FRICTIONAL ELECTRICITY.
[Illustration: Fig. 159. Franklin and his kite.]
Of all the agents with which man is acquainted, not one can afford a
greater source of wonderment to the ignorant, of meditation to the
learned, than the effects of that marvellous force pervading all matter
called electricity. We look at matter endowed with life, and matter
wanting this divine gift, with some degree of interest, depending on our
various tastes and occupations; we know at a glance a bird, a beast, or
a fish; we observe with pleasure and admiration the wonderful changes of
nature, and know that a few seeds thrown into the broken clods and
well-tilled earth may become either the waving, golden corn-field or in
time may grow from the tender little shrub to the stately forest-tree;
we know all these things because they belong to the visible world, and
are continually passing before our eyes: but in looking at the visible,
we must not forget and ignore the invisible. It may with truth be [Page
174] stated that the greatest powers of nature are all concealed, and if
any truth would lead us from Nature to Nature's God, it is the fact that
no visible, solid, tangible agent can work with so much force and power
as invisible electricity. Many centuries passed away since the
commencement of the Christian era, before the human mind was prepared to
appreciate this great power of nature; other forces had claimed
attention, and the difference in the presence or absence of two of the
imponderable agents, heat and light, as derived from the sun, in the
effects of the change of the seasons, and other common facts, had led
philosophers to speculate early upon their nature; but electricity, from
its peculiar properties, long escaped observation, and it was not until
the beginning of the eighteenth century (about 1730) that any material
facts had been discovered in this science, when Mr. Stephen Grey, a
pensioner of the Charterhouse, discovered what he termed _electrics_ and
_non-electrics_, and also the use of insulating materials, such as silk,
resin, glass, hair, &c.; and it is obvious that, until the latter fact
was discovered, the science would remain in abeyance, because there
would be no mode of preserving the electrical excitement in the absence
of non-conductors of this force.
The year 1750 was remarkable for Volta's discoveries and Dr. Franklin's
identification of the electricity of the machine with the stupendous
effects of the thunderstorm. Sir Humphry Davy, in 1800, with his
commanding genius, threw fresh light upon the already numerous
electrical effects discovered. In 1821, Faraday commenced his studies in
this branch of philosophy; which he has since so diligently followed up,
that he has been for some years, and is still the first electrician of
the age. From the commencement of the present century, discoveries have
succeeded each other in regular order and with the most amazing results;
and now electricity is regularly employed as a money-getting agent in
the process of the electrotype and electro-silvering and gilding; also
in the electric telegraph; and in a few years we may possibly see it
commonly employed as a source of artificial light.
The nature of electricity, says Turner, like that of heat, is at present
involved in obscurity. Both these principles, if really material, are so
light, subtle, and diffuse, that it has hitherto been found impossible
to recognise in them the ordinary characteristics of matter; and
therefore electric phenomena may be referred, not to the agency of a
specific substance, but to some property or state of common matter, just
as sound and light are produced by a vibrating medium. But the effects
of electricity are so similar to those of a mechanical agent, it appears
so distinctly to emanate from substances which contain it in excess, and
rends asunder all obstacles in its course so exactly like a body in
rapid motion, that the impression of its existence as a distinct
material substance _sui generis_ forces itself irresistibly on the mind.
All nations, accordingly, have spontaneously concurred in regarding
electricity as a material principle; and scientific men give a
preference to the same view, because it offers an easy explanation of
phenomena, and suggests a natural language intelligible to all.
[Page 175]
There are five well-ascertained sources of electricity, and three which
are considered to be uncertain. The five sources are friction, chemical
action, heat, magnetism, peculiar animal organisms. The three uncertain
sources are contact, evaporation, and the solar rays.
_First Experiment._
A stick of sealing-wax or a bit of glass tube, perfectly dry, rubbed
against a warm piece of flannel, has elicited upon its surface a new
power, which will attract bits of paper, straw, or other light
materials; and after these substances are endowed with the same force, a
repellent action takes place, and they fly off. One of the most
convenient arrangements for making experiments with the attractive and
repellent powers of electricity is to fix with shell-lac varnish round
discs of gilt paper, of the size of a half-crown, at each end of a long
straw that is supported about the centre with a silk thread, which may
hang from the ceiling or any other convenient support. (Fig. 160.)
