ELECTRICITY.
370. =Origin of the Term.=--The ancients observed that when certain
substances were rubbed together singular phenomena were produced.
One of these substances was amber, and as the Greek name for this
is ηλεκτρον, the power which is thus excited into action has been
called electricity.
[Illustration: Fig. 257.]
371. =Attraction and Repulsion in Electricity.=--One of the most
common effects of electricity is attraction. If we rub a tube or
rod of glass with woolen or silk it will attract light articles,
such as cotton, feathers, lint, etc., so that they will adhere to
it. But repulsion is also an effect of electricity under certain
circumstances. In order that the explanation of these two opposite
effects may be clear to you, I will detail some of the experiments
which exhibit both. Suppose that we have a pith ball, A, Fig. 257,
suspended by a silk thread, B, from a stand, C. I must premise
that silk does not readily let electricity pass over it, or is a
non-conductor, and therefore any electricity communicated to the
pith ball will remain there unless something be brought in contact
with it or very near it. If now you rub a glass tube, thus exciting
electricity upon it, and then bring it near the ball, it will
attract the ball to it, and then in a moment repel it, so that it
will stand off from the tube and retreat from it if you follow the
ball with the tube. Why is this? It is supposed that there is a
subtile fluid on the electrified glass, some of which passes to the
ball as it touches the glass, so that the ball and the glass are
in a similar condition. But the particles of the fluid repel each
other; and this is the reason that the ball is repelled from the
glass as soon as it becomes charged with a part of the electricity
of the glass. For the same reason if two pith balls hanging from
a standard become electrified from a glass tube or rod they will
repel each other, for they are in the same electrical condition.
372. =Vitreous and Resinous Electricity.=--Suppose now that you
rub a rod of sealing-wax with woolen or silk and hold it near a
pith ball which has been electrified from glass. It will attract
the ball. The reason is that an electricity is excited on the
sealing-wax of a different kind from that which is excited on the
glass. The former is called _resinous_, and the latter _vitreous_
electricity. They are supposed to be two fluids, which have a
strong attraction for each other, while, on the other hand, the
particles of either fluid are repellent to each other. It is this
attraction between the two fluids which causes in the case just
stated the sealing-wax to attract the ball to itself. We can
illustrate this attraction in another way. Take two pith balls and
electrify them, the one from glass and the other from sealing-wax.
Brought near together they will attract each other, because they
have two unlike electricities. This, you see, is just the reverse
of the effect produced in an experiment cited at the conclusion
of § 371, in which the electricities were alike in the two pith
balls. Again, if you bring the rubbed sealing-wax near to the ball
electrified from glass, the ball will be attracted, and the same
effect will follow if you bring the rubbed glass near to the ball
electrified from sealing-wax.
373. =Franklin's Theory.=--In § 372 is developed the theory now
commonly received in regard to electricity. The theory of Franklin
was different. He supposed that there is but one electric fluid,
and that all bodies are in their usual state charged with a certain
portion of it, some having more than others, according to their
capacity for electricity. While a body is in its usual state there
is no manifestation of electricity. The fluid is in a quiescent
condition, because its particles are prevented from repelling
each other by the attraction which exists between them and the
particles of the substance. But this quiescence can be disturbed
by friction and other causes. Thus if a glass rod be rubbed with
a piece of silk, the natural equilibrium is disturbed, the glass
having an excess and the cloth a deficiency of electricity.
The glass is therefore said to be _positively_ and the cloth
_negatively_ electrified. The equilibrium can be restored in the
case of a positively electrified body by having its excess drawn
off, and in the case of a negatively electrified body by having
its deficiency made up by receiving electricity from other bodies.
Though this theory is discarded, the terms positive and negative
derived from it are retained, being applied to the two fluids[6] or
electricities, and they are often designated by the two signs + and
-.
