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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.
Clark cells use a zinc amalgam anode and a mercury cathode in a saturated aqueous zinc sulphate electrolyte, yielding a reference EMF of 1.4328 Volts. (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 electrodes 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.
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