demonstration, and as the whole science rests upon inferences derived
from these, the reader would do well to perform the following simple
experiments for himself. Apparatus is represented in Fig. 253, but the
only essential portion is a straw, B, suspended from any convenient
support by a very fine filament of _white silk_. To one or both ends of
the straw a little disc of gilt paper, or a small ball of elder-pith or
of cork, should be attached, so that the straw may be balanced
horizontally. Now rub on a piece of woollen cloth a bit of sealing-wax,
or a stick of sulphur, or a piece of amber, or a penholder, paper-knife,
or comb made of ebonite, and immediately present the substance to the
ball at the end of the straw. It will be first attracted to the rubbed
surface, but after coming into contact with it, repulsion will be
manifested and the ball will separate, and may be chased round the
circle by following it with the excited body. The attraction of light
bodies by amber after it has been rubbed appears to be the one solitary
electrical observation recorded by the ancients, but it has given its
name to the science, ελεκτρον being the Greek name for amber. The cause,
then, of this property is named _electricity_, and bodies which exhibit
it are said to be _electrified_. The reader will remark that these words
_explain_ nothing: they are used merely to _express_ a certain state of
matter and the entirely unknown cause of that state. Let the pith or
cork ball at the end of the straw be again charged with electricity, by
bringing it into contact with a piece of sealing-wax or ebonite which
has just been electrified by friction. In this condition it will, as we
have just seen, be repelled by the substance which charged it, and on
trial it will be found to be repelled also by all the substances we have
named, after they have been excited by friction. But if, while still
charged with the electricity communicated to it by contact with
sealing-wax, sulphur, ebonite, or amber, we present to it a warm and dry
glass tube which has just been rubbed with dry silk, we shall find that
the ball will be strongly attracted. After contact with the glass,
repulsion will take place, and the ball will refuse again to come into
contact with the excited glass. In this condition, however, it will be
immediately attracted by rubbed sealing-wax or ebonite, and so on
alternately: the ball when repelled by the wax is attracted by the
glass, and when repelled by the glass is attracted by the wax.
These simple experiments prove that, whatever electricity may be, there
are two kinds of it, or, at least, it manifests two opposite sets of
forces. The electricity evolved by the friction of glass with silk was
formerly called _vitreous_ electricity, and that shown by excited resin,
sealing-wax, amber, &c., was named _resinous_ electricity. These names
have now been respectively replaced by the terms _positive_ and
_negative_. It must be understood that these terms imply no actual
excess or defect, but are purely distinguishing terms, just as we speak
of the _up_ and _down_ line of a railway, without implying an
inclination in one direction or the other. A fact of great importance in
electrical theory is discovered when the substances in which electricity
is developed are carefully examined: it is found that one kind is never
produced without the other simultaneously appearing. Thus, the silk
which has been used for rubbing the glass in the above experiments will
be found to exhibit the same electricity as sealing-wax or ebonite. And,
further, the _quantities_ of positive and negative electricity evolved
are always found to be equal, or equivalent to each other; that is, if
they are put together they completely neutralize or destroy each other’s
effects. We have used the word “quantity,” implying that electricity can
be measured. No doubt, whatever electricity may be, there may be more or
less of it; but can we measure an imponderable, invisible, impalpable
thing, incapable of isolation? What we really measure when we say that
we measure electricity is the attractive or repulsive force: we balance
this against some other force (that of gravitation, for example), and we
say, so much weight lifted represents so much electricity.