374. =Upon What the Kind of Electricity Excited Depends.=--It
depends on what a substance is rubbed with whether vitreous or
resinous electricity is excited in it. Thus smooth glass rubbed
with woolen cloth or silk will be positively electrified; while
if it be rubbed upon the back of a cat it will exhibit negative
or resinous electricity. So, also, if a resin, as gumlac or
sealing-wax, be rubbed with silk or woolen cloth, it will be
charged with resinous electricity, but it will be charged with
vitreous or positive if it be rubbed with sulphur. The terms
vitreous and resinous are therefore incorrect, for they are based
upon the idea that one kind of electricity is always excited on
glass, whatever the friction may be made with, and that the other
kind is always excited on resins. The most decided illustration of
the incorrectness of these terms we have in the fact, that while
smooth glass rubbed with silk or woolen cloth becomes charged with
positive (vitreous) electricity, roughened glass rubbed with the
same gives us negative (resinous) electricity. Below I give a table
of substances, any one of which has positive electricity developed
on it when it is rubbed with any substance below it on the list,
and negative when rubbed with any substance above it:
1. Cat-skin.
2. Polished glass.
3. Woolen cloth.
4. Feathers.
5. Wood.
6. Paper.
7. Silk.
8. Sealing-wax.
9. Amber.
10. Roughened glass.
11. Sulphur.
375. =Conductors and Non-Conductors.=--Electricity passes over
the surface of some substances very readily; while over others
it moves with very great difficulty, and therefore very slowly
and sparingly. The former are termed conductors, and the latter
non-conductors. As in the case of heat, so with electricity
there are no substances which are wholly non-conducting. The
best of all the conductors are the metals, those least liable
to oxydation being the most perfect. Next come charcoal, water,
living substances, flame, smoke, steam. The best non-conductors
are gumlac and gutta-percha. Then come amber, resins, sulphur,
glass, silk, wool, hair, feathers, cotton, paper. Non-conductors
are sometimes called _insulators_, from the Latin word _insula_,
as they serve to confine electricity within certain bounds, and
prevent its escaping. Thus in the experiments with pith balls,
already cited, the silk threads by which they are suspended prevent
the electricity from escaping from them. So the glass knobs on
which the wires of the telegraph rest are insulators, preventing
the electric fluid from escaping down the poles into the ground.
[Illustration: Fig. 258.]
376. =Electricity Always on the Surface.=--There is a marked
difference between heat and electricity in the manner in which they
are disposed of. Heat pervades all the particles of substances, and
in its conduction spreads through them, while electricity in its
ordinary movements operates altogether on the surface. A hollow
ball, therefore, can contain as much electricity as a solid, and
a hollow conductor of electricity is just as effectual as a solid
one. The following experiment exhibits in a very striking manner
this disposition of electricity to occupy the surface alone: Let
_a_, Fig. 258, be a metallic ball supported by a glass stand,
_b_; and let _c c_ be metallic caps which will just cover the
ball, having non-conducting handles, either glass or gumlac. Now,
after having charged the ball with electricity, let the caps held
by the insulating handles be carefully placed over the ball. On
withdrawing them it will be found that the electricity of the ball
has all passed to the outer surface of these caps.
377. =Electrics and Non-Electrics.=--It will be observed, on
looking over the list of conductors and non-conductors, that among
the non-conductors are those substances in which electricity is
easily excited by friction, such as glass, amber, silk, etc. These
were therefore called electrics. The conductors, on the other hand,
were called non-electrics, it being supposed that electricity could
not be excited with them. But this has been found not to be true.
For example, if a metal be insulated by being placed on a pillar
of glass or of gumlac, so that the electricity, when excited, can
not pass off readily, its generation can be made manifest. It is
probably true that every substance is more or less an electric, it
being difficult to make this manifest in the case of conductors,
because the electricity passes off as fast as it is generated.
378. =Electricity Every Where Active.=--I have said that there is
electricity in all substances, each having its own capacity for it,
but that in the usual condition of substances the electricity is in
a state of equilibrium, and therefore of quiet. We see this quiet
disturbed whenever there is a thunder-storm, when we rub glass or
silk, or a cat's back, or when we work an electrical machine. But
the active state of electricity is not limited to such palpable
demonstrations as these. Electricity is undoubtedly in action every
where and always, although we can seldom appreciate and measure
its action. Wherever there is motion there is a disturbance of
the equilibrium of electricity, and a consequent return to this
equilibrium. And this change from the one state to the other must
be the constant cause of important changes and operations in the
world around us, and in our own bodies. Let us look at some of the
indications of this universality electrical action. The friction
of any electric upon another awakens it. The friction of the belts
upon the drums in cotton factories does it quite freely. Every
stroke of India rubber upon paper as you erase a pencil mark
excites electricity. The blowing of air upon glass does the same.
