Electric Reversal.
It now remains to be proved that the "radiation product" exhibits a change of sign of electric touch. The sensitiveness of certain substances belonging to each of these two classes is very great. On the other hand, in the transition from one class to the other, substances are met with the sensitiveness of which is rather feeble. The experimental verification of the hypotheses mentioned above seemed at first very difficult, as the reversed action was likely to be masked by the stronger normal action of the still unconverted portion of the substance. It however occurred to me that if slightly sensitive substances were taken, then the direct and reversed actions were likely to be obtained with less difficulty. For this reason I took for my first experiments arsenic, which is moderately negative. It is however possible, though the adjustments are difficult, to exhibit the reversed actions even with strongly sensitive substances, and as a type of such actions that of iron will be taken as an example.
Observations with Arsenic Receiver.—Experiment I.—A receiver was made with freshly powdered arsenic; the critical distance was found to be 25 cm.; that is to say, when the radiator was moved from 1 to 25 cm. there was always produced an increase of resistance, while beyond this distance there was a diminution of resistance; the critical distance, 25 cm., may therefore be taken as an approximate measure of the negative character of the specimen. As has been said before, if through any cause the substance becomes more negative, the critical distance will be increased; but if the substance tends towards the positive direction by becoming less negative, then the critical distance will be decreased. The receiver was now continuously subjected to radiation for ten minutes. After this it was found that the receiver gave a diminution or positive reaction, even when the radiator was brought close to the receiver. The action of radiation has thus reversed the sign of electric touch.
Experiment II.—Any arbitrary length of exposure labours under the defect that what is observed is the final effect, the intermediate effects not being taken into account. In order to observe the intermediate effects, a very laborious series of observations is necessary. The experiment was therefore modified in the following manner:—A fresh receiver was subjected to radiation, and observations at intervals of fifteen seconds were taken to test the nature of reaction of the sensitive substance. The first action of radiation on the fresh specimen was a great increase of resistance—so very great that the current was reduced to zero; it was no longer possible to make any further observation without re-establishing the current. This was done by a very gradual increase of pressure, effected by means of a fine micrometer screw which moved the compressing electrode in a perfectly parallel manner. There
