Measurement Of Low Resistance and the ac Resistance of Superconductors

Abstract

The resistance of highly conductive materials is measured by an induction method that does not require a direct electrical connection to the test specimen. Instead, the test sample is used as a short‐circuited secondary winding of a transformer. The method is suited to specimens where lead correction errors would introduce difficulty, or where the temperature or field environment requires that the sample be located at some distance from the instruments. Resistances of the test specimen from 10⁻⁵ to 10⁻² Ω are suitable for this method. It has been applied to determining the resistance of hard superconductors at 4.2° and at frequencies of 100 to 30 000 cps. Various niobium‐zirconium alloys all followed a characteristic resistivity curve with ρ proportional to f^0.5 at high frequencies and ρ proportional to f^1.7 at f<500 cps.