Patent GB-560411-A (GB)
Title: Device for measuring alternating current, voltage and power
IPC: G01R17/10
Filed: 1942-09-24
Abstract: 560,411. Current, voltage, and power measurements. NEWTON, E. P., (Tolefonaktiebolaget L. M. Ericsson). Sept. 24, 1942, No. 13467. A specification was laid open to inspection under Section 91 on Feb. 17, 1943. [Class 37] Alternating current is measured by means of a bridge circuit through which is passed a part of the alternating current as well as direct current to bring the bridge into equilibrium and including a resistance variable with the temperature, the bridge circuit being connected with resistances in such manner that it can be connected in the A.C. circuit either in series with a resistance for measuring the voltage or in parallel with another resistance for measuring the current, or in series with the former and in parallel with the latter resistance for measuring the power. A resistance R2 having a high temperature coefficient, e.g. a thin platinum wire, is connected in bridge circuit with three resistances R1, R3, R4 of negligible temperature coefficient. A galvanometer G across one diagonal of the bridge then indicates zero for one particular value of direct current supplied to the other diagonal. Resistances R5, R6, R7, R8 are connected as shown in a D.C. circuit between terminals L1. A reversing switch O having contacts 1 .... 6, switches a .... f, and resistances R9 .... R14 are connected as shown, the resistances R13, R14 being adjustable for different measuring ranges. The direct current between the terminals L1 is adjusted until the galvanometer G indicates zero and this current is measured by measuring the voltage V0 across the resistance R8. An A.C. voltage to be measured is applied to terminals 7, 8, the switch O is thrown to the left, and the D.C. between terminals L1 is adjusted until the galvanometer G again indicates zero. This current is measured by measuring the voltage V1 across the resistance R8. The effective value Ve of the A.C. voltage is then Ve= K#(V0<2> - V1<2>) where K is a constant depending on R10. An alternating current to be measured is passed through the resistance R13 between terminals 9, 10, switches c, d are closed, and the switch O is thrown to the left. The D.C. in the circuit L1 is then adjusted so that the galvanometer G indicates zero and is measured by measuring the voltage V1 across the resistance R8. The effective value Ie of the current to be measured is then Ie = C#(V0<2> - V1<2>) where C is a constant depending on R13. Power may be measured by supplying the voltage of the circuit to the terminals 7, 8 and the current to the terminals 9, 10. The switches a, b, c, e are closed and the switch O thrown to the left. The D.C. between terminals L1 is adjusted until zero reading is obtained and is measured by reading the voltage V1 across the resistance R8. The switch O is then thrown to the right and the D.C. is again adjusted to give zero reading and is measured by reading the voltage V2. The power is then P=KC(V1<2> - V2<2>) where K and C are constants depending on RIO and R13. The switch O may be replaced by a switch which changes the phase angle of the voltage instead of the current through 180‹ when measuring the power. In a modification two bridge circuits are so connected that when measuring alternating current and voltage one bridge circuit only is traversed by A.C. proportional to the alternating voltage or current applied, while when measuring power both bridge circuits are traversed by A.C. proportional to the current and the voltage, which in one bridge circuit have their phase angle changed through 180‹. For a direct measurement of power the power consuming instrument is connected between the terminals 7, 9 and the power supply to the terminals 7, 10 so that the current passes through the resistance R13 and the bridge circuit in parallel therewith. The switches a, d, e are closed and the power is calculated as described above.