Michael Faraday was the son of a Yorkshire blacksmith, and was born
in London on September 22, 1791. At the age of twenty he became
assistant to Sir Humphry Davy, whose lectures he had attended at
the Royal Institution. Here he worked for the rest of his laborious
life, which closed on August 25, 1867. The fame of Faraday, among
those whose studies qualify them for a verdict, has risen steadily
since his death, great though it then was. His researches were of
truly epoch-making character, and he was the undisputed founder of
the modern science of electricity, which is rapidly coming to
dominate chemistry itself. Faraday excelled as a lecturer, and
could stand even the supreme test of lecturing to children.
Faraday's "Experimental Researches in Electricity" is a record of
some of the most brilliant experiments in the history of science.
In the course of his investigations he made discoveries which have
had momentous consequences. His discovery of the mutual relation of
magnets and of wires conducting electric currents was the beginning
of the modern dynamo and all that it involves; while his
discoveries of electric induction and of electrolysis were of equal
significance. Most of the researches are too technical for
epitomisation; but those given are representative of his manner and
methods.
_I.--Atmospheric Magnetism_
It is to me an impossible thing to perceive that two-ninths of the
atmosphere by weight is a highly magnetic body, subject to great changes
in its magnetic character, by variations in its temperature and
condensation or rarefaction, without being persuaded that it has much to
do with the variable disposition of the magnetic forces upon the surface
of the earth.
The earth is a spheroidal body consisting of paramagnetic and
diamagnetic substances irregularly disposed and intermingled; but for
the present the whole may be considered a mighty compound magnet. The
magnetic force of this great magnet is known to us only on the surface
of the earth and water of our planet, and the variations in the magnetic
lines of force which pass in or across this surface can be measured by
their action on small standard magnets; but these variations are limited
in their information, and do not tell us whether the cause is in the air
above or the earth beneath.
The lines of force issue from the earth in the northern and southern
parts and coalesce with each other over the equatorial, as would be the
case in a globe having one or two short magnets adjusted in relation to
its axis, and it is probable that the lines of force in their circuitous
course may extend through space to tens of thousands of miles. The lines
proceed through space with a certain degree of facility, but there may
be variations in space, _e.g._, variations in its temperature which
affect its power of transmitting the magnetic influence.
Between the earth and space, however, is interposed the atmosphere, and
at the bottom of the atmosphere we live. The atmosphere consists of four
volumes of nitrogen and one of oxygen uniformly mixed and acting
magnetically as a single medium. The _nitrogen_ of the air is, as
regards the magnetic force, neither paramagnetic nor diamagnetic,
whether dense or rare, or at high or low temperatures.
The _oxygen_ of the air, on the other hand, is highly paramagnetic,
being, bulk for bulk, equivalent to a solution of protosulphate of iron,
containing of the crystallised salt seventeen times the weight of the
oxygen. It becomes less paramagnetic, volume for volume, as it is
rarefied, and apparently in the simple proportion of its rarefaction,
the temperature remaining the same. When its temperature is raised--the
expansion consequent thereon being permitted--it loses very greatly its
paramagnetic force, and there is sufficient reason to conclude that when
its temperature is lowered its paramagnetic condition is exalted. These
characters oxygen preserves even when mingled with the nitrogen in the
air.
Hence the atmosphere is a highly magnetic medium, and this medium is
changed in its magnetic relations by every change in its density and
temperature, and must affect both the intensity and direction of the
magnetic force emanating from the earth, and may account for the
variations which we find in terrestrial magnetic power.
We may expect as the sun leaves us on the west some magnetic effect
correspondent to that of the approach of a body of cold air from the
east. Again, the innumerable circumstances that break up more or less
any average arrangement of the air temperatures may be expected to give
not merely differences in the regularity, direction, and degree of
magnetic variation, but, because of vicinity, differences so large as to
be many times greater than the mean difference for a given short period,
and they may also cause irregularities in the times of their occurrence.
