expression is
the same for a plane or a hemispherical surface.

APPLICATION OF FORMULAE TO MEASUREMENTS
BY ELECTROMETER. — When there are two discs, at

168 ELEMENTS OF STATIC ELECTRICITY.

different potentials, near each other, as A and B in the
electrometer, the attraction of each for the other is
equal ; the air being the dielectric between them.
Hence the force, exerted at any point between them,
equals the force on both surfaces, represented b}T ^TIQ;
and tends to draw the movable disc C towards B.
But this force is also equal to the difference of poten-
tial, divided by the distance between the discs. Hence
when x represents difference of potential, and d the

or
distance, the resultant force, at any point, equals — .

x x

Hence 4 ?*(>=— , and Q =
a

Now if the surface of the movable disc O be repre-
sented by s, its attractive force will equal s Q : hence
the total attractive force equals 2 TCQ x s Q = 2 it s if.

nr

And substituting for Q its value, -7 — — , we have

4t Tt Ct

/ x V 0 x2 sx2

2^M A - -j)=^nS 1X 2^2=Q - ^2'

\4 it d' 16 a* d2 Sad2

Now since the attractive force equals the weight w,
multiplied by the acceleration produced by gravity,

s x2

represented by #, we have w g = - — : therefore x

"

n

= d — ^ (1), which expresses x in absolute meas-

ure. But x represents the potential of the first body
tested by the electrometer.

By a similar process the potential, 2, of the second

body is expressed by the equation, z = h I— ^ (2).

Subtracting (2) from (1), we have x — z = (d — Ji)
n w g

J

ELECTROMETERS. 169

By substituting figures for the letters in the second
member of this equation, the difference of potential, of
any two bodies we wish to test, may be expressed
arithmetically. ,—

I O 7t W C/

The expression J ^- is constant ; since it rep-
resents the attraction of the disc B for (7, when the
Ley den jar is at the constant potential, V: while the
expression (d — A) is variable; representing the differ-
ence of distance, required by the variable difference of
potential, expressed by x — z.

THOMSON'S QUADRANT ELECTROMETER. — This in-
strument, invented by Sir William Thomson, is highly
esteemed for its great sensitiveness. It is represented
by Fig. 55, and consists of a frame supporting a Leyden
jar, which resembles an inverted glass shade, with a
brass cover, to which the principal parts are. attached.

These consist of the idiostatic gauge and replenisher,
already described, and the quadrants and needle, and
parts connected with them.

The jar contains strong sulphuric acid : which forms
the inner coating, keeps the interior free from moist-
ure, and forms a perfect connection with the needle,
without friction. The outer coating consists of strips
of tin-foil, connected with the cover and supporting
frame. The upper part of the jar incloses the quad-
rants and needle ; protecting the needle from currents
of air, and permitting its movements to be seen.

Fig. 56 is an enlarged view of the needle and quad-
rants. The needle is a thin, flat piece of aluminium,
shaped like a figure 8 ; represented by the dotted lines
in Fig. 56 ; and seen edgewise in its place at w, in Fig.
55. Through its center passes a piece of stout platinum
wire to which it is attached, and which terminates

170 ELEMENTS OF STATIC ELECTRICITY.

above in a small, T- shaped piece of metal : to which
are attached, at the extremities of the cross piece, two
fibers of unspun silk ; by which the needle is suspended

Fig. 55 — Thomson's Quadrant Electrometer.

from a projecting arm, supported, in the upper part of
the instrument, on a vertical glass rod. When the
needle is at rest, in the fixed position between the
quadrants, as shown in Fig. 56, the silk fibers hang
parallel to each other, and the cross piece, below, is

ELECTROMETERS.

171

then parallel to the projecting arm above. But in any
other position, each fiber is at an angle with its vertical
position, and the needle slightly elevated : conse-
quently the force of gravity tends constantly to turn
the needle, without friction, back to its fixed position.
This mode of suspension is termed bifilar.

A platinum weight, suspended in the sulphuric acid
by a fine platinum wire, from the lower end of the stiff
wire below the needle, keeps
the needle in position, and in
contact with the inner coat-
ing.

The wire, above and below
the needle, is inclosed in fixed
guard tubes ; the lower one
shown at w : which screen it
from external electric influ-
ence; and furnish a COimeC- Fig. 56-Quadrants and Needle.

tion, by which the charge is given to the inner coating.

The needle is inclosed within four brass quadrants :
which, if joined, would form a circular box. They are
separated from each other, and from the needle, as
shown in Fig. 56 : and opposite pairs, A and A', B and
Bf, are connected by fine wires ; and all supported at
the same level ; and insulated, by glass rods attached
to the cover.

Three of them are permanently attached, but the
fourth can be moved in and out horizontally ; guides,
and a spring and counteracting screw, being arranged
to keep it in position, and regulate its movement.

Above the needle, and attached to its supporting
wire, is a small concave mirror t; by which a ray of
light is reflected on a scale, placed in front of it, at a

172 ELEMENTS OF STATIC ELECTRICITY.

distance of about 36 inches. This scale is shown in
Fig. 57. Behind it is a lamp, the light from which
comes through a vertical slit in a screen : above which
is a horizontal screen, which cuts off the direct rays
from below ; while the angle of reflection brings the
ray from the mirror directly on the scale, where it
appears as a small spot of light. Another screen,
placed at an angle,
cuts off the direct
rays from above.

As the mirror turns
with the needle, the
reflected ray becomes
a long pointer ; mov-
ing without friction :
by which the slight-
est movement of the
needle is indicated on

Fig. 57— Scale, Lamp, and Screen.
In the instruments

first constructed, the needle was suspended by a single
fiber of silk ; and a small magnet attached to the back
of the mirror : which, by the attraction between it and
a large magnet, placed outside the jar, as shown in Fig.
55, controlled and limited the movements of the needle ;
the attraction of the magnets tending constantly to
bring it back to its fixed position, where the spot of
light rests on the zero of the scale. But the bifilar sus-
pension is now preferred ; rendering the use of magnets
unnecessary.

At I and m. Fig. 55, are seen the chief electrodes ;
used to connect the opposite pairs of quadrants with
bodies whose potential is to be tested : and at p is the

ELECTROMETERS. 173

charging electrode, used to connect the replenisher with
the inner coating of the Ley den jar. One pair of quad-
rants, A Af, Fig. 56, is connected with the electrode ?,
and the other pair, B B', with the electrode m.

MODE OF USING THE QUADRANT ELECTROMETER.
— The Leyden jar is connected with the replenisher by
the electrode />, and charged to a certain constant po-
tential, F", as indicated by the gauge ; and its constancy
maintained during the experiment: and the needle,
being connected with its inner coating, has therefore
the same constant potential V.

By means of the electrodes I and m, a connection is
then made between the opposite pairs of quadrants, and
any two bodies \vhose difference of potential is required ;
one of which is usually the earth. Suppose the earth
connection to be made with the electrode m; then, if
the potential of the other body is higher than that of
the earth, the needle will move round from the higher
to the lower potential ; that is, from A A' to B Bf: but
if it is lower, the movement will be from. B Bf to A Af:
and the difference of potential will be indicated on the
scale by the movement of the spot of light, to the right
or left from zero ; and may be considered practically
correct, within certain limits. In this way the re-
quired potentials are compared with the constant poten-
tial V; and the results determined in absolute measure.

In the Helmholtz quadrant electrometer the quad-
rants are maintained at the constant potential ; and the
bodies whose potential is required are connected with
the needle.

There are various styles of Thomson's electrometers :
both of the attracted-disc and quadrant instruments.
Some of them are portable, and much simpler than

174 ELEMENTS OF STATIC ELECTRICITY.

those already described; the replenisher, gauge, and
Leyden jar, being omitted; also the bifilar attachment
in the quadrant instrument; the movements of the
needle being controlled by the torsion of a fine wire.
And, in the attracted-disc electrometer, the position of
the discs is sometimes reversed; the attracting disc
being placed above, in the portable style.

CHAPTER XII.
THE ELECTRICITY OF THE EARTH AND ATMOSPHERE.

POTENTIAL AND EARTH CURRENTS.

TERRESTRIAL and atmospheric electricity are so in-
timately related, that to obtain a correct knowledge of
either requires the consideration of its relations to the
other.

Viewing electricity as a universal property of mat-
ter, its existence in the earth and atmosphere follows
as a necessary consequence. Hence, we are to study
its phenomena in this connection, rather than to ac-
count for its origin. These phenomena pertain chiefly
to difference of potential between different parts of the
earth's surface; different parts of the atmosphere; and
between the earth's surface and the atmosphere.

This \lifference of potential results from various
causes. We have already seen how difference of po-
tential may be produced, artificially, by various instru-
ments, which are combinations of different substances,
having different degrees of electric resistance and con-
ductivity. By similar methods nature, on a grand
scale, produces results of which ours are but feeble im-
itations.

ILLUSTRATIONS FROM THE THERMOPILE. — In the
thermopile we have an illustration of the method by
which difference of potential is produced by heat. This

176 ELEMENTS OF STATIC ELECTRICITY.

instrument is a combination of metal bars, whose con-
ductivity for heat and electricity varies greatly. A
number of these bars, arranged in compact form, and
properly insulated, are soldered together in an alter-
nating series: so that a current of electricity, passing
through them, has to pass from one metal to the other.
They are folded together, and mounted in such a man-
ner, that heat may be applied to one set of junctions;
while the opposite, alternate set, is cooled.

In this way, instruments are constructed, in which a
very slight difference of temperature, between the op-
posite sets of junctions, creates a perceptible difference
of electric potential: and powerful batteries are con-
structed in the same manner.

The earth may be regarded as an immense battery of
this kind; being composed of heterogeneous materials,
whose conductivity for heat and electricity varies
greatly: and which are "subjected to great extremes of
temperature, at opposite junctions, fulfilling exactly
the conditions of the thermopile.

The ocean, a vast, homogeneous conductor, is sepa-
rated into different parts by the great continents; whose
conductivity differs from it greatly: the five great
divisions of the ocean, and the two continents, con-
stituting an alternating series of conductors, of differ-
ent conductivities.