So, also, does the blowing off of steam from an engine. Electricity
has been excited even upon ice by rubbing it when cooled down to
13° below zero. Experiments upon the air have shown that there is
usually some free electricity in it, the atmosphere being generally
in a positive state, especially when the air is dry and clear.
It is constantly generated from one source and another. It is
generated every where by evaporation. Every gust of wind, causing
friction of the particles of the air upon various substances,
generates it. Motion of every kind probably generates it. Chemical
action, as you will see in another part of this chapter, generates
it every where. It is generated also in the operations of life, and
in some animals there are special organs--electrical batteries--for
the generation of this agent.
[Illustration: Fig. 259.]
379. =Induction.=--A remarkable influence is exerted by an
electrified body upon another body in its usual state when brought
near it, and this influence is called induction. I will illustrate
this by Fig. 259. Let A be a metallic ball standing on a glass
pillar, and charged with positive electricity. Let B be a metallic
cylinder supported upon two glass pillars. Now if A be placed near
B, but not near enough for the electric spark to pass from it to
B, it will destroy the equilibrium of the two electricties in B,
the negative electricity being accumulated at the end near A,
and the positive at the remote end. This is because the positive
electricity in A repels its like in B and attracts the unlike
fluid. You observe that there is a pair of pith balls suspended
at each end of B, and also at the middle. The two balls at the
positive end repel each other because they are charged with the
same electricity, and so with the balls at the negative end. But
the balls hanging from the middle are not affected, because they
are on middle ground between the two electricities. Here is no
communication of electricity from A to B, but only an influence
upon the quiescent balanced electricities of B. Accordingly, if the
surplus electricity of A be discharged by putting the hand or any
good conductor upon it the influence will cease, the equilibrium in
B will be restored, and the pith balls will all hang straight down.
The same effect will be produced if A be withdrawn to a distance
from B, and the influence will be renewed if A be brought near
again.
[Illustration: Fig. 260.]
If instead of one conductor we use two, B and C, Fig. 260, and have
them in contact, we shall have the negative electricity on B and
the positive on C. Now if we withdraw C from B we may have the two
electricities separate, B being charged with the negative and C
with the positive.
[Illustration: Fig. 261.]
380. =Electrical Machine.=--You are now prepared to see how the
common electrical machine operates. There are two kinds--the plate
and the cylindrical. The plate machine, Fig. 261, has at _p_ a
large plate of glass, and at _r_ a rubber which consists of two
brass plates lined with leather which is stuffed, the pressure of
which upon the glass is regulated by a screw. Above this rubber is
a brass ball, _d_, and a brass chain connects the rubber and the
ball with the floor, or, in other words, with the earth. At _c_ is
what is called the prime conductor--a hollow brass cylinder with
rounded ends, having attached to it a rod with points, as seen at
_a_. There is a similar rod attached to it on the other side of the
glass plate. The different parts of the instrument are supported on
glass pillars, _g g g_, standing on a wooden platform. The lower
part of the plate is covered with a case of silk, which, being a
non-conductor, prevents the electricity on the glass from being
lost in the air, and also serves to keep the plate free from dust.
The rubber is covered with an amalgam of tin, zinc, and mercury,
this being found very effectual in exciting electricity. The
operation of the machine is this: As the plate revolves positive
electricity is collected upon the glass, and negative electricity
upon the rubber. The former, as it comes to the points at _a_, goes
to them and passes on by the rods to the prime conductor, while
the latter passes from the rubber by the chain to the earth. The
points at _a_ are of great service in collecting the electricity,
because the fluid is always much more ready to go to points than to
conductors of a blunt shape.
[Illustration: Fig. 262.]
The cylinder machine is represented in Fig. 262, _a a_ being a
glass cylinder, which can be turned rapidly by the multiplying
wheel, _b b_. At _c_ is a piece of silk, and on the rear part of
the cylinder is the rubber. At _d_ is the prime conductor.
381. =Experiments.=--Many experiments may be tried with the
electrical machine. I will cite a few of them:
If pith balls be attached to the prime conductor, as seen in Fig.
261, they will stand out from each other as soon as the machine
is worked, because they are both charged with the same kind of
electric fluid.
Let a small figure with its head covered with hair be placed upon
the prime conductor. As soon as the conductor becomes charged
with electricity the hair stands out, as represented in Fig. 263,
for the same reason that the pith balls diverged in the previous
experiment.
