M. Buff, described in Art. 271, shews the resistance of a cubic metre of glass in Ohms at different temperatures.
| Temperature. | Resistance. |
| 200°C | 227000 |
| 250° | 13900 |
| 300° | 1480 |
| 350° | 1035 |
| 400° | 735 |
369.] Mr. C. F. Varley[1] has recently investigated the conditions of the current through rarefied gases, and finds that the electromotive force is equal to a constant together with a part depending on the current according to Ohm's Law, thus
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 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 establishing this state of polarization. When the maximum polarization is established, 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, described in Art. 57, 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 constant quantity. The electromotive force corresponding to this constant quantity may be regarded as the intensity of polarization of the electrodes.