If we try to electrify a piece of metal by holding it in the hand and
rubbing it against woollen cloth, silk, or other substance, we shall
fail in the attempt: no signs of electricity will thus be shown by the
metal. Hence bodies were formerly divided into two classes—those which
could be electrified by friction, and those which could not. It was
afterwards found, however, that there was no real ground for this
division, but that, on the contrary, _no two bodies can be rubbed
together, even if they are made of the same substances, without positive
electricity appearing in one, and an equivalent quantity of negative
electricity in the other_. The real difference between bodies which
prevents the manifestation of electricity in many cases depends upon the
fact that electricity is able to traverse some substances with great
facility, while others prevent its passage. Thus, if we suspend
horizontally a hempen cord by white silk attached to the ceiling, so
that the hempen cord comes in contact with nothing but the silk, we
shall find, on presenting a piece of excited ebonite to one end of the
cord, that electric attraction of light bodies will be manifested at the
other. If a silk cord be substituted for the hempen one, no such effect
will be observed. The hemp is, therefore, said to be a _conductor_, and
the silk a _non-conductor_. Again, if we substitute for one of the silk
threads suspending the cord a piece of twine, or a wire, we shall fail
to obtain any electric manifestations at the remote end, because the
electricity will be carried off into the earth by the conducting powers
of these substances. On the other hand, filaments of glass or ebonite
may be used, instead of the silk, with the same effect: they do not
allow the electricity to run through them to the ground, and are
therefore termed, like the silk, _insulators_ of electricity. The
distinction of bodies into conductors on the one hand, and into
non-conductors or insulators on the other, is of paramount importance in
the science and in all its applications. This distinction, however, is
not an absolute one: there is no substance so perfect an insulator that
it will not permit any electricity to pass, and there is no conductor so
perfect that it does not offer resistance to the passage. Substances may
be arranged in a list which presents a gradation from the best conductor
to the best insulator. The metals are by far the best conductors, but
there is great relative diversity in their conductive power. Silver,
copper, and gold are much the best conductors among the metals, iron
offering eight times, and quicksilver fifty times, the resistance of
silver. Coke, charcoal, aqueous solutions, water, vegetables, animals,
and steam are all more or less conductors, while among the substances
called insulators may be named, in order of increasing insulating power,
india-rubber, porcelain, leather, paper, wool, silk, mica, glass, wax,
sulphur, resins, amber, gum-lac, gutta-percha, and ebonite. It will now
be obvious why the electricity developed by the friction of a piece of
metal fails to manifest itself under ordinary circumstances, as, for
instance, when held in the hand: the metal and the body being both
conductors, the electricity escapes. But if the piece of metal be held
by an insulating handle of glass or ebonite, the electrified condition
may easily be observed.
_THEORY OF ELECTRICITY._
The few elementary facts which have been pointed out are absolutely
necessary for the foundation of what is sometimes termed the theory of
electricity, but which is properly no theory,—at least, not a theory in
the same sense as gravitation is a theory explaining the motions of the
planets, or even in the sense in which the hypothesis of the ether and
its movements explains the phenomena of light. It is absolutely
necessary to have a conception of some kind which may serve to connect
in our minds the various phenomena of electricity, if it were only to
enable us the more easily to talk about them. In default of any
supposition which will shadow forth what actually occurs in these
phenomena, we have recourse to what has been aptly termed _a
representative fiction_: we picture to ourselves the actions as due to
_imaginary fluids_—fluids which we know _do not exist_, but are as much
creations of the mind as Macbeth’s air-drawn dagger; not, however, like
his “false creation,” proceeding from “the heat-oppressed brain,” but
intellectual fictions, consciously and designedly adopted for the
purpose of enabling us the better to think of the facts, to readily
co-ordinate them, and to express them in simple and convenient language.
Non-scientific persons hearing this language usually mistake its
purport, and imagine that the actual existence of an “electric fluid” is
acknowledged. The accounts which appear in the newspapers of the damage
done by thunderstorms are often amusing from the objectivity which the
reporter attributes to the “electric fluid.” It is described, perhaps,
as “entering the building,” “passing down the chimney,” then “proceeding
across the floor,” “rushing down the gas-pipes,” “forcing its way
through a crevice, and then streaming down the wall,” &c., in terms
which imply the utmost confidence of belief in the existence of the
“fluid.” With this intimation that the hypothesis of electric fluids is
merely, then, a “_façon de parler_,” the reader will not be misled by
the following brief explanation of the elementary facts in the language
of the theory.
