The generation of electricity by a dynamo is based on a principle of magnetism called induction. When the lines of force that pass from the north to the south pole of a magnet are cut by a wire there is produced or induced in the wire a current of electricity. That is, if we take a loop or coil of wire which has no current in it and a magnet which also has no current, and move the loop or coil between the poles, as shown in Fig. 83, a momentary current is produced. If a series of loops or coils are used instead of one loop, a current may be generated continuously. This method of generating electric current is called induction.

The strength of a current in electromotive force set up by induction depends upon: (1) the strength of the magnet, (2) the number of turns of wire in the coil or loop, and (3) the speed with which the magnetic lines of force are cut, that is, the speed at which the coil rotates.

228. Direction of an Induced Current.

- The direction of an induced current depends upon two factors: (1) the direction of the motion of the wire, and (2) the direction of the magnetic lines of force.

A very valuable method of determining the direction of current used in practical life is called Fleming's Rule.

Place the thumb, forefinger, and center finger of the right hand so as to form right angles to each other. If the thumb points in the direction of the motion of the wire, and the forefinger in the direction of the magnetic lines of force, the center finger will point in the direction of the induced current.

It is very important to know the direction of the current in revolving a loop of wire between the poles of a magnet in order to understand the working of a dynamo.

Examine Fig. 83 and notice, the loop of wire between the poles of the magnet. If the loop is rotated to the right, as indicated by the arrow head, the wire XB moves down during the first half of the revolution. According to Fleming's Rule, the current would be directed from B to X. The wire YA would move up during the first half of the revolution and the current flow from A to Y. As the result of the first half of the revolution, the current would flow in the direction AYBX. Fig. 83. - Magnetic Field. Showing loop of wire rotating between the north (N) and south (S) poles of a magnet.

Repeat the reasoning for the second half of the revolution. Notice that for every complete revolution, the current reverses its direction twice. It is accordingly called alternating current. As the strength of the current depends upon the number of lines of force cut, so the induced electromotive force starts at zero, goes to a maximum, and then back to zero in the first half-turn. That is, the induced electromotive force reaches its maximum when the loop is in a horizontal position because it cuts the most lines of force at this position. It cuts the least number of lines of force at the beginning and at the end of each half-vertical revolution.