Yet again, the atmosphere diminishes in density upwards, and this
diminution will affect the transmission of the electric force.
The result of the _annual variation_ that may be expected from the
magnetic constitution and condition of the atmosphere seems to me to be
of the following kind.
Since the axis of the earth's rotation is inclined 23° 28' to the plane
of the ecliptic, the two hemispheres will become alternately warmer and
cooler than each other. The air of the cooled hemisphere will conduct
magnetic influence more freely than if in the mean state, and the lines
of force passing through it will increase in amount, whilst in the other
hemisphere the warmed air will conduct with less readiness than before,
and the intensity will diminish. In addition to this effect of
temperature, there ought to be another due to the increase of the
ponderable portion of the air in the cooled hemisphere, consequent on
its contraction and the coincident expansion of the air in the warmer
half, both of which circumstances tend to increase the variation in
power of the two hemispheres from the normal state. Then, as the earth
rolls on its annual journey, that which was at one time the cooler
becomes the warmer hemisphere, and in its turn sinks as far below the
average magnetic intensity as it before had stood above it, while the
other hemisphere changes its magnetic condition from less to more
intense.
_II.--Electro-Chemical Action_
The theory of definite electrolytical or electro-chemical action appears
to me to touch immediately upon the absolute quantity of electricity
belonging to different bodies. As soon as we perceive that chemical
powers are definite for each body, and that the electricity which we can
loosen from each body has definite chemical action which can be
measured, we seem to have found the link which connects the proportion
of that we have evolved to the proportion belonging to the particles in
their natural state.
Now, it is wonderful to observe how small a quantity of a compound body
is decomposed by a certain quantity of electricity. One grain of water,
for instance, acidulated to facilitate conduction, will require an
electric current to be continued for three minutes and three-quarters to
effect its decomposition, and the current must be powerful enough to
keep a platina wire 1/104 inch in thickness red hot in the air during
the whole time, and to produce a very brilliant and constant star of
light if interrupted anywhere by charcoal points. It will not be too
much to say that this necessary quantity of electricity is equal to a
very powerful flash of lightning; and yet when it has performed its full
work of electrolysis, it has separated the elements of only a single
grain of water.
On the other hand, the relation between the conduction of the
electricity and the decomposition of the water is so close that one
cannot take place without the other. If the water be altered only in
that degree which consists in its having the solid instead of the fluid
state, the conduction is stopped and the decomposition is stopped with
it. Whether the conduction be considered as depending upon the
decomposition or not, still the relation of the two functions is equally
intimate.
Considering this close and twofold relation--namely, that without
decomposition transmission of electricity does not occur, and that for a
given definite quantity of electricity passed an equally definite and
constant quantity of water or other matter is decomposed; considering
also that the agent, which is electricity, is simply employed in
overcoming electrical powers in the body subjected to its action, it
seems a probable and almost a natural consequence that the quantity
which passes is the equivalent of that of the particles separated;
_i.e._, that if the electrical power which holds the elements of a grain
of water in combination, or which makes a grain of oxygen and hydrogen
in the right proportions unite into water when they are made to combine,
could be thrown into a current, it would exactly equal the current
required for the separation of that grain of water into its elements
again; in other words, that the electricity which decomposes and that
which is evolved by the decomposition of a certain quantity of matter
are alike.
This view of the subject gives an almost overwhelming idea of the
extraordinary quantity or degree of electric power which naturally
belongs to the particles of matter, and the idea may be illustrated by
reference to the voltaic pile.
The source of the electricity in the voltaic instrument is due almost
entirely to chemical action. Substances interposed between its metals
are all electrolytes, and the current cannot be transmitted without
their decomposition. If, now, a voltaic trough have its extremities
connected by a body capable of being decomposed, such as water, we shall
have a continuous current through the apparatus, and we may regard the
part where the acid is acting on the plates and the part where the
current is acting upon the water as the reciprocals of each other. In
both parts we have the two conditions, _inseparable in such bodies as
these_: the passing of a current, and decomposition. In the one case we
have decomposition associated with a current; in the other, a current
followed by decomposition.