In the natural state all bodies contain an indefinite quantity of an
imponderable subtile matter, which may be called “neutral electric
fluid.” This fluid is formed by a combination of two different kinds of
particles, positive and negative, which are present in equal quantities
in bodies not electrified; but when there is in any body an excess of
one kind of particles, that body is charged accordingly with positive or
negative electricity. Both fluids traverse with the greatest rapidity
certain substances termed _conductors_; but they are retained amongst
the molecules of _insulating_ substances, which prevent their movement
from point to point. When one body is rubbed against another, the
neutral electric fluid is decomposed—the positive particles go to one
body, the negative with which these positive particles were before
united pass to the other body. The particles of the same name repel each
other, but particles of opposite names attract each other; and it is
this attraction which is overcome when the electricities are separated
by friction or in any other manner.
It will be observed that the above is nothing but the statement of the
elementary facts in the language of the hypothesis. This system of the
two fluids readily lends itself to the explanation of nearly all the
phenomena presented in what is termed _static electricity_—that is, in
those phenomena where the actions are conceivably due to a more or less
permanent separation of the fluids. The grand discoveries in electricity
turn, however, upon quite another condition, namely, one in which the
two hypothetical fluids must be imagined as constantly combining, and
here the utility of the hypothesis is less marked. Inasmuch, however, as
there can be no doubt regarding the identity of the agent operating in
the two sets of circumstances, the facts of _dynamical electricity_ must
still be expressed in the same language, with the aid of any additional
conceptions which may give us more grasp of the subject.
_ELECTRIC INDUCTION._
In all electrical phenomena an inductive action occurs, which resembles
that which we have already indicated with regard to magnetism. Thus, if
we take an insulated metallic conductor in the uncharged state, and
bring it near an electrified body, we shall find that the conductor,
while still at a considerable distance, will give signs of an electrical
charge. Suppose we have a cylindrical conductor, and that we present one
end of it to the electrified body, but at such a distance that no spark
shall pass, we shall find, if the charge on the electrified body be
strong and the conductor be brought sufficiently near, that on bringing
the finger near the insulated cylinder, a spark passes. While the
cylinder continues in the same position with regard to the electrified
body, no further sparks can be drawn from it; but if the distance
between the two bodies be increased, the insulated cylinder will be
found to have another charge of electricity, which will again produce a
spark. And by repeating these movements we may obtain as many sparks as
we desire by these mechanical actions, without in the least drawing upon
the charge on the original electrified body. The electrophorus is a
device for obtaining electricity by this plan, and several rotatory
electrical machines have lately been invented which yield large supplies
of electricity by a similar inductive action.
[Illustration:
FIG. 254.
_The Gold-leaf Electroscope._
]
It is found that in such a case as that we have above supposed, if the
electrified body is charged with positive electricity, the uncharged
conductor brought near it has its electricities separated—the negative
attracted and held by the attraction of the positive charge in the parts
of the cylinder nearest the inducing body; while the corresponding
quantity of positive electricity is driven towards the most remote parts
of the insulated conductor. It is this last which gives the spark in the
first case, and if it be not thus withdrawn from the conductor, it
re-combines with the negative electricity when the conductor is
withdrawn from the neighbourhood of the electrified body, and the
conductor then reverts to the natural or unelectrified state. But the
contact of a conducting body with the conductor while it is under the
influence of the electrified body withdraws only positive electricity,
the negative—being held, as it were, by the attraction of the positive
electricity of the charged body—is not thus removed, and in this
condition it is sometimes called _disguised_ or _dissimulated_
electricity—a term the propriety of which is doubtful. The excess of
negative “fluid” which the conductor thus acquires shows itself,
however, only when the inducing body has been withdrawn. Precisely
similar effects will take place, _mutatis mutandis_, if the electrified
body has a negative charge. A demonstration of inductive effects is
readily afforded in the action of the gold-leaf _electroscope_, Fig.
