Electrical Engineering ⇒ Topic : E.m.f of cadmium standard cell

Emf

The most convenient reference standard of emf is the Weston Cadmium standard cell, shown in section in Fig. (a). The positive electrode is mercury and the negative electrode is an amalgam of mercury and cadmium. The electrolyte is a saturated solution of cadmium sulphate in water with the addition of a smallquantity of sulphuric acid. Mercurous sulphate acts as a depolarizer. When thiscell is manufactured according to the specification of the International Electrochemical Commission, its emf is exactly 1.01859 V at 20°C. Consequently, one volt can be taken as 1/1.01859 of the emf of a cadmium cell at 20°C. The emf of the cell falls by about 40 microvolts per degree rise in temperature. Since the internal resistance of the cell is about 1000 Ω, this cell should be used only as a standard reference of emf and not as a source of electrical energy.

                                         FIGURE (a) The cadmium standard cell