So long ago as 1836 it was noticed by Prof. Page of Salem, that,
whenever a current of electricity was made to flow in a coil of wire,
another current in the opposite direction was induced in a coil that was
parallel with the first; and also, when the current in the first was
broken, another current in the second coil would flow in the opposite
direction to the former one. These currents, which are called secondary
currents, are very transient. No current at all flows save at the
instant of making or breaking the current. In this respect, we are
reminded of the behavior of the soft iron within the coil, which gives
origin to a current of electricity when it is made to approach a magnet
or recede from it, but gives no current so long as it is still.
These secondary currents were investigated by Prof. Henry, resulting in
the discovery of many curious and interesting phenomena. It will be
sufficient here for me to refer to what are called induction coils,
which are developments of the principles involved in electro-magnetism
and electro-induction. Imagine a rod of soft iron of any size to be
wound with a coil of wire, the ends of the wire to be so left that they
may be connected with a galvanic battery. Around this coil let another
coil be wound of very fine and well-insulated wire; the terminal wires
of it to be left adjustable to any distance from each other. Now, upon
making connection with a battery to the primary coil, there will be two
results produced simultaneously. First, the soft iron will be rendered
magnetic; and, second, a current of electricity will be generated in the
secondary coil; and the strength of this secondary current is very much
increased by the inductive action of the soft iron that has been made a
magnet. When the battery current is broken, the iron loses its
magnetism, and a current of electricity is again started in the
secondary coil in the opposite direction. The energy of this derived
current is so great that it will jump some distance through the air, and
thus is apparently unlike the electricity that originates in a battery.
An induction coil made by Mr. Ritchie for the Stevens Institute at
Hoboken, N.J., has a primary coil of 195 feet of No. 6 wire. The
secondary coil is over fifty miles in length, and is made of No. 36
wire, which is but .005 of an inch in diameter. This instrument has
given a spark twenty-one inches in length, with three large cells of a
bichromate battery.
Mr. Spottiswood of London has just had completed for him the largest
induction coil ever made. It has two primary coils, one containing
sixty-seven pounds of wire, and the other eighty-four pounds, the wire
being .096 inch in diameter. The secondary coil is two hundred and
eighty miles long, and has 381,850 turns. This coil is made in three
parts, the diameter of the wire in the first part being .0095 inch; of
the second part, .015; and the third part, .011. With five Grove cells
this induction coil has given a spark forty-two inches long, and has
perforated glass three inches thick.
The electricity thus developed in secondary coils is of the same
character as that developed by friction; and all of the experiments
usually performed with the latter may be repeated with the former, many
of them being greatly heightened in beauty and interest. Such, for
instance, are the discharges in vacuo in Geisler tubes, exhibiting
stratifications, fluorescence, phosphorescence, the production of ozone
in great quantity, decomposition of chemical compounds, &c.
The electricity developed by friction upon glass, wax, resin, and other
so-called non-conductors, has heretofore been called static electricity,
for the reason that when it was once originated upon a surface it would
remain upon it for an indefinite time, or until some conducting body
touched it, and thus gave it a way of escape. Thus, a cake of wax if
rubbed with a piece of flannel, or struck with a cat-skin or a fox-tail
becomes highly electrified, and in a dry atmosphere will remain so for
months. Common air has, however, always a notable quantity of moisture
in it; and, as water is a conductor of electricity, such damp air moving
over the electrified surface will carry off very soon all the
electricity.
Again, the electricity developed through chemical action in a battery
and through the inter-action of magnets and coils of wire has been
called dynamic electricity, inasmuch as it never appeared to exist save
when it was in motion in a completed circuit. This, however, is not
true; for if one of the wires from a galvanic battery be connected with
the earth, and the other wire be attached to a delicate electrometer, it
will be found that the latter gives evidence of electrical excitement in
the same manner as it does for the electricity developed by friction in
another body. This is sometimes called _tension_, and is very slight for
a single cell; but in a series of cells it becomes noticeable in other
ways. Thus when the terminals of a single cell are taken in the hands,
no effect is perceived: if, however, the terminals of a battery
consisting of forty or fifty cells be thus taken, a decided shock is
felt, not to be compared though with the shock that would be felt from
the discharge of a very small Leyden jar. The shock from several hundred
cells would be very dangerous.
It was formerly doubted that the electricity would pass between the
terminals of a battery without actual contact of the terminals. Gassiot
first showed that the spark would jump between the wires of a battery of
a large number of cells before actual contact was made. Latterly Mr. De
La Rue has been measuring the distance across which the spark would
jump, using a battery of a large number of cells.