The surface of the continents, composed of rock and
soil, of lakes, rivers, and sandy deserts, presents a great
diversity of material, of widely different conductivity.

In the torrid and frigid zones, we have the opposite
extremes of temperature; which, in the thermo-electric
battery, are produced by exposing one set of junctions
to the heat of a lamp furnace ; while the opposite set is

POTENTIAL AND EARTH CURRENTS. 177

cooled with ice. Similarly also in the diurnal revolu-
tion of the earth, opposite sides are subjected daily to a
constantly changing temperature. And, in its annual
revolution, we have the same result in the changing sea-
sons; which also produce great changes in the conduct-
ing character of the surface ; from the frozen, snow-
clad surface of winter, to the verdure-clad surface of
summer.

DLURNAL AND SEASONAL VARIATION. — The change
of electric potential produced by these causes in the
earth, induces the opposite potential in the atmosphere;
which, by its lower strata, is insulated from it. Hence,
in observations made on the potential of the earth and
atmosphere, we find, as we should be led to expect,
daily maxima and minima potential, and also seasonal
maxima and minima.

In several series of observations, made by different
observers in Europe, both on the continent and in the
British Isles, these maxima and minima were carefully
noted: and it was found, that, in winter, the daily
maxima occur at about 10 A. M. and 7 P. M.; in sum-
mer at about 8 A. M. and 10 P. M.; and in spring and
autumn, at about 9 A. M. and 9 P. M. The daily min-
ima occur, in summer, at about 3 P.M., and midnight;
but the daily winter minima are not given with suf-
ficient definiteness to be reliable.

From this we see, that the daily maxima, occurring
soon after sunrise and sunset, correspond to the hours
of greatest change of temperature ; while the daily
minima occur at the hours when temperature is most
constant.

The seasonal maximum occurs in winter, and the
seasonal minimum in summer: the maximum about

12

178 ELEMENTS OF STATIC ELECTRICITY.

January, and the minimum in May and June. They
are doubtless due, in part, to the different conduct-
ivity of the earth's surface in summer and winter,
as already mentioned ; and also to the dry winter
atmosphere, when atmospheric insulation is high, as
compared with the damp atmosphere of spring and
early summer, when it is low ; the greatest minimum
occurring in the months when our atmosphere, in the
north temperate zone, is most heavily laden with vapor.
At this season the earth is covered with green, suc-
culent herbage ; wet with frequent showers, and laden
at night with heavy dews ; forming a conducting sur-
face, which offers but slight resistance to electric trans-
mission.

Towards the close of summer, the grain ripens, the
showers become less frequent, the dews lighter, and a
vast expanse of dry straw and stubble, with a parched
soil beneath it, offering high electric resistance, takes
the place of the former conducting surface. As fall ad-
vances, and the grass becomes dry and withered, and
the trees shed their leaves, there is a constant increase
of this surface resistance, and a corresponding increase
of electric potential, till the winter maximum is reached.

While this difference of conductivity in the land
surface is taking place, the conductivity of the water
surface remains practically constant : hence the period
of minimum potential corresponds to that in which the
difference of conductivity, between the land and water
surfaces, is least; while the period of maximum poten-
tial corresponds to that in which it is greatest ; point-
ing clearly to this difference as a probable cause.

In addition to the changes of electric potential, in-
duced in the atmosphere by these changes in the elec-

POTENTIAL AND EARTH CURRENTS. 179

trie condition of the earth's surface, its electricity is
doubtless affected, directly, by conditions similar to
those which affect the earth's electricity.

In its combination of dry air arid watery vapor ; the
one, an insulator, and the other, a conductor ; separate
parts heated and cooled alternately, twice in twerrty-
four hours, we have thermo-electric conditions similar to
those already noticed in the earth's surface ; though
the resulting electric disturbance is, perhaps, less in-
tense, as the composition of the atmosphere is nearly
uniform, while that of the earth's surface presents great
diversity.

DIFFERENCE OF POTENTIAL BETWEEN ATMOS-
PHERIC STRATA. — Another cause of atmospheric elec-
tric disturbance is found in the great difference of
electric resistance between the upper and lower atmos-
pheric strata; caused by the density below and rarity
above. This resistance makes the dense lower stratum,
where most of our observations are made, an excellent
insulator ; while the rarity of higher strata facilitates
electric transmission ; a constant decrease of resistance
taking place, from the lower to the higher, till a point
is reached, where it is reduced to that of the ordinary
Geissler tube ; while, in still higher strata, the resist-
ance increases, on account of the extreme rarity of the
air : which equals, and finally exceeds, that of the best
vacuum tubes.

The existence of a corresponding difference of elec-
tric potential has been proved by numerous experi-
ments ; among which may be noted the following : —

From an elevated position, a metal - pointed arrow
was shot upward to a vertical height of 250 feet: a con-
ducting cord, connected with it, and properly insulated,

180 ELEMENTS OF STATIC ELECTRICITY.

communicated with an electroscope at its lower extrem-
ity. As the arrow rose, the electroscope showed a
steadily increasing difference of potential, till the
indications equaled the full capacity of the instrument.

The arrow was then shot horizontally, at an eleva-
tion of about three feet, but no change of potential was
indicated ; proving that the indications resulted from a
difference of potential existing in the atmosphere, and
were not due to the friction of the arrow in passing
through the air.

The difference of potential, in this experiment, was
between the earth and atmosphere : but the following
experiment was entirely independent of the earth.
During a balloon ascent, a conductor, 170 feet in length,
was lowered into the air ; a ball being attached to its
lower end, and its upper end connected with an elec-
troscope. The indications showed a marked difference
of potential between the upper and lower strata.

As the balloon moved with the wind, the friction
between the ball and the air could not have been suf-
ficient to affect the electroscope perceptibly ; so that,
in this instance, as in the former, the indications of the
instrument must be attributed to a difference of po-
tential existing in the atmosphere.

The series of observations already referred to, and
numerous others of a similar character, prove that the
potential of the atmosphere is almost invariably positive
with reference to that of the earth.

THE ATMOSPHERE AS A LEYDEN JAE. — It is evi-
dent that we have, in the atmosphere and on the earth's
surface, the same conditions which exist in the Ley den
jar — tvvo conducting surfaces insulated by a dielectric ;
the stratum of least resistance forming the upper con-

POTENTIAL AND EARTH CURRENTS. 181

ducting surface; the earth's surface, the lower one; and
the dense lower stratum, the dielectric. And, as in
the Leydeii jar, any change of potential in either sur-
face produces the opposite electric condition in the
other surface.

The upper surface, being insulated, corresponds to
the inner coating; and the lower uninsulated surface,
to the outer coating. But since those surfaces are of
vast extent, any limited area of upper surface would be
connected with a conducting surface at its outer edges ;
through which connection electricity would be repelled
from this area, or attracted to it, as the potential of the
surface below it had a greater or less intensity. But
the earth connection, of the lower surface, would be
exactly the same as that of the outer coating of the
Leyden jar.

We live and move on the outer coating of this Ley-
den jar; on a surface practically equipotential within
limited areas ; and hence do not perceive electric action
taking place, no matter how highly charged the jar
may be, except when the tension becomes strong
enough to overcome the resistance of the dielectric, or
to render prominent or visible the action on either
side of it:

This surface then, which we call neutral, is really a
charged surface ; but, like the outer coating of a
charged Leyden jar, quiescent, till brought into action
by connection with the inner coating, or by induction
between the two.

ASCENDING AND DESCENDING CURRENTS. — We
have seen how air currents are produced by the action
of an electric machine, and how light bodies vibrate
between electrodes connected with opposite surfaces of

182 ELEMENTS OF STATIC ELECTRICITY.

a charged Leyden jar. Now since a constant difference
of potential is proved to exist between the earth's
surface and the atmosphere, and between upper and
lower atmospheric strata, we must conclude that
ascending and descending currents result from this
difference : and that the clouds, and the invisible vapor
diffused through the air, are, like the air, subject to this
constant electric movement. But, there being also a
horizontal movement, due to the winds, the resultant
of the two movements is a series of curves, ascending
and descending, as the body of air and vapor moves
over areas of high or low potential.

The air and vapor in contact with the earth, becom-
ing electrified to the same potential as the earth's sur-
face, are repelled, and attracted upward by the force
resulting from difference of potential in the stratum of
least resistance above. Similarly the air and vapor
above are repelled, and attracted downward in conse-
quence of the difference of potential below.

The morning and evening maxima, occurring at
opposite points in the rational horizon, show that two
electric waves traverse the surface daily from east to
west, as the earth revolves from west to east. And, at
points about equally distant from these waves, follow
the two daily minima. During the maxima the ascend-
ing and descending currents must acquire a great
increase, both in volume and in acceleration of move-
ment: while the minima, preceding and following,
create horizontal movements between the areas of high
and low potential ; producing resultant curves, similar
to those due to the winds, but recurring in regular
succession. In fact these currents are themselves
electric winds.

POTENTIAL AND EARTH CURRENTS. 183

The rarefying of the air from heat, at the time of the
morning maximum, must increase and accelerate the as-
cending current, while its condensation from cold, at the
evening maximum, similarly affects the descending cur-
rent ; gravity in each case supplementing electric force.

COSMEC ELECTRIC INFLUENCE. — Assuming that
electricity is a universal force, acting through matter
in different forms, as a universal medium, it follows
that electric induction is universal. Hence induction
between our planet and the other members of the solar
system, especially the sun and moon, must affect the
electric condition of the earth and atmosphere.

It is considered a well established principle, that the
tides are due to the attraction of the sun and moon,
attributed to gravity. But the daily electric maxima
and minima indicate that there are electric tides, coin-
cident with the ocean tides, due to the electric induc-
tion of the sun and moon : that an electric impulse
follows the earth's movement, as different portions of
its surface are successively exposed to this influence
during its daily rotation, producing electric currents
in both the land and water surface ; and perhaps also
tidal waves in the ocean and atmosphere.