Let us apply this in support of my surmise respecting the enormous
electric power of each particle or atom of matter.
Two wires, one of platina, and one of zinc, each one-eighteenth of an
inch in diameter, placed five-sixteenths of an inch apart, and immersed
to the depth of five-eighths of an inch in acid, consisting of one drop
of oil of vitriol and four ounces of distilled water at a temperature of
about 60° Fahrenheit, and connected at the other ends by a copper wire
eighteen feet long, and one-eighteenth of an inch in thickness, yielded
as much electricity in little more than three seconds of time as a
Leyden battery charged by thirty turns of a very large and powerful
plate electric machine in full action. This quantity, although
sufficient if passed at once through the head of a rat or cat to have
killed it, as by a flash of lightning, was evolved by the mutual action
of so small a portion of the zinc wire and water in contact with it that
the loss of weight by either would be inappreciable; and as to the water
which could be decomposed by that current, it must have been insensible
in quantity, for no trace of hydrogen appeared upon the surface of the
platina during these three seconds. It would appear that 800,000 such
charges of the Leyden battery would be necessary to decompose a single
grain of water; or, if I am right, to equal the quantity of electricity
which is naturally associated with the elements of that grain of water,
endowing them with their mutual chemical affinity.
This theory of the definite evolution and the equivalent definite action
of electricity beautifully harmonises the associated theories of
definite proportions and electro-chemical affinity.
According to it, the equivalent weights of bodies are simply those
quantities of them which contain equal quantities of electricity, or
have naturally equal electric powers, it being the electricity which
_determines_ the equivalent number, _because_ it determines the
combining force. Or, if we adopt the atomic theory or phraseology, then
the atoms of bodies which are equivalent to each other in their ordinary
chemical action have equal quantities of electricity naturally
associated with them. I cannot refrain from recalling here the beautiful
idea put forth, I believe, by Berzelius in his development of his views
of the electro-chemical theory of affinity, that the heat and light
evolved during cases of powerful combination are the consequence of the
electric discharge which is at the moment taking place. The idea is in
perfect accordance with the view I have taken of the quantity of
electricity associated with the particles of matter.
The definite production of electricity in association with its definite
action proves, I think, that the current of electricity in the voltaic
pile is sustained by chemical decomposition, or, rather, by chemical
action, and not by contact only. But here, as elsewhere, I beg to
reserve my opinion as to the real action of contact.
Admitting, however, that chemical action is the source of electricity,
what an infinitely small fraction of that which is active do we obtain
and employ in our voltaic batteries! Zinc and platina wires
one-eighteenth of an inch in diameter and about half an inch long,
dipped into dilute sulphuric acid, so weak that it is not sensibly sour
to the tongue, or scarcely sensitive to our most delicate test papers,
will evolve more electricity in one-twentieth of a minute than any man
would willingly allow to pass through his body at once.
The chemical energy represented by the satisfaction of the chemical
affinities of a grain of water and four grains of zinc can evolve
electricity equal in quantity to that of a powerful thunderstorm. Nor is
it merely true that the quantity is active; it can be directed--made to
perform its full equivalent duty. Is there not, then, great reason to
believe that, by a closer investigation of the development and action of
this subtile agent, we shall be able to increase the power of our
batteries, or to invent new instruments which shall a thousandfold
surpass in energy those we at present possess?
_III.--The Gymnotus, or Electric Eel_
Wonderful as are the laws and phenomena of electricity when made evident
to us in inorganic or dead matter, their interest can bear scarcely any
comparison with that which attaches to the same force when connected
with the nervous system and with life.