254, in which two strips of gold-leaf are suspended within a glass case
from wire passing through the top, and terminated in a metal plate. This
instrument is often used for showing the existence of very small
electric charges. Let a stick of sealing-wax be rubbed and held, say, a
foot or more from the plate of the electroscope, the leaves will diverge
with negative electricity. The sealing-wax being retained in the same
position, touch the plate for an instant with the finger. This will
remove the negative charge, but the _positive_ electricity will be
retained on the plate by the attraction of the negative of the
sealing-wax. Now remove the sealing-wax, when the dissimulated charge
will spread itself over the whole insulated metallic portion of the
electroscope, and the leaves will diverge with a strong charge of
_positive_ electricity. If an excited glass tube is brought near the
electroscope, the leaves will now diverge still more; if the sealing-wax
is replaced in its former position, the leaves will collapse. In all
these cases the electrified body parts with none of its own electricity
by developing electrical effects in the neighbouring bodies.
The inductive actions we have described take place through the air,
which is a non-conductor, and such actions may be made to take place
through any other non-conductor. With solid non-conductors, such as
glass, gutta-percha, &c., the inducing body may be brought very near to
the conductor on which it is to act; for the intervening solid
substance, or _dielectric_, as it has been appropriately called, opposes
a resistance to the combination of the opposite electricities, and the
inductive effects are greatly intensified by the approximation. Faraday
discovered that the amount of inductive action with a given charge is
also dependent upon the nature of the dielectric, and that the electric
forces act upon the particles of the dielectric, circumstances which are
of the greatest importance, as we shall presently find, in practical
telegraphy. The most familiar instance of induction is probably well
known to the reader in the Leyden jar, Fig. 255, which is simply a
wide-mouthed bottle of thin glass, covered internally and externally
with tin-foil to within a few inches of the neck. The inner coating
communicates by means of a rod and chain with a brass knob. Such a jar
admits of the accumulation of a larger quantity of electricity than the
conductor of a machine will retain. A very few turns of the machine will
suffice usually to charge the conductor to the fullest extent; but if it
be put in communication with the knob of a jar, a great many more turns
will be required to attain the same charge in the conductor, and the
excess of electricity represented by these additional turns will have
accumulated within the jar—an effect due to the “dissimulated”
electricity of its exterior.
[Illustration:
FIG. 255.—_The Leyden Jar._
]
Everybody knows the result when a metallic communication is established
between the exterior and the interior of a charged Leyden jar. There is
a very bright spark, a snap, and the jar is “discharged.” Everybody
knows, also, the sensation experienced when his body takes the place of
the metallic communication, or forms part of the circuit through which
the communication takes place. Everybody knows that the shock then felt
may also be experienced at the same moment by any number of persons who
join hands, under such conditions that they also form a part of the line
of communication. Such facts irresistibly suggest the notion of
something passing through the whole chain, and this notion is in perfect
harmony with the hypothesis of the “fluids,” for we have only to suppose
that it is one or both of these which rush through the circuit the
instant the line of communication is complete. It was one of Franklin’s
discoveries that the electrical charges of the Leyden jar do not reside
in the metallic coatings; for he made a jar with removable inside and
outside coatings, which, properly taken from the glass, showed no signs
of electrification, yet when replaced the jar was found to be again
highly charged. This would seem to show that the charge clings to, or
penetrates within, the glass.
_DYNAMICAL ELECTRICITY._
Let us take a vessel containing water, to which some sulphuric acid has
been added, Fig. 256, and in the liquid plunge a plate of copper, C, and
a plate of _pure_ zinc, Z, keeping the plates apart from each other. As
it is not easy to obtain zinc perfectly free from admixture of other
metals, an artifice is commonly resorted to for obtaining a surface of
pure metal, by rubbing a plate of the ordinary metal with quicksilver,
which readily dissolves pure zinc, but is without action on the iron and
other metals with which the zinc is contaminated, while the quicksilver
is not acted upon by the diluted acid, but is merely the vehicle by
which the pure zinc is presented to the liquid. Under the conditions we
have described, no action will be perceived, no gas will be given off,
nor will the zinc dissolve in the acid. If the electrical condition of
the portion of the copper-plate which is out of the liquid be examined
by means of a _delicate_ electroscope, it will be found to possess a
very weak charge of _positive_ electricity, and a similar examination of
the zinc plate will show the existence on it of a feeble charge of
_negative_ electricity. If the two plates be made to touch each other,
or if a wire be attached to each plate, as shown in the figure, and the
wires be brought into contact outside of the vessel, an action in the
liquid is immediately perceptible at the surface of the _copper_ plate,
when a multitude of small bubbles of hydrogen gas will at once make
their appearance, and the gas will be given off continuously from the
copper plate so long as there is metallic contact through the wires, or
otherwise, between the two plates, or until the acid is saturated with
zinc—for in this action the zinc is dissolving, and, in consequence,
liberating hydrogen, which strangely makes its appearance, not at the
place where the chemical action really occurs, namely, at the surface of
the zinc which is in contact with the acid, but at the surface of the
copper which is not acted upon by the acid.
