A direct reading AC differential ohm-meter circuit for electron transport measurements in the standard four terminal configuration is described. This circuit can easily detect changes in resistance of 10 -5 Omega in a 1 Omega resistor. Power dissipation of a fraction of a microwatt in the sample makes the apparatus suitable for use in a dilution refrigerator. The value of the sample resistance can be read directly in ohms and small changes in this value are obtained as a voltage which is linearly related to the change in resistance. This voltage can be simply calibrated and used for recording directly on a plotter or as input for a control system or data acquisition system.
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Cochrane et al. (1982) studied this question.
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