[Illustration: Fig. 263. Fig. 264.]
So, also, if you place on the conductor a figure having attached to
it strips of tissue-paper, they will diverge in the manner shown in
Fig, 264.
[Illustration: Fig. 265.]
Let a metallic plate, _a_, Fig. 265, be suspended by a chain to
the prime conductor, and another plate, _b_, be supported upon a
conducting stand. If figures of paper or pith be placed between
these plates as the machine is worked they will move about briskly
between the plates, being alternately attracted and repelled by the
communication of the electricity.
[Illustration: Fig. 266.]
The experiment represented in Fig. 266 is a very beautiful one.
Let _a b_ be a brass rod with an arch, _g_, by which it can be
suspended from the end of the prime conductor. To this rod are
suspended three bells, the two outer ones by chains, and the middle
one by a silk thread; also two clappers, _d_ and _e_, by silk
threads. The middle bell has a chain, _f_, connecting it with the
table--that is, with the earth. The operation of the apparatus is
this: As soon as the outer bells become electrified they attract
the clappers. These, on touching the bells, receive a portion of
their electricity, and are repelled. They therefore strike against
the middle bell, to which they impart the electricity which they
received from the outer bells. They swing back again then in the
same state that they were in at first, and now are attracted again
by the outer bells. This goes on so long as the electricity is
communicated.
[Illustration: Fig. 267.]
Let there be pasted upon a slip of glass a continuous line of
tin-foil, going back and forth, as represented in Fig. 267, and
let there be a ball, G, connected with one end of, the foil. The
word light is made upon it by cutting out with a sharp knife little
portions of the foil. If now with your finger on one end of the
line of foil at _a_, you present the ball G to the prime conductor,
the electric fluid will run along the whole length of the line from
G to _a_. In doing this the letters are beautifully illuminated, a
spark being produced at each interruption of the line. So rapid is
the passage of the electricity that the whole appears to the eye
simultaneously illuminated.
[Illustration: Fig. 268.]
382. =The Insulating Stool.=--This consists of a wooden top, _a_,
Fig. 268, supported by glass legs, _c c_. It can be made simply by
boring holes in the four corners of a piece of board sufficiently
large to admit the necks of bottles. Many amusing experiments can
be tried with this. A person standing upon it can be highly charged
with electricity by holding a chain connected with the prime
conductor. The hair will rise up as represented in Fig. 263, and he
can give electric shocks to other persons from any part of his body.
[Illustration: Fig. 269.]
383. =Electricity Discharged from Points.=--I have already, in
giving an account of the electrical machine, spoken of the
readiness with which electricity is received by points. It is
discharged from them with equal readiness; so that, if a metallic
point be attached to the prime conductor, the electricity will be
carried off into the air nearly as fast as it is received upon the
conductor. And as it passes off it creates a current in the air as
it strikes upon it. The reaction of the air upon the electrical
currents can be very prettily exhibited with the apparatus
represented in Fig. 269, which consists of a cap, A, resting upon
the point of a rod, and having pointed wires branching out from
it in a wheel-like arrangement. You observe that the points are
all bent one way. If this apparatus be set upright upon the prime
conductor, the wheel can be made to revolve rapidly by working
the machine. As the reaction of the air against the gases issuing
from the rocket makes it rise, so the same reaction against the
electricity issuing from these points causes the circular motion.
If electricity be discharged from a point in a darkened room it
appears like a brush of light, as represented in Fig. 270.
[Illustration: Fig. 270.]
[Illustration: Fig. 271.]
384. =Leyden Jar.=--The Leyden jar, Fig. 271, is so called because
it was contrived at Leyden. It was suggested by an accidental
result of an experiment tried there with the electrical machine.
