Basic Radio is a free introductory textbook on electronics based on tubes. See the editorial for more information....


Author: J.B. Hoag

Over one hundred years ago, Michael Faraday performed a simple, most interesting experiment. He made up a coil of a few turns of wire and connected its ends to an instrument which would detect an electric current. A small bar magnet was placed near the coil along its axis.

Fig. 3 H. The principle of induction

When all was connected as in Fig. 3 H, Faraday plunged the magnet into the coil and found that the pointer of the current meter gave a sizeable kick. When the magnet was pulled out, the meter deflected in the opposite direction; when the magnet was held still, there was no flow of current; when the magnet was moved slowly, the current was small, and when it was inserted or pulled out of the coil quickly, the current " induced " in the coil circuit was large. Faraday then tried holding the magnet still and moving the coil. The results were the same; so he concluded that whenever a conductor, such as a copper wire, is moved through a magnetic field, or whenever a magnetic field moves across a conductor, an induced electromotive force is set up which can drive a current through a completed electrical circuit, and in an amount which is greater the shorter the time of motion. He also showed that the induction is greater when a stronger magnetic field is used. The stronger the field and the faster the lines of force of the magnetic field cut the conductor, the greater the e.m.f and current. That is Faraday's law of induction. Very small voltages and electrical currents are set up in the metal parts of an airplane as it moves through the earth's magnetic field.

Last Update: 2010-11-21