[Illustration: Fig. 160. A. The glass pillar support. B. Straw with
discs, hanging by a silk thread.]
The varnish is easily prepared by placing four or eight ounces of
shell-lac in a bottle, and pouring enough pyroxylic spirit (commonly
termed wood naphtha) upon the lac to cover it. After a short time, and
by agitation, solution takes place. In a variety of ways friction is
proved to be a source of electricity, and forms a distinct branch of the
science, under the name of _frictional_ electricity.
_Second Experiment._
The nature of chemical action has been already explained, and is alluded
to here as a source of electricity of which the proof is very simple. A
piece of copper and a similar-sized plate of zinc have attached to them
copper wires; these plates are placed opposite to, but do not touch each
other, in a vessel containing water acidulated with a small quantity of
sulphuric acid. When the wires are brought in contact, a current of
electricity circulates through the arrangement, but has no power to
attract bits of paper, straw, &c. In order to ascertain whether the
current of electricity passes or not, a piece of covered copper wire is
bent several times round a magnetic needle, so that it has freedom of
motion inside the core or hollow formed by twisting the copper wire.
This arrangement, properly constructed, is called the galvanometer
[Page 176] needle, and is invaluable as a means of ascertaining the
passage of electricity derived from chemical action. (Fig. 161.)
[Illustration: Fig. 161. A. The galvanometer needle. B. Vessel
containing weak acid and the zinc and copper plates. The arrows show the
path of the electric current.]
When the wires leading from the metal plates are connected with the
extremities of the coil in the galvanometer, the needle is deflected or
pushed aside to the right hand or to the left, according to the
direction of the current.
_Third Experiment._
The third source of electricity is heat, and the effect of this agent is
well shown by twisting together a piece of platinum and silver wire, so
as to form one length. If the silver end is attached to any screw of the
galvanometer, and the platinum end to the second screw, no movement of
the magnetic needle takes place until the heat of a spirit-lamp is
applied for a moment to the point of juncture between the silver and
platinum wires, when the magnetic needle is immediately deflected.
[Illustration: Fig. 162. A. The galvanometer needle, with wires
attached. S, S. Silver wire joined to P, P, the platinum wire. The heat
of the spirit-lamp is applied at the point of juncture, +.]
_Fourth Experiment._
The fourth source of electricity--viz., magnetism--requires a somewhat
more complicated arrangement; and a most delicate galvanometer needle
must be provided, to which is attached the extremities of a long spiral
coil of copper wire covered with cotton or silk. Every time a bar magnet
is introduced inside the coil, so that the conducting wire cuts the
magnetic curves, a deflection of the galvanometer needle takes place,
[Page 177] and the same effect is produced on the withdrawal of the
magnet, the needle being deflected in the opposite direction.
The magnetic spark can be obtained by employing a magnet of sufficient
power; and the arrangement for this purpose is very simple. A cylinder
of soft iron is provided, and round its centre are wound a few feet of
covered thin copper wire, one end of which is terminated with a copper
disc well amalgamated, and the other end, after being properly cleaned
and coated with mercury, is brought into contact with the disc. Directly
this cylinder is laid across the poles of the magnet, and as quickly
removed, the point and disc, from the elasticity of the former, separate
for the moment, the contact is broken between the point and disc, and a
brilliant but tiny spark is apparent.
[Illustration: Fig. 163. A B. Horse-shoe magnet. C. Cylinder of soft
iron. D. Coil of copper wire and contact breaker.]
