Clark cell

The Clark cell, invented by English engineer Josiah Latimer Clark in 1873, is a wet-chemical cell (colloquially: battery) that produces a highly stable voltage usable as a laboratory standard.

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Chemistry

Clark cells use a zinc, or zinc amalgam, anode and a mercury cathode in a saturated aqueous solution of zinc sulphate, with a paste of mercurous sulphate as depolarizer.

Construction

Original cell

Clark's original cell was set up in a glass jar in a similar way to a gravity Daniell cell. The copper cathode was replaced by a pool of mercury at the bottom of the jar. Above this was the mercurous sulphate paste and, above that, the zinc sulphate solution. A short zinc rod dipped into the zinc sulphate solution. The zinc rod was supported by a cork with two holes - one for the zinc rod and the other for a glass tube reaching to the bottom of the cell. A platinum wire, fused into the glass tube, made contact with the mercury pool. When complete, the cell was sealed with a layer of marine glue.

H-form cell

The H-form cell was introduced by Lord Rayleigh in 1882. It was set up in an H-shaped glass vessel with zinc amalgam in one leg and pure mercury, surmounted by a layer of mercurous sulphate paste, in the other. The vessel was filled, nearly to the top, with zinc sulphate solution. Electrical connections to the zinc amalgam and the mercury were made by platinum wires fused through the lower ends of the legs.

Characteristics

The cell yields a reference EMF of 1.4328 Volts at a temperature of 15°C (288 Kelvin). Reference cells must be applied in such a way that no current is drawn from them.

The design had two drawbacks - a rather large temperature coefficient of -1.15 mV/°C, and corrosion problems caused by the platinum wires alloying with the zinc amalgam connections where they enter the glass envelope.

Clark cells were later made obsolete by the more temperature-independent Weston cell design.

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