619,085. Frequency modulation. PHILIPS LAMPS, Ltd. Nov. 15, 1946, No. 33953. Convention date, May 6, 1943. [Class 40 (v)] In an oscillator frequency modulated by a variable reactance other than a reactance valve, an amplitude modulated voltage derived from the oscillatory circuit is rectified and negatively fed back to the modulation input so as to obtain a substantially linear relation between the modulating signal and the frequency of the oscillatory circuit. A reactor 4, whose inductance may be varied by modulating signals applied to the grid of valve 8, is in parallel with the resonant circuit 2, 3 of an oscillator 10. Changes in reactance in accordance with the modulation effect changes the current in the primary of transformer 5, 6, from which an amplitude modulated output is obtained. This output is rectified and fed back to the modulation input. To compensate for the component of the current through the reactor 4 that is due to the power losses of the coil-represented as a parallel resistor 13-a current is fed back from the oscillator 10, via condenser 12 and resistor 14, which is substantially equal and opposite to this component. In a modification, Fig. 5 (not shown), the oscillating circuit includes a generator of oscillations of a constant frequency and amplitude to ensure a linear variation of current in the variable reactor as its reactance changes. A filter passing only oscillations of the generator frequency is included in the output circuit of the feed back transformer. In a further modification, Fig. 3, (not shown), the variable reactor is in series with the main oscillator coil, the amplitude modulated feed back voltage being developed across the variable reactor. In another embodiment, Fig. 4 (not shown), a potential variable condenser replaces the variable inductance, the feed back voltage being taken from a transformer whose primary forms a small inductance in series with the condenser. Specification 609,970 is referred to.