TheWeston cell orWeston standard cell is awet-chemical cell that produces a highly stablevoltage suitable as a laboratory standard forcalibration ofvoltmeters. Invented byEdward Weston in 1893, it was adopted as the International Standard forEMF from 1911 until superseded by theJosephson voltage standard in 1990.
Theanode is anamalgam ofcadmium withmercury with acathode of pure mercury over which a paste of mercurous sulfate and mercury is placed. Theelectrolyte is asaturated solution ofcadmium sulfate, and thedepolarizer is a paste ofmercurous sulfate.
As shown in the illustration, the cell is set up in an H-shaped glass vessel with the cadmium amalgam in one leg and the pure mercury in the other. Electrical connections to the cadmium amalgam and the mercury are made byplatinum wires fused through the lower ends of the legs.
Reference cells must be applied in such a way that no current is drawn from them.
The original design was a saturated cadmium cell producing a1.018638 V reference and had the advantage of having a lowertemperature coefficient than the previously usedClark cell.[1]
One of the great advantages of the Weston normal cell is its small change of electromotive force with change of temperature. At any temperaturet between0 °C and40 °C,
This temperature formula was adopted by the London conference of 1908[2]
The temperature coefficient can be reduced by shifting to an unsaturated design, the predominant type today. However, an unsaturated cell's output decreases by some 80 microvolts per year, which is compensated by periodic calibration against a saturated cell.
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