Page:Elementary Text-book of Physics (Anthony, 1897).djvu/350

336 of copper is immersed in the cupric sulphate, and a plate of zinc in the zinc sulphate. The copper is the positive pole, the zinc the negative pole. When the circuit is made, and the current passes, zinc is dissolved, the quantity of zinc sulphate increases and that of the cupric sulphate decreases, and copper is deposited on the copper plate. To prevent the destruction of the cell by the consumption of the cupric sulphate, crystals of the salt are placed in the solution. The electromotive force of this cell is evidently due to the loss of energy in the substitution of zinc for copper in the solution of cupric sulphate.

The secondary cell of Planté is an example of a cell made directly by electrolysis, as has been assumed in the preliminary discussion. The electrodes are both lead plates, and the electrolyte dilute sulphuric acid. When a current is passed through the cell, the oxygen evolved on the anode combines with the lead to form peroxide of lead, which coats the surface of the electrode. When the cell is inserted in a circuit, a current is set up, the peroxide is reduced to a lower oxide, and the metallic lead of the other plate is oxidized.

Cells of this sort, which have been constructed directly by coating lead plates with the proper oxides of lead, are called storage cells. They may be put in condition for use by sending a current through them in the proper direction. The sulphate of lead formed plays an important part in the operation of these cells.

The Latimer-Clarke standard cell is of great value as a standard of electromotive force. The positive pole consists of pure mercury, which is covered by a paste made by boiling mercurous sulphate in a saturated solution of zinc sulphate. The negative pole consists of pure zinc resting on the paste. Contact with the mercury is made by means of a platinum wire. As no gases are generated, this cell may be hermetically sealed against atmospheric influences. According to the measurements of Rayleigh, the electromotive force of this cell is very constantly 1,435 • 108 C. G. S. electromagnetic units at 15° Cent.