aerial method of
conversation. So far, they had not the least understanding, beyond a
general idea, of how the thing was done. Of the details by which Mr.
Chadwick had worked out this radical departure in telephony, they knew
nothing.

"Well, what did you think of it, boys?" asked Mr. Chadwick when they
returned to the workshop.

"Wonderful, beyond anything I could have imagined," declared Jack.

"How far will it work?" asked Tom.

"That's just the point," said Mr. Chadwick. "That's where I'm at sea.
I need a metal of greater conductivity than any attainable to get real
results. The carbon that I am using does not throw off enough radio
activity to produce a sufficient number of electric impulses to the
atmosphere."

Jack and Tom looked puzzled.

"You don't understand me I see," said Mr. Chadwick.

"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 aerial."

"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."

"That's it exactly," said Mr. Chadwick. "Now we are ready to go a step
further. Now, as this metal disc is attracted or released by the
current coming over the wire, it compresses or rarefies the air
between it and the ear-drum of t