NUMEROUS attempts have been made to explain the phenomena of
electricity. As a general thing, these phenomena are so utterly unlike
other phenomena that have been explained and are easily intelligible,
that it has quite generally been taken for granted, until lately, that
something very different from ordinary matter and the laws of forces
applicable to it must be involved in the phenomena themselves.
Consequently the term _imponderable_ was applied to it,--something that
was matter minus some of the essentials of matter; and as it was
apparent that, whatever it was, it moved, apparently flowed, from one
place to another, the term _fluid_ was applied to it, a term descriptive
of a certain form of matter. Imponderable fluid was the descriptive name
applied to electricity. Newton supposed that an excited body emitted
such a fluid that could penetrate glass. When the two facts of
electrical attraction and repulsion had to be accounted for, two
theories were propounded,--one by Benjamin Franklin, the other by Dufay.
Franklin supposed that electricity was a subtle, imponderable fluid, of
which all bodies contained a certain normal quantity. By friction or
otherwise this normal quantity was disturbed. If a body received more
than its due share, it was said to be positively electrified: if it had
less than its normal quantity, it was said to be negatively electrified.
Franklin supposed this electric fluid to be highly self-repulsive, and
that it powerfully attracted the particles of matter.
According to Dufay, there are two electric fluids, opposite in tendency
but equal in amount. When associated together in equal quantities, they
neutralize each other completely. A portion of this neutral compound
fluid pervades all matter in its unexcited state. By friction or
otherwise this compound fluid is decomposed, the rubber and the body
rubbed exchanging equal quantities of opposite kinds with each other,
leaving one of them positively, the other negatively electrified. These
two fluids were supposed to be self-repulsive, but to attract each
other: so that, if two bodies be charged with either positive or
negative electricity, such bodies would mutually repel each other; but
if one was charged with positive, while the other was charged with
negative electricity, the two bodies would mutually attract each other.
Either of these two theories may be used to illustrate the phenomena,
and so have done good service in systematizing the facts. It is evident
that both of them cannot be true, and it is in the highest degree
probable that neither of them is true.
Some have supposed that there was a kind of electric atmosphere about
every atom of matter; and still another theory, now advocated by Edlund
of Stockholm, assumes that electricity is identical with the ether by
which radiant energy, light and heat, is transmitted.
Before a correct judgment can be formed of the nature of any force, it
is necessary to know what it can do, what kind of phenomena it can
produce. Let us, then, take a brief survey of what electricity can do.
1st, It can directly produce _motion_, through the attractions and
repulsions of electrified bodies,--as indicated by electrometers, the
rotation of the fly-wheel, the deflection of the galvanometer needle. It
has been proved by the mathematical labors of Clausius, and confirmed by
experiment, that, when electricity performs any mechanical work, so much
electricity is lost, annihilated as electricity.
2d, It can directly produce _heat_, as shown by passing a sufficient
quantity of electricity through a fine platinum wire: the wire becomes
heated, and glows, and it may even be fused by the intensity of the
heat. The heat developed in the so-called electric arc is so great as to
fuse the most refractory substances. If a current of electricity from a
battery be sent through a thermo-pile, one of the faces of the pile will
be heated. The heat of the spark from a Leyden jar may be made to ignite
gunpowder, and dissipate gold into vapor. The heat produced by lightning
is seen when a live tree is struck by a powerful flash: the sap of the
tree is instantly converted into steam of so high a tension as to
explode the tree, scattering it in small fragments over a wide area. The
tips of lightning-rods often exhibit this heating effect, being fused by
the passage of too great a quantity of electricity.
In the early part of the present century it was demonstrated by Count
Rumford, and also by Sir Humphry Davy, that heat was but a form of
molecular motion. Since then the exact relations between the motion of a
mass of matter and the equivalent heat have been experimentally
determined by Joule, so that the unit of heat may be expressed in the
motion of a mass of matter. This is deducible from a more general law,
known as the conservation of energy. The application in this place is,
that whenever heat appears through electric action, as in the
above-mentioned places, we know that it still is only _motion_ that is
the product, only that this motion is now among the molecules of the
body, instead of the motion of the whole body in space, as when a
pith-ball moves, or a galvanometer-needle turns.
3d, It can directly produce _light_. This is seen in every spark from an
electric machine, in the flash of lightning, and in the electric light.
It has been shown in numberless ways, that there is no essential
difference between light and heat, and that what we call light is only
the active relation which certain rays of radiant energy have to the
eyes. In order to make this plain, suppose that a beam of light, say
from the sun, be permitted to fall upon a triangular prism of glass: at
once it is seen that the beam is deflected, and instead of appearing a
spot of white light, as it did before it was deflected, it now appears
as a brilliant band of colors, which is called the solar spectrum. If
now this spectrum be examined as to the distribution of heat, by moving
a thermo-pile through it from the blue end towards the red end, it will
be noticed that the galvanometer-needle will be but slightly deflected
at the blue end; but, as the thermo-pile is moved, the deflections are
greater until it is past the red end, where the heat is greatest. On
this account it has been customary to say that the red end of the
spectrum was the heating end. With various pieces of mechanism the rays
may be separated from each other, and measured; and then it appears that
a red ray of light has a wave length of about 1/37000 in., and the
violet ray about 1/60000 in. The rays beyond the red have also been
measured, and found to be greater in length uniformly as one recedes
from the visible part of the spectrum.
