Weston cell

The Weston cell, invented by Edward Weston in 1893, is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters. It was adopted as the International Standard for EMF in 1911.

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Chemistry

The anode is an amalgam of cadmium with mercury, the cathode is of pure mercury, the electrolyte is a solution of cadmium sulphate and the depolarizer is a paste of mercurous sulphate.

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 by platinum wires fused through the lower ends of the legs.

Characteristics

The original design was a saturated cadmium cell producing a convenient 1.0183 Volt reference and had the advantage of having a lower temperature coefficient than the previously used Clark cell. (Reference cells must be applied in such a way that no current is drawn from them.)

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 periodical calibration against a saturated cell.

Patent

• U.S. Patent 494,827 , "Voltaic cell"

See also

• Primary cell terminology

Sources

• Practical Electricity by W. E. Ayrton and T. Mather, published by Cassell and Company, London, 1911, pp 198-203