Page:Collected Physical Papers.djvu/198

178 work there had been but little fatigue with the attendant change of sensibility.

Relation between the Intensity of Radiation and the Conductivity-variation.—The resistance of the receiver being not very large, the external resistance of the shunted galvanometer and of the cell are not negligible in comparison; the change of deflections is, therefore, not proportional to the variation of resistance. To interpret the absolute values of the deflections, a resistance box was substituted for the receiver, keeping the rest of the circuit just as before. In this way the absolute values of the resistances corresponding to particular deflections were found. Some of these are given on the left-hand side of fig. 36.

The galvanometer deflections, when the radiator was at distances of 40, 25 and 15 cm., were 23, 33 and 42 divisions respectively. Owing to the comparative steadiness of the last two deflections there is no uncertainty about them; but on account of the fluctuation in the deflection when the radiator is at a distance of 40 cm., it is difficult to find the exact value of the deflection; the mean of the various deflections gives twenty-three divisions. The absolute values of resistances corresponding to these deflections are 180, 380, and 1020 ohms. The original resistance being 20 ohms, the variations due to the different radiation intensities are 160, 360, and 1000 ohms.

The intensities of radiation at the above distances may approximately be taken as proportional to $1⁄40^{2}$, $1⁄25^{2}$, $1⁄15^{2}$, or as 14 : 36 : 100. The corresponding molecular effects as measured by the increase of resistance are found to be as 16 : 36 : 100.