We have seen that when a charged sphere is placed
near the end of a cylinder, or of the longer axis of a
spheroid, the electricity of the cylinder or spheroid is
either repelled or attracted by induction, according as
the potential of the sphere is positive or negative, \vith
reference to that of the other body ; and that this effect
is intensified when two charged spheres, at different
potentials, are placed at opposite ends of the cylinder,
or longer axis of the spheroid. If both are placed at
the same end, the inductive effect is a mean between

184 ELEMENTS OF STATIC ELECTRICITY.

the two effects. But if one be placed opposite the
center of the cylinder or spheroid, so that its action is
at right angles to that of the other, the intensity of
action at the ends is diminished.

In the sun, moon, and earth, these conditions are
exactly fulfilled as to shape and position ; and, prob-
ably also, as to difference of potential. The earth is
an oblate spheroid, whose longer axis lies east and west ;
pointing nearly to the apparent path of the sun and
moon. Hence at the full moon, the new moon, and
the quarters, we must have the same inductive effects
as in the experiment with the spheroid and the two
spheres. The earth, at full moon, is between the sun
and moon, and receives the highest inductive effect.
At new moon they are on the same side of it, and
nearly in line, and their effect, if at different potentials,
is lessened : while, at the quarters, when the induction
from each is at right angles to that of the other, it is at
its minimum. Hence we should expect to find, as in the
ocean tides, electric neap and spring, ebb and flood tides.

Very little is known of the relative inductive influ-
ence of the sun and moon on the earth. Judging from
the analogy of the ocean tides, we might infer that the
induction of the moon is greatly in excess of that of
the sun. But in estimating effects produced by gravity,
the two principal factors are mass and square of dis-
tance; whereas, in estimating inductive electric effects,
the various agencies by which electricity is generated
must also be taken into account.

The nearness of the moon to the earth causes its
effect on the ocean tides to be much greater than that
of the sun, though its mass, as compared with the mass
of the sun, is only as 1 to 26,400,000. But, in consid-

POTENTIAL AND EARTH CURRENTS. 185

ering the electric influence of the two bodies, we find
that the lunar surface is that of a dead world, abso-
lutely quiescent, so far as we know; while the solar
surface, to a great depth, is in a state of the most
violent agitation. From which we must infer a great
difference of electric potential in favor of the sun.
And observation indicates that this state of agitation
affects the earth's electricity ; while we have no obser-
vations of electric effects produced by the moon.

Certain electric phenomena on the earth are found
to coincide with certain solar phenomena. These con-
sist in violent oscillations of the magnetic needle
during prominent solar disturbances, indicated by the
sun spots. And it is found that the periods of max-
imum solar disturbance, which occur once in eleven
years, are noted for corresponding maxima in those
perturbations of 'the magnetic needle.

To make this clear, it should be stated that magnet-
ism is produced, artificially, by the circulation of an
electric current around a conductor capable of being
magnetized, at right angles to its length; as by a
current circulating in a coil of wire, round a bar of
iron or steel. And, conversely, a magnet generates an
electric current in such a coil.

It is known that the earth is a great natural magnet;
having north and south magnetic poles, which exercise
a directive force on the magnetic needle ; and it seems
highly probable, that its magnetism is the result of
electric waves, or impulses, circulating round it from
east to west as has been shown; giving rise to electric
currents ; and due to difference of temperature, and to
solar and lunar influences : and that the perturbations
of the magnetic needle, coincident with solar disturb-

186 ELEMENTS OF STATIC ELECTRICITY.

ances, are the result of corresponding disturbances in
these electric movements.

OBSERVATIONS ON TELEGRAPH LINES. — The tel-
egraph affords special facilities for observing many of
the phenomena pertaining to terrestrial and atmospheric
electricity, by means of its long lines of nearly uniform
conductivity, insulated in the air, having earth con-
nections at points remote from each other, and extend-
ing, in the United States, chiefly, either at right angles
to the magnetic meridian, or parallel with it.

These facts have been recognized; and, within the
last five years, observations have been made, on a
limited scale, in the United States, and in Europe.
These observations have been somewhat desultory and
local ; no general, extended, well established system
having yet been instituted.

During the fall and winter of 1883-84, a series of
observations was made on a line belonging to the
Postal Telegraph Co.; extending, at first, from New
York City to Meadville, Pa.; 509 miles by wire, 325
direct; but subsequently completed to Chicago; 1058
miles by wire, 725 direct. The observations from Oct.
18 to Nov. 20, 1883, were between New York and Mead-
ville; and the subsequent observations, which were con-
tinued during November and December, 1883, and part
of February, 1884, were between New York and Chicago.

The line consisted of a large copper wire having a
steel core ; thus combining conductivity and strength ;
and the object of the observations was to ascertain the
relations of the electric current to difference of temper-
ature. They were made daily, at both ends of the line,
at the hours when it was least occupied with other
business, 8 to 8.30 A.M., 5 to 5.30 and 11 to 11 30 P.M.

POTENTIAL AND EARTH CURRENTS. 187

The line being disconnected from the batteries, and
connected with the earth at both ends, the current was
obtained from the earth alone, independent of any artifi-
cial source: and its strength and direction, as indicated by
the galvanometer, were noted, and also the temperature.

It was found, that the general direction of the cur-
rent was from a region of high to one of low temper-
ature, though frequent reversals of current were
observed. And as the east, from longer exposure to
the sun's heat, would have a higher temperature than
$he west, at the time of the morning observation, the
prevailing current, at this hour, was found to be from
east to west. As these conditions of temperature
would be reversed in the evening, the observations at
that hour showed a corresponding reversal, and a '
prevailing west to east current. While the observa-
tions near midnight, when another reversal of temper-
ature is at hand, showed that the current then was
fluctuating and uncertain.

The deflection of the galvanometer needle varied
from 0 to 57°; the morning average being 11.4°, the
evening average 14.3°, the average near midnight 7.3°,
and the general average 11°. The difference of tem-
perature, between the points of observation, varied
from 0 to 37°; the morning average being 14.5°, the
evening average 9.5°, the average near midnight 10.3°,
and the general average 11.4°.

When the earth connection was severed, at either
station, the current was reduced to a minimum ; cor-
responding to the probable leakage along the line ;
proving that it was an earth current, and not an
atmospheric current.

If a similar east and west line were extended rouna

188 ELEMENTS OF STATIC ELECTRICITY.

the globe, we may reasonably infer that similar results
would be observed 011 every part of it; and hence,
that east and west currents are constantly traversing
the earth, as it revolves from west to east.

This will be more fully understood, when we con-
sider, that, during the diurnal revolution of the earth,
the sun occupies practically a fixed position with ref-
erence to it : so that from the earth's heated hemi-
sphere, electric currents are constantly flowing, from a
central point where the sun's rays are vertical, in
opposite directions, towards a point within the cooler
hemisphere, opposite to the sun.

But the diurnal revolution of the earth brings any
limited area of its surface, surrounding an observer,
alternately into each of these currents. So that, while
they have a fixed direction with reference to the sun,
and to the earth, as a whole; they become, alternately,
east or west currents, with reference to such an area.

From noon to midnight this area would be in the
west to east current ; and, from midnight to noon, in
the east to west current; an equatorial point, on the
observer's meridian, passing the point from which the
currents diverge, at noon ; and reaching the point
towards which they converge, at midnight.

At both these hours, the temperature, at equally
distant points in the observer's latitude, reaching from
his position, east and west to the sensible horizon, is
nearly the same : and the noon and midnight minima
of electric potential are the result.

At sunset and sunrise the temperature on similar
quadrants of the observer's latitude, east and west of his
position, attains its maximum difference ; and the even-
ing and morning maxima of electric potential occur.

POTENTIAL AND EARTH CURRENTS. 189

It will be observed that while the observer's position
reaches the point of highest temperature at noon, the
point of lowest temperature is reached at sunrise. For
the heating of any given area begins at sunrise,
increases till noon, as the sun's rays become more
vertical; and declines from that hour till sunset, as
the rays become less vertical ; while the cooling is
constant from sunset to sunrise. So that the morning
difference of temperature, between east and west
regions, is greater than the evening difference ; and we
should expect to find a corresponding increase of elec-
tric potential, at the morning maximum.

But the series of telegraphic observations given
shows the reverse ; which may result from the fact that
the line on which the observations were made, has the
Atlantic ocean at its eastern terminus, and the interior
of the continent at its western. And, as change of
temperature is much slower on a water surface than on
a land surface, the difference of temperature between the
Atlantic on-the east, receiving the sun's rays first, and the
interior on the west, would be less in the morning than in
the evening, when these relative positions are reversed.

As the distance between heated and cooled regions
alternately increases or diminishes during the earth's
diurnal revolution, electric resistance increases or di-
minishes in the same ratio, and increase or decrease of
current intensity is a corresponding result : and electric
maxima and minima, and also reversal of current, must
follow from this cause, as well as from difference or
equality of temperature. But as increase or decrease
of distance is coincident with increase or decrease of
difference of temperature, the two causes intensify each
other's effects.

CHAPTER XIII.
THE ELECTRICITY OF THE EARTH AND ATMOSPHERE.

THE AURORA.

THE relations of the aurora to terrestrial and atmos-
pheric electricity present a problem of the deepest inter-
est and importance, whose satisfactory solution must
render clear many questions now involved in doubt and
obscurity. Hence, during the last fifty years, it has
been carefully observed, and a number of important facts
in regard to it ascertained. The laws which govern it
are still far from being understood, and much con-
flict of opinion exists in regard to many points; but its
electric origin may be regarded as fully established.

This phenomenon occurs in zones surrounding the
northern and southern magnetic poles. And obser-
vations have been chiefly confined to its occurrence in
the north. The northern aurora is known as the
aurora borealis, the southern as the aurora australis,
while the term aurora polaris, or simply the aurora, is
applied to either.

In the United States it is usually first seen
at from 8 to 10 P. M., though often beginning much
later : and it continues from three to four hours. Its
occurrence during the day, also, is probable ; though
it can only be inferred from coincident effects; the
brilliancy of the daylight rendering it invisible.

THE AURORA. 191

Some of the great auroras have been seen ft^r several
nights in succession ; their occurrence during the inter-
vening days also being highly probable.