_Fifth Experiment._
The fifth mode of procuring electricity would require the assistance of
an electrical eel, a fine specimen of which (forty inches in length) was
exhibited at the Adelaide Gallery some years ago. Various experiments
were made with this animal, and the author had the pleasure of
witnessing all the ordinary phenomena of frictional electricity,
illustrated by Dr. Faraday, with the assistance of the animal
electricity derived from this curious creature. Recent experiments have,
however, proved that the electric current is induced through the agency
of the nervous [Page 178] system. This important fact has been
communicated by M. Dubois-Raymond, whose experiment is thus recorded:--A
cylinder of wood is firmly fixed against the edge of a table; two
vessels filled with salt and water are placed on the table, in such a
position that a person grasping the cylinder may, at the same time,
insert the fore-finger of each hand in the water. Each vessel contains a
metallic plate, and communicates, by two wires, with an extremely
sensitive galvanometer. In the instrument employed by M. Dubois-Raymond,
the wire is about 3¼ miles in length. The apparatus being thus arranged,
the experimenter grasps the cylinder of wood firmly with both hands, at
the same time dipping the fore-finger of each hand in the saline water.
The needle of the galvanometer remains undisturbed; the electric
currents passing by the nerves of each arm, and being of the same force,
neutralize each other. Now, if the experimenter grasp with energy the
cylinder of wood with the right hand, the left hand remaining relaxed
and free, immediately the needle will move from west to south, and
describe an angle of 30°, 40°, and even 50°; on relaxing the grasp, the
needle will return to its original position. The experiment may be
reversed by employing the left arm, and leaving the right arm free: the
needle will, in this case, be deflected from west to north. The
reversing of the action of the needle proves the influence of the
nervous force. The conditions, it may be added, essential to the success
of the experiment are: 1st, Great muscular and nervous energy; 2nd, The
contraction of only one arm at a time; 3rd, Dryness and cleanliness of
skin; and 4th, Freedom from any kind of wound on the immersed part.
_Sixth Experiment._
In making electrical experiments of the simplest kind, it soon becomes
apparent that certain substances, such as glass, sealing-wax, &c.,
retain the condition of electrical excitement; whilst other bodies, and
especially the metals, seem wholly incapable of electrical excitation:
hence the classification of bodies into conductors and non-conductors of
electricity. This arrangement is not strictly correct, because no
substance can be regarded as absolutely a conductor, or _vice versâ_. It
is better to consider these terms as meaning the two extremes of a long
chain of intermediate links, which pass by insensible gradations the one
into the other. In the manufacture of electrical apparatus, glass is of
course largely employed, and this substance, with brass and wood,
constitute the usual materials. One of the most instructive pieces of
apparatus is the electroscope, which can be made with a gas jar, a cork,
a piece of glass tube, brass wire and ball, or a flat disc of brass,
with some Dutch metal, or still better, gold leaf. The latter is first
cut into strips by retaining the leaf between a sheet of well-glazed
paper and cutting through the paper and the copper or gold leaf,
otherwise it would be impossible to cut the metal, on account of its
excessive thinness, except with a gilder's knife and cushion. The cork
is next fitted to the gas jar, and perforated with a hole to admit the
glass tube, which must be thoroughly dry, and [Page 179] is best coated
both inside and out with the shell-lac varnish described at page 175.
Some dry silk is wound round the brass wire, so that it remains fixed
and upright in the glass tube, the end outside the jar having a ball, or
still better, a flat disc of brass attached, and the other extremity
being split so as to act like a pair of forceps, to retain a piece of
card to which the gold leaves are attached. By removing the cork, tube,
and brass wire bodily from the neck of the gas jar, and then in a
perfectly still atmosphere carefully bringing the card, slightly wetted
with gum at the extremity, on two of the cut gold leaves, they may be
stuck on, and the whole is again arranged inside the dry gas jar, and
forms the important instrument called the electroscope. (Fig. 164.) With
the help of this arrangement, a number of highly instructive experiments
are performed.
[Illustration: Fig. 164. A. The brass wire, with flat disc outside, and
forceps holding gold leaf B inside the jar. C C. The glass tube.]
_Seventh Experiment._
First, the difference between conductors and non-conductors is admirably
shown by rubbing a bit of sealing-wax against a piece of woollen cloth
or flannel; on bringing the wax to the brass disc of the electroscope
the gold leaves no longer hang quietly side by side, but stand out and
repel each other, in obedience to the law "_that bodies similarly
electrified repel each other_." If the brass cap is touched whilst the
leaves are in this electrical state, they fall again to their original
position, showing that sealing-wax, after being excited, retains its
electrical condition, as also the gold leaves, because they are
supported on glass, or what is termed _insulated_--_i.e._, cut off from
conducting communication with surrounding objects. When, however, the
sealing-wax is passed through a damp hand, or the brass disc of the
electroscope touched, the electricity is conveyed away to the earth,
because the human body is a conductor of electricity.
