The number of experiments that have been made on the Röntgen rays is already very large, and much has been learned from them regarding the properties of these rays. There has been, however, as yet, but little done to determine their energy. Indeed, the only accurate work which has been carried out in this direction is that of E. Dorn (Wied. Ann., Bd. 63, p. 160), who measured the heat produced by Röntgen rays falling on a metal plate. This investigation is, however, not complete, as Dorn did not determine the number per second or the duration of the discharges of the Röntgen rays.
On the suggestion of Professor E. Wiedemann (Erlangen), in whose laboratory I have been working, I have tried to investigate this subject; and, although I have not had time to complete my experiments, as I now find I have to return to India much earlier than I expected, I shall describe the methods adopted and the results obtained, in the hope that they may not be without interest.
I first sought to determine in absolute measure the quantity of light produced on a screen of barium platino-cyanide by Röntgen rays. For this purpose I used an optical bank with the barium platino-cyanide screen and the Röntgen lamp at one end, the source of light which I used as a standard of comparison at the other end, and between them a movable photometer. The arrangement of the apparatus is shown in the following diagram.