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


Author: J.B. Hoag

When a steady current flows through a conductor, a stationary magnetic field is produced in the surrounding space. The strength of the field is great near the wire and weaker farther away.

If the field is moved, an e.m.f. will be induced in a nearby conductor in an amount proportional to the rate at which the field cuts the conductor. The e.m.f. induced in a more distant conductor will be less because the magnetic field is weaker. Between the two conductors a difference of potential is created and hence an electric field is established.

It is not necessary that conductors be specified. The electric field will be established between any two points in space, whether they are separated from each other by a vacuum, an air column, an insulator, or a conductor.

An electric field E accompanies a moving magnetic field, just as a magnetic field H accompanies a moving electric field. The fields, if in motion, are always associated together. It can be shown that they are at right angles to each other, that both are at right angles to their motion, and that they contain equal amounts of energy. They are spoken of jointly as an electromagnetic field.

In 1864, Clerk Maxwell said that light, passing from a source to an observer, consisted of electric and magnetic fields in motion. He predicted that there should be other electromagnetic waves of comparatively low frequency. These were found in the laboratory work of Heinrich Hertz in 1887, and are now called radio waves.

Last Update: 2009-11-01