Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/463

366.] {| align=center at 22°C (Hg=1) || Percentage increment of conductivity for 1°C.
 * colspan=4|Resistance of Mixtures of Sulphuric Acid and Water at 22°C in terms of Mercury at 0°C.  MM. Kohlrausch and Nippoldt.
 * Specific gravity at 18°5 || Percentage of H2SO4 || Resistance
 * Specific gravity at 18°5 || Percentage of H2SO4 || Resistance
 * Specific gravity at 18°5 || Percentage of H2SO4 || Resistance
 * 0.9985 || 0.0 || 746300 || 0.47
 * 1.00 ||0.2 || 465100 ||0.47
 * 1.0504 || 8.3 || 34530 || 0.653
 * 1.0989 ||14.2 || 18946 || 0.646
 * 1.1431 || 20.2 || 14990 || 0.799
 * 1.2045 || 28.0 || 13133 || 1.317
 * 1.2631 || 35.2 || 13132 || 1.259
 * 1.3163 || 41.5 || 14286 || 1.410
 * 1.3547 || 46.0 || 15762 || 1.674
 * 1.3994 || 50.4 || 17726 || 1.582
 * 1.4482 || 55.2 || 20796 || 1.417
 * 1.5026 || 60.3 || 25574 || 1.794
 * }
 * 1.2631 || 35.2 || 13132 || 1.259
 * 1.3163 || 41.5 || 14286 || 1.410
 * 1.3547 || 46.0 || 15762 || 1.674
 * 1.3994 || 50.4 || 17726 || 1.582
 * 1.4482 || 55.2 || 20796 || 1.417
 * 1.5026 || 60.3 || 25574 || 1.794
 * }
 * 1.4482 || 55.2 || 20796 || 1.417
 * 1.5026 || 60.3 || 25574 || 1.794
 * }
 * 1.5026 || 60.3 || 25574 || 1.794
 * }

On the Electrical Resistance of Dielectrics.

366.] A great number of determinations of the resistance of gutta-percha, and other materials used as insulating media, in the manufacture of telegraphic cables, have been, made in order to ascertain the value of these materials as insulators.

The tests are generally applied to the material after it has been used to cover the conducting wire, the wire being used as one electrode, and the water of a tank, in which the cable is plunged, as the other. Thus the current is made to pass through a cylindrical coating of the insulator of great area and small thickness.

It is found that when the electromotive force begins to act, the current, as indicated by the galvanometer, is by no means constant. The first effect is of course a transient current of considerable intensity, the total quantity of electricity being that required to charge the surfaces of the insulator with the superficial distribution of electricity corresponding to the electromotive force. This first current therefore is a measure not of the conductivity, but of the capacity of the insulating layer.

But even after this current has been allowed to subside the residual current is not constant, and does not indicate the true conductivity of the substance. It is found that the current continues to decrease for at least half an hour, so that a determination