266 OXIDATION AND REDUCTION ELEMENTS, chap.
short time, as with bells, telephones, in telegraphy, etc. In scientific work, too, accumulators have been of great service, both for producing comparatively large currents and for obtaining high potentials. For the latter purpose a large number of small elements is used ; these elements are con- structed with small preparation tubes containing sulphuric acid and strips of lead. In charging, a large number of elements is arranged in parallel, '^rhilst on discharging all the cells are connected in series. Batteries of this sort, which, on account of the smallness of the electrodes, have only a small capacity, are frequently used for the study of electrical phenomena in gases, insulators, and poor conductors.
Certain disadvantages also attend the use of accumulators. The lead of which the electrodes consist must be used in fairly large pieces if any degree of rigidity is to be obtained, and this, of course, means a considerable weight. Further- more, lead is very easily attacked chemically. In order to avoid these objections, attempts have been made to use lead containing small amounts of foreign metals; 4 per cent, of antimony (so-called Julien metal) and other metals have been tried.
Every reversible element is in a certain sense an accumu- lator. For instance, if a current is impressed through a Daniell cell in the direction opposite to that of its own electromotive force, zinc is deposited from the zinc sulphate, and copper dissolves to copper sulphate. When left to itself, the process takes place in the opposite direction. In this case, however, the reversibility is more of a theoretical than of a practical nature, because, on account of diffusion of the two electrolytes, copper is deposited on the zinc. This gives rise to a vigorous " local action," whereby the zinc is rapidly destroyed, and there is an evolution of hydrogen in the short-circuited element consisting of zinc, deposited copper, and sulphuric acid (or sulphate solution). A similar dis- turbance vitiates the usefulness of the copper element (which it was hoped would prove a valuable accumulator), because
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