Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

From: Duff, A. Wilmer, A Text-Book of Physics, 5th ed., P. Blakiston's Son & Co., Philadelphia (1921) pp. 418-9.
© Copyright 1998 R. Paselk

473. Standard Cells for E.M.F. Determinations. - In calibrations with the potentiometer it is necessary to have a "normal" or "standard" cell of known and constant e.m.f. The two cells used universally for this purpose are the cells devised by Latimer Clark and by Edward Weston. A form of the Clark cell is shown in Fig. 339. The positive pole is mercury (Hg), in contact with a paste of mercurous sulphate (Hg2SO4), and the negative pole is zinc in contact with a solution zinc sulphate. When this cell is made strictly according to the specifications fixed by the national physical laboratories, it has an e.m.f. of 1.434 volts at 15°C. and for a temperature t , an e.m.f. of [1.434 - 0.0012 (t - 15)] volts.
The Weston cell is exactly like the Clark cell except that the zinc is replaced by cadmium, and the zinc sulphate by cadmium sulphate. Its e.m.f. in the standard form is 1.0190 volts, and it has the great advantage of having practically no change of e.m.f., with temperatures. No appreciable current should be taken from a standard cell, as the accompanying chemical actions cause more or less permanent changes in the cell and its e.m.f.


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