Abstract
This paper describes an instrument designed to measure the ratio of resistance between a standard stable resistor and a test resistor. The ratio measurement is reproducible to within two parts in 105. Since the measurement is made upon a comparative basis extreme precautions regarding absolute temperature control of the testing chamber need not be taken, provided that both the test and standard specimens are at the same temperature and are subjected to identical thermal gradients. The measuring bridge comprises the standard and test specimens which are linked to the primary and secondary windings of a current transformer. The current in the test resistor can be changed in definite proportions by a ratio transformer. The resistance ratio of the two resistors is given directly by the setting on this device when the circuits are balanced. The design of the test specimen is such that it can be removed from the testing chamber and replaced, after heat treatment, without incurring errors owing to changes in contact resistance and thermal e.m.fs.
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