The strength of the current induced in a circuit cutting the lines of force of a magnet is called its pressure, voltage, or electro-motive force (expressed shortly E.M.F.). It may be compared with the pounds-to-the-square-inch of steam. In order to produce an E.M.F. of one volt it is calculated that 100,000,000 lines of force must be cut every second.
The voltage depends on three things:—(1.) The strength of the magnet: the stronger it is, the greater the number of lines of force coming from it. (2.) The length of the conductor cutting the lines of force: the longer it is, the more lines it will cut. (3.) The speed at which the conductor moves: the faster it travels, the more lines it will cut in a given time. It follows that a powerful dynamo, or mechanical producer of current, must have strong magnets and a long conductor; and the latter must be moved at a high speed across the lines of force.
A SIMPLE DYNAMO.
In Fig. 67 we have the simplest possible form of dynamo—a single turn of wire, w x y z, mounted on a spindle, and having one end attached to an insulated ring C, the other to an insulated ring C1. Two small brushes, B B1, of wire gauze or carbon, rubbing continuously against these collecting rings, connect them with a wire which completes the circuit. The armature, as the revolving coil is called, is mounted between the poles of a magnet, where the lines of force are thickest. These lines are supposed to stream from the N. to the S. pole.
In Fig. 67 the armature has reached a position in which y z and w x are cutting no, or very few, lines of force, as they move practically parallel to the lines. This is called the zero position.
Fig. 67.
Fig. 68.