property of an electric current which enables
it to pass from one conductor to another conductor through the air.
Another singular and interesting thing is that the current so
transmitted across spaces changes its direction of flow, and,
furthermore, the tension of such a current may be changed by
transmitting it from one conductor to another.

LOW AND HIGH TENSION.--In order to effect this latter change--that is,
to convert it from a low tension to a high tension--coils are used, one
coil being wound upon the other; one of these coils is called the
primary and the other the secondary. The primary coil receives the
current from the battery, or source of electrical power, and the
secondary coil receives charges, and transmits the current.

For an illustration of this examine Fig. 70, in which you will note a
coil of heavy wire (A), around which is wound a coil of fine wire (B).
If, for instance, the primary coil has a low voltage, the secondary
coil will have a high voltage, or tension. Advantage is taken of this
phase to use a few cells, as a primary battery, and then, by a set of
_Induction Coils_, as they are called, to build up a high-tension
electro-motive force, so that the spark will jump across a gap, as shown
at C, for the purpose of igniting the charges of gas in a gasoline
motor; or the current may be used for medical batteries, and for other
purposes.

[Illustration: _Fig. 70._ INDUCTION COIL AND CIRCUIT]

The current passes, by induction, from the primary to the secondary
coil. It passes from a large conductor to a small conductor, the small
conductor having a much greater resistance than the large one.

ELASTIC PROPERTY OF ELECTRICITY.--While electricity has no resiliency,
like a spring, for instance, still it acts in the manner of a cushion
under certain conditions. It may be likened to an oscillating spring
acted upon by a bar.

Referring to Fig. 71, we will assume that the bar A in falling down upon
the spring B compresses the latter, so that at the time of greatest
compression the bar goes down as far as the dotted line C. It is obvious
that the spring B will throw the bar upwardly. Now, electricity appears
to have a kind of elasticity, which characteristic is taken advantage of
in order to increase the efficiency of the induction in the coil.

[Illustration: _Fig. 71._ ILLUSTRATING ELASTICITY]

THE CONDENSER.--To make a condenser, prepare two pine boards like A,
say, eight by ten inches and a