Page:Outlines of Physical Chemistry - 1899.djvu/207

 sealed with paraffined corks and blackened on the outside to avoid the action of light. The electromotive force of a Clark at 15° is 1*4384 volt. As the temperature rises the electromotive force decreases by 0*001 volt per degree.

IP. Ostwald recommends the following one volt element which has a very small temperature-coefficient. The arrangement of the cell is shown in fig. 39/ and it consists of zinc amalgam, solution of zinc chloride (density 1*409), mercurous chloride, and mercury. A more dilute zinc solution produces a current of higher potential, and a more concentrated one gives a lower potential. The element is compared with a standard Clark, and water or zinc chloride added until the potential is exactly one volt. The temperature-coefficient is smaller than + 0*00007 (the electromotive perature).

Soubces op Electricity. — In order to obtain a current of low intensity use is made of one or more LeclancM elements, the electromotive force of which is 1*2 to 1*3 volt and fairly constant. For stronger currents which are to be used for a considerable time accumulators render the best service. Thermo-electric piles might be used advantageously, but the majority of these are too easily destroyed by superheating. The new Gulcher pile, however, seems not to be so easily overheated.

For a description of conductors, keys, contacts, com- mutators, &c, works on Electricity must be consulted ; and here we shall pass on to Measurement Instruments.

The tangent galvanometer may be used for directly determining the intensity of fairly strong currents.

The ordinary galvanometer reveals the presence and the direction of a current, and within certain limits gives indications of the intensity of the current. In electro- chemical work the sensitiveness of the instrument is

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