0.1 ppm four-terminal resistance bridge for use with a dilution refrigerator

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Abstract
Measurement of electrical resistance of pure metal samples at low temperatures poses special problems because the resistance is usually very small and must be measured very precisely to be useful. We present a tested design for a bridge which in several significant respects is superior to previous ones. At ultralow temperatures it permits the measurement of resistance to 0.1 ppm in resistors from ∼1 mΩ to 0.1 μΩ, or lower. Indeed, relatively high precision measurements can be made over the whole temperature range from ∼20 mK to near 7 K. Finally, a new technique is developed which eliminates most of the problems of making a comparison (or standard) resistor and at the same time eliminates the necessity of an extrapolation to zero temperature in order to obtain ρ0, as is conventionally done.