AURORAL ARCHES, CORONA, AND STREAMERS. —
It first appears, usually, as a low arch of light, in the
direction of the pole, resembling the dawn of day;
whence its name, aurora, the morning. This arch is
often accompanied by a low bank of clouds, lying
under it, next the horizon. As the arch slowly rises
streamers of light, differing in color, size, and brilliancy,
dart up through it ; extending from the horizon to
a considerable height above the arch; their color
varying from a pale white to a light red; though yel-
low, green, and blue tints have also been observed;
the prevailing tints differing more or less in different
localities.

These streamers appear to radiate from a central
region below the horizon, cutting the arch vertically,
at right angles, as shown in Fig. 58. The streamers
sometimes appear to rise from widely separated points
in the horizon; and, as the aurora increases in size and
brilliancy, they culminate at the zenith, as shown in
Fig. 59, forming a corona of more or less prominence ;
one of the most prominent being shown in Fig. 60.

By comparing the three cuts, it will be seen, that if
the center of the corona shown in Fig. 59, or Fig. 60,
were below the horizon, the appearance would be the
same as in Fig. 58. So that, supposing the observer
placed below the horizon, under the center from which
the streamers seem to emanate, he would see the
corona above him, as in Figs. 59 and 60. And, con-
versely, an observer in the latitude of Paris, looking at
the corona, observed in latitude 70° N., Fig. 60, would

THE AURORA. 193

see only the upper part of its southern half, corre-
sponding to the aurora shown in Fig. 58.

In the aurora shown in Fig. 61, seen from the Vega,
in latitude 65° N., we have an arch formation without
streamers. A series of concentric arch segments, more
or less perfect, is seen ; the outer one less than a semi-
circle, and the most perfect of the inner ones greater
than a semicircle ; the central one, a double arch, with
the nucleus of a second double arch above the junc-
tion. From an inspection of the figure, it is evident,
that the perfect arches would appear as complete circu-
lar belts to an observer under the central point near
the horizon.

Difference of longitude, as well as latitude, must also
modify the appearance : as that portion of the arch
which appears to one observer as its summit, appears
to another, at a distant east or west point in the same
latitude, as its east or west base. And, supposing the
first observer placed in the magnetic meridian which
coincides with the center of the aurora, the effect of
perspective would cause it to assume a different appear-
ance to him, from that seen by the other observer,
viewing it from a different angle. A streamer, seen
from one position, would appear foreshortened; while
at a different angle it would appear elongated: to one
observer it might appear as a narrow ray, to another as
a broad band.

Hence, we may infer, that we see in the arch, rising
from the horizon, the outer edge of a circular belt
of electric light, with its varied phenomena of arches,
streamers, rays, and coronae, covering a large area,
parallel to the earth's surface, and extending, as it
increases in size, from a region surrounding the pole,

THE AURORA. 195

towards the equator : and that its different aspects, at
different times and places, and its different phases, as
seen at the same time by observers at different points,
are greatly modified by its position with reference to
the position of the observer.

Fig. GO— Auroral Corona Observed at Bossekop, Lat. 70° N.

AURORAL MOVEMENT, CURTAIN FORMATION. — A
peculiar feature of the aurora is the continual move-
ment visible in every part. A streamer darts up
rapidly from the horizon, increasing in size and brill-
iancy; and as rapidly fades away. Along one part of
the arch a series of streamers form in rapid succession,
giving the impression of an undulatory, horizontal
movement, at right angles to the vertical movement of
the rising streamers : and, as the intensity of this phase
decreases, a similar movement, at some distant point,
rises and declines in a similar manner. At times there
occurs a curtain formation, composed of parallel rays ;
appearing either as a single curtain, as shown in Fig.
62, or as a series of curtains, hung one behind the
other, showing only their lower margins, as in Fig. 63;

THE AURORA. 197

undulatory movements occurring, transverse to the
apparent vertical position of the rays, like the move-
ments of a banner floating in the breeze.

This appearance is doubtless greatly modified by
perspective: the rays which are apparently vertical,

Fig. 02— Auroral Curtain Formation, Observed at Bossekop, Lat. 70° N.

being- horizontal; and probably emanating from the
edge of an arch; producing the single curtain shown in
Fig. 62 ; or from the edges of several concentric arches,
like those shown in Fig. 61 ; producing the series of

Fig. 63— Auroral Curtain Formation, Observed at Bossekop, Lat. 70° N.

curtains shown in Fig. 63. It is also evident from this,
that in the formation of coronse, the appearance is
probably often due to the edge of the arch, with the
streamers emanating from it, reaching the zenith of
the observer.

198 ELEMENTS OF STATIC ELECTRICITY.

AURORAL BANDS. — Sometimes a single streamer
spans the heavens from west to east like a band. The
author saw such a one at Chicago, Oct. 5, 1882.
Appearing at about 10.30 P. M., near the horizon, a
little north of west, it extended, within ten minutes,
to the eastern horizon, passing near the zenith : and
remained visible for more than half an hour. Its
apparent width was about four degrees, and its color a
light red.

The signal service record, for the same date, de-
scribes an aurora, "seen generally throughout New
England, as far south as Washington, and, in the
northwest, from 10 30 p. M. till after midnight ; reach-
ing an altitude of 90°, and covering 90° of the horizon."
Its different colors, in different localities, were "white,
blue, yellow, and crimson. Beams, arches, waves, stream-
ers, and patches of light were visible ; and, at Wash-
ington, frequent flashes of lightning, at the edge of the
dark segment."

HEIGHT OF THE AURORA. — Great diversity of opin-
ion has existed in regard to the height of the aurora
above the earth. A great altitude has been assigned
to it by some, who argue that the same aurora could
not otherwise be visible to observers thousands of miles
apart : while others assign to it a low altitude ; main-
taining that these different observers do not see the same
aurora, but different ones, occurring at the same time:
since the appearance seen by one, often differs greatly
from that seen by another. But, since differentials
of the same aurora may be visible to different observ-
ers, it is evident, that one of low altitude, and great
extent, might be seen at points as widely remote from
each other as the eastern and western continents ; the

THE AURORA. 199

electrified stratum of the atmosphere surrounding the
polar area, like a circular belt.

The weight of evidence is now in favor of the low
altitude ; sixty-nine miles above the surface being con-
sidered a fair estimate. But strict accuracy is not
attainable ; since it is impossible for any two observers,
at opposite ends of a base line of sufficient length, to
fix with certainty on the same point, so as to make an
angular measurement. But we can estimate the prob-
able height at which atmospheric resistance would be
sufficiently reduced to produce the auroral phenomena ;
and we have already seen that this plane of least
resistance must lie between the dense strata below
and the region of high vacuum above; both of which
oppose electric movement. Hence the height, given
above, may be approximately correct ; and yet subject,
doubtless, to variation, resulting from difference of
atmospheric pressure ; low pressure diminishing resist-
ance, and depressing the auroral plane, and high pres-
sure producing the opposite effect.

GEOGRAPHICAL POSITION OF THE AURORA. — Ob-
servation shows that the aurora is confined to com-
paratively narrow belts. It is never seen at the
equator, and is rarely visible in the northern hemi-
sphere south of latitude 40°: while in higher northern
latitudes, it is seen to the south of the observer; and
decreases in frequency and brilliancy, assuming appar-
ently a more southerly position, as the observer moves
farther north.

In Fig. 64, we have a chart, giving the results of
observations made in the northern hemisphere, by dif-
ferent European observers ; which shows that this
auroral belt is about 30° in width. Its southern limit,

200

ELEMENTS OF STATIC ELECTRICITY.

. Isothasmen.
.Magnetic Meridians.

Fig. 64 — Chart showing Isochasmen or Lines of Equal Auroral Frequency.
(From Petermann's Mittheilungen, 20 Band, 1874— IX.)

THE AURORA. 201

in the western hemisphere, is shown at Lat. 22° N., Long.
75° W. from Greenwich ; and its northern limit, on
the same meridian, at Lat. 58° N. In the eastern
hemisphere, its southern and northern limits, on the
same meridian, are between 47° N. and 77° N.

The increased width and number of the lines, towards
the northern limit, show a great increase in the frei
qnency, brilliancy, and duration of the auroras in that
region.

It is also found, that the position of this auroral belt
varies at different seasons of the year; reaching its
southern limit near the equinoxes, and its northern
limit near the solstices.

The results given in the above chart must be regarded
as approximate, rather than strictly accurate ; as the data
on which they are based were more or less imperfect.

CAUSES OF THE AURORA. — Having now examined
the various phases of auroral phenomena, and their
location, we are prepared to investigate more fully the
causes by which they are produced.

The earth has already been described as a thermo-
electric battery, and the atmosphere as a Ley den jar;
the one a generator and the other an accumulator; and,
in the combination of the two, we may look for the
principal cause of the aurora.

We have seen that electric movement is from higher
to lower temperature, producing earth currents on east
and west lines, governed by the earth's rotation, and
by solar and lunar influence. But the greater differ-
ence of temperature between the equatorial and polar
regions must produce north and south currents of far
greater energy than these east and west currents.

It has also been shown, that a change of potential, in

202 ELEMENTS OF STATIC ELECTRICITY.

any portion of the earth's surface, must produce a
corresponding change in the stratum of least resistance,
in the atmosphere above it; and that a transfer of
electricity must occur between this electrified atmos-
pheric area, and the surrounding atmosphere, lying in
the same horizontal plane, either from it or to it, as the
earth's surface below is positive or negative.

We have, in the aurora, the exact fulfillment of all
these conditions. A high earth potential, in the polar
regions, must result from the currents flowing in from
the warm region ; and produce, by induction, a corre-
sponding negative potential in the atmosphere. And,
in the belts where these ice-bound polar regions join
the warmer region, the principal electric action must
take place; producing the auroral arches of white light :
while the electricity radiating in opposite directions,
north and south from the arch, produces the streamers,
beams, rays, bands, and coronas ; as the electric action
at different points has greater or less intensity, or meets
with varying resistance.

