CHAPTER XII.
HOW ELECTRICITY IS GENERATED BY INDUCTION.

99. Electromagnetic Induction. We have seen that a magnet has the power to act through space and induce another piece of iron or steel to become a magnet. A charge of static electricity can induce a charge upon another conductor. We have now to see how a current of electricity in one conductor can induce a current in another conductor, not in any way connected with the first, and how a magnet and a coil can generate a current.

Fig. 99. Fig. 100.

100. Current from Magnet and Coil. If a bar magnet, Fig. 99, be suddenly thrust into a hollow coil of wire, a momentary current of electricity will be generated in the coil. No current passes when the magnet and coil are still; at least one of them must be in motion. Such a current is said to be induced, and is an inverse one when the magnet is inserted, and a direct one when the magnet is withdrawn from the coil.

101. Induced Currents and Lines of Force. Permanent magnets are constantly sending out thousands of lines of force. Fig. 100 shows a bar magnet entering a coil of wire; the number of lines of force is increasing, and the induced current passes in an anti-clockwise direction when looking down into the coil along the lines of force. This produces an indirect current. If an iron core be used in the coil, the induced current will be greatly strengthened.

Fig. 101.

It takes force to move a magnet through the center of a coil, and it is this work that is the source of the induced current. We have, in this simple experiment, the key to the action of the dynamo and other electrical machines.

102. Current from two Coils. Fig. 101 shows two coils of wire, the smaller being connected to a cell, the larger to a galvanometer. By moving the small coil up and down inside of the large one, induced currents are generated, first in one direction and then in the opposite. We have here two entirely separate circuits, in no way connected. The primary current comes from the cell, while the secondary current is an induced one. By placing a core in the small coil of Fig. 101, the induced current will be greatly strengthened.