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 some of the copper oxide dissolves in the alkali and diffases to the zinc. In this respect mercury takes up an exceptional position (see p. 251). Attempts have, therefore, been made to use mercury as the positive pole plate of accumulators, but no practical success has been attained by this. Since in lead accumulatoi's no fear of disturbances due to diffusion need be entertained, the distance between the plates may be made very small, and in this way the internal resistance reduced to a minimum. According to Streintz {20), lead peroxide conducts like a metal, and this also tends to reduce the internal resistance of lead accumulators.

In this latter respect aluminium stands in sharp contrast to lead. At the ordinary temperature an anode of this metal becomes covered with a film of oxide, which offers such a large resistance that the passage of the current is almost entirely stopped. Based on this property, Graetz {22) has constructed a commutator of some theo- retic interest In a trough

filled with a salt solution, in which are a platinum and an aluminium electrode, the current can only pass in the direc- tion from the platinum through the solution to the aluminium, on account of the property mentioned. If we introduce into the circuit of an alternating current two such troughs arranged as in Fig. 49, the current goes through each branch almost entirely in the direction indicated by the arrows.

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