experiment, namely, by the sudden change of flux through a coil, the terminals of which are connected to the plug. Electromagnetic induction is also the working principle of all dynamo machines, but here we do not want to produce sparks at given times, but a sustained electric current flowing under a definite and steadily maintained e.m.f. Hence coils must go into and come out of action in regular rotation, and this condition is fulfilled by the part in the dynamo called the armature.
Let, in Fig. 14, A be a cylindrical piece of iron capable of revolving between the poles N S; and let this cylinder be wound with a coil C1, C2, the wires passing across the face and along the sides of the cylinder. One end of this coil is attached to an insulated metal ring R1, and the other to a similar ring R2. Metal springs B1 and B2, technically termed "brushes," press against these rings for the purpose of maintaining electrical continuity between the revolving coil and the fixed points of attachment of the external circuit, which by way of example we may take to contain an incandescent lamp. In the position shown, the full flux from the magnets passes through the coil, but. no e.m.f. is generated, because at that particular moment