Confirmatory evidence of this view is found in the
fact, shown by the chart on page 200, that within the
torrid and north frigid zones, where a comparatively
even temperature exists, the aurora is not seen ; and
also in the shifting position of the auroral belt with
change of temperature, as already mentioned.

The east and west earth currents must also exercise
their inductive influence, giving rise, probably, to the
transverse undulations observed in the streamers and
curtain formations. And the resultants of these cur-
rents, and the north and south currents, are seen in the
bands and streamers which often assume a diagonal
direction, northwest and southeast, or otherwise.

THE AURORA. 203

The stratum, in which these phenomena occur, must
have a certain degree of thickness; its upper surface
merging into the region of high vacuum, and its lower
surface into that of greater density ; resistance increas-
ing upwards and downwards from a central plane.
Hence, different phases of electric action must occur at
different altitudes; corresponding to the different aspects
of electric transmission in high and low vacua, seen in
laboratory experiments, as described in Chapter X :
which may account for the common auroral appearance,
shown in Fig. 58, where the arch seems to form a back-
ground for the streamers. And, as there is often a
series of concentric arches, as shown in Fig. 61, it is
easy to see how streamers might radiate from one arch,
across the plane of another arch, at a different altitude.
And, if one was below, and the other above the horizon,
the appearance would be the same as in Fig. 58.

Now, since the causes here assigned are in constant
operation, we may infer that there should be a constant
aurora ; though it does not follow, that it should be
everywhere constantly visible. And from the great
number of auroras observed in the course of the year,
in different parts of the auroral belts, especially in the
northern part of the northern belt, it is reasonable to
infer, that, with a more perfect system of observation,
auroras, of greater or less magnitude, would be seen, at
one or more points, every night in the year.

It is also probable that this electric action may be
constant, without being always sufficiently intense to
attract attention : and that the aurora is the result of
its increased intensity.

Other atmospheric phenomena, not usually recognized
as belonging to the aurora, may also be due to this

204 ELEMENTS OF STATIC ELECTRICITY.

electric action. The peculiar band and arch formation
of cirro-stratus clouds often strongly resembling auroral
bands and arches, has, by many observers, been attrib-
*ited to similar electric action ; though doubtless occur-
ring at a much lower altitude than that of the aurora.

The existence of strong earth currents during the
prevalence of auroras, and of those violent perturba-
tions, known as ''electric storms," are well established
facts, proved by observations on telegraph lines. Dur-
ing the aurora of Feb. 4, 1872, visible over an area
embracing 30° of latitude, and 150° of longitude, these
currents and perturbations were observed on all the
lines within this area, both land and submarine ; being
strongest on those having a southeast and northwest
direction.

The following description of the auroral storm of
Nov. 17, 1882, is condensed from the Signal Service
Reports : " Beginning a little before daylight, it was
known at first by its interference with telegraphy.
For three hours not a wire of the Western Union Tel-
egraph Company could be worked. Late in the after-
noon, the trouble seemed to decrease ; and, at night,
there was a brilliant aurora prevailing over the eastern
half of North America, the Atlantic, and northwestern
Europe; and all telegraphic service was interrupted.
Cables to Europe, and wires to Chicago, could not be
worked; annunciators in telephone offices dropped;
the switch-board in Albany, N. Y., was ignited; the
switch-board and wires at Chicago were burned ; and an
incandescent lamp was illuminated at St. Paul, Minn.
A message was sent from Bangor, Me., to North Sid-
ney, C. B., 710 miles, by the earth current alone, with-
out the batteries ; the current being as strong as that

AURORA. 205

from 100 cells. And the short line from Boston to
Dedham, ten miles, showed the disturbing influence as
much as the longer lines."

In these observations, as in those cited in Chapter XII,
it has been found that whenever the earth connection is
severed, at either end of the line, the current immedi-
ately ceases ; proving it to be an earth current, and
not a current in the atmosphere.

The increased intensity of current, on lines having a
southeast and northwest direction, noticed during the
'aurora of Feb. 4, 1872, is confirmatory evidence of the
existence of resultant currents, as explained on page
202.

The hours at which maximum and minimum effects
were observed, during the aurora of Nov. 17, 1882,
correspond exactly to the hours of maxima and minima
potential, and current intensity, already cited. A max-
imum having occurred during the three morning hours,
beginning just before daylight; a minimum late in the
afternoon, and a maximum again after sunset.

Another cause of the aurora is found in the move-
ment of warm air from the torrid to the frigid zones,
and of cold air, at a lower altitude, from the frigid zones
to the tonid. The meeting and intermingling of these
opposite currents, at different temperatures, must give
rise to strong electric action in the atmosphere, similar
to that already described as taking place in the earth,
and coincident with it. And this action must occur in
the stratum next the earth, far below that assigned to
the aurora; its intensity increasing with the density of
the atmosphere, and hence being greatest at the earth's
surface.

This becomes evident, when we consider, that the

206 ELEMENTS OF STATIC ELECTRICITY.

greater part of the mass of the atmosphere lies near the
earth's surface ; being included, probably, within the
first nine miles ; while the auroral stratum is supposed
to have an altitude of sixty-nine miles. Hence this
atmospheric electric action would be supplementary to
that of the earth, already described; and would have
an east and west as well as a north and south direction,
as described on page 182.

The influence of the sun and moon, already referred
to, must intensify the effects produced by other causes :
so that we should expect to find maximum and miiv
imum auroral effects, corresponding to an increase or
decrease of intensity, in solar or lunar influence. Ob-
servation has shown, that such an auroral maximum
occurs, during the recurrence, once in eleven years, of
the -period of the maximum solar disturbance; that
auroras are then more frequent and brilliant than at
other times : and we may reasonably infer, that future
observation will show the existence of electric maxima
and minima, analogous to the tides, and auroral effects
corresponding to them.

CHAPTER XIV.

THE ELECTRICITY OF THE EARTH AND ATMOSPHERE.

LIGHTNING AND THUNDER.

FORMATION OF THUNDER CLOUDS. — Our investiga-
tion of this subject thus far has been confined chiefly
to the electricity of the earth and its inductive effect
on the atmosphere; we are- now to investigate the elec-
tricity of the atmosphere and its inductive effect on the
earth.

We have seen, in the Topler machine, how electric-
ity is generated by the mutual friction and induction of
insulated conductors, put in motion by mechanical force ;
and collected in accumulators which acquire different
potentials, and between which a discharge finally takes
place, attended with a flash and report. Something
analogous to this occurs in the atmosphere. The clouds
are large conductors, insulated in the air, moved by the
winds, acting inductively on each other and on the
earth, and, in other respects, fulfilling the same condi-
tions found in the machine.

As the vapor forming these clouds rises from the earth,
it must have, when generated, the same electric potential
as that part of the earth from which it rises, and hence
the same difference of potential which has been shown
to exist in different parts of the earth's surface.

The air laden with this rising vapor, moving along in

208 ELEMENTS OF STATIC ELECTRICITY.

currents, and brought into contact with elevated parts of
the surface, and with trees, buildings, and other elevated
objects, must generate electricity by friction, much in the
same way as the carriers on the revolving plate of the
machine. And, as the vapor forms into clouds, they be-
come the accumulators of this electricity, in the same way
that it is accumulated by the plates and Leyden jars of
the machine. And this concentration of electricity in
the clouds raises their electric potential; and makes
them the nuclei to which the rising vapor is attracted in
consequence of its lower potential.

Each infinitesimal drop of vapor is a sphere with its
electric charge on the surface ; and as these drops
coalesce, and form larger ones in the cloud, the charge
on each new drop accumulates on the surface ; and as the
increase of volume is greatly in excess of the increase
of surface, the electric surface density must increase
in nearly the same ratio ; the volume representing
electric quantity, which is thus condensed on a reduced
surface, producing a corresponding increase of intensity.

Thus as a large body of invisible vapor forms first
into light fleecy clouds; and these collect into denser
masses; there is a constant reduction of volume, and
increase of electric intensity; till the fully formed
thunder cloud is the result.

DISCHARGE BETWEEN CLOUDS. — Two or more such
clouds, formed in different localities, often many miles
apart, and electrified in this manner, must, almost inevi-
tably, be at different electric potentials. And when car-
ried to\vards each other by opposite atmospheric currents,
at different altitudes, and brought within the sphere of
mutual electric influence, strong inductive effects are
produced ; their approach is accelerated by attraction,

LIGHTNING AND THUNDER. 209

and, when brought within proper distance, a discharge
takes place from the cloud of higher to that of lower
potential : just as a similar discharge takes place be-
tween the sliding electrodes of the machine : and the
result is chain lightning, of which the spark of the ma-
chine is an exact type.

The distance, through which this discharge takes
place, depends on the quantity and intensity of the
charge, and the difference of potential between the
clouds. It may be any distance, from a few yards to
several miles. Observation on discharges between
clouds overhanging fixed localities, as two mountain
peaks, shows that they are sometimes from three to five
miles or more in length.

We have seen how sparks, eight to ten inches in
length, are produced by the machine ; and have tested
their energy. If we compare such a discharge to that
produced between two clouds, whose magnitude and
potential, as compared with those of the machine, are
almost infinite, we can form some adequate conception
of the enormous energy of the lightning.

When the line of discharge is concealed by inter-
vening clouds, and we see only the illumination result-
ing from it, the phenomenon is known as sheet light-
ning. We have the same result, when the spark from
the machine, occurring in a dark room, is concealed.
Hence, we may reasonably infer, that the discharge be-
tween the clouds, like that between the electrodes of the
machine, would always present the appearance of chain
lightning, if the line of discharge were always visible.

The contorted and bifurcated discharges, known as
zigzag lightning, and forked lightning, like similar dis-
charges in the machine, are doubtless due to differences

210 ELEMENTS OF STATIC ELECTRICITY.

of resistance in the air, to the induction of surrounding
clouds, and to the mutual repulsion of the molecules of
air and vapor within the line of discharge ; which, be-
iug electrified to the same potential, tend to separate and
form resultant lines, under the influence of forces act-
ing at right angles to each other.