It consists of a glass jar coated upon the inside and the outside
to near the top with tin-foil, and having a metallic rod passing
through the cork, with one end touching the inner coating, and the
other surmounted by a brass ball or knob. The jar is charged by
holding the knob near to the prime conductor while the machine
is worked. The electricity passes by the metallic rod to the
inside coating of the jar, and accumulates there. This is positive
electricity. In the mean time there is an accumulation of negative
electricity on the outside coating. But how is this? It is by the
repulsion of positive electricity for itself, and its attraction
for its opposite, negative electricity. As you hold the jar in your
hand positive electricity is repulsed from its outside through your
arm earthward, while negative electricity is attracted to it by the
positive which is within. The two fluids get as near to each other
as possible. They are prevented from coming actually together by
the non-conducting quality of the glass. If a slip of tin-foil were
made to connect the inside foil with the outer, there would be no
accumulation of electricity on the inside, for as fast as it passed
from the prime conductor to the inside it would pass out over the
bridge of foil to the outside, and down your arm and body to the
earth.
[Illustration: Fig. 272.]
If there were no communication of the outside with the earth the
jar would not be charged. No electricity would pass to it, because
the positive electricity which is on the outside can not be driven
off, and no negative electricity can be received. To make this
plain, suppose that the jar, _a_, Fig. 272, having a bent rod, is
suspended to the prime conductor, _b_. Here you have the inside
tin-foil connected with the source of positive electricity. But
the outside is insulated. No electricity can pass from it or to
it. It has both positive and negative electricity, but they are in
equilibrium. If there were a preponderance of negative electricity
there, it would attract positive electricity to it as near as
possible, and so the latter would enter the jar from the conductor.
But there is no such preponderance, and so, though a little may
enter--a spark or two--there will not be enough to charge the jar
sensibly, because there is no attraction in that direction. But
bring now another jar, _c_, near to the outside coating of _a_,
and there is a movement at once in the electricities. The positive
electricity has a chance now to pass off from the outside of _a_ to
the inside of _c_, leaving therefore a preponderance of negative
electricity on the outside of _a_, which exerts an attractive
influence on the positive electricity of the conductor drawing it
to the inside of the jar.
[Illustration: Fig. 273.]
385. =Discharge of the Leyden Jar.=--The jar may be discharged by
making a communication between the inside and outside by means of
any conductor. It may be done with the discharging-rod (Fig. 273).
This has two slender metallic rods, with brass knobs at their ends,
and jointed at _a_, so that the knobs can be separated to different
distances. The handle is glass, so that as the electricity passes
through the rods none of it may be communicated to the hand. In
discharging the jar one knob is placed upon the outside foil, and
the other is brought near to the knob of the jar. The two fluids
now rush together from their attraction, and in doing so a bright
flash is produced, going from the knob of the jar to that of the
discharging-rod, and with this a report. You can yourself be the
conductor to discharge the jar. If, having one hand upon the
outside of the jar, you bring the other near its knob, the fluids
meet in you as they do in the discharging-rod, and a shock will be
experienced in proportion to the amount of charge in the jar. Any
number of persons can together receive the same shock. To do this
they must join hands, and the person at one end of the row must
touch the knob of the jar while the person at the other end has his
hand upon the outside.
You may touch either the knob of the jar or the outside coating
_separately_, and the power that is in it remains quiet; but the
moment that you touch both it bursts forth, because a bridge is
made upon which the two fluids can meet.
In a dry air the charge in the jar can be retained for some
time, the communication between the two electric fluids being
very slow through the medium of air. It is otherwise when there
is much moisture in the air, for water is a good conductor. For
this reason, if you let the moisture from your breath come upon
the jar between the outside coating and the rod, the jar will be
discharged soon, though imperceptibly, the moisture making a medium
of communication between the inner and outer electricities.
[Illustration: Fig. 274.]
386. =The Electrical Sportsman.=--In this contrivance, Fig. 274,
the discharge of the Leyden jar is very prettily exhibited. The
jar, _c_, has a rod with two branches. On the end of one of these,
B, are suspended pith balls cut in the shape of birds. On the other
is a knob by which the jar can receive its charge from the prime
conductor. After it is charged it is placed on the stand with its
knob, _b_, near the gun, _a_, of a metallic figure. The suspended
birds, you observe, stand out from each other, because they are
charged with the same fluid, positive electricity, and therefore
are repellent. Now when the chain, _e_, which is connected with
the outside of the jar, is made to touch the foot of the metallic
image, the connection between the inside and outside of the jar is
established. Of course there is an instantaneous flash between _a_
and _b_, and the birds, losing their electricity, fall, and hang
as they did before the jar was charged.
[Illustration: Fig. 275.]
387. =Electrical Battery.=--By combining together a number of
jars, having the insides all connected together, as seen in Fig.
