This form of electric motor is used largely in England in the form of an indicator. It is very easily made and if you have an old electro-magnet will cost practically nothing.
Illustration: Electric Motor
A large soft-iron wheel is mounted on an axle with a pulley-wheel on one end and a circuit breaker on the other end. The teeth on the circuit-breaker must be the same number as on the soft-iron wheel.
The electro-magnet is mounted so that its core is level with the axle and in a line with the wheel. One wire from it is attached to one binding screw and the other end is grounded to the iron frame that supports it. This frame is connected to the frame supporting the wheel. A small brush presses on the circuit-breaker and is connected to the other binding screw.
In the diagram A represents the iron wheel; B, the brush; C, the circuit breaker; D, the magnet. The wire connecting the two frames is shown by a dotted line.
To start the motor, attach your battery to the screws and turn the wheel a little. The magnet attracts one of' the teeth on the wheel, but as soon as it is parallel with the core of the magnet the circuit is broken and the momentum of the wheel brings another tooth to be attracted.
To reverse the motor reverse the connections and start the wheel the other way. Be sure that the frames are screwed down well or the motor will run jerkily and destroy the connections. --Contributed by F. Crawford Curry, Brockville, Ontario.