Observation shows, that there is usually a succession
of discharges between the two clouds, similar to the
repeated discharges from a Holtz machine : in which,
after the initial charge, electricity is generated by in-
duction alone. This action begins when the edges
of the two clouds, at different altitudes, approach
within discharging distance, and come into vertical
line ; and the effect of induction is to accumulate
the electricity of the cloud of higher potential at
the end nearest to the other cloud, while the elec-
tricity of the latter is repelled to the remote end ;
just as a similar effect is produced by the mutual
approach of two differently charged conducting plates
or cylinders ; the difference of potential between the
adjacent parts being thus greatly increased.

The discharge produces a momentary equilibrium,
which is again disturbed by induction, as larger areas
of the two clouds approach more closely: the residual
becoming the initial for a new charge, further conden-
sation taking place, and fresh supplies of electricity flow-
ing in from the surrounding atmosphere. In this way
the series of discharges continues, till the clouds unite,
and complete equilibrium takes place.

When several such clouds, at different potentials and
different altitudes, collect in each other's vicinity ; as
is usually the case in a thunder storm of much magni-
tude ; the mutual inductive effect is greatly intensified.

LIGHTNING AND THUNDER. 211

Suppose three clouds, arranged in a series, end to
end, and so graduated as to potential, that the central
cloud is at a mean between the other two. Let a dis-
charge take place from the cloud of highest potential
to the central one ; a second discharge must quickly
follow, from the central cloud to the one of lowest po-
tential : since the first discharge has greatly increased
their difference of potential. This second discharge
would renew the difference of potential between the
first and central clouds, and prepare the way for another
series of similar discharges.

The most careless observer cannot fail to have noticed
such series of discharges, following each other in
rapid succession, in different parts of the sky, during a
violent thunder storm.

Observation also shows, that during a thunder show-
er, there is always an increase of rain-fall, and an en-
largement of the drops, within a few seconds after each
electric discharge ; the time being just sufficient for
the rain to descend, if it left the cloud at the moment
of the discharge. From which we may infer, that con-
densation is a result of the discharge ; that, in the mo-
mentary equilibrium which follows it, the small drops,
which were before kept apart by mutual repulsion,
from being highly charged and at the same potential,
now coalesce, and form the large drops ; which, being
too heavy to be sustained in the atmosphere, fall.

THUNDER. — As the spark from the machine is the
type of lightning, so the snap represents tlmnder ;
which is undoubtedly due to the same cause — the sud-
den and intense vibratory motion of the air, in the line
of discharge, producing violent undulations in the sur-
rounding air. A cause which will appear sufficiently

212 ELEMENTS OF STATIC ELECTRICITY.

adequate, when we consider the results which must fol-
low from the rush of the enormous energy of a thunder
cloud, along a line, perhaps five miles in length, in an
infinitesimal fraction of a second.

And here, as in the case of the spark, it is quite un-
necessary to suppose the passage of any material sub-
stance through the air, producing partial vacuum and
collapse, or the occurrence of anything in the nature of
an explosion, producing similar results. It is more in
accordance with the known laws of electric movement,
to suppose that the electric energy has used the air as
the medium in which to travel; and thus produced the
vibratory motion.

Common observation shows, that in explosions where
the expenditure of energy must often be far less than
in the electric discharge between clouds, the vacuum
and collapse shatter window-glass in the vicinity; while
the heaviest thunder produces only a slight tremor in
adjacent buildings ; proving that such vacuum and col-
lapse cannot result from an electric discharge.

The succession of reports accompanied by a continu-
ous rumble, heard so frequently during a thunder storm,
has been considered, by some observers, as a series of
echoes from a single report; and by others, as a num-
ber of separate reports, from discharges occurring si-
multaneous]}T, at different distances from the observer,
and heard in the order of their distance.

An echo requires the intervention of an extended
surface, as a wall or its equivalent; and observation
shows, that the under surface of a dense thunder cloud
is of this character, being remarkably uniform, though
its upper surface may be quite the reverse : and it is also
evident, that this under surface, resting on tho denser

LIGHTNING AND THUNDER. \^O^ Zl3

£L

strata of air, and sustaining the weight of the ~
air and vapor above, must have greater density than
the upper surface. Hence we may reasonably infer,
that this surface, and that of the earth below it, fulfill
the conditions necessary for a series of echoes.

The hypothesis of simultaneous discharges, at differ-
ent distances, may also be true in certain instances : as
it is quite possible that such simultaneous discharges
frequently occur. But the succession of reports, often
following each other with marked regularity, and steadi-
ly decreasing in volume and intensity, is not fully ex-
plained by this hypothesis, while it is entirely in ac-
cordance with the character of a series of echoes.

The re-adjustment of electric energy between differ-
ent parts of a large cloud, which must follow the pri-
mary discharge, gives rise to numerous minor discharges;
whose sound, mingling with that from the larger air
waves, causes the rumble ; analogous to the crackling
sound from similar minor discharges in the machine. A
premonitory rumble, from a similar cause, often precedes
the heavier discharge ; just as the crackling precedes
the discharge of the machine.

If the cloud were a perfectly homogeneous conductor,
like a metal cylinder, this could not occur. But as it
is a mass of vapor, composed of drops insulated from
each other by air spaces, each particular drop having its
own electric charge ; and different parts of the cloud
having different densities, and hence differing in con-
ductivity and resistance; and condensation, with increase
of potential, following the discharge, as already shown,
such minor discharges, with the accompanying roar and
rumble, are inevitable. Also the development of the
residual, after the primary discharge, which, in a large

214 ELEMENTS OF STATIC ELECTRICITY.

cloud, must iii itself have great energy, greatly intensi-
fies these effects.

DISCHARGE FROM THE CLOUDS TO THE EARTH.—
We have already seen that the potential of the atmos-
phere, and hence of the clouds, is almost invariably
positive with reference to that of the earth. Hence the
earth's surface under a thunder cloud, and all objects
connected with it, become negatively electrified by in-
duction, to the same degree that the cloud is positive ;
electricity, equal to the charge of the cloud, being re-
pelled from the earth's surface to its interior. A result of
this difference of potential is a strong attraction between
the earth and cloud, by which the cloud is drawn towards
the earth; and, unless its potential is reduced by discharge
into another cloud, a discharge to the earth is inevitable,
whenever, from reduction of distance, the resistance of
the air becomes less than the electric tension of the cloud.

When there are two clouds at different altitudes, and
a discharge takes place from the upper to the lower
cloud, the difference of potential between the latter and
the earth, being thus increased, the liability of a dis-
charge from it to the earth is increased in the same ratio.

If there are elevated objects, such as trees and build-
ings, on the surface below, the resistance between them
and the cloud is less than that of the surrounding flat
surface ; not only on account of reduced distance, but
also on account of the points and angles which they
present. Hence, we find, that trees, flag-staffs, tele-
graph poles, church spires, chimneys, and projecting
corners of roofs are much more frequently struck by
lightning than flat surfaces.

Good conductors, such as tin gutters, metal cornices,
and ornamental iron work, also offer far less resistance

LIGHTNING AND THUNDER. 215

than imperfect conductors, like wood, brick, and stone;
both from their superior conduct! vity, and their projecting
edges and points ; and when connected with a building
and not connected by a metallic conductor with the earth,
greatly increase the liability of the building, both to re-
ceive the electric discharge, and to sustain injury from it,
by making the building its terminus instead of the earth.

DISCHARGE FROM THE EARTH TO THE CLOUDS. —
As already shown, the electricity of a large cloud, like
that of a cylinder, may be so distributed by the prox-
imity of one end to another cloud, at a lower potential,
or to an elevated portion of the earth's surface, that the
potential of this end shall be higher than that of the
remote end. The potential of the earth's surface, be-
neath it, must also be similarly affected by induction, in
reverse order; being negative where the cloud is positive,
and positive where the cloud is negative. If, under
these circumstances, the difference of potential between
the negative end of the cloud and the earth becomes
greater than the resistance of the air, a discharge from
the earth to the cloud must occur; the discharge in this,
as in all other cases, being from higher to lower potential.

These conditions are similar to those of the three
clouds already referred to : so that a discharge from
the positive end to another cloud, or to the earth, may
increase the difference of potential between earth and
cloud at the negative end.

The resistance of the earth, also, over such an exten-
sive area, retards the restoration of surface equilibrium
after the discharge from the positive end; and increases
the liability of the return discharge from the earth to
the cloud, in the ratio of this resistance to that of the
vapor of the cloud.

216 ELEMENTS OF STATIC ELECTRICITY.

In this case, as in that of a discharge from the clouds
to the earth, elevated objects reduce the resistance, es-
pecially if they are good conductors, or furnished with
sharp angles or points; and become the electrodes
through which the discharge takes place.

LIGHTNING RODS. — Franklin first proposed the
lightning rod. The identity of lightning and elec-
tricity, strange to say, was unknown, till, by the erec-
tion of a metal rod at his suggestion, and subsequently
by his well known kite experiment, sparks were drawn
from the cloud, Leyden jars charged, and various similar
laboratory experiments, previously known to electric
science, performed by means of atmospheric electricity.

The first lightning rod was erected, May 10, 1752, a
month previous to the kite experiment, by M. Dalibard,
in France, according to the plan proposed by Franklin
for testing the identity of lightning and electricity:
and sparks similar to those from the electric machine
were drawn from it.

The identity of lightning and electricity having been
established, Franklin showed how the rod could be used
as a means of protecting buildings. The result is the
lightning rod, as we now have it, in its numerous forms.
And though ignorance, greed, and dishonesty have cast
their shadow upon it, yet thousands of well con-
structed rods, standing as the silent guardians of life
and property, sufficiently attest its value.

The proper construction of lightning rods was re-
cently investigated by a conference of leading English
scientists, specially appointed for that purpose : among
whom were several eminent electricians. And, after
three years of thorough investigation, during which
practical information was collected from all parts of

LIGHTNING AND THUNDER. 217

the world, a code of rules for the construction and
erection of lightning rods, or conductors, was adopted
December 14, 1881; which is substantially as follows: —

RULES FOR THE CONSTRUCTION^ AND ERECTION OF
LIGHTNING CONDUCTORS.