275, with metallic rods, and the outsides connected together in
a similar manner, we have what is termed an electrical battery.
By such an arrangement we can accumulate a large amount of
electricity, which can be discharged in the same way essentially as
in the case of the single jar.
388. =Light of Electricity.=--The light produced by electricity is
not occasioned by any thing like combustion. It depends obviously
upon the resistance which is offered to its passage. Thus when the
electric fluid passes through air from the prime conductor to the
knob of the Leyden jar it causes a flash of light, but when it
arrives at the knob the flash ceases. What is the reason of the
difference? In both cases it has the resistance of the air, for
when it comes to the knob it passes over the _surface_ of the knob
and rod; but in the latter case it is so diffused in its conduction
over the metallic surface that it meets with much less resistance
from the air. By experiments with the air-pump it is found that the
denser the air is the more vivid is the spark; and if electricity
be passed through a glass vessel from which the air has been mostly
exhausted we have the streams of light seen in the aurora borealis,
which are so strikingly in contrast with the vivid flashes of the
lightning. In the experiment, § 381, in which the word light is
made by the passing electricity, we have a striking illustration of
the production of the spark by the resistance of the air. If the
foil were one continuous surface the electricity would be diffused
over it without giving any light. It is only where the electric
fluid has to leap through the air from one portion of foil to
another that the light is seen.
389. =Sound of Electricity.=--The report of electricity is a sort
of crack or snap from the sudden condensation of the air by the
rapid passage of the fluid. The rolling of thunder is occasioned by
the reverberation of the first sound among the clouds. The nearer
the flash is to us the more like a crack is its first sound as it
comes to our ears.
390. =Mechanical Injuries from Electricity.=--When any great amount
of electricity meets in its passage with any imperfect conductor
it does much violence to it. Thus it rends wood, scatters water,
breaks glass, etc. Various experiments have been tried illustrating
the manner in which mechanical injuries result from electricity.
Thus if it be made to pass through a card or several leaves closely
pressed together, there is a burr on each side of such a character
as to show that two forces moving in opposite directions have made
their passage.
391. =Heat Produced by Electricity.=--Electricity always produces
in its passage some amount of heat, probably by its mechanical
effect. When it is diffused over a large conducting surface the
heat is not sufficient to be observable; but if it be confined to
the surface of a small wire the heat may be sufficient to melt or
even burn it. Various effects can be produced by the heat thus
caused by the passage of electricity. Gunpowder may be exploded
by it. Alcohol and ether may be readily ignited by it, especially
the latter. Gas can sometimes be lighted by pointing the finger to
an opened burner after walking across the room two or three times
briskly, rubbing the feet upon a thick carpet.
[Illustration: Fig. 276.]
392. =Franklin's Discovery.=--It had very early been conjectured
that the electricity produced by the electrical machine is
identical with lightning; but it was reserved for our countryman
Franklin to prove the fact. A tall spire which was being erected
in Philadelphia in 1752 he conceived might be used in his
investigations, but before it was completed the sight of a boy's
kite in the air suggested to him another plan. He made a kite
by stretching a silk handkerchief over a frame, and sent it up
as he saw a thunder-shower rising, his only companion being his
son. Having raised the kite, he attached to the end of the hempen
string a key, and also a silk ribbon, by which he insulated his
apparatus, as seen in Fig. 276. He now watched with much anxiety
the result. A cloud arose, which he supposed, from its appearance,
was well charged with electricity, and yet no effect was seen.
Franklin began to despair; but he soon saw some loose fibres of
the hempen string bristling up, and, applying his knuckle to the
key, received just such a spark as he had often received from the
conductor of an electrical machine. The discovery was made, and
Franklin was at once overcome with emotion at the thought of the
immortality which it would give his name. He felt very much as
Archimedes did when, after making one of his grand discoveries as
he lay in a bath, he went home saying all the way, Εὕρηκα! Εὕρηκα!
The fame of the discovery, made in a manner so simple and yet so
original, spread every where, and prompted to many experiments by
other philosophers. One, Professor Richman of St. Petersburg, fell
a victim to his investigations. While he was attending a meeting of
the Academy of Sciences he heard the sound of distant thunder, and
hastened home to make some observations with an apparatus which he
had erected. While doing this a charge of electricity flashed from
the conducting rod, and piercing his head killed him instantly. His
assistant, who stood near, was struck down, and remained senseless
for some time, and the door of the room was torn from its hinges.
