qual to
that from a battery of fifty Grove cells.

SECONDARY CURRENTS.

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