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52 for the separation of the two liquids. The porous pot forms the inner vessel into which the acid and zinc are placed, whilst an outer vessel is provided for the reception of the copper plate, and the solution of copper sulphate. It is advisable to amalgamate the zinc with mercury so as to protect it against attack by the acid when the cell is not working. When it is working no protection is possible, for the electrochemical action must be going on as long as the current flows. By this action oxygen is carried to the zinc, and this is thereby dissolved, forming with the sulphuric acid zinc sulphate. Thus the electrical energy given by the cell to the external circuit is obtained at the cost of the chemical energy liberated in the oxydation of the zinc and its conversion into sulphate. The e.m.f. of the Daniell cell is quite constant; it is a little over one volt.

Since Daniell's time many types of depolarising cells have been invented, zinc being generally one of the metals employed. The current passes from the zinc through the liquid to the other plate, which may be of copper, as in the Daniell and Meidinger cell, or of platinum as in Grove's, or of carbon as in Bunsen's and others. Since the current