393. =Lightning-Rods.=--It was the discovery of Franklin which
led to the custom of attaching lightning-rods to buildings. The
object of a lightning-rod is to conduct any electricity in a cloud
that may come over the building down into the ground. For this
purpose the rod should terminate in the air in points, as these,
as you saw in § 380, so readily receive the electric fluid. The
rod should be separated from the house by wooden supports, and it
should pass so far into the ground as to have its end in the midst
of continual moisture. The points should be gilt, in order to
preserve from corrosion, or they may be made of silver or platina.
Lightning is very apt to go down in chimneys, as smoke is a very
good conductor; and therefore it is well to have the rods go up by
chimneys, especially if they are to have fire in them during the
summer. Lightning-rods often undoubtedly are of service when there
is no obvious passage of the lightning down them, by quietly and
continuously receiving electricity upon their points, and passing
it down into the earth.
[Illustration: Fig. 277.]
394. =Galvanic or Voltaic Electricity.=--This form or mode
of electricity I will barely notice here, reserving its full
consideration for Part Second, where it appropriately belongs.
The history of its discovery is interesting. The first dawning of
Galvanism is to be found in an experiment noticed by Sulzer, a
citizen of Berlin, in 1767. He states that if a piece of zinc be
placed under the tongue, and a piece of silver upon it, on being
brought in contact a metallic taste is perceived, and a shock is
felt by the tongue. Sulzer attributed the effect to some vibratory
motion occasioned by the contact of the metals, and, satisfied with
this fanciful explanation, pursued the inquiry no farther. The
statement excited but little notice until other facts of a similar
character were brought out in 1790 by Galvani, professor of Anatomy
at Bologna. He observed that the legs of some frogs, which had
been obtained for his invalid wife, were convulsed, when near an
excited electrical machine, on touching the nerves with a knife.
In contrast with the example of Sulzer, he was led to examine the
matter further. He found that the effect was produced when no
electricity was communicated from the machine, by establishing a
connection between the nerves and the muscles by some conductors.
For example, when a strip of zinc was placed in contact with the
nerve which goes to the lower extremities, and a strip of copper in
contact with the legs, on bringing the two together at the other
end the legs would be convulsed, being drawn up, as represented in
Fig. 277 (p. 306). But Galvani did not get at the true explanation.
He supposed this to be an exhibition of animal electricity,
regarding the muscles as being a sort of Leyden jar, the nerve
being the medium of communication with the inside.
[Illustration: Fig. 278.]
395. =Volta's Pile.=--The observations of Galvani awakened much
interest in all scientific minds, and of course there was much of
inquiry, observation, and experiment. Professor Volta, of Pavia,
went a step farther than Galvani toward the true explanation, in
referring the effects to the contact of dissimilar metals, and he
was led by this view of the subject to construct his _pile_ or
battery--called after him the voltaic pile--the object of which
was to produce a much greater amount of electricity than could
be obtained by the contact of only two pieces of metal. The pile
is made of circular pieces of copper, zinc, and cloth, the cloth
being moistened with salt-water. They are arranged as represented
in Fig. 278. First a disk of copper is laid down, then upon this
one of zinc, then one of cloth, and so on in the same order, the
top of the pile ending in a plate of zinc. If you touch one end of
the pile with a moistened finger and the other end with a finger
of the other hand, you will feel a shock like that from a Leyden
jar. The communication between the two ends of the pile may be made
by wires, as seen in the figure. Volta afterward changed this to
the form of a cup battery, the plates of metal being immersed in a
series of cups in a mixture of sulphuric acid and water. There have
been various improvements from time to time, but the arrangement is
in all the different batteries essentially the same. Although Volta
accomplished so much he did not arrive at the truth in full. His
"contact theory," as it is called, so long received as the true
theory, gradually gave way to the true explanation, viz., that the
electricity produced is owing to chemical action.
396. =Difference Between Frictional and Voltaic Electricity.=--The
electricity produced by the friction of the electrical machine is
more intense than that of the voltaic battery. Voltaic electricity,
on the other hand, is much more abundant, and is more continuous
and lasting. As it is therefore more steady and more easily
controlled than frictional electricity, it is used in the working
of the Telegraph.