s called a
_conduction_ current. Remember these terms.

In this connection there is also another thing which you will do well to
bear in mind. In Fig. 15 you will notice a core (C) and an insulated
wire coil (B) wound around it. The current, through the wire (B), as
shown by the darts (D), moves in one direction, and the induced current
in the core (C) travels in the opposite direction, as shown by the darts
(D).

[Illustration: _Fig. 16._ DIRECTION OF INDUCTION CURRENT]

PARALLEL WIRES.--In like manner, if two wires (A, B, Fig. 16) are
parallel with each other, and a current of electricity passes along the
wire (A) in one direction, the induced current in the wire (B) will move
in the opposite direction.

These fundamental principles should be thoroughly understood and
mastered.

CHAPTER IV

FRICTIONAL, VOLTAIC OR GALVANIC, AND ELECTRO-MAGNETIC ELECTRICITY

THREE ELECTRICAL SOURCES.--It has been found that there are three kinds
of electricity, or, to be more accurate, there are three ways to
generate it. These will now be described.

When man first began experimenting, he produced a current by frictional
means, and collected the electricity in a bottle or jar. Electricity, so
stored, could be drawn from the jar, by attaching thereto suitable
connection. This could be effected only in one way, and that was by
discharging the entire accumulation instantaneously. At that time they
knew of no means whereby the current could be made to flow from the jar
as from a battery or cell.

FRICTIONAL ELECTRICITY.--With a view of explaining the principles
involved, we show in Fig. 17 a machine for producing electricity by
friction.

[Illustration: _Fig. 17._ FRICTION-ELECTRICITY MACHINE]

This is made up as follows: A represents the base, having thereon a flat
member (B), on which is mounted a pair of parallel posts or standards
(C, C), which are connected at the top by a cross piece (D). Between
these two posts is a glass disc (E), mounted upon a shaft (F), which
passes through the posts, this shaft having at one end a crank (G). Two
leather collecting surfaces (H, H), which are in contact with the glass
disc (E), are held in position by arms (I, J), the arm (I) being
supported by the cross piece (D), and the arm (J) held by the base piece
(B). A rod (K), U-shaped in form, passes over the structure here thus
described, its ends being secured to the base (B). The arms (I, J) are
both electrically connected wi