Clark cell

Lore & Background

The Clark cell was invented by English engineer Josiah Latimer Clark in 1873. In 1893, the International Electrical Congress defined its output at 15 °C as 1.434 volts, and this definition became law in the United States in 1894. The cell was later supplanted by the Weston cell in 1905.

The original cell was set up in a glass jar similar to a gravity Daniell cell, with a pool of mercury at the bottom serving as the cathode, above which was a paste of mercurous sulfate as depolarizer, then a saturated aqueous solution of zinc sulfate. A short zinc rod dipped into the solution and was supported by a cork. An H-form variant was introduced by Lord Rayleigh in 1882, using an H-shaped glass vessel with zinc amalgam in one leg and mercury with mercurous sulfate paste in the other.

The cell yields a reference EMF of 1.4328 volts at 15 °C. It had two drawbacks: a rather large temperature coefficient of −1.15 mV/°C, and corrosion problems caused by platinum wires alloying with the zinc amalgam connections where they enter the glass envelope.

Reader's Guide

The Clark cell was notable for producing a highly stable voltage, which led to its adoption as a voltage standard. In 1893, the International Electrical Congress defined its output at 15 °C as 1.434 volts, and this definition became law in the United States in 1894. However, the design had limitations: a large temperature coefficient of −1.15 mV/°C made it sensitive to temperature changes, and corrosion issues arose from platinum wires alloying with zinc amalgam connections.

Reference cells like the Clark cell must be applied in such a way that no current is drawn from them. In 1905, the Clark cell was supplanted as a voltage standard by the more temperature-independent Weston cell, which offered better stability and fewer practical problems. Despite its replacement, the Clark cell played a key role in the early standardization of electrical measurements.