The Electrical Resistance of Dielectrics

234*.] 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 greater 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 of the resistance deduced from the current will give a greater value if a certain time is allowed to elapse than if taken immediately after applying the battery.

Thus, with Hooper’s insulating material the apparent resistance at the end of ten minutes was four times, and at the end of nineteen hours twenty-three times that observed at the end of one minute. When the direction of the electromotive force is reversed, the resistance falls as low or lower than at first and then gradually rises.

These phenomena seem to be due to a condition of the gutta-percha, which, for want of a better name, we may call polarization, and which we may com- pare on the one hand with that of a series of Leyden jars charged by cascade, and, on the other, with Ritter’s secondary pile.

If a number of Leyden jars of great capacity are connected in series by means of conductors of great resistance (such as wet cotton threads in the experiments of M. Gaugain), then an electromotive force acting on the series will produce a current, as indicated by a galvanometer, which will gradually diminish till the jars are fully charged.

The apparent resistance of such a series will increase, and if the dielectric of the jars is a perfect insulator it will increase without limit. If the electro- motive force be removed and connexion made between the ends of the series, a reverse current will be observed, the total quantity of which, in the case of perfect insulation, will be the same as that of the direct current. Similar ef- fects are observed in the case of the secondary pile, with the difference that the final insulation is not so good, and that the capacity per unit of surface is immensely greater.

In the case of the cable covered with gutta-percha, &c., it is found that after applying the battery for half an hour, and then connecting the wire with the external electrode, a reverse current takes place, which goes on for some time, and gradually reduces the system to its original state. These phenomena are of the same kind with those indicated by the ‘resid- ual discharge’ of the Leyden jar, except that the amount of the polarization is much greater in gutta-percha, &c. than in glass.

This state of polarization seems to be a directed property of the material, which requires for its production not only electromotive force, but the pas- sage, by displacement or otherwise, of a considerable quantity of electricity, and this passage requires a considerable time. When the polarized state has been set up, there is an internal electromotive force acting in the substance in the reverse direction, which will continue till it has either produced a reversed current equal in total quantity to the first, or till the state of polarization has quietly subsided by means of true conduction through the substance.

The whole theory of what has been called residual discharge, absorption of electricity, electrification, or polarization, deserves a careful investigation, and will probably lead to important discoveries relating to the internal structure of bodies.

235*.] The resistance of the greater number of dielectrics diminishes as the temperature rises.

Thus the resistance of gutta-percha is about twenty times as great at 0°C as at 24°C. Messrs. Bright and Clark have found that the following formula gives results agreeing with their experiments. If r is the resistance of gutta- percha at temperature T centigrade, then the resistance at temperature T + t will be

R = r × 0.8878t , the number varies between 0.8878 and 0.9.

Mr. Hockin has verified the curious fact that it is not until some hours after the gutta-percha has taken its temperature that the resistance reaches its corresponding value.

The effect of temperature on the resistance of india-rubber is not so great as on that of gutta-percha. The resistance of gutta-percha increases considerably on the application of pressure.

The resistance, in Ohms, of a cubic metre of various specimens of gutta- percha used in different cables is as follows∗ . Name of Cable. Red Sea . . . . . . . . . . . . . . . . . . Malta-Alexandria . . . . . . . . . . Persian Gulf . . . . . . . . . . . . . . Second Atlantic . . . . . . . . . . . Hooper’s Persian Gulf Core Gutta-percha at 24°C . . . . . . 0.267 × 1012 to 0.362 × 1012 1.23 × 1012 1.80 × 1012 3.42 × 1012 74.7 × 1012 3.53 × 1012 236*.] The following table, calculated from the experiments of M. Buff† , shews the resistance of a cubic metre of glass in Ohms at different tempera- tures: Temperature. 200°C 250° 300° 350° 400° Resistance. 227000 13900 1480 1035 735 237*.] Mr. C. F. Varley‡ has recently investigated the conditions of the current through rarefied gases, and finds that the electromotive force E is equal to a constant E0 together with a part depending on the current according to Ohm’s Law, thus E = E0 + RC.

For instance, the electromotive force required to cause the current to begin in a certain tube was that of 323 Daniell’s cells, but an electromotive force of 304 cells was just sufficient to maintain the current. The intensity of the

Jenkin’s Cantor Lectures.

[Annalen der Chemie und Pharmacie, bd. xc. 257 (1854).] ‡ Proc. R. S., Jan. 12, 1871. 232 current, as measured by the galvanometer, was proportional to the number of cells above 304. Thus for 305 cells the deflexion was 2, for 306 it was 4, for 307 it was 6, and so on up to 380, or 304 + 76 for which the deflexion was 150, or 76 × 1.97.

From these experiments it appears that there is a kind of polarization of the electrodes, the electromotive force of which is equal to that of 304 Daniell’s cells, and that up to this electromotive force the battery is occupied in estab- lishing this state of polarization. When the maximum polarization is estab- lished, the excess of electromotive force above that of 304 cells is devoted to maintaining the current according to Ohm’s Law. The Law of the current in a rarefied gas is therefore very similar to the law of the current through an electrolyte in which we have to take account of the polarization of the electrodes.

In connexion with this subject we should study Thomson’s results∗ , in which the electromotive force required to produce a spark in air was found to be proportional not to the distance, but to the distance together with a con- stant quantity. The electromotive force corresponding to this constant quantity may be regarded as the intensity of polarization of the electrodes. 238*.] MM. Wiedemann and Rühlmann have recently† investigated the passage of electricity through gases. The electric current was produced by Holtz’s machine, and the discharge took place between spherical electrodes within a metallic vessel containing rarefied gas. The discharge was in gen- eral discontinuous, and the interval of time between successive discharges was measured by means of a mirror revolving along with the axis of Holtz’s machine. The images of the series of discharges were observed by means of a heliometer with a divided object-glass, which was adjusted till one image of each discharge coincided with the other image of the next discharge. By this method very consistent results were obtained. It was found that the quantity of electricity in each discharge is independent of the strength of the current and of the material of the electrodes, and that it depends on the nature and density of the gas, and on the distance and form of the electrodes. ∗ † [Proc. R. S., 1860, or Reprint, chap. xix.] Berichte der Königl. Sächs. Gesellschaft, Oct. 20, 1871.