This paper presents an improved version of the low temperature direct current comparator described originally by Harvey. The comparator described herein exhibits a resolution of 1.5 nA middot;turn and provides ratios which have uncertainties of less than 4 parts in 1010. An alternate approach to the shielding of the windings is discussed and a preliminary comparator based on this concept is shown to have equally good performance. The latter comparator is probably better suited to the attainment of large current ratios.
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