The standard-cell comparator. A specialized potentiometer (Reprinted from Bureau of Standards Journal of Research, vol 11, pg 211-233, 1933)

The standard cell plays a very important role in the maintenance of the ele ctrical units and in correlating the units of the various national laborato ries. Modern standard cells have obtained such a high degree of reproducibi lity and permanence as to warrant the use of apparatus of the utmost precis ion and reliability in their intercomparison. The paper describes a new pot entiometer developed especially for this purpose. Although it actually meas ures the small difference between the known emf of a reference cell and tha t of the cell under test, it contains a simple mechanical computing feature which automatically adds this small difference algebraically to the emf of the reference cell and thereby indicates directly the value of the emf und er measurement. The design of the instrument is such that no readjustment o f its coils will be required when the impending changes in the ohm and the volt are accomplished. The new instrument has been given the distinctive na me, "standard-cell comparator".