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. [Illustration: 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. [Illustration: 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. The fact that it is a reverse current is shown by the actions of the galvanometer for it will be noticed that the needle swings in the opposite direction this time. It will also be noticed that no current is produced when the coil and magnet are stationary. Current is only generated when the coil and magnet are approaching one another or moving apart suddenly. This is because it is only then that the magnetic field is changing. The field is strongest nearest the magnet, and therefore if either the magnet or the coil of wire is moved, the strength of that part of the field which intersects the coil is changed. Induced currents can only be generated by a _changing_ magnetic field. ¹ See chapter on Measuring Instruments. CHAPTER VI ELECTRICAL UNITS The Ampere There are certain terms used in the electrical field to distinguish various properties and qualities of the electrical current with which it is well for the young experimenter to acquaint himself.