GB783523A - Improvements in or relating to apparatus for obtaining measurements of electric power - Google Patents

Abstract

783,523. Power measurement. GENERAL ELECTRIC CO., Ltd. April 28, 1955 [May 4, 1954], No. 12953/54. Class 37. Apparatus for measuring electric power comprises a bridge having a temperature-sensitive resistive element in one arm, a conductive path supplying the measurement power to at least the temperature sensitive arm of the bridge, an unbalance indicator, a source of current energizing one diagonal of the bridge which is adjustable for bridge balance with zero power supplied to the resistive element, and means for effecting a discrete non-continuous adjustable step change in the amplitude of the bridge energization current such that following such step change the bridge is again balanced at a required value of input power dependent on the magnitude of the change but independently of the standing value of energization current. Fig. 1 shows apparatus for standardizing the power output of a noise generator connected over input terminals 1, 2 to output terminals 3, 4 over series resistances 5, 6 of a T attenuator, whose shunt path comprises capacitance 7 in series with a diagonal of a Wheatstone bridge'8 containing fixed resistances 9, 10, 12 and thermistor resistance 11; the remaining diagonal containing unbalance detector 15 and being decoupled to earth by capacitances 13, 14. The first bridge diagonal is energized from a D.C. (or lowfrequency A.C.) source 19 over fixed resistance 17 in series with variable resistance 18, which may be shunted by fixed resistance 20 through normally open pressel switch 21. Initially, the noise generator is disconnected, switch 21 is closed and resistance 18 is adjusted for balance. Thereafter, switch 21 is opened and the noise generator reconnected, after which the level of input power is adjusted to a level at which the bridge is rebalanced by variation of thermistor resistance by the additional heat dissipated therein. At balance, the attenuator network matches the input and output impedances, and the output power at terminals 3, 4 is accurately standardized. Mathematical theory is given, and it is shown that the circuit constants may be chosen so that the output power level is independent of the value of bridge energization current set initially by adjustment of resistance 18, but is determined by the value of resistance 20. Resistance 18 may be given a positive temperature coefficient to render the measurements independent of ambient temperature changes. In a modification (Fig. 2) the thermistor element 25 of a bridge comprising resistances 28, 29, 30 and unbalance indicator 26 may take the form of a bolometer heated by H.F. power derived from a waveguide or coaxial line 27, while the bridge is as before 'energized from a source of D.C. or low-frequency A.C. over resistance 31 in series with resistance 32 in parallel with which resistance 33 may be connected by a pressel switch, the resistance values being chosen so that the bridge is initially balanced with the R.F. power. source disconnected and the pressel switch closed, and also balanced at the required level of input power with such switch open. Resistance 20 and switch 21 (Fig. 1) may be replaced by plural resistances each corresponding to a distinct power level which are connectible in parallel with resistance 18 by a multiposition switch, whereby on initial balancing without power input the resistance corresponding to the required output level may be connected, and thereafter disconnected for measurement. Alternatively, the multiple switchable resistances are replaceable by a permanently connected power level calibrated variable resistance (Fig. 3, not shown) which is adjusted for the required power level during the initial balancing with the power source disconnected, after which it is reset to its maximum value, and the power source reconnected and adjusted to restore balance, at which the output power is at the required level.