condition, that of Fig. 56, because it
won't tell us anything. On the other hand why use the tube under the
first conditions where we need a large plate battery? If we can get the
same result, that is an indication when the other station is signalling,
by using a small battery let's do it that way for batteries cost money.
For that reason we shall confine ourselves to the study of what takes
place under the conditions of Fig. 57.

We now know that when a signal is being sent by the distant station the
current in the plate circuit of our audion at the receiving station is
greater, on the average. We are ready to see what effect this has on the
telephone receiver. And to do this requires a little study of how the
telephone receiver works and why.

[Illustration: Fig 58]

I shall not stop now to tell you much about the telephone receiver for
it deserves a whole letter all to itself. You know that a magnet
attracts iron. Suppose you wind a coil of insulated wire around a bar
magnet or put the magnet inside such a coil as in Fig. 58. Send a stream
of electrons through the turns of the coil--a steady stream such as
comes from the battery shown in the figure. The strength of the magnet
is altered. For one direction of the electron stream through the coil
the magnet is stronger. For the opposite direction of current the magnet
will be weaker.

[Illustration: Fig 59]

Fig. 59 shows a simple design of telephone receiver. It is formed by a
bar magnet, a coil about it through which a current can flow, and a thin
disc of iron. The iron disc, or diaphragm, is held at its edges so that
it cannot move as a whole toward the magnet. The center can move,
however, and so the diaphragm is bowed out in the form shown in the
smaller sketch.

Now connect a battery to the receiver winding and allow a steady stream
of electrons to flow. The magnet will be either strengthened or
weakened. Suppose the stream of electrons is in the direction to make it
stronger--I'll give you the rule later. Then the diaphragm is bowed out
still more. If we open the battery circuit and so stop the stream of
electrons the diaphragm will fly back to its original position, for it
is elastic. The effect is very much that of pushing in the bottom of a
tin pan and letting it fly back when you remove your hand.

Next reverse the battery. The magnet does not pull as hard as it would
if there were no current. The diaphragm is therefore not bowed out so
much.

Sup