But when the radiator was placed at a distance of 15 cm. (see (c) ), the fluctuations almost disappeared, and the galvanometer deflection was held rigid as long as the radiation was kept on; in fact, we have here an effect which physiologists describe as "tetanic." On the cessation of radiation there was immediate recovery. It will be noticed how extraordinarily consistent are the succeeding values of response. The resultant effect being due to the additive effects of numerous flashes, an occasional failure of an individual flash has little or no importance. The series of responses in (d) was taken after half an hour, and it will be noticed how very consistent they are among themselves, and how similar to those in (c), showing that even after half an hour's continuous work there had been no 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, and the variation 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. 5.
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 uncertainity 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 at least be taken as proportional to 1402, 1252, 1152, or as 14 : 36 : 100. The corresponding molecular effects as measured by the increase of resistance are found to be as 16 : 36 : 100.
It will thus be seen how accurately the indications of the Ag′ receiver measure the intensity of radiation. Further progress in the study of different phenomena connected with electric radiation has been seriously hampered owing to the want of means for measurement of intensity of electric radiation. But this difficulty, as will be seen from the above, is not insuperable.
The strict proportionality of molecular effect can only be taken as
