TABLE OF CONTENTS An electric current going through a conductor generates a magnetic field, as already mentioned on pages 13 and 103 (and probably was known to the reader previously). A fast change in this field will generate a voltage in any stationary wire that is in the field, as mentioned on page 104. If the field is very quickly increasing and decreasing periodically, in other words it is a high frequency alternating field, there is something that might be surprising about the way this field goes a long distance as a "wave," and it can induce a voltage in a stationary wire that is far away. If the frequency is very roughly a million times per second (1 megahertz, or "MHz"), and it has been focused to form a parallel-sided beam, a moderately strong current in the original wire (the transmitting antenna) can send the beam 12,000 miles around the equator to be detected electrically in the other wire (the receiving antenna) on the other side of the world. This, of course, is a radio wave. The thing that makes the field strong enough to be detectable so far away is the extremely fast change in transmitting current, in order to go from zero to full intensity and back again in less than a millionth of a second. The effect would be much less at lower frequency, which is why radio usually involves MHz frequencies.
Although Heinrich Hertz is given credit for developing the early theory of radio and...
