undred cycles a second would give you the
impression of a musical note. As the frequency is made still larger you
have merely the impression of a higher-pitched note until we get up into
the thousands of cycles a second. Then, perhaps about twenty-thousand
cycles a second, you find you hear only a little sound like wind or like
steam escaping slowly from a jet or through a leak. A few thousand
cycles more each second and you don't hear anything at all.

You know that for radio-transmitting stations we use audion oscillators
which are producing alternating currents with frequencies of several
hundred-thousand cycles per second. It certainly wouldn't do any good to
connect a telephone receiver in the antenna circuit at the receiving
station as in Fig. 60. We couldn't hear so high pitched a note.

[Illustration: Fig 60]

Even if we could, there are several reasons why the telephone receiver
wouldn't work at such high frequencies. The first is that the diaphragm
can't be moved so fast. It has some inertia, you know, that is, some
unwillingness to get started. If you try to start it in one direction
and, before you really get it going, change your mind and try to make it
go in the other direction, it simply isn't going to go at all. So even
if there is an alternating current in the coil around the magnet there
will not be any corresponding vibration of the diaphragm if the
frequency is very high, certainly not if it is above about 20,000 cycles
a second.

The other reason is that there will only be a very feeble current in the
coil anyway, no matter what you do, if the frequency is high. You
remember that the electrons in a coil are sort of banded together and
each has an effect on all the others which can move in parallel paths.
The result is that they have a great unwillingness to get started and an
equal unwillingness to stop. Their unwillingness is much more than if
the wire was long and straight. It is also made very much greater by the
presence of the iron core. An alternating e. m. f. of high frequency
hardly gets the electrons started at all before it's time to get them
going in the opposite direction. There is very little movement to the
electrons and hence only a very small current in the coil if the
frequency is high.

If you want a rule for it you can remember that the higher the frequency
of an alternating e. m. f. the smaller the electron stream which it can
set oscillating in a given coil. Of course, we m