In like manner, beyond the blue end the wave lengths become shorter and
shorter; and in each of these directions the spectrum that is invisible
is much longer than the visible one. Now, it has also been found that
where a prism of glass or other material is used to produce a spectrum,
it distributes the rays very unevenly; that is, towards the red end of
the spectrum they are very much crowded, while towards the blue end they
are more dispersed. Hence, if one were measuring the heating power of
such a spectrum, many more rays would fall upon an equal surface of the
thermo-pile at the red end than at the blue end; therefore the
indications of the galvanometer would be fallacious. Before any thing
definite could be known about the matter, it would plainly be necessary
to work with an equal dispersion of all the rays. This was effected a
few years ago by Dr. Draper of New York. He took the spectrum produced
by diffraction instead of refraction, and measured that. In that way it
was found that the heating power of the spectrum is equal in every part
of it; and hence the pictures in treatises on physics that represent the
heating power of the spectrum to be concentrated at the red end is not
true save where the spectrum is irregularly produced. As for vision, the
mechanical structure of the eye is such that radiant vibrations having a
wave length between 1/37000 in. and 1/60000 in. can affect it, while
longer or shorter wave lengths can not. Such waves we call light, but it
is not at all improbable that some animals and insects have eyes adapted
to either longer or shorter wave-lengths; in which case, what would be
perfectly dark to us would be light to them. It is a familiar enough
fact, that many animals, such as dogs, cats, rats, and mice, can see in
the night. Some horses may be trusted to keep in the road in a dark
night, when the driver cannot see even the horse itself. This has
usually been accounted for by saying that their eyes are constructed so
as to collect a greater number of luminous rays. It is much better
explained by supposing their eyes to be constructed to respond to
wave-lengths either greater or less than those of mankind.
A ray of light, then, consists of a single line of undulations of a
definite wave length, such that if it falls upon the eye it will produce
sight; if it falls upon a thermo-pile it heats it by just the same
quantity that another wave-length would heat it; if it falls upon matter
in unstable chemical relations, it will do chemical work, depending upon
the kinds of matter. A red ray is as effective for some substances as a
violet ray is for others. The statement, then, so often lately made to
do certain analogical work, namely, that a ray of light consists of
three distinct parts, which may be separated from each other, and are
called heat, light, and chemical properties, is simply untrue. What a
ray will do, depends upon what kind of a structure it falls on; and when
it has done that work, of whatever kind it may be, it ceases to exist as
a ray.
If, therefore, electricity can directly produce light, it is simply
producing _motion_, as in the case of heat, the motion being of such a
sort that the eyes of men are affected by it.
4th, It can produce _magnetism_. A current of electricity passing
through a coil of wire makes such a coil a magnet, which will set itself
in the direction of the magnetic meridian of the earth; and, if a bar of
soft iron be placed in the coil, it becomes the familiar electro-magnet;
and, if hardened steel be put in it, it becomes a permanent magnet.
This leads to the inquiry as to what magnetism is. We know that it can
produce motion by its moving at a distance a piece of iron or another
magnet. It will also sustain a mass of matter against gravity or some
other contrary force. Through such mechanism as magneto-electric
machines it produces electricity in great abundance, which again can be
used to produce any of the effects of electricity,--moving bodies by
attraction or repulsion, generating heat or light, or again making a
magnet. But as all of these are but varied forms of motion, either of a
mass as a whole, or molecular, can it be doubted for an instant, that
what we call magnetism is but some form of motion? Must it not be either
some form of matter, or some form of motion? If it were a form of
matter, then a magnet would only be permanent so long as it was not
used; for use implies consumption of the force; and, if this be matter
in any form, then in a given mass of matter there can be but a definite
quantity of such magnetic matter, and consumption must lessen that
quantity. As a matter of fact, there is no perceptible lessening of the
power of a magnet when it is properly used. It is also a matter of fact,
that neither motion of a mass, nor electrical effects, nor any other,
can be produced by the action of a magnet alone. It is only when some
form of motion has been added to its own property, that we get any kind
of an effect from it: hence all effects due to its action are
_resultants_ of two forces, one of them being common motion of a mass of
matter, and the other the energy of the magnet. Hence we infer that a
magnet is a mechanism of such a structure as to change the direction and
character of the motion which acts upon it. When the wheel of a common
electrical machine is turned, the product is electricity,--a force very
different from that which originates it. Ordinary mechanical motion
_goes in_; electricity _comes out_, the latter being a modified motion
due to the physical structure of the machine. In like manner, a magnet
may be considered as a machine by means of which mechanical motion may
be converted into some other form of motion. It is evident that
molecular structure is chiefly concerned in this. If a bar of iron that
exhibits no evidence of magnetism whatever be subjected to torsion, it
will immediately become a magnet with poles dependent upon the direction
of the twist. This developed magnetism will re-act upon a coil of wire,
and so move a galvanometer needle. If the bar be permitted to recover
its original condition, it will lose its magnetism, which will at once
re-appear upon twisting the rod again. Now, when the rod is twisted, it
is evident that there is a molecular strain in certain directions
throughout the mass. The converse experiment illustrates the same thing.
