h a switch. The contacts, C1 to C5, lie in the path of
the switch lever, and are connected through binding posts T1 to T6 with one
terminal of their respective cells. The cells are coupled up in series to
one another, and one terminal of the series with binding posts T0 and T6.
By moving the lever, any number of the cells can be put in circuit with T7.
The button under the head of the lever should not be wide enough to bridge
the space between any two contacts. Change the order of the cells
occasionally to equalize the exhaustion.

[Illustration: FIG. 50.--Adjustable resistance for controlling current.]

Resistance.--With accumulators, a "resistance" should be included in the
circuit to regulate the flow of current. The resistance shown in Fig. 50
consists of a spiral of fine German silver wire lying in the grooved
circumference of a wood disc. One of the binding posts is in connection
with the regulating lever pivot, the other with one end of the coil. By
moving the lever along the coil the amount of German silver wire, which
offers resistance to the current, is altered. When starting the motor use
as little current as possible, and open the resistance as it gets up speed,
choking down again when the necessary speed is attained.

General.--All the three fittings described should for convenience be
mounted on the same board, which itself may form the cover of the box
holding the dry cells or accumulators.

SOME SUGGESTIONS.

Instead of dry cells or accumulators a small foot or hand operated dynamo
generating direct, not alternating current, might be used. Its life is
indefinitely long, whereas dry cells become exhausted with use, and
accumulators need recharging from time to time. On occasion such a dynamo
might prove very convenient.

Anyone who possesses a fair-sized stationary engine and boiler might
increase the realism of the outdoor track by setting up a generating
station, which will give a good deal of extra fun.

XIV. A SIMPLE RECIPROCATING ENGINE.

Figs. 51 and 52 illustrate a very simple form of fixed-cylinder engine
controlled by a slide valve.

An open-ended "trunk" piston, similar in principle to that used in gas
engines, is employed; and the valve is of the piston type, which is less
complicated than the box form of valve, though less easily made steam-tight
in small sizes. The engine is single-acting, making only one power stroke
per revolution.

The cylinder is a piece of brass tubing;