Humboldt State University

Robert A. Paselk Scientific Instrument Museum

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Standard Cell

The Epply Laboratory

Humboldt S-T-C; c.1927

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Unsaturated Weston cells such as this example are the most common voltage standards in normal laboratory use. 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), Hg2SO4Hg.
Some contemporary/early descriptions of the standard cell and its use are provided below:


The cell stands 7 1/2" high to the top of the binding posts, 6 3/8" to the top of the bakelite case lid. The case itself is a 3 3/8 x 1 7/8 x 3 5/8" hollow bakelite tube, setting on a bevel edged 4 3/8 x 2 13/16 x 1/2" base, and with a 3 5/8 x 2 1/8 x 1/4" top with binding posts marked - & + in raised symbols. There is a black and nickel plated brass plaque pinned to the front, approx. as shown in the drawing below:

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The cell is sealed on the bottom with a light brown wax? plug stamped "E" and has the remains of an apparent BKH decal on top. (The cell is marked, in barely visible hand printing, on the back: 1.0189 8/8/27, apparently as calibration data by the original user.)

Paul Weigold, curator of the Electrical exhibit for the Vintage Radio & Communications Museum of Connecticut (711 Main St. East Hartford Conn 06108), has kindly supplied a scan of the original manufacturer's test data for this cell.


HSTC Instrument Collection

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HSTC (1921-34)
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HSC (1935-1953)
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HSC (1954-1973)

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© R. Paselk
Last modified 30 August 2010