he 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 aerial 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
5,000 or 8,000 or more."

"What do you mean by frequency?" asked Tom.

"Speaking in reference to radio telephony it means the number of
electrical vibrations per second required to produce a certain sound.
In electric currents 100 per second is a low frequency current,
100,000 per second is spoken of as high frequency. In early
experiments with radio telephony it was found that the chief
difficulty lay in obtaining a current of sufficiently high frequency
to transmit the human voice, the currents used in wireless telephony
being much too weak for this purpose.

"I had, therefore, to invent my own alternator, which is attached to
that gasoline motor. There is a similar one in the shed from which you
just talked with me."

"But why does radio telephony require a stronger current than wireless
telegraphy?" Tom wanted to know.

"Because, up to the present, no way has been found of utilizing in
radio telephony the entire energy of the electric waves sent out,"
replied Professor Chadw