6 Bumstead — Heating Effects produced by Rontgen Rays. 



three levelling screws. Through this plate passed the tube 

 which led to the P„0 5 bulb, pump, and McLeod gauge. The 

 whole was covered with a heavy, cylindrical, brass casting, 

 12'5 cm in internal diameter, 29 cm high, and with walls l--i cm 

 thick. This heavy metal case was found to he necessary owing 

 to the sensitiveness of the radiometer to thermal disturbances ; 

 with a blackened glass bell-jar, which was first tried, the zero 

 was so unsteady that nothing could be done with the instru- 

 ment. As a further protection against thermal disturbance, 

 the cover and base were surrounded with cotton-wool and a felt 

 jacket drawn over the whole. The case was provided with 

 two windows, 2*9 cm in diameter ; one of these (A, fig. 1) was 

 covered with sheet aluminium l-2 ram thick and served to admit 

 the beam of Rontgen rays ; the other was of glass for observing 

 the deflections. Both were put on with sealing wax and the 

 joints covered with soft wax. The inside of the cover was 

 painted with lampblack in alcohol, with a little shellac to make 

 it stick. The bottom of the cover was turned flat, and the 

 joint between it and the brass plate was surrounded by a mix- 

 ture of equal parts by weight of vaseline, paraffin and rubber, 

 which, after being put on, was glazed over with a small gas 

 flame. When all the joints were carefully made, little diffi- 

 culty was experienced in maintaining the vacuum for consider- 

 able periods ; the rise in pressure due to leakage was usually 

 less than 0-002 mm per day. 



In the preliminary experiments for testing the working con- 

 ditions of the radiometer, the ebonite wheel and its metal 

 strips were replaced by a light ebonite frame which carried 

 two strips of platinum foil (one opposite each vane), through 

 either of which a known current could be sent. By this means 

 one could readily find the pressure of maximum sensitiveness, 

 and compare different quartz fibers and different forms of the 

 suspended system. The best pressure appeared to be between 

 0*03 and 0*08 mm , and within this region the variation of sen- 

 sitiveness with pressure was slow ; in the subsequent work a 

 pressure between these two limits was usually employed. 

 From a knowledge of the resistance of the platinum strips and 

 of the current employed, it appeared that a deflection of one 

 millimeter (scale distance, 196 cm ) corresponded to an emission 

 of about 0*04 ergs per second from each square centimeter of 

 platinum surface. The deflections were also found to be pro- 

 portional to the energy generated in the strips. Of course the 

 radiometer action depends primarily on the temperature of the 

 surface, and with a surface of different emissivity, the deflection 

 for a given emission of energy will be different. 



The Rontgen bulb finally employed was a very large one 

 made by Muller and obtained from Isenthal & Co. The diam- 