POINTS AND UPPER TERMINALS. — As the point of
the upper terminal, from its peculiarly exposed position,
is liable to be fused by a heavy charge, it should not
be sharper than a cone whose height is equal to the
radius of its base. But, to secure the peculiar advan-
tages derived from sharp points, three or four such
points made of copper, each about six inches long,
should be attached to a copper ring ; which should be
screwed or soldered to the terminal, about twelve inches
below its highest point. And all points should be so
platinized, gilded, or nickel-plated, as to resist oxidation.

The number of terminals required, their height above
the building, and the number of conductors connected
with them, depends on the size and style of the build-
ing, and the conductivity of the material of which it is
constructed.

All elevated parts, such as turrets and spires, should
be protected by terminals : and especially chimneys,
whose liability to receive a discharge is greatly increased
by the heated air and soot.

Factory chimneys should have a copper band round the
top; with stout, sharp, copper points, each about twelve
inches long, projecting from it at intervals of two or three
feet, and specially guarded against oxidation. And the
conductor, attached to this band, should be attached to
all bands and metallic masses in or near the chimney.

SPACE PROTECTED. — No definite rule can be given

218 ELEMENTS OF STATIC ELECTRICITY.

as to the space protected by a conductor ; as opinion
and practice vary in regard to it: but there is no well
authenticated instance of a building furnished with a
property constructed conductor, having been injured
by lightning within a conical space, having the point
of the upper terminal for its apex, and the radius of
whose base equaled the height of the conductor.

ATTACHMENT TO BUILDING. — The evidence against
the use of glass or other material, in order to insulate
the conductor, is overwhelming ; and insulation may
be regarded as unnecessary and mischievous. The.
attachment to the building should be made with metal
fastenings ; which should be of the same metal as the
conductor itself, to prevent corrosion from galvanic
action. They should be of adequate strength : and
each should support its proper proportion of the weight.
They should not compress or distort the conductor; and
should allow free play for its expansion and contraction.

As far as practicable, it is desirable that conductors
be connected with extensive masses of metal belonging
to the building, both internal and external; except
soft metal pipes, which, from low conductivity for heat
and electricity, are liable to fusion. Gas-pipes, es-
pecially, should not be so connected on account of
liability to ignition of the gas by an electric spark,
resulting from fusion of the pipe, or from bad joints :
but the inlet and outlet pipes of large gas meters should
always be electrically connected with each other, as a
protection against such accidents from the electric
resistance of joints; which is sometimes greatly in-
creased by india-rubber packing.

Church bells, inside well protected steeples, need
not be connected with the conductor.

LIGHTNING AND THUNDER. 219

ORNAMENTAL IRON WORK. — All vanes, finials,
ridge iron work, and similar ornamental metal work,
should be connected with the conductor : and it is not
absolutely necessary to use any other point than that
afforded by such ornamental work ; provided the con-
nection be perfect, and the mass of iron considerable.
As, however, there is risk of derangement through re-
pairs, it is safer to have an independent upper- terminal.

MATERIAL FOR CONDUCTOR. — The best material for
a conductor is copper ; its weight not less than six
ounces per foot run ; and its conductivity not less than
ninety per cent, of that of pure copper. It may be
used either in the form of tape, or of wire cable, in
which 110 wire should be less than No. 12 B. W. G.
Iron may be used, but its weight should not be less
than 2i poundsj^er foot run. And all iron conductors,
whether galvanized or not, should be painted, as a
protection against oxidation. Copper conductors may be
painted or not according to architectural requirements.

FORM OF CONDUCTOR. — The form of the conductor
does not seriously affect its conductivity : and great ex-
tent of surface in proportion to mass is not essential : but
sectional area of mass is highly essen tial,and should al ways
be sufficient to carry the heaviest charge without dan-
ger of fusion of the conductor, or division of the current.

The rod is desirable for long upper terminals, on
account of its rigidity ; but the necessity of frequent
joints, and the difficulty of avoiding disfigurement of
the building, are serious objections to its use for the
body of the conductor.

Tubes are liable to the same objections ; their larger
diameter, and the collars necessary for their joints, ren-
dering them more conspicuous and undesirable.

220 ELEMENTS OF STATIC ELECTRICITY.

Twisted wire cables have the advantage of compara-
tive freedom from joints ; but their interstices afford a
lodgment for smoke, dirt, and water; especially if
small wires are used: which are also less capable of
resisting oxidation than large wires.

Tape has the special advantages of requiring but few
joints; of their being easily made, where necessary;
and of being flat and flexible, so that it can be adapted
to the outlines of a building, or countersunk in it and
painted over, so as not to be conspicuous.

Conductors should not be bent abruptly round sharp
corners : and in no case should the length of conductor
between the two points of a bend be more than one-
half greater than the straight line joining them. When>
practicable, the conductor may pass straight through a
projection; the hole being made large. enough to allow
it to pass freely, without compression.
x The reasons for these precautions are found in the
liability to discharge from a sharp angle, or across a
short space in a bend.

JOINTS. — The most fruitful source of danger in con-
ductors is from bad joints. Screwed, scarfed, or riveted
joints, however well made, are certain to rust and cor-
rode in time ; introducing nodes of resistance, at which
the electric charge is liable either to fuse the conductor,
or to leave it and enter the building.

No joint is electrically perfect that is not metallically
continuous, and as absolutely free from resistance as
any other part of the conductor : and careful soldering,
in addition to the screwing, scarfing, or riveting, is the
only certain means of securing this, which has borne
the test of experience.

EARTH CONNECTION. — A good earth connection, for

LIGHTNING AND THUNDER. 221

the lower terminal, is of the utmost importance ; and
in a majority of cases of injury to buildings from badly
constructed conductors, such injury is traceable to
imperfect earth terminals.

The terminal should connect with damp earth, at a
sufficient depth below the surface, to insure permanent
dampness, and hence permanent conductivity. • And,
to render this connection more complete, it should
bifurcate below the surface ; and be connected by sol-
dering, with a mass of metal, buried in the earth. The
hole, in which this mass is buried, should be filled to the
surrace with cinders or coke, to facilitate the percolation
of water; and any available drainage of pure water,
from rain water pipes or otherwise, connected with it.

The metal mass may be of copper or galvanized iron,
having about eighteen square feet of surface. And
where permanently damp earth is not available, it
should consist of three or four hundred pounds of iron.

Where the use of large iron water or gas mains is
available, a connection by a copper strip, can be made
with them ; no risk being incurred by such connection,
as in the case of internal supply pipes.

INSPECTION. — Periodical inspection, and careful elec-
tric testing, are requisite to maintain the system in
efficient order ; as points may corrode or become fused,
joints become electrically imperfect, connections be-
come severed above or below ground, or other im-
perfections occur, from alterations in the building, and
the carelessness or ignorance of occupants or workmen.

The author has, on his house, a copper tape conductor,
constructed in accordance with these principles, and
erected twenty-three years ago ; and neither the house,

222 ELEMENTS OF STATIC ELECTRICITY.

nor the conductor, has ever received the slightest injury
from lightning; while numerous instances of damage
to buildings and conductors have occurred in the vicin-
ity. Which, considering the length of time, the
exposed position, and the repeated thunder storms of
great severity, which have occurred, is strong negative
evidence of the value of the conductor, and the correct-
ness of the rules here given.

SILENT DISCHARGE. — The protection afforded by a
lightning conductor does not consist, so much, in its
being the avenue by which a destructive discharge may
pass harmlessly between the earth and cloud ; as in
preventing its occurrence, by a gradual, silent discharge
through the points of the conductor; by which the
accumulated energy is reduced, before it can acquire
sufficient tension to overcome the resistance of the air,
and produce a full, sudden, disruptive discharge.

This is strikingly illustrated by the gradual, silent
discharge of a large, powerfully charged Leyden bat-
tery, through the point of a cambric needle ; and is
confirmed by the brush discharge, often observed,
during thunder storms, on the points of lightning
conductors, and on the tips of the masts and yard-arms
of ships.

As a building must be regarded, electrically, as an
elevated part of the earth's surface, the importance of
as perfect an electric connection between it and the
conductor, as practicable, is apparent, in order to
secure the full benefit of protection in the manner de-
scribed; which is impaired by the resistance caused by
the use of insulators.

It is also apparent, that the conductor affords equal
protection whether the discharge is from the cloud to

LIGHTNING AND THUNDER. 223

the earth, or from the earth to the cloud ; as in either
case, the discharge will follow the path of least resist-
ance ; which is always through the conductor, when
properly constructed.

HEAT LIGHTNING. — The phenomenon, known as
heat lightning, is probably nothing more than the or-
dinary electric discharge from clouds invisible to the
observer, and so distant that the thunder is inaudible.
Such lightning is generally observed at night, near the
horizon ; and close observation will show, either the
existence of clouds, indistinctly visible in the darkness,
or the probability of the discharge occurring from
clouds below the horizon.

Its existence, independent of clouds, is claimed from
the fact, that it has been observed when no thunder
storm had occurred within a radius of one hundred
miles. But, not only lightning, but clouds are often
visible at greater distances. On the level surface
round Chicago, the author has frequently observed
heavy thunder storms, eighty miles distant, as shown
by subsequent reports, when both clouds and lightning
were distinctly visible, though the thunder was not
audible.

TORNADOES. — As an electric origin has been claimed
for tornadoes, it is proper to remark, in conclusion,
that recent investigation has demonstrated that they
are chiefly due to currents of air, generated by differ-
ences of atmospheric temperature and pressure, and
modified by other causes: and while electricity may
intensify their force, it cannot be considered as their
primary cause.

224 ELEMENTS OF STATIC ELECTRICITY.

NOTE REFERRED TO ON PAGE 118.

The brush from K makes its appearance first, and
increases in length till the brush from V appears; after
which it decreases in the same ratio as the brush from
V increases, till the discharge occurs, when both dis-
appear. This is sufficiently explained by increase and
decrease of difference of potential at different points.
As the potential of the revolving plate A increases, the
difference of potential between the inside coating of the
jar (7, and that part of A which receives the charge
from it through the comb K, decreases, as indicated by
the decrease in brush-length, till the potential of both
is the same, when the brush disappears.

