In 1831, Michael Faraday, a famous English chemist and physicist, discovered that if a magnet be suddenly plunged into a hollow coil of wire, a momentary current of electricity is generated in the coil. As long as the magnet remains motionless, it induces no current in the coil, but when it is moved back and forth, it sets up the currents. The source of electrical energy is the mechanical work done in moving the magnet.
Fig. 86.—Showing how a Current of Electricity may be induced by a Bar Magnet and a Coil.
The medium which changes the mechanical energy into electricity is the magnetic field which we have already seen exists in the neighborhood of a magnet.
A current of electricity produced in a coil in such a manner is said to be an induced current and the phenomenon is that known as magnetic induction.
Magnetic induction is met in the dynamo, induction coil, telephone, transformer, some forms of motors, and a number of other electrical devices.
Fig. 87.—A Horseshoe Magnet and a Coil arranged to produce Electric Currents by Induction.
A simple experiment in which electricity is produced by magnetic induction may be performed by winding a number of turns of fine insulated wire around the armature or keeper of a horseshoe magnet, leaving the ends of the iron free to come in contact with the poles of the permanent magnet. Connect the ends of the coil to a sensitive galvanometer,[1] the ends of the armature being in contact with the poles of the horseshoe magnet as shown in Figure 87.
Keeping the magnet fixed, suddenly pull off the armature. The galvanometer will show a momentary current. Suddenly bring the armature up to the poles of the magnet; another momentary current in the reverse direction will flow through the circuit.