"No, I must say I don't," said Jack; "you see----"

"It's pretty technical," broke in Tom.

"Well, then I'll try to explain to you, in simple language, the
general principles of radio telephony," said Mr. Chadwick. "In the
first place you know, of course, from your wireless studies, that an
electric wave sent into the air will travel till it strikes something,
such as an aërial."

"To use the old illustration, an electric impulse sent into the air
spreads out in all directions just like the ripples from a stone
chucked into a mill-pond," said Jack.

"That's it," said Mr. Chadwick. "Now then, as you also know the wire
telephone works by a metal disc in the receiver, vibrating in exactly
the same way as does the microphone in the transmitter. According to
the vibrations of the voice of the person sending the spoken message,
the electric current along the wire, acted upon by the microphone in
the transmitter, increases or decreases. This increasing and
decreasing current acts on a thin metal disc or diaphragm in the
receiver which is held to the ear of the person listening to the
message."

"That's plain sailing so far," said Jack. "For instance, when you say
'Hullo' over a phone, the microphone or transmitter gets busy and
records it in electrical impulses and shoots it all along the wire
where the receiver picks it up and wiggles the metal disc inside it to
just the same tune."