_Eighth Experiment._
When a brass wire is rubbed and brought to the electroscope, the leaves
do not move, in consequence of the electricity passing away to the earth
through the body as fast as it is generated: it is just like pouring
water into a leaky cistern; but if the brass wire is tied to a long
stick of sealing-wax, and this latter held in the hand whilst the wire
is rubbed with a bit of flannel, then the gold leaves of the
electroscope are affected, on account of the insulation of the metal, as
every substance which can be rubbed (even fluids, as water) produces
electricity.
[Page 180]
_Ninth Experiment._
An insulating stool is merely a piece of strong square board, supported
on glass legs, which should be well varnished. If the assistant stands
on this stool and touches the disc of the electroscope, no movement of
the leaves takes place until his coat is briskly struck with a piece of
dry silk or skin, when the usual repulsion occurs.
[Illustration: Fig. 165. Assistant standing on the insulating stool and
touching the disc of the electroscope whilst being struck with a dry
handkerchief.]
_Tenth Experiment._
If a little powdered chalk is placed inside a pair of bellows, and then
forcibly ejected on to the disc of the electroscope, the friction of the
particles of chalk against the inside of the nozzle of the bellows and
against the disc of the instrument soon liberates sufficient electricity
to cause the gold leaves to stand out and repel each other.
_Eleventh Experiment._
Whilst the leaves of the electroscope are repelled from each other by
the application of a bit of rubbed sealing-wax, they may be again caused
to approach each other on bringing a dry glass tube previously rubbed
with a silk-handkerchief; because the electricity obtained from
sealing-wax is different from that procured from glass: the former is
called _resinous_ or _negative_ electricity, the latter _positive_ or
_vitreous_ electricity. Either, separately, is _repulsive_ of its own
particles, but _attractive_ of the other. [Page 181] No electrical
excitation can occur without the separation of these two curious states
of electricity, and electrical quiescence takes place when the two
electricities are brought together; hence the fall of the gold leaves
repelled by rubbed wax when the excited glass is brought towards the
disc of the electroscope. This experiment may be reversed by repelling
the leaves first with the excited glass, and then bringing the rubbed
wax, when the same effect takes place.
_Twelfth Experiment._
To show the important elementary truth, that in all cases of electrical
excitation the two kinds of electricity are generated, take a dry roll
of flannel, and holding it as lightly as possible, rub it against a bit
of wax. If the flannel is brought to the electroscope, the leaves repel
each other, and they immediately fall when the wax is now approached,
because the flannel is in the positive or vitreous state of electricity,
whilst the sealing-wax is in the negative or resinous condition.
_Thirteenth Experiment._
Any kind of friction generates electricity. A little roll of brimstone
placed in a dry mortar and powdered, and then thrown on to the
electroscope, quickly causes the repulsion of the leaves.
_Fourteenth Experiment._
A sheet of dry brown paper laid on a flat surface, and vigorously rubbed
with a piece of india-rubber, produces so much electricity that sparks
and flashes of light are apparent in a dark room when it is lifted from
the table; and it affects the leaves of the electroscope very
powerfully, so much so that care must be taken to apply it very
carefully to the disc, or the violence of the repulsion may cause the
fracture of the gold leaves, and then a great deal of time is wasted
before they can be put on again.
_Fifteenth Experiment._
A dry wig or bunch of horse-hair when combed becomes electrical, and
likewise affects the leaves of the electroscope.
_Sixteenth Experiment._
Two dry silk ribbons, the one white and the other black, passed rapidly
together through the fingers, exhibit sparks and flashes of light when
drawn asunder, and also cause the gold leaves to repel each other.
_Seventeenth Experiment._
Much instructive amusement is afforded by testing the gold leaves when
separated from each other during either of the former experiments,
[Page 182] with an excited piece of sealing-wax. If the electricity
produced is negative, they repel each other further when the excited wax
is approached; if positive, they fall when the excited wax is brought
near them.
