On the variations of wave-length of the oscillations generated by three-electrode thermionic tubes due to changes in filament current, plate voltage, grid voltage, or coupling

Abstract

It is well known that a circuit containing a condenser and inductance coil can be maintained in oscillation at a frequency near its natural frequency by aid of a three-electrode thermionic vacuum-tube and suitably connected batteries. It is also known that although the frequency of the oscillations depends mainly upon the magnitudes of the inductance and the electrical capacity it is also affected by the resistance in the oscillatory circuit, by the voltages of the various batteries in use, by the temperature of the filament supplying the electrons, by other properties of the vacuum tube, and by the coupling between portions of the circuit associated with the grid and the anode. It is again well known that when one such vibrating circuit is caused to induce current in an independent oscillating circuit the induced current beats with the local oscillatory current, the beat frequency being equal to the difference between the frequencies of the induced and the local oscillations. This is applied, for instance, in the so-called auto-heterodyne or endodyne method of reception familiar in wireless telegraphy, where the difference of the high frequencies is arranged to be of acoustic frequency so as to operate the telephone receiver connected with the local oscillatory circuit. In this application the operator varies the pitch of the sound made by the beats by altering the capacity of his tuning condenser. Starting with the variable condenser adjusted to an extreme position such that the local frequency is, say, 10,000 per second lower than the frequency induced by the distant apparatus, a very shrill note is heard in the telephone, and as the local condenser is diminished in capacity, the pitch falls continuously through all the audible octaves till the beats are so slow, say 30 per second, that they cease to form a note.