That is what the audion does in the transmitting set of a radio telephone.
In my sixth letter I told you something of how the audion works.
The audion is a glass bulb like an electric light bulb which has in it a thread, or filament, of metal.
This audion is in a feed-back circuit, the oscillation frequency of which depends upon the condenser C and the inductance L.
The audion amplifiers each consisted of a number of tubes operating in parallel.