_Eighteenth Experiment._
When fresh, dry, ground coffee is received on to the disc of the
electroscope, as it falls from the mill, powerful electrical excitation
is displayed, and this is sometimes so apparent, that the particles
cling around the lower part of the mill or to the sides of the cup or
basin held to catch it.
_Nineteenth Experiment._
After playing a tune on a violin, hold the bow (well rosined) to the
electroscope, when the usual divergence of the leaves will be apparent.
_Twentieth Experiment._
Cut some chips from a piece of wood with a knife attached to a glass
handle, and as they fall on to the electroscope the leaves are repelled.
_Twenty-first Experiment._
Warm a piece of bombazine by the fire and then draw out some of the
threads (which are of two kinds--viz., silk and wool), and place them on
the electroscope, when divergence of the leaves immediately takes place.
_Twenty-second Experiment._
Put upon the same leg a worsted stocking and over that a silk one, if
the latter is now quickly rubbed all over with a dry hand and near the
fire, and then suddenly slipped off, the sides repel each other, and the
silk stocking retains very much the same shape as if the leg still
remained in it, and of course collapses as the electricity passes away.
_Twenty-third Experiment._
Electrical machines consist only in the better arrangement of larger
pieces of glass and a more convenient mechanical contrivance for rubbing
them, and are of two kinds--viz., the cylinder and plate machines; it is
usual to give directions for the manufacture of an electrical machine
from a common bottle, and doubtless such rude instruments have been
made, but as Messrs. Elliott Brothers, of 30, Strand, now supply
excellent small machines at a very low cost, it is hardly worth while to
incur even a small expense for an instrument that must at the best be a
very imperfect one and frequently out of order. (Fig. 166.)
[Page 183]
[Illustration: Fig. 166. A cylinder electrical machine.]
[Illustration: Fig. 167. The ordinary plate electrical machine.]
Plate machines are somewhat more expensive than cylinder ones, but at
the same time are more quickly prepared for experiments, and Mr.
Hearder, of Plymouth, states, that the secret in obtaining the greatest
amount of electricity from a cylinder machine, is to keep the inside of
the glass absolutely clean, dry, and free from dust. Sometimes the glass
of which electrical machines are made is wholly unfit for electrical
[Page 184] purposes, in consequence of the decomposition of the surface
from imperfect manufacture and the liberation of the alkali. (Figs. 167,
168.)
[Illustration: Fig. 168. Woodward's double plate electrical machine,
giving a much larger quantity of electricity than Fig. 167.]
_Twenty-fourth Experiment._
Cylinder and plate machines are furnished with proper rubbers, and
before using the instrument it is usual to remove them, and after
carefully cleaning the glass with a dry silk handkerchief before a fire,
the rubbers are scraped with a paper-knife to remove the old amalgam,
and fresh applied by first melting the end of a tallow candle slightly,
and after passing this over the rubber, the finely powdered amalgam is
now dusted on to it. Electrical amalgam is prepared by fusing one part
of zinc with one of tin, and then agitating the liquid mass with two
parts of hot mercury placed in a wooden box; when cold it should be
carefully powdered and kept in a well-stoppered bottle for use. When the
amalgam has been applied, the rubbers are again screwed in their places,
and the machine when turned (if the atmosphere is tolerably dry) will
emit an abundance of bright sparks.
_Twenty-fifth Experiment._
Attraction and repulsion are shown on a larger scale, with the
assistance of electrical machines, by placing a fishing rod (the last
joint of [Page 185] which is made of glass) in an erect position, and
attaching to the extremity a long tassel of paper from which a thin wire
passes to the prime conductor of the electrical machine; on turning the
instrument, the strips of paper all stand out and repel each other.
(Fig. 169.)
[Illustration: Fig. 169. A A. The glass joint of the fishing-rod, from
which the last joint, carrying the paper tassel, B, projects. C. The
electrical machine.]
[Illustration: Fig. 170. A. Prime conductor. B. Upper brass-plate. C.
Lower ditto. The figures are seen between B and C.]
