Page:Motors and motor-driving (1902).djvu/320

288 An accumulator may be looked upon as a reversible primary battery.

It has already been assumed that everybody is familiar with the common or horseshoe magnet. It will also be assumed that everybody is acquainted with the ordinary galvanic battery, but it may not be amiss to recapitulate its leading features.

If a plate of pure zinc and a plate of pure carbon be immersed in dilute sulphuric acid and connected outside the vessel in which they are placed by a metal wire, an electric current will flow along the wire from the point at which it is connected with the carbon to the point at which it touches the zinc. The current, however, will not flow for long. Soon after the wire connection between the two plates has been made, it will be observed that the surface of the carbon plate becomes covered with a layer of bubbles of gas, which increase in quantity till the whole plate is covered and the bubbles disengage and rise to the surface. The bubbles are hydrogen gas (due to the decomposition of the acidulated water by the zinc), and their formation on the carbon plate stops the further production of the electric current, and is known as polarisation. In the ordinary Bunsen battery, the carbon plate is set up inside a porous vessel containing nitric acid—a powerful oxidising material—the zinc remaining immersed in dilute sulphuric acid. The nitric acid oxidises or burns up the disengaged hydrogen, and by so doing produces additional electric energy; and therefore the voltage of a Bunsen cell is higher than that of a plain zinc carbon combination. If we could keep all the contents of the porous pot from escaping outside it, we could restore a Bunsen battery to its original condition by passing an electric current in the opposite way to that originally produced. This would reproduce the nitric acid which had reacted with the hydrogen and redeposit on the zinc plate the zinc that had been dissolved by the sulphuric acid, in the same way as a metallic deposit is produced by electroplating. In this manner the battery would be restored to its original condition. This cannot be done in practice because