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The Electric Conductivity of Pure Water


THE difficulties besetting the preparation of water free from the last traces of dissolved impurity cannot be better illustrated than by the attempts which have been made to ascertain the electric conductivity of the pure liquid. At the outset it has to be remembered that the conductivity of water is exceedingly small. As the result of the most recent observations it has been found that one millimetre of water has at 0° almost the same resistance as 40,000,000 kilometres of copper of the same cross-section; consequently a copper wire having the same resistance and sectional area as one millimetre of water would be long enough to encircle the earth one thousand times. From the difficulty of preventing the introduction of small quantities of dissolved material into the water, and from the large diminution which such impurities exercise upon the resistance, there is probably no physical constant for which such widely varying values have been given as for the electric conductivity of water. If the conductivity of mercury be taken as 1010, prior to 1875, the following values had been ascribed to water by the observers named:—80, Pouillet; 70, Becquerel; 15, Oberbeck;. 4-5, Rosetti; 216, Quincke; and 1˙33, Magnus. In 1875, Kohlrausch succeeded in reducing the observed conductivity to 0˙71, or a value only 1/120th of that given by Pouillet. The large diminution thus brought about was no doubt due, for the most part, to the improved methods employed in obtaining purer samples of water. In Kohlrausch's experiments pains were taken not only to remove organic matter and any volatile alkaline or acid impurities from the water, but also to ensure that in its subsequent treatment contact with glass was avoided the purified water being distilled through a platinum condenser into a platinum resistance-cell. The next important modificacation in the treatment of the water was again introduced by Kohlrausch in 1884. The whole of the above measurements had been made upon water distilled under ordinary conditions, and thus in presence of air; he therefore proceeded to ascertain what alteration in conductivity took place when the water was rendered air-free. For this end he employed a glass apparatus resembling in construction the so-called “water-hammer.” A glass bulb of some 150 c.c. capacity, which served as a retort, was connected by a glass tube with a small glass receiver fitted with platinum electrodes. In this receiver the resistance of the water was measured by the use of a galvanometer and a continuous current, as the latter was so feeble that no appreciable effect was produced by polarisation. The glass connecting-tube was provided with a vertical branch, through which water, or liquids 10 clean the apparatus, could be introduced. Having admitted a quantity of purified water into the bulb, the vertical tube was then connected with a mercury air-pump, the pump set in action, and the water repeatedly shaken. A flask of cooled sulphuric acid was also put into communication with the evacuated enclosure to absorb water vapour, and thus promote partial distillation of the water. When dissolved gases had been removed, the vertical tube was sealed, and water was then distilled from the bulb into the receiver, the former being immersed in a bath at a temperature of 30° to 40°, and the latter in a cooling mixture at from o° to –8°, the temperature being kept as low as possible in order to diminish the solvent action of water on the glass. The value obtained in this way for the conductivity at 18° was 0˙25, or a number which is practically only one-third of that given by water distilled in air.


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RODGER, J. The Electric Conductivity of Pure Water. Nature 51, 42–43 (1894).

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