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Page:Popular Science Monthly Volume 19.djvu/563

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From the ease with which secondary batteries can be constructed of very low resistance, so that they will give for a short time what practical electricians call a quantity current, they have been for some time in use for certain special purposes, principally for heating the wire of the galvanic écraseur in surgical practice. By a secondary battery is meant a galvanic battery which, as at first put together, has no tendency to give a current at all; but, if a current of electricity be passed through it of sufficient tension to decompose the fluids which it contains, will give a current in the opposite direction, due to the recombination of the separated parts of the decomposed fluid. The older forms consisted of two plates of platinum, preferably coated with spongy platinum immersed in a weak mixture of sulphuric acid and water, the action in this case being that the charging current decomposes the water (either directly or as the result of a chemical action set up by decomposing the acid first) into oxygen and hydrogen, which gases are absorbed by the platinum plates, the oxygen by one and the hydrogen by the other. When the charging battery is removed, the secondary battery will give a powerful current until all the oxygen and hydrogen absorbed by the plates are recombined in the form of water. It was afterward found that satisfactory results could be got from plates of lead treated in the same way. Their employment, of course, reduced the first cost of the apparatus. M. Planté then produced his secondary battery, in which he obtained great surface, and consequently low internal resistance, and large current, by rolling into a spiral form two lead plates separated by pieces of insulating material placed between them at intervals. He further succeeded in greatly increasing the time for which the battery would yield a given current, or its capacity, by adopting an elaborate process for the "formation" of the plates, which consisted in charging the battery and discharging it, varying the direction of the exciting current, and leaving the battery undisturbed between the charging and discharging for gradually increasing intervals of time. This process added enormously to the expense of the apparatus, which was also too bulky and heavy.

M. Faure, however, has succeeded in increasing the capacity of the battery, and getting rid of the long and delicate process of formation. His battery, like M. Planté's, consists of two plates of lead rolled together into a spiral, but he coats each plate with a thin layer of red lead (one of the oxides of that metal), kept in its place by a piece of absorbent felt, which also keeps the two plates from touching. This felt is saturated with the weak acid. The effect of the exciting current in this case is to deposit spongy lead on one plate and to convert the red oxide on the other into puce-colored oxide which contains more oxygen than the red form; no doubt, also, the spongy lead at a late period of the charging becomes saturated with hydrogen. When the battery is now set in action, the spongy lead becomes reoxidized to red lead and the puce-colored oxide reduced to the same salt.