5.2.2 - Vacuum tube oscillator
Schematic of the valve oscillator circuit developed by
1 - E.H. Armstrong
2 - Edwin Colpitts

The vacuum tube oscilator generated continuous waves at a single frequency, unlike its predecessor, the spark gap, thus permitting communication over a single channel. Using positive feedback and the triode as an energy pump, it provided a sign-wave output at the resonant frequency of its tuned circuit. Several individuals independently invented the circuit in the beginning of 1910, notably Reginald A. Fessenden, Edwin Armstrong and Edwin Collpits.