The Measurement of Magnetic Fields of Medium Strength by Means of a Magnetron

Abstract

Use of magnetron to measure magnetic fields of strength 0.1 to 500 gauss.— The Magnetron is a special symmetrical type of kenotron, with a straight axial filament and cylindrical anode. When this is placed in a magnetic field parallel to its axis, the radial paths of the electrons are curved into arcs of circles, and for fields stronger than the critical for the given voltage, do not reach the anode at all; or, for a given field, as the voltage is decreased, the current suddenly begins to decrease rapidly at a critical voltage. Two methods are described. (1) The magnetron is connected in series with a high resistance and a source of voltage, and in parallel with a standard voltmeter. Then for a field $H$ at the center of the magnetron, parallel to its axis, there is a definite reading $V$ of the voltmeter, given by the equation $H=\frac{6.72 V^{\frac{1}{2}}}{R}$, when $R$ is the radius of anode. This method may be used for 20 to 500 gauss. (2) For weak fields, $H<\mathrm{}20\mathrm{}$ gauss, the magnetron is connected in series with a milliammeter, (which may be the voltmeter used in (1) with its resistance shortcircuited) and a source of voltage, and surrounded by a close-fitting solenoid. The solenoid is connected, through tungsten filament lamps, to the same source of voltage as the anodes, and the lamp resistance adjusted so that the magnetic field of the solenoid is the "critical" field of the magnetron. The combination of magnetron and solenoid is then calibrated, and the calibration is found to be practically independent of voltage fluctuations. The accuracy of the first method is ¼ per cent with calibration or for relative values over a small range without calibration, using commercial power supply and portable meters. The accuracy of the second method is about 1 per cent under good conditions.