In like manner the potential of that part of the plate
A, passing the comb L, continues to increase till it
equals the potential of the inside coating of the jar D;
and this charged surface, passing on to the comb H, the
surplus of charge which D, from increase of potential
rejects, escapes through II to the comb V, and from V
to that part of the plate A between V and K, as indi-
cated by the increase of brush-length from V.

This process is greatly intensified by the inductive
effect of the high potential of the lower part of inductor
T, and low potential of the upper part of inductor Jf,
by which electricity is repelled from the corresponding
lower part of the plate A to its corresponding upper
part.

INDEX.

Absolute Electrometer, Thomson's, 161-169.

Accumulators, 72-91.

Amber, 1.

Atmosphere, the, as a Leyden jar, 180, 181.

Atmospheric potential, 177-180.

strata, difference of potential between,
179, 180.

currents, 181-183.

Attraction and repulsion, 1-4, 15, 40-42.
Aurora, the, 190-206.

, height of the, 198, 199.

, geographical position of the, 199-201.

, causes of the, 201-206.

, tubes, 146, 147.

Auroral arches, coronas, and streamers, 191-
195.

movement, curtain formation, 195-197.

bands, 198.

Bag experiment, 60.
Balanced rod, the, 2.
Bath, electric, 142, 143.
Battery, the Leyden. 79, 80.
Bells, electric, 102, 103, 125, 126.
Brush discharge, 117, 118, 134, 137.

Charge denned, 22.

, multiplication of, in Topler machine,
121, 122.

, variation of, 67.

Charged surfaces, formulae for, 167.
Chime, electric, for frictional machine, 102,
103.

, for Topler machine. 125. 126.
Condensation, surface, 55-58.
Condi-users, 74.

15

Conductivity for heat and electricity com-
pared, 37, 38.
Conductors and non-conductors, 4-6.

, hollow, 58, 59-66.
Conservation of energy, the, 23-26.
Convection, 66, 67.
Cosmic electric influence, 183-186.
Coulomb's torsion balance, 156-161.
Currents, atmospheric, 181-183.

, earth, 186-189, 204, 205.
Cylinder, electrified, 48, 69.

Avith points, 70.

Dielectric denned, 50.

, required thickness of, 74.
Disc, electrified, 71.
Discharge, appai-ent time of, 126-128.

, brush, 117, 118, 134, 137.

between clouds, 208-211.

from the clouds to the earth, 214, 215.

from the earth to the clouds, 125, 126.

, disruptive, 88.

, silent, 89.

, spontaneous, 88.

through book, 81-84.
Discharger, 76.

, universal, 87, 88.
Dual theory, the, 40-42.

Earth currents, 186-189, 204, 205.
Ebonite, 1, 6, 53, 54.
Electricity, the nature of, 23-42.

of the earth and atmosphere, 175-223.

generated by the friction of metals, 132,

133.

Electrics, 4.
Electric bath, 142, 143.

226

INDEX.

Electric movement, 13-16.

potential, 10-11.

transmission in vacua, 146-154.

wind, 104, 105, 143.
Electrometers, 155-174.

, attracted disc, 161.
Electrometer, Thomson's absolute, 161-1C9.

, mode of using the absolute, 166-169.

, Thomson's quadrant, 109-174.

, mode of using the quadrant, 173, 174.
Klectrophorus, the, 92-96.
Electroscope, the gold leaf, 16-18.

, the pith ball, 2 3.

, charged by induction, 44.
Energy, the conservation of, 23-26.

, radiant, 31.
Ether, 31-33.
Equipotential, 55.
Experiments with the Topler machine ,125-145.

F

Farad iy's hollow cube, 65.
Faradic current, 141.
Figures, Lichtenberg's, 89-91.
Force, 1.

, lines of, 55.
Form, influence of, 67.
Formulae for charged surfaces, 167.

, application of, to measurement by elec-
trometer, 167-169.
Fracture of Leyden jar, 88, 140.
Friction, mutual effects of, 18-21.
Frictional electricity, 8, 9.

machine, 96-100.

G

Gauge, idiostatic, for electrometer, 63.
Gas lighting, 143-145.
Geissler tubes, 147, 148.
Generators, electric, 92-124.
Glass for Leydeu jars, 77.

illuminated by electricity, 151.

, required thickness of, for insulation, 74.

, specific inductive capacity of, 53, 54.
Gravity and electiicity compared, 13, 14.
Gunpowder, method of exploding by elec-
tricity, 87.

IT

Heat and electricity compared, 13, 14, 33, 37,

Heat, light, and electricity compared, 26-31.
Heat lightning, 223.

Heating effects of electricity in high vacua,

153, 154.

Hollow conductors, 58-66.
Hollow cube, Faraday's, 65.
Holtz machine, the, 108-110, 122-124.
Holtz and Topler machines compared, 122-

124.

Holtz, Dr. W., correspondence with, 123, 124.
Hydro-electro machine, Armstrong's, 105-

107.

Idiostatic gauge for electrometer, 163.
Image plates, 103, 104.
Induction, 43-54.

, theory of, 48, 49.

varies inversely as square of distance, 46,

47.
Inductive capacity, specific, 51-54.

influence of dielectric, 49-51.
Influence machines, 108.
Insulator defined, 6.
Intensity, electric, 6-8.

Jar, the Leyden, 75-91.

Jar D,in Topler machine, higher potential of,
139, 140.

Leyden jar, the, 75-91.

, charged by cascade, 77-79.

, discharged through book, 81-84.

, electromotive force of, 77.

, fractured by overcharge, 88, 140.

, glass suitable for, 77.

, Lane's unit, 101, 102.

, residual charge of, 84, 85.

, spontaneous discharge of, 88.

, the atmosphere as a, 180, 181.

, with movable coatings, 85, 86.
Leyden battery, the, 79, 80.

, Tyndall's experience with, 87.
Lichtenberg's figures, 89-91.
Light, heat, and electricity compared, 26-31,
138.

, polarized and electricity, 28-31, 3fi.
Lightning and thunder, 207-223.
Lightning conductors, 216-221.

, attachment of, 218.

, earth connection of, 221.

, form Of, 219, 220.

, inspection of, C21.

INDEX.

227

Lightning conductors, joints of, 220.

, material lor, 219.

.points lor, 217.

, silent discharge of, 222.

, space protected by, 217, 218.

, test of copper tape, 221, 222.
Lines of force, 55.

M

Machine, Armstrong's hydro-electric, 105-

107.

described by Xoad, 100.
, frictional, 96-100.
. the Holtz, 108-110.
, the Topler, 110-122.
Machines compared, Iloltz and Topler, 122-

1-24.

, influence, 108.

Measurement of energy, 100-102.
Medical treatment by electricity, 142, 143.
Metals electrified by friction, 4,5, 132.
Metal screen, inductive action of, 152, 153.
Mode of action of the f rictional machine, 99,

100.

of the Holtz machine, 122, 123.
of the Topler machine, 115-124.
Multiplication of charge in Toplei machine,
121, 122.

N

Nature of electricity, 23-42.
Negative charge, 22.

potential, 12, 13, 21.

sign, 13.

Non-conductors, 4, 5, 6.
Non electrics, 4.

O

O/one, generation of, 131.

I'ail experiment, CO-65.
Pane, the charged, 72-74.
Plates, image, 103. 104.
Points, air current from, 104, 105.

, influence of, C9, 70.
Polarized light and electricity, 28-31,
Proof plnne, 58, 59.
Positive and negative, 12, 13, 21.

sign, 13.

Potential, atmospheric, 177-180.
and earth currents, 175-189.
, el.-ctric, 10-22,
, difference of, 11, 12.
, difference of, between atmospheric

strata, 179, 180.
, diurnal and seasonal variation of, 177-

179.

of jar P, in Topler machine. 139, 140.
. reversal of, in Tbpler machine, 120, 140.
, zero, 13, 65, 66.

Power, transmission of, by static electricity,
128,129.

Q

Quadrant electrometer, Thomson's, 169-174.
Quantity and intensity, 6-8.

Radiant energy, 31.
matter, 154.

Replenisher for electrometer, 164.

Repulsion, 1-4, 15, 16, 1C9.

Residual charge, 84, 85.

Reversal of potential in Topler machine, 120,
140.

Rotation of Topler machine, direct and re-
versed, 138, 139.

Rotary movement in Aracua, 149-151.

s

Silent discharge, 89.

Source of electric supply of the Topler ma-
chine, 129-132.

Spark, the, its direction, subdivision, and
color, 133-138.

, and snap, 39, 40.
Specific inductive capacity, 51-54.
Spheres, electrified, 68, 69.
Spheroid, electrified, 70, 71.
Spontaneous discharge, 88.
Static electricity defined, 8, 9.
Surface condensation , 55-58.

, thickness of electrified, 66.

transmission , 58.

Telegraph lines, observations on, 186-183, 204,

205
Tides, electric, 183, 184.

228

INDEX.

Time of electric discharge, 126-123.
Thermopile, illustrations from the, 175, 176.
Thickness of electrified surface, 66.
Thunder, 211, 214.

clouds, formation of, 207-208.
Topler machine, the, 110-122.

, the four-plate, 114.

, experiments with, 125-145.

, mode of action of, 115-124.
Tornadoes, 223.
Torsion balance, Coulomb's, 156-161.

, inaccuracy of the, 159 101.
Transmission, electric, in vacua, 146-154.

of power by static electricity, 118, 12'J.

, surface, 58.
Tubes, Geissler, 147, 148.
Tube, vacuum, 146, 147.

u

Universal discharger, 87, 88.
Unit jar, Lane's, 101, 102.

Vacua, electric transmission in, 32, 146-154.

, electric transmission in low, 146-149.

, electric transmission in high, 149-154.

, rotary movement in high, 149-151.
Vacuum tube, 146, 147.

w

Wave theory, the, 31-37.
Whirl, the electric, 104.
Wind, electric, 104, 105, 143.

Zero potential. 13. C5, 66.

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