1. The trolley system, with overhead cable attached to insulators on posts,
to carry the current one way, the rails being used as the "return." This
system has the disadvantages associated with a wire over which the human
foot may easily trip with disastrous effect.
2. That in which one of the wheel rails is used for taking the current to
the motor, and the other as the return. The objection to the system is that
the wheels must be insulated, to prevent short circuiting; and this,
besides causing trouble in construction, makes it impossible to use the
ordinary model rolling stock. To its credit one may place the fact that
only two rails are needed.
3. The third and, we think, best system, which has an insulated third rail
as one half of the circuit, and both wheel rails as the return, the motor
being kept in connection with the third rail by means of a collector
projecting from the frame and pressing against the top of the third rail.
The last, for reasons of convenience, is placed between the wheel rails. We
will assume that this system is to be employed.
[Illustration: FIG.
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