Bumstead — Heating Effects produced by Hontgen Rays. 5 



side the case ; it thus serves as a sort of torsion head for con- 

 trolling the zero of the radiometer. This very elegant device is 

 borrowed from the work of Nichols and Hull on the pressure 

 of radiation. 



The metals whose heating effects were to be compared were 

 mounted upon an ebonite disc (W, figs. 1 and 2) 6 mm thick 

 and 7'8 cm in diameter. Three rectangular holes, 3*6 Xl'4: cm , 

 were cut in this disc, at an angular distance from each other of 

 120°. Across these holes strips of the metals under investiga- 

 tion were fastened with a little soft wax ; the strips were 2 cm 

 long and l cm wide and their inner edges were 4 mm apart. Up 

 to the present, I have had time to compare only two metals, 

 lead and zinc. The lead strips were O30 mm thick and the zinc 

 strips 0'82 mm , these thicknesses being chosen because they 

 gave nearly the same absorption of the Rontgen rays used, as 

 will appear later. The thinner lead strips were blocked up 

 from the wheel on small bits of card, so that the surface of 

 lead and zinc toward the radiometer vanes were in the same 

 plane. The arrangement of the metal strips on the ebonite 

 disc was as follows : across one of the rectangular openings 

 two lead strips were placed ; this was for testing the balance 

 of the radiometer when both vanes were influenced by the same 

 metal ; across the second opening a lead and a zinc strip were 

 placed side by side, while the third opening also contained a 

 lead and a zinc strip but in reversed order. The shaft which 

 carried the ebonite disc was provided with a ratchet wheel 

 with sixty teeth ; and the disc could be rotated from one posi- 

 tion to another by an electromagnet inside the radiometer case. 

 The current for this purpose was led in by means of wires 

 passing through small glass tubes sealed with sealing-wax, into 

 holes in the base-plate of the instrument. Certain marks on 

 the edge of the ebonite disc which could be seen through the 

 observing window enabled one to be sure that the disc was in 

 proper position in any given case. All the metal strips, before 

 being mounted on the disc, were covered on both sides with 

 leaf aluminium, which was held by the thinnest possible layer 

 of soft wax, put on when the metal was hot. This was to give 

 both metals, the same surface, so that the loss of heat from the 

 surface for a given rise in temperature might be the same in 

 both cases. The ebonite disc was also covered with aluminium 

 leaf to avoid electrostatic effects ; to the same end a small 

 quantity of an impure radium salt was put inside the case in a 

 small open dish. All the metal strips were connected to the 

 shaft of the wheel by thin copper wires ; the shaft in turn was 

 connected to the case and to earth. 



The portions of the apparatus hitherto described stood upon 

 a brass base-plate turned flat and smooth, which rested on 



