Use/History: Unsaturated Weston cells, such as this
example, are the most common voltage standards in normal laboratory
use. This specimen represents a style going back to at least
1929. These cells show a small temperature coefficient and so
are normally preferred over the saturated Weston cell. However
their e.m.f. decreases by about 0.08 mv per year, and thus must
be calibrated periodically against a saturated Weston cell at
a defined temperature. The Weston cell may be diagramed as:
Cd(Hg) | CdSO4(aq), Hg2SO4
| Hg.
Weston invented and patented the saturated cadmium cell in
1893. It had the advantage of being less temperature sensitive
than the previous standard, the Clark cell. It also had the advantage
of producing a voltage very near to one volt: 1.0183 V. In 1911
the Weston Saturated Cadmium Cell became the International Standard
for electromotive force. Weston waved his patent rights shortly
afterword so anyone was allowed to manufacture it.1
Early/contemporary descriptions of the features of the standard
cell and its use are provided below:
Description: The circular base of the cell is 4 3/16"
dia. and the cell is 3 7/8" high. There is a metal tag on
the top of the cell which reads: WESTON STANDARD CELL / MODEL
4 NO. 19866 / WESTON INSTS., DIV. OF DAYSTROM, INC., / NEWARK,
N.J., U.S.A. There is a decal under the cell: Dec. 21 '59. Includes National Bureau of Standards
Certificate, (80k) dated March 1, 1960, and Weston
Instruments Certificate, (132k) dated February 26, 1960 for
this cell: number 19866. (Humboldt College decal: 30642, now
removed.)
