"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 the person to whose oral cavity it is
held. In this way the sensation of the same sound as was spoken at the
transmitter end is reproduced at the receiver end. In other words, the
transmitter jerks and jumps just as the needle of a phonograph does in
traveling over a record, and transmits these jerks and jumps over the
wire to the metal disc which by aërial pressure on the ear drums of
the receiver of the message, causes the aural membrane to translate
the words, or vibrations along the nerves, to the brain.

"Following up this line," said Mr. Chadwick, "we find that the problem
in radio telephony is the same as that met with in ordinary wire
telephony. That is to say, we are required to cause a distant metal
disc to repeat every inflection of the transmitter. But in the case of
radio telephony the result is to be obtained by Hertzian waves,
instead of by a current passing through an insulated wire."

"The same sort of waves that are employed in wireless telegraphy?"
asked Tom.

"Just the same, only in radio telephony we are confronted by a problem
not met with in wireless telegraphy. We have not only to transmit
sound, such as isolated dots and dashes, but to send through the air
every rise and fall and inflection of the human voice just as it is
recorded in the minute lines of a phonographic record.

"Experiments have shown that articulation, that is, understand, a
speech, depends upon overtones and upper harmonies of a frequency of