Much information about the density of electron states in a superconductor may be obtained using electron tunneling techniques. It is of particular interest to measure the normalized dynamic conductance of superconducting tunnel junctions as a function of applied voltage. This quantity is the ratio of the dynamic conductance (di/dv)s when one or both metallic members of the tunnel junction are in the superconducting state, to the dynamic conductance (di/dv)n when both members are normal. A new method has been developed which enables measurements to be made of (di/dv)s/(di/dv)n to a few parts in ten thousand. With this method only a bridge circuit, a galvanometer amplifier, and an oscilloscope are used. The galvanometer amplifier has a passband from dc to a few cps and an input noise voltage of about 3×10-8 rms V. The circuits of the bridge and amplifier are presented and analyzed. The operation of the circuits for measuring the characteristics of low resistance specimens (≲500Ω) is described.
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