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 Chadwick. "Only the variations in the waves can be
detected, or transformed into sound at the receiving end of a radio
telephone system. Therefore an immense amount of electrical energy has
to be manufactured in order that the voice vibrations may register
their variations as powerfully as possible."

"What percentage of the electrical energy manufactured by a high
frequency alternator can be transformed into variations of sound?"