GB541942A - Improvements in or relating to bridge arrangements for the comparison of electrical resistances - Google Patents

Abstract

541,942. Resistance measurements. BLUMLEIN, A, D. June 17, 1940, No. 10471. [Class 37] A bridge for comparing low resistances is arranged so as to reduce errors due to contact resistances. The bridge comprises ratio arms AC, AD, an arm BD containing the standard resistance Ro, and an arm Bc containing the resistance R1 which is to be compared with the resistance Ro. An alternating current source having a frequency of about 1000 cycles per second is connected between the points AB, and a voltage detector P such as a pair of telephones with a thermionic amplifier is connected between the points C, D. If it is desired to adjust the resistance R1 to equality with the standard Ro, the ratio arms AC, AD are chosen to be of equal impedance and the resistance R1 is adjusted until the voltage between the points C, D is a minimum. Contact resistances r2, r6 in series with the detector P do not affect the balance of the bridge and may be neglected. Contact resistances r1, r5 in series with the ratio arms AD, AC have negligible effect on the balance of the bridge provided they are small compared with the impedance of the arms AD, AC. The error due to the resistances r1, r5 may therefore be reduced by increasing the impedance of the arms AC, AD. The shunting effect of stray capacitances may be reduced by forming these arms as two tightly coupled inductances L1, L2 and earthing their common point, so that each arm has a high impedance but the voltage set up across each arm when the bridge is in balance is small. In order to reduce the errors due to contact resistances r4, r8 in series with the resistances Ro, R1, one end of each of the resistances Ro, R1 is connected by one pair of leads to the ends of a resistance R3 which is provided with an adjustable slider B. The same ends of each of the resistances R1, Ro are connected by a separate pair of leads to a tapped resistance R2 which is small in relation to the resistance R3. The resistance R3 is adjusted fo bring the bridge into balance and the position of the slider B in relation to the mid-point of the resistance R3 then indicates whether the resistance R1 is higher or lower than the standard Ro. The resistance R1 is then changed by adjusting the soldered connection and the procedure continued until balance occurs with the slider B at the mid-point of the resistance R3. As the value of the resistance R1 approaches equality with the standard Ro, the resistance R2 is decreased to give greater sensitivity. A resistance may be inserted between the generator G and the point A of the bridge and the voltage across this resistance applied to the moving plate of a differential condenser, the two fixed plates of which are connected to the points C, D. The current component in quadrature with the in-phase currents in the arms BC, BD may thus be equalized and the voltage minimum across the points C, D sharpened.