It has been found, that when a rod of iron is made magnetic by the
action of a current of electricity circulating about it, and at the same
time passing longitudinally through it, the rod is slightly lengthened
and twisted in a direction that depends upon the direction of the
current. Moreover, if a permanent magnet be heated to a red heat, its
magnetism is destroyed; for such a heat allows the molecules to freely
arrange themselves without any external constraint. Also, if a permanent
magnet be suspended so as to give out a musical sound when it is struck,
the magnetism will be much weakened by making it thus to vibrate. In
this case, as in the other, the vibrations affect every molecule, and so
enable them to re-adjust themselves to the positions they held before
being magnetized. The same thing happens when a bar of iron is made
magnetic through the inductive action of the earth. When this bar is
held in the direction of the magnetic dip, it becomes but very slightly
magnetized; but, if it be so held that when it is struck with a hammer
it will ring, that is, give out a musical sound, it will at once become
decidedly magnetic. Evidently the earth's action tends to set the
molecules of the mass in a new position, but cohesion prevents them from
assuming it. When the molecules are made to vibrate, they can assume
such new positions more readily. The molecules of a magnet, then, are
differently arranged from those in an unmagnetized piece of iron or
steel; and, for every new arrangement of the molecules of a mass of any
kind, we always have some new physical property developed. The same
identical substance may appear as charcoal, coke, plumbago, anthracite
coal, and diamond. Hence a magnet is a machine in which other forces
acting upon it are transformed in character, and re-appear as
attractions and repulsions of other kinds of matter: this transformation
cannot take place, and hence magnetism cannot become apparent, only upon
the condition of another force acting in concert with it; and, if at any
time it may seem to be acting without such external force, it is done at
the expense of the heat it has absorbed, and therefore the magnet must
at such time be losing temperature proportional to the work done. This I
have discovered to be true by making a magnet to exert its force in
front of a thermo-pile, which uniformly exhibits a cooled face under
such conditions. What the particular form of the motion may be that we
call magnetism, is not yet made out; but that it is some form of motion,
is very evident. The following experiments may throw some light upon it.
Last August Mr. Kerr read a paper before the British Association of
Science, in which was detailed the following experiment: The pole of an
electro-magnet was nicely polished so as to reflect light like a mirror.
A beam of sunlight was permitted to fall upon it, and be reflected to a
convenient place for examination. A current of electricity was sent
through the coil, which of course rendered the iron magnetic; and it was
noticed that the light that was reflected from the pole was circularly
polarized: that is, the motion of a ray, instead of being a simple
undulatory movement, was now made to assume such a motion as the water
from a garden-hose has when the nozzle is swung round in a circle while
the water is escaping from it. After reading the account of it, it
occurred to me that the converse experiment might be tried; that is to
say, the effect of a circularly polarized beam of light upon a piece of
steel. By concentrating a large beam of ordinary plane polarized light
with a quartz lens, and passing it through a quarter wave-plate at the
proper angle, a powerful beam of circularly polarized light was
obtained. At the focus of this beam a fine cambric needle without
magnetism was placed so that the light passed it longitudinally. Ten
minutes' exposure was sufficient to make it decidedly magnetic. Hence I
infer that the motions which we call magnetic attractions and repulsions
may be quite analogous to such helical motions; also, that these motions
exist in ether, and evidently may be either right-handed or left-handed.
Wind up on a pencil a piece of wire twelve or fifteen inches long,
making a loose spiral. Bring the two ends of the spiral together; and
note first that one is twisted to the right, the other to the left. If
they be twisted into each other, they will advance very easily; but if a
right-handed spiral were to be interlocked with another like it, and
both turned in the direction of their spiral, they would separate
rapidly. Applying this conception to a magnet, we might suppose that
such spiral motions will be set up in the ether by the magnet, and that
such motions re-acting upon ordinary matter affect it as attraction and
repulsion; and thus we should have at least a conceivable mechanical
explanation of the phenomenon.
[Illustration: FIG. 5.]
There are numberless experiments which might be given to further exhibit
the relation of mass motion to magnetism, but a single one more must
suffice. No rotation of a magnet upon its own axis can produce any
effects upon a current that is exterior to it; but if a loop of wire be
kept stationary adjacent to a magnet, as in Fig. 5, while the magnet
revolves, a current of electricity is produced; and if the magnet be
kept stationary, and the loop revolves, a current will also be produced,
but in the opposite direction. Here, as in all the other cases, no
electricity is originated, save when motion is imparted to one or other
of the parts. This experiment is due to Faraday.
From all these cases we can come to but one conclusion, that both
electricity and magnetism are but forms of motion; electricity being a
form of motion in ordinary matter, for it cannot be made to pass through
a vacuum, while magnetism must be a form of motion induced in the ether,
for it is as effective in a vacuum as out of it; electricity always
needing some material conductor, magnetism needing no more than do
radiant heat and light.