[Illustration:
FIG. 256.—_A Voltaic Element._
]
It is known that when we establish a metallic communication between two
bodies charged with equivalent quantities of positive and negative
electricities respectively, these combine and neutralize each other, and
all signs of electricity vanish. It is obvious that the contact of the
two wires has this effect, as the signs of electric charge which were
before discoverable in each of the plates are no longer found while the
wires are in contact. But the charges reappear the instant the contact
is broken, the chemical action ceasing at the same time. If the wire
connecting the two plates outside of the vessel be carefully examined,
it will be found, so long as the chemical action is going on, to be
endowed with new and very remarkable properties. If this wire be
stretched horizontally over a freely suspended magnetic needle, and
parallel to it, the needle will be deflected from its position, and, if
the wire be placed very near it, will point nearly east and west,
instead of north and south. Now, this effect is produced by any part
whatever of the wire, and it instantly ceases if the wire be cut at any
point. These facts at once suggest the idea of its being due to
something flowing through the wire, so long as metallic continuity is
preserved. This idea is much strengthened when we find that the action
of the connecting wire upon the magnetic needle is quite definite—or, in
other words, there are indications which correspond with the notion of
direction. For when the wire, which we shall still suppose to be
stretched horizontally _above_ the needle and parallel to its direction,
is so connected with the plates immersed in the acid that the portion
which approaches the south-pointing pole of the needle proceeds from the
copper plate, while the portion above the north pole is in connection
with the zinc plate, then the north end of the needle will always be
deflected towards the west—whereas, if the connections be made in the
contrary manner, the deflection will be in the opposite direction; and
if the wire be below the needle, the contrary deflections will be
observed with the same connections. The discovery of the action of such
a wire on the magnetic needle was made by Œrsted in 1819, and it is a
discovery remarkable for the wonderful extent of the field which it
opened out, both in the region of pure science and in that of practical
utility.
[Illustration:
FIG. 257.—_Ampère’s Rule._
]
Since by such experiments as those just mentioned the notion of a
_current_ is arrived at, the mind recurs to the fiction of the “fluids,”
and pictures the “positive fluid” as rushing in one direction, and the
“negative fluid” in the other, to seek a re-combination into “neutral
fluid.” But we must never lose sight of the fact that these ideas are
consciously adopted as representative fictions to help our thoughts—just
as John Doe and Richard Roe, imaginary parties to an imaginary lawsuit,
used to be named in legal documents, in order to explain the nature of
the proceedings. Failing, then, to find anything really flowing along
our wire, it is still absolutely necessary, seeing there is something
definite in its action, to assign a direction to the supposed current;
and it has been agreed that we shall represent the current as flowing
from the positively charged body to the negatively charged body—that is,
in the case we have been considering, from the copper to the zinc
through the wire. When this conventional representation has been
adopted, the action on a magnetic needle can easily be defined and
remembered by an artifice proposed by Ampère. In Fig. 257, let N S
represent the magnetized needle, N being the pole which points towards
the north, and S the south pole. Let C be the end of the wire connected
with the copper plate, and Z that connected with the zinc. The current
is therefore supposed to flow in the direction indicated by the arrows
in a wire above the needle and in the wire placed below. Now, suppose
that a man is swimming in the current in the same direction it is
flowing, _and with his face towards the needle, then the north pole of
the needle will always be deflected towards his left_. With the
direction of current represented in the figure, the pole, N, will be
thrown forward from the plane of the paper, or towards the spectator.