Page:Electrical Engineering Volume 1.djvu/300

1732 same as the difference between the heats of formation of zinc nitrate and copper nitrate.

2654. It naturally follows, that with given metals for the anode and cathode, the E. M. F. should be the same, whatever salt of the two metals be used as electrolyte and depolarizer, respectively. This is borne out in practice, as experiments have shown the E. M. F. under these circumstances to be practically the same.

In order, then, to obtain a high E. M. F., it is necessary to use as an anode a metal whose salts have a high heat of formation, and as a cathode a metal whose salts have a low heat of formation, just as in the other classes of cells.

For commercial use, the same considerations apply as to the other classes; that is, the materials used in the cell must be easily and cheaply obtained, even if they do not result in the highest possible E. M. F. The cells which best realize this condition are the Daniell cell and its derivatives.

2655. The Daniell cell uses for the anode, zinc; for the electrolyte, a solution of (usually) zinc sulphate, ZnSO$4$; for the cathode, copper; and for the depolarizer, a solution of copper sulphate, CuSO$4$. Sometimes, in setting up the cell, dilute sulphuric acid is used instead of the zinc sulphate, but this soon forms a solution of zinc sulphate; hence, the result is the same as if the zinc sulphate were used originally. The E. M. F. of the Daniell cell is given several values by different investigators, ranging from 1.059 to 1.079 volts. The London Post Office uses this cell as a standard, and calls its E. M. F. 1.07 volts.

The original form of the Daniell cell consisted of a glass jar, into which the zinc, in the form of a cylinder, was placed. Inside the zinc was a porous cup containing the cathode, a strip of sheet copper. The porous cup was filled with the CuSO$4$ solution and the outer jar with the ZnSO$4$ solution.

2656. To prevent the gradual weakening of the depolarizer, it is usual to put a considerable amount of copper sulphate crystals (commonly known as blue vitriol) into the