Bumstead — Heating Effects produced by Rontgen Bays. 15 



that is until the window has reached its