A primary cell consists of a vessel containing a liquid known as electrolyte and the two dissimilar metal plates known as electrodes immersed in the liquid.

A simple voltaic cell consists of a glass vessel containing copper and zinc plates immersed in dilute solution of sulphuric acid, as shown in. When pure zinc and copper plates are placed in the dilute solution of sulphureic acid, the zinc plate immediately becomes strongly charged with negative electricity and the copper plate with positive electricity so a potential difference is set up between these two plates and now if these plates are connected by a wire, an electric current will flow form copper plate to zinc plate in the external circuit. The copper is known to be electropositive to the zinc with respect to the external circuit i.e. copper plate acts as a positive plate or positive electrode and zinc plate acts as a negative plate or negative electrode.

It is not necessary that every primary cell may have plates of copper and zinc and electrolyte of dilute sulphuric acid solution. If a cell consists of carbon and lead electrodes immersed in a common salt solution, a potential difference will be set up between these two plates and on connecting the terminals of these plates, current will flow from carbon plate to lead plate in the external circuit.

If two plates, one of copper and another of zinc are immersed in distilled water, it will be observed that no potential difference is set up between the plates but if distilled waterish replaced by ordinary water, an appreciable potential difference will be set up, between the two plates of copper and zinc.

Simmilarly when two copper or two zinc plates are immersed in dilute solution of sulphuric acid (H₂SO₄), no potential difference is set up.

Hence to set up potential difference between the two plates immersed in an electrolyte, the following conditions are necessary.

(i)    Both the plates must be good conductors and of different materials.

(ii)    Electrolyte must be good conductor and containing acid or alkali or salt.

Since in these cells the energy is obtained after the primary action of placing the plates in the electrolyte so these cells are called the primary cells.

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