_Twenty-sixth Experiment._
Suspend from the prime conductor by a chain a circular brass plate and
under this place another supported by a brass adjusting stand. If pith
figures of men and women are placed on the lower plate, they rise
directly the machine is turned, although sometimes, in consequence of
irregularity in the adjustment of the centre of gravity, they perversely
dance on their heads instead of the usual position; out of half a dozen
figures, one only perhaps will be found to dance well, by alternately
jumping to the upper plate and falling to the lower one to discharge the
excess of electricity; and indeed the experiment will be found to
succeed better with one or two only on the plate instead of a number, as
they cling together and impede each other's movements. (Fig. 170.)
[Page 186]
_Twenty-seventh Experiment._
An assistant provided with a wig of well-combed hair presents a most
ridiculous appearance when standing on the insulating stool and
connected by a wire with the prime conductor of the electrical machine,
every hair, when not matted together, standing out in the most absurd
manner, when the machine is put in motion.
_Twenty-eighth Experiment._
Whilst standing on the stool, sparks may be obtained from his body, and
if some tow is tied over a brass ball, and moistened with a little
ether, and presented to the tip of his finger, a spark flies off which
quickly sets fire to the inflammable liquid.
[Illustration: Fig. 171. A A A. A ring of brass wire supported on a
glass pillar inside which the spiral tube, B, revolves, and produces
beautiful and ever-changing circles of light, when connected with the
conductor, C, of the electrical machine.]
_Twenty-ninth Experiment._
If small discs of tinfoil, cut out with a proper stamp, are pasted in
continuous lines over plate glass, or spirally round glass tubes, a
very [Page 187] pretty effect is produced when they receive the sparks
from the electrical machine, and the passage of the electricity from one
disc to the other produces a vivid spiral or other line of light. When
the tube is mounted in a proper apparatus, so as to revolve whilst the
sparks pass down the spiral tube, the effect of the continuous electric
sparks is much heightened. (Fig. 171.)
_Thirtieth Experiment._
A great variety of experiments, depending on the proper arrangement of
discs of tinfoil on various tubes of coloured glass are manufactured,
and some in the form of windmills, the sails being made luminous by the
passage of the electricity. The names of illustrious electricians,
beautiful crescents, stars, and even profile portraits, have been
produced in continuous streams of electric sparks.
_Thirty-first Experiment._
When an electrified body is brought towards another which is not
electrical, the latter is thrown into the opposite state of electricity
as long as the excited body remains in its neighbourhood; and this
condition of electrical disturbance, set up without any contact or
supply of electricity, is called _induction_, and involves a vast number
of interesting facts, which are thoroughly discussed in Dr. Noad's
excellent work on electricity, but can only be briefly alluded to here.
[Illustration: Fig. 172. The lengths of brass wire supported on glass
rod pillars indented by blowpipe, so as to retain the brass wires with
the pith balls hanging from each series, the letters P and N mean
Positive and Negative, and the signs for these terms are placed above.
The letters P and N are painted on the blocks which support the glass
rods.]
If a number of lengths of brass wire, supplied with balls at the
extremities, are supported on glass legs and arranged in a line, with a
little pith ball attached to a thread hanging from each end of the
length of brass wire, the effect of induction is shown very nicely; and
when an excited glass rod is brought towards one end of the series, the
rising of the pith balls to each other betrays the change which has
occurred in the electrical state of the brass wires by the mere
neighbourhood of the excited glass tube. The glass tube is electrified
positively, and attracts the negative electricity from the brass wire
towards the end nearest to it; [Page 188] the other extremity of the
brass wire is found to be in the positive state, and this re-acting on
the next, and so on throughout the lengths, completes the electrical
disturbance in the whole series. (Fig. 172.)
_Thirty-second Experiment._
[Illustration: Fig. 173. A A. Large circular tin or brass disc with
turned-up edge half an inch deep, and containing the resinous mixture B,
which is rubbed with the warm flannel. C C. The upper plate supported by
the glass handle D, a pith ball attached to a wire shows the electrical
excitation, and the spark is supposed to be passing to the hand E.]
