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 Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

. In 1893, the output of the Clark cell at 15 °C was defined by the

International Electrical Congress The International Electrical Congress was a series of international meetings, from 1881 to 1904, in the then new field of applied electricity. The first meeting was initiated by the French government, including official national representatives, le ...

as 1.434 volts, and this definition became law in the United States in 1894. This definition was later supplanted by one based on the Weston cell.

Chemistry

Clark cells use a

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

, or zinc amalgam,

anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

and a mercury

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...

in a saturated

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in water ...

zinc sulfate Zinc sulfate is an inorganic compound with the formula ZnSO4. It forms hydrates ZnSO4·''n''H2O, where ''n'' can range from 0 to 7. All are colorless solids. The most common form includes water of crystallization as the heptahydrate, with the che ...

, with a paste of mercurous sulfate 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 Daniell cell is a type of electrochemical cell invented in 1836 by John Frederic Daniell, a British chemist and meteorologist, and consists of a copper pot filled with a copper (II) sulfate solution, in which is immersed an unglazed earthenw ...

. The

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

cathode was replaced by a pool of mercury at the bottom of the jar. Above this was the mercurous sulfate paste and, above that, the zinc sulfate solution. A short zinc rod dipped into the zinc sulfate 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 Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

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 John William Strutt, 3rd Baron Rayleigh ( ; 12 November 1842 – 30 June 1919), was an English physicist who received the Nobel Prize in Physics in 1904 "for his investigations of the densities of the most important gases and for his discovery ...

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 sulfate paste, in the other. The vessel was filled, nearly to the top, with zinc sulfate 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

volt The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...

s at a temperature of 15 °C (288 K). 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 A temperature coefficient describes the relative change of a physical property that is associated with a given change in temperature. For a property ''R'' that changes when the temperature changes by ''dT'', the temperature coefficient α is def ...

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. In 1905, Clark cells were supplanted as a voltage standard by the more temperature-independent Weston cell.

Sources

* ''Practical Electricity'' by W. E. Ayrton and T. Mather, published by Cassell and Company, London, 1911, pp 198–203

References

{{Galvanic cells Battery types Mercury (element)