Page:Text-book of Electrochemistry.djvu/276

 tine syphon or wet thread containing 0'1-normal KCl. Any nnpolarisable electrode may be used as a normal electrode. Since the polarisation diminishes rapidly after the circuit is broken, it should be measured as soon as possible (within 0*01 second) after the current is stopped : the measure- ment can best be made with an electromagnetic tuning-fork. Accumulators. — The accumulators constructed by Plante {16) in 1859 are a particular kind of secondary elements. The simplest form consists of two lead pole plates immersed in a 15 to 30 per cent, solution of sulphuric acid. When a current is conducted through this element, hydrogen is separated at one side, and oxygen at the other, which gradually oxidises the positive plate to lead peroxide. When this process has taken place for some time, the current is reversed, so that the lead peroxide is reduced to lead oxide, which, with the sulphuric acid, forms lead sulphate, and this, by further reduction, leads to a spongy mass of lead. At the same time the surface of the other plate becomes covered with a film of lead peroxide. By repeatedly reversing the direction of the current, the lead peroxide permeates deeper and deeper into the positive plate, which becomes more porous. This successive charge and discharge necessary to " form " the accumulator plates requires a very long time, and formerly about a year was spent on this process. Chemical and mechanical means were afterwards introduced for treating the lead plates, so that the change into spongy lead was so far accelerated that the process may now be carried out in about fourteen days, or even less. In order to still further aid the "forming" of the plates, Faure {17) introduced the process of mechanically fixing litharge, or red lead, upon the lead plates. This succeeds well, for both litharge and red lead form a sort of cement with sulphuric acid, which (on account of the formation of lead sulphate) assumes a solid consistency, and, according to the process of Sellon and Volckmai' (/5), a mixture of this sort is brought into properly disposed cuts on the lead plates. Finely divided lead, moistened with water and sulphuric acid,

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