th a metal fence,
drain-pipe, etc.

XI. A RECIPROCATING ELECTRIC MOTOR.

The electric motor to be treated in this chapter illustrates very prettily
the attractive force of a hollow, wire-wound bobbin on a movable core, when
the electric current is passed through the wire. If one inserts the end of
an iron rod into the coil, the coil exerts a pull upon it, and this pull
will cease only when the centre of the rod is opposite the centre of the
coil. This principle is used in the "electric gun," which in its simplest
form is merely a series of powerful coils arranged one behind another on a
tube through which an iron or steel projectile can pass. The projectile
closes automatically the circuit of each coil in turn just before reaching
it, and breaks it before its centre is halfway through the coil, being thus
passed along from one coil to the other with increasing velocity.

Our motor is essentially a very inefficient one, its energy being small for
the current used, as compared with a revolving motor of the usual kind. But
it has the advantage of being very easy to make.

[Illustration: FIG. 37.--Electric reciprocating engine and battery.]

How it works.--The experimental engine, constructed in less than a couple
of hours, which appears in Fig. 38, consists of a coil, C, strapped down by
a piece of tin to a wooden bedplate; a moving plunger, P, mounted on a
knitting-needle slide rod, SR; a wire connecting rod, SR; a wooden crank,
K; and a piece of knitting-needle for crank shaft, on which are mounted a
small eccentric brass wipe, W, and a copper collar, D. Against D presses a
brass brush, B1 connected with the binding post, T1; while under W is a
long strip of springy brass against which W presses during part of every
revolution. T2 is connected to one end of the coil winding, and T1 through
a 4-volt accumulator or three dry cells, with the other end of the coil.
When W touches B2 the circuit is completed, and the coil draws in the
plunger, the contact being broken before the plunger gets home. The crank
rotates at a very high speed if there is plenty of battery power, all the
moving parts appearing mere blurs.

CONSTRUCTION.

The coil is made by winding 4 oz. of No. 32 cotton-covered wire (price 6d.
to 8d.) on a boxwood reel 2 inches long and 1-1/2 inches in diameter, with
a 9/16-inch central hole. Before winding, bore a hole for the wire through
one end of the reel, near the central part, and mount the reel