Daniell cell

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Early 20th-century engraving of a gravity cell.  Note the distinctive crowfoot shape of the zinc anode.
Early 20th-century engraving of a gravity cell. Note the distinctive crowfoot shape of the zinc anode.

The Daniell cell (var. sp. Daniel cell), also called the gravity cell or crowfoot cell was invented in 1836 by John Frederic Daniell, who was a British chemist and meteorologist. The Daniell cell was a great improvement over and is somewhat safer than the voltaic cell used in the early days of battery development. The Daniel cell's theoretical voltage is 1.1 volts and the chemical reaction is:

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

The Daniell cell proper consists of a central zinc anode dipping into a porous earthenware pot containing zinc sulphate solution. The porous pot is, in turn, immersed in a solution of copper sulphate contained in a copper can, which acts as the cell's cathode. The use of a porous barrier prevents the copper ions in the copper sulphate solution from reaching the zinc anode and undergoing reduction. This would bring the battery to equilibrium without it driving a current. In other words, it would make the battery dead.

In the 1860s, a Frenchman named Callaud invented a variant of the Daniell cell which dispensed with the porous barrier. Instead, a layer of zinc sulphate sat on top of a layer of copper sulphate, the two kept separate by their differing densities. The zinc anode was suspended in the top layer whilst the copper cathode sat in the bottom layer. A layer of oil was often added on top to prevent evaporation. This variant was known as the gravity cell, and sometimes the crowfoot cell due to the distinctive shape of the electrodes. This arrangement was less costly for large multicell batteries but could not be moved and was vulnerable to loss of integrity if too much electric current was drawn, which would cause the layers to mix.


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