system is known as frequency modulation or FM, the principle be-
hind the AM (amplitude modulation) radios to be found in practi-
cally every home. These days, many FM broadcasting stations add
low transmitters to their signals which, in common with normal sig-
nals, are adjustable through a transmitter, but are of a very high fre-
quency. While the frequency of normal signals varies from 20 to
30