The existence of animals able to give the same concussion to the living
system as the electrical machine, the voltaic battery, and the
thunderstorm being made known to us by various naturalists, it became
important to identify their electricity with the electricity produced by
man from dead matter. In the case of the _Torpedo_ [a fish belonging to
the family of Electric Rings] this identity has been fully proved, but
in the case of the _Gymnotus_ the proof has not been quite complete, and
I thought it well to obtain a specimen of the latter fish.
A gymnotus being obtained, I conducted a series of experiments. Besides
the hands two kinds of collectors of electricity were used--one with a
copper disc for contact with the fish, and the other with a plate of
copper bent into saddle shape, so that it might enclose a certain
extent of the back and sides of the fish. These conductors, being put
over the fish, collected power sufficient to produce many electric
effects.
SHOCK. The shock was very powerful when the hands were placed one near
the head and the other near the tail, and the nearer the hands were
together, within certain limits, the less powerful was the shock. The
disc conductors conveyed the shock very well when the hands were wetted.
GALVANOMETER. A galvanometer was readily affected by using the saddle
conductors, applied to the anterior and posterior parts of the gymnotus.
A powerful discharge of the fish caused a deflection of thirty or forty
degrees. The deflection was constantly in a given direction, the
electric current being always from the anterior part of the animal
through the galvanometer wire to the posterior parts. The former were,
therefore, for the time externally positive and the latter negative.
MAKING A MAGNET. When a little helix containing twenty-two feet of
silked wire wound on a quill was put into a circuit, and an annealed
steel needle placed in the helix, the needle became a magnet; and the
direction of its polarity in every cast indicated a current from the
anterior to the posterior parts of the gymnotus.
CHEMICAL DECOMPOSITION. Polar decomposition of a solution of iodide of
potassium was easily obtained.
EVOLUTION OF HEAT. Using a Harris' thermo-electrometer, we thought we
were able, in one instance, to observe a feeble elevation of
temperature.
SPARK. By suitable apparatus a spark was obtained four times.
Such were the general electric phenomena obtained from the gymnotus, and
on several occasions many of the phenomena were obtained together. Thus,
a magnet was made, a galvanometer deflected, and, perhaps, a wire heated
by one single discharge of the electric force of the animal. When the
shock is strong, it is like that of a large Leyden battery charged to a
low degree, or that of a good voltaic battery of, perhaps, one hundred
or more pairs of plates, of which the circuit is completed for a moment
only.
I endeavoured by experiment to form some idea of the quantity of
electricity, and came to the conclusion that a single medium discharge
of the fish is at least equal to the electricity of a Leyden battery of
fifteen jars, containing 3,500 square inches of glass coated on both
sides, charged to its highest degree. This conclusion is in perfect
accordance with the degree of deflection which the discharge can produce
in a galvanometer needle, and also with the amount of chemical
decomposition produced in the electrolysing experiments.
The gymnotus frequently gives a double and even a triple shock, with
scarcely a sensible interval between each discharge.
As at the moment of shock the anterior parts are positive and the
posterior negative, it may be concluded that there is a current from the
former to the latter through every part of the water which surrounds the
animal, to a considerable distance from its body. The shock which is
felt, therefore, when the hands are in the most favourable position is
the effect of a very small portion only of the electricity which the
animal discharges at the moment, by far the largest portion passing
through the surrounding water.
This enormous external current must be accompanied by some effect within
the fish _equivalent_ to a current, the direction of which is from the
tail towards the head, and equal to the sum of _all these external_
forces. Whether the process of evolving or exciting the electricity
within the fish includes the production of the internal current, which
is not necessarily so quick and momentary as the external one, we cannot
at present say; but at the time of the shock the animal does not
apparently feel the electric sensation which he causes in those around
him.
The gymnotus can stun and kill fish which are in very various relations
to its own body. The extent of surface which the fish that is about to
be struck offers to the water conducting the electricity increases the
effect of the shock, and the larger the fish, accordingly, the greater
must be the shock to which it will be subjected.