If an insulated brass rod (such as has been described in the last
experiment) is touched by the finger whilst under induction, it remains
permanently electrified on the removal of the disturbing electrified
body; and it is on this principle that the useful electrical machine
called the Electrophorus is constructed. This _constant_ electrical
machine--for it will remain in action during weeks and months if kept
sufficiently dry--was invented by Volta in the year 1774, and has been
brought to great perfection by Mr. Lewis M. Stuart, of the City of
London School; so that with a little additional apparatus the whole of
the fundamental principles of electricity can be demonstrated. It
consists of a flat brass or tin circular dish about two feet in diameter
and half an inch deep, which is filled with a composition of equal parts
of black rosin, shell-lac, and Venice turpentine; the rosin and the
Venice turpentine being first melted together, and the shell-lac added
afterwards, care of course being taken that the materials do not boil
over and catch fire, in which case the pot must be removed from the
heat, and a piece of wet baize or other woollen material thrown over it.
Another tin or brass circular plate of twelve inches diameter, and
supported in the centre with a varnished glass handle nine inches long,
is also provided, and the resinous plate being first excited by several
smart blows with a warm roll of flannel, the plate held by the glass
handle is now laid upon the centre of the resinous one, and if removed
directly afterwards, does not afford the electric spark; but if, whilst
standing upon the excited resinous plate, it is touched, and then
removed by the glass handle, a powerful electric spark is obtained; and
this may be repeated over and over again with the like results, provided
the plate with the glass handle is touched with the finger just before
lifting it from the resinous plate. (Fig. 173.) [Page 189] The
electricity excited on the resinous plate is not lost, and by induction
sets up the opposite condition in the plate with the glass handle. The
resinous plate, being excited with negative electricity, disturbs the
electrical quiescence of the upper plate, and positive electricity is
found on the surface touching the resinous plate, and negative
electricity on the upper one, so that when it is removed without being
touched, the two electricities come together again, and no spark is
obtained; but if, as already described, the upper plate is touched
whilst under induction, then positive electricity appears to pass from
the finger to the negative electricity on the upper side of the plate,
when the two temporarily neutralize each other, and then, when the plate
is removed, the excess of electricity derived from the earth through the
finger becomes apparent. Induction requires no sensible thickness in the
conductors, and can be just as well produced on a leaf of gold as on the
thickest plate of metal; and it should be remembered that non-conductors
do not retain their state of electrical excitation when the disturbing
cause is removed, whereas conductors possess this power, and this fact
brings us to the consideration of the Leyden jar.
_Thirty-third Experiment._
If one side of a dry glass plate is held before and touches a brass ball
proceeding from the prime conductor of an electrical machine whilst in
action, the other side is soon found to be electrical; this does not
arise from the conduction of the electricity through the particles of
the glass, but is produced by induction, the side nearest the ball being
in the positive state, and the other side negative: as glass is a
non-conductor of electricity, the effect is much increased by coating
each side with tinfoil, leaving a margin of about two inches of
uncovered glass round the covered portion, then, if one side of such a
plate is held to the prime conductor of the electrical machine, and the
other connected with the ground, a powerful charge is accumulated; and
if the opposite sides are brought in contact with a bent brass wire, a
loud snapping noise is heard, and the two electricities resident on
either side of the glass come together with the production of a
brilliant spark, or if the hands are substituted for the bent brass
wire, that most disagreeable result is obtained--viz., an _electric
shock_; hence these glass plates are sometimes fitted up as pictures,
and when charged and handed to the unsuspecting recipient, he or she
receives the electric discharge to the great discomfort of their nervous
system.
Mica is sometimes substituted for glass, and the late Mr. Crosse, the
celebrated electrician, constructed a powerful combination of coated
plates of this mineral. It consisted of seventeen plates of thin mica,
each five inches by four, coated on both sides with tinfoil within half
an inch of the edge. They were arranged in a box with a glass plate
between each mica plate, all the upper sides were connected by strips of
tinfoil to one side of the box, and all the under surfaces in the same
manner with the opposite extremity of the box. They were charged like an
ordinary Leyden battery.
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_Thirty-fourth Experiment._
If the glass plate coated with tinfoil is charged, and then placed
upright on a stand, it may be slowly discharged by placing a bent wire
on the edge with the extremities covered with pith balls. The wire
balances itself, and continues to oscillate with noise until the
electricities of the two surfaces neutralize each other. (Fig. 174.)
[Illustration: Fig. 174. A A. Glass plate or stand coated with tinfoil
on each side, B. C. Wire with pith balls oscillating during the
discharge of the glass plate.]
_Thirty-fifth Experiment._
It is easy to imagine the glass plate of the last experiment rolled up
into the more convenient form of the Leyden jar, which consists of a
glass vessel lined both inside and out with tinfoil, leaving some two or
three inches of the glass round the mouth uncovered and varnished with
shell-lac; a piece of dry wood is fitted into the mouth of the jar,
through which a brass wire and chain are passed, and the end outside is
fitted with a ball. The Leyden jar is charged by holding the ball to the
prime conductor of the electrical machine until a sort of whizzing noise
is heard, caused by the excess of electricity passing round the
uncovered part of the jar and not through it, as the smallest crack in
the glass of the Leyden jar would render it useless. Electricity is
sometimes called a fluid, and the fact of collecting it like water in a
jar, helps us to understand this analogy. The noise, the bright spark,
or the shock are obtained by grasping the outside with one hand and
touching the ball with a brass wire held in the other. (Fig. 175.)
[Illustration: Fig. 175. The Leyden jar and brass wire discharger.]
_Thirty-sixth Experiment._
The jar is silently discharged if the balls are removed from the
discharger and points used instead; so, also, the whole of the
electricity produced by an electrical machine in full action may be
readily drawn off by a pointed conductor, such as a needle, placed at
the end of a brass wire. Electricity passes much more rapidly through
points than rounded surfaces, hence the reason why all parts of
electrical apparatus are free from sharp points and rough asperities.
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_Thirty-seventh Experiment._
Extremely thin wires may be burnt by passing the charge of a large
Leyden jar through them. The show jars, called specie jars, usually
decorated and placed in the windows of chemists' shops, make excellent
Leyden jars, when not too thick; and with two of the largest, all the
interesting effects produced by accumulated electricity may be
displayed. To pass the discharge through wires, nothing more is required
than to strain them across a dry mahogany board, between two brass wires
and balls, and if a sheet of white paper is placed under them, most
curious markings are produced by the fine particles of the deflagrated
metal blown into the surface of the paper. An arrangement of two or more
Leyden jars is usually called a Leyden Battery, just as a single cannon
is spoken of as a gun, whilst two or more constitute a battery. (Fig.
176.)
[Illustration: Fig. 176. A. Mahogany board with a sheet of white paper
and three pairs of brass wires and balls fixed in the wire, three on
each side. The thin wires are stretched between the balls, and the lower
one is in course of deflagration. B B. Charged large Leyden battery of
two jars; the arrows indicate the path of the electricity.]
_Thirty-eighth Experiment._
Little models of houses, masts of ships, trees, and towers are sold by
the instrument makers, and by placing a long balanced wire on the top of
the pointed wire of a large Leyden jar, having one end furnished with
wool to represent a cloud, a most excellent imitation of the effects of
a charged thunder-cloud is produced. The mechanical effect of a flash of
lightning has been analysed, and it has been stated, in one instance,
that the power developed through fifty feet was equal to a 12,220
horse-power engine, or about the power of the engines of the _Great
Eastern_, and that the explosive power was equal to a pressure of three
hundred millions of tons. (Fig. 177.)
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[Illustration: Fig. 177. A. Charged Leyden jar with balanced wire and
wool at B, representing a thunder-cloud. C. The obelisk overturned with
the discharge. D. Another model of the gable end of a house; the square
pieces of wood fly out when the continuity of the conductor is broken.]
It was the learned but humble minded Dr. Franklin who established the
identity between the mimic effects of the electrical machines (such as
have been described), and the awe-inspiring thunder and lightning of
nature. A copper rod, half an inch thick, pointed and gilt at the
extremity, and carried to the highest point of a building, will protect
a circle with a radius of twice its length. The bottom of the rod must
be passed into the earth till it touches a damp stratum.
[Illustration: Fig. 178. A storm.]
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