5.2.3 - Constant - current modulator
Schematic of the constante-current modulator circuit

In 1913, R. A. Heising invented the first modulator circuit capable of working efficiently.As per in the eschematic, with inductor L preventing any change in the total plate current drawn by V1 and V2, the modulating signal was applied to the plate circuit of the RF oscilator so that audio-frequency variations at the modulator´s output would produce similar variations in the oscilator´s plate current.