The Daniell cell is an early form of electrochemical cell that was invented in 1836 by the British chemist and meteorologist John Frederic Daniell. It consists of two half-cells, each containing different electrolytes and connected by a salt bridge.

The Daniell cell consists of a copper electrode immersed in a solution of copper sulfate (CuSO₄) in one half-cell, and a zinc electrode immersed in a solution of zinc sulfate (ZnSO₄) in the other half-cell. The two half-cells are connected by a salt bridge, which is usually a tube filled with an electrolyte solution such as potassium nitrate (KNO₃).

In operation, the zinc electrode oxidizes, releasing electrons and forming zinc ions (Zn²⁺). The electrons flow through an external circuit to the copper electrode, where reduction takes place. At the copper electrode, copper ions (Cu²⁺) in the solution gain electrons and are reduced to form copper atoms, which deposit onto the electrode surface. This maintains a flow of electrons in the external circuit.

The Daniell cell was a significant advancement in early battery technology and was widely used in telegraphy and early electrical experiments. It provided a stable source of electrical energy due to the separate half-cells and the use of different electrolytes, which prevented the mixing of ions and the build-up of polarization.