Precision resistance-comparison bridge using current transformers

The transformer ratio-arm bridge described is unusual in that it employs toroidal current transformers in combination with diode rectifiers in place of the more commonly used voltage transformers. The two resistors are compared using 5c/s alternating squarewave currents, obtained by rectifying modulated 1000c/s currents. The 1000c/s currents are provided through separate current transformers, one of which is adjustable to obtain a balanced condition. The circuits of the resistive elements being compared are isolated from earth, and the measurements are independent of the impedances of the leads and of thermoelectric effects. The transformer and overall instrument design is described, and results obtained on production bridges are discussed. Typical accuracy is within ±0.001% over the range of resistances normally encountered in platinum-resistance thermometry (0.1¿500¿).