n casing) between which a spark has to pass. As soon
as H touches S the circuit is completed. The core becomes a powerful
magnet with external lines of force passing from one pole to the other
over and among the turns of the secondary coil. H is almost
instantaneously attracted by the core, and the break occurs. The lines
of force now (at least so it is supposed) sink into the core, cutting
through the turns of the "secondary," and causing a powerful current to
flow through them. The greater the number of turns, the greater the
number of times the lines of force are cut, and the stronger is the
current. If sufficiently intense, it jumps any gap in the secondary
circuit, heating the intermediate air to a state of incandescence.

THE CONDENSER.

The sudden parting of H and S would produce strong sparking across the
gap between them if it were not for the condenser, which consists of a
number of tinfoil sheets separated by layers of paraffined paper. All
the "odd" sheets are connected with T, all the "even" with T^1. Now,
the more rapid the extinction of magnetism in the core after "break" of
the primary circuit, the more rapidly will the lines of force collapse,
and the more intense will be the induced current in the secondary coil.
The condenser diminishes the period of extinction very greatly, while
lengthening the period of magnetization after the "make" of the primary
current, and so decreasing the strength of the reverse current.

TRANSFORMATION OF CURRENT.

The difference in the voltage of the primary and secondary currents
depends on the length of the windings. If there are 100 turns of wire in
the primary, and 100,000 turns in the secondary, the voltage will be
increased 1,000 times; so that a 4-volt current is "stepped up" to 4,000
volts. In the largest induction coils the secondary winding absorbs
200-300 miles of wire, and the spark given may be anything up to four
feet in length. Such a spark would pierce a glass plate two inches
thick.

It must not be supposed that an induction coil increases the _amount_ of
current given off by a battery. It merely increases its pressure at the
expense of its volume--stores up its energy, as it were, until there is
enough to do what a low-tension flow could not effect. A fair comparison
would be to picture the energy of the low-tension current as the
momentum of a number of small pebbles thrown in succession at a door,
say 100 a minute. If you went on pelting the do