MAGNETIC MEASUREMENTS AT LOW FLUX DENSITIES 57 



The effect of contact resistance is minimized by changing few, or 

 preferably no, contacts between a-c. and d-c. readings. This is 

 facihtated by supplying current to one corner of the bridge through 

 the sliding contact of the slide wire resistance used for fine balancing. 

 This excludes contact resistance errors from resistance determinations 

 in which both a-c. and d-c. balances fall within the range of the slide 

 wire, and thus increases the bridge accuracy for small values of effective 

 resistance. Usual precautions as to clean and positive contacts are 

 sufficient for larger resistance measurements. 



TO 



OSCILLATOR 



Fig. 1 — Diagram of inductance comparison bridge suitable for measurement of 



magnetic core coils. 



The a-c. supply to the bridge should, of course, be a sine wave, and 

 the bridge transformers should be designed for minimum distortion. 

 The frequency should be known accurately, and the voltage should be 

 constant during any set of measurements. Rheostats are required to 

 permit accurate adjustment of bridge current, and they must be 

 designed and shielded to avoid stray coupling with the bridge. A suit- 

 able thermocouple is provided for measuring the current into the 

 bridge, from which measurement the current through the test coil can 

 be readily determined, since the bridge has equal ratio arms. 



A distortion-free amplifier and a filter circuit tuned to the measuring 

 frequency, are essential for magnifying the bridge unbalance current 

 so as to permit precise measurements. Such unbalances may be 

 detected by a vibration galvanometer at frequencies below say 200 ~ 



