Page:Collected Physical Papers.djvu/168

148 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 ones passing unnoticed. 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 the 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