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- From: Duff, A. Wilmer, A Text-Book of Physics, 5th
ed., P. Blakiston's Son & Co., Philadelphia (1921) pp. 418-9.
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- © Copyright 1998 R. Paselk
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- 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.
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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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- © R. Paselk
- Last modified 22 July 2000
