Page:Collected Physical Papers.djvu/157

Rh became less and less. When the distance was increased to 25 cm. the action was reduced to zero, when this was further increased to 30 cm. there was a diminution of resistance, showing that 25 cm. is, in this case, the critical distance. The receiver continued to exhibit a diminution until the radiator was removed to a distance of 70 cm., when the radiation intensity was too feeble to affect the receiver. Now this critical distance may approximately be regarded as a measure of the sensibility of the substance. In this particular case the electric touch has a negative sign. If by any means (some of which will be described later on) the substance becomes more sensitive, i.e., more negative, the critical distance will be increased. On the contrary, if the sensitiveness becomes less (the substance tending towards positive direction) the critical distance will be decreased. The application of this principle is of importance as affording a means of determining the variation of sensitiveness under different conditions.

Experiments with Osmium Receiver.—Substances which are feebly positive give a diminution of resistance when the radiator is close to the receiver, and an increase of resistance when the radiator is beyond the critical distance. Thus the critical distance for an osmium receiver (whose normal action is moderately positive) was found to be about 250 cm. At the reduced distance of 50 cm. the response was +150 divisions; this was diminished to +4 when the distance was increased to 200 cm. But at a distance of 300 cm. when the intensity of radiation was so feeble as to be below the critical, the response was reversed to −3 divisions.