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.
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