GB767266A - Improvements in or relating to electrical frequency measuring instruments - Google Patents

Abstract

767,266. Frequency measurement. ELLIOTT BROS. (LONDON), Ltd. June 15, 1955 [March 15, 1954], No. 7524/54. Class 37. An instrument for checking or measuring the frequency of an A.C. source comprises a dynamometer instrument whose moving coil is energized from the source through a series impedance such that the current therein leads the supply voltage, and whose fixed coil is energized through a series impedance such that the current lags the voltage; the impedance magnitudes being variable and calibrated in response to frequency so that at the frequency to be checked or measured the sum of the phase angles between the coil currents and the source voltage is 90 degrees and the deflection of the instrument is zero. Moving coil M of the instrument is energized from the source TT through series variable resistance R2 shunted by capacitance C, while fixed coil F is energized through series resistance R1 and inductance L. Corresponding currents I1 lagging the supply voltage by 61 and I2 leading it by 62 flow in the coils, and when #1 + #2 = 90 degrees deflection is zero so that the impedances may be proportioned for zero deflection at the source frequency whose correct value is to be checked. #1 and . 62 may be 45 degrees so that deviations are symmetrically indicated. Resistance R2 may be continuously variable and calibrated in source frequency while inductance L and capacitance C may be simultaneously-variable in steps by a ganged switch calibrated in discrete frequency ranges ; the frequency being indicated when the resistance and impedances are adjusted to zero deflection of the instrument, which may be of the kind described in Specification 765,682 incorporating switching to introduce the required variable impedance components in series with the fixed and moving coils across the source.