two half ring magnets put together, there will be a double pole of one kind where the current enters, and another double pole of the opposite kind where the current goes out. If the brushes be arranged to slip from one coil of the wire w to another, then if the whole wire-wound ring c be turned round, the magnetic poles will remain constant in space, although the ring turns round. If such a ring be mounted between the poles of such an electromagnet m, as shown in fig. 2, there will be a permanent pull on one of its pairs of poles in one direction and a permanent pull
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Fig. 3
on the other pair of poles in the opposite direction. Now the position of these poles in space is permanent, and depends upon the position, as already explained, of the brushes b and b1 . The result is that perpetual rotary pull or torque is produced in the ring and it turns round. The amount of torque produced is dependent upon the amount of magnetism present, that is upon the strength of the poles N and S, and the strength of the poles produced in the ring c; and this depends on the strength of the current flowing, and the
