302 II. A. Bumstead — Rontgen Bays in Lead and Zinc. 



strips were measured, and thus a known quantity of energy 

 could be developed in either strip and the corresponding de- 

 flection of the radiometer obtained. The loss of heat through 

 the electrodes was so rapid, however (especially in the case of 

 the zinc), that measurable deflection could not be got with the 

 Rontgen rays. I accordingly substituted for each strip a 

 five-barred grid, carefully cut from the same materials, each 

 bar being one millimeter wide ; one electrode-clamp held the 

 beginning of the first bar, the other the end of the fifth bar, so 

 that the current passed through the five bars in series. This 

 served the purpose although the deflection produced by the 

 rays falling on the zinc were still too small for very accurate 

 measurement. The resistance of the zinc grid was 6"5xl0" 6 

 ohms and of the lead 64X10 -5 ohms; currents of 10 to 12 

 milliamperes were used in the lead and from 35 to 50 milliam- 

 peres in the zinc. The deflections produced by either grid 

 were found to be proportional to the quantity of heat developed 

 in it, but the sensitiveness of the two grids was very different. 

 Thus in one experiment, which may serve as an example of 

 many which were made, the energy necessary to produce a 

 deflection of l cm was : with the zinc grid, 26 - 8 ergs per second ; 

 with the lead grid, 7"82 ergs per second. When the Rontgen 

 rays fell upon the zinc, the deflection of the radiometer 

 was 2*6 cm ; when on the lead, 10*8 cm . These measurements 

 give 70 ergs per second in the zinc and 84 ergs per second in 

 the lead ; the ratio of the two is l - 2. This is a much smaller 

 difference than was obtained in the Cambridge experiments ; 

 and, what is more significant, any errors (due, for example, to 

 a gain of heat by both strips from other portions of the appa- 

 ratus struck by the rays, or to imperfect screening of one strip) 

 would favor the lead on account of its greater sensitiveness. 

 So that the difference would be less than 20 per cent, rather 

 than more. 



A more careful consideration of the conditions of the orig- 

 inal experiments showed that it was not impossible that the 

 result obtained was due to escape of heat by conduction through 

 the ebonite disc which supported the strips, the aluminium leaf 

 which covered the disc (to prevent electrical effects), and the 

 copper wires by which the strips were earthed. The lead strips 

 had also, between them and the disc, pieces of cardboard to 

 bring their front surface into the same plane with those of the 

 thicker zinc strips. I accordingly repeated the experiments 

 with the following modifications : the cardboard was left out 

 and the zinc strips sunk into recesses cut in the disc; the 

 aluminium foil was scraped away from the vicinity of the ends 

 of the strips ; and the strips were earthed by manganin wires 

 0'05 mm in diameter and 3 em long ; also the strips were covered 



