Patent GB-713649-A (GB)
Title: Improvements in or relating to electrical measuring instruments
Inventor: DALY CHARLES TREVOR
IPC: G01R19/03
Filed: 1951-05-01
Abstract: 713,649. Current measurements. STANDARD TELEPHONES & CABLES, Ltd. April 30, 1952, [May 1, 1951.] No. 10171/51. Class 37. An arrangement for measuring the R.M.S. value of a current of irregular wave form by causing the current to act on a temperature dependent resistor in the circuit of a current of regular wave form and observing the change in the value of the latter current comprises a thermionic amplifier with positive and negative feed-back paths in one or other of which the temperature dependent resistor is connected. As shown in Fig. 2, an oscillator VI with an automatic level control provided by rectifying the output from a tuned transformer T2 has its output connected to a meter and to a bridge 5 with one of its arms R6 constituted by the temperature dependent resistor. A reading is taken at Mi with the bridge output short circuited and is adjusted if necessary by a potentiometer Pi. The short circuit is then removed and the bridge output is applied as negative feed-back to the grid circuit. The arm R 9  is then adjusted until M I again shows its former reading. In a modification, Fig.  3 , valves V1, V 2  constitute a negative feed-back voltage amplifier. Negative feed-back coupling is provided by resistors R8, R 9  from the anode of V2 via a coupling capacitor C5, R 9  also being the cathode resistor for Vi. A directly heated thermistor R6 has its cold resistance equal to that of a resistor R7 so that there is ample positive feedback initially to cause vigorous oscillation at about 100 kc/s. The 100 kc/s. voltage developed across R6 causes its resistance to drop until it is approximately equal to R 9  and there is then just sufficient positive feed-back to maintain oscillation. Fig. 4 shows the basic circuit of Fig. 3, the resistors R6 ... R 9  forming a bridge circuit fed across the vertical diagonal with the oscillator voltage Vo and having a slight unbalance output Vi across the horizontal diagonal, this being the input to the valve VI. The wave-form to be examined is applied across the thermistor R6 at input terminals 9, 10 and causes a reduction of its resistance. The voltage Vo is reduced in proportion to the input voltage so that the total power dissipated in the thermistor R6 remains constant and maintains the bridge in balance. The reduction in the voltage V 0  is proportional to the R.M.S. value of the input voltage. The Provisional but not the Complete Specification includes a further arrangement, Fig. 5, in which the complex wave-form to be measured is balanced against its equivalent D.C., which is a measure of the R.M.S. value of the complex wave at the input. A bridge comprises two similar indirectly heated thermistors R6, R 9 , equal ratio arms R 7 , R8, and a battery I2 across the diagonal 13, 14. The current to be measured is applied to the heater R6H and raises the temperature of the thermistor R6 by an amount proportional to the R.M.S. value of the current. The resulting change of resistance of the thermistor unbalances the bridge and causes an unbalance voltage to appear across the diagonal 15, 16. This is fed to the input terminals 17, 18 of a D.C. amplifier 19, the output of which is fed to the heater R 9 H of the thermistor R 9  and tends to rebalance the bridge. The D.C. current is measured by a moving-coil ammeter A and is proportional to the R.M.S. value of the input current to the heater R6H. The two thermistors R6, R 9  may be mounted in a common envelope to prevent errors from changes in ambient temperature. Specification 682,189, [Group XL(c)], is referred to.