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The EMF's of standard cells are now being transferred between laboratories over a 1½-km cable with a precision of 4 parts in 108 to provide an instantaneous comparison of the 2e/h and ϒp´ experiments being carried out at the two facilities. This is accomplished by transferring a constant current that produces a 1-V drop across standard resistors located at both ends of the cable.
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Williams et al. (1974) studied this question.
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