Bumstead — Heating Effects produced by Rontgen Rays. 13 



It will be seen that the agreement is as good as could be 

 expected from the very rough nature of the determinations. 



Returning now to the experiment plotted in fig. 3 it is to be 

 observed that the steady deflection for lead is nearly twice as 

 great as that for zinc. The fractions of the incident rays 

 absorbed by the two strips were determined by means of the 

 electroscope with the pieces of metal from which the strips 

 had been cut ; the pieces of lead and zinc were placed behind 

 a sheet of aluminium of the same thickness as the window of 

 the radiometer. It was found that the lead absorbed 79 per 

 cent and the zinc 78 per cent of the rays which got through 

 the aluminium ; in this particular case the spark gap in the 

 automatic adjuster of the Rontgen bulb was 7*5 cm long. 



Thus (unless there has been some error in the experimental 

 method or in the interpretation of the result) it appears that 

 for practically equal absorptions of Rontgen rays in lead and 

 in zinc, about twice as much energy is generated in the lead 

 as in the zinc. It may be said at once that essentially the 

 same result was obtained in all the experiments, with rays of 

 different hardness, with both positions of the strips and with 

 the radiometer vanes sometimes between the window and the 

 strips, as well as in the position indicated in fig. 1. Before 

 giving, however, the numerical results of all the experiments, 

 it will, I think, be conducive to clearness to consider the pos- 

 sible sources of error (so far as they have occurred to me) and 

 their influence upon the results. 



Possible Sources of Error. 



1. After it had been found that the lead strips predomi- 

 nated, and before the rates of heating and cooling of the strips 

 had been worked out, an effect was observed which caused 

 considerable perplexity. The slight drift of the zero-point 

 due to the warming up of the room after the observer entered 

 was found to be always in the direction indicating a repulsion 

 from the lead ; its direction reversed with the reversal of the 

 order of the strips. As the aluminium window was the only 

 part much exposed to changes of temperature, the effect of 

 slightly warming it was investigated. When an incandescent 

 lamp, or a warm soldering iron, was held six inches from the 

 aluminium window, the radiometer was deflected away from 

 the lead strip — at first slowly, then more and more rapidly 

 until it went off the scale and usually bumped against the zinc 

 strip. As this was in the same direction as the effect when 

 both strips were exposed to the Rontgen rays, it appeared that 

 the whole effect might be spurious. By putting the lamp 

 about two feet away from the window, less violent effects were 

 produced, and it was possible to follow the progress of the 



