Abstract :
The paper describes the results of experiments made to determine the factors controlling the production and wavelength of those oscillations, usually termed electronic, which are produced in positive-grid triodes and the frequency of which depends primarily on the inter-electrode time of transit of the electrons. The frequency of the oscillations obtained in this way is so high (of the order of 109 cycles per sec.) that it is not possible at present to measure the dynamic characteristics of the valve used to produce them. It is shown in the paper that oscillations similar in character to the electronic oscillations obtained with positive-grid triodes can be obtained with the split-anode magnetron, at magnetic field intensities greater than the critical field at which the anode current decreases rapidly with increase in magnetic field. The frequency of these electronic oscillations is proportional approximately to the ratio of the anode potential and the magnetic field intensity applied to the magnetron, and by suitable choice of this ratio the frequency of the oscillations can be made low enough for ordinary radio-frequency measurements to be made on valve impedance in the neighbourhood of oscillation. This procedure was followed in the experiments described in the paper; and the results obtained at a series of frequencies between 6 and 15 megacycles per sec. indicate that, in the case of the magnetron, the valve reactance changes sign in the neighbourhood of these electronic oscillations. An analysis is given of the type of electrical circuit equivalent to the magnetron valve required to reproduce the type of reactance and resistance variation obtained experimentally. It is shown that this equivalent circuit is of the series form having negative inductance, negative capacitance, and negative resistance components, and that this type of equivalent circuit could be produced if the inter-electrode transit time of the electrons within the valve were comparable with the- period of oscillations generated by the valve. Oscillations without any marked change in valve reactance were also obtained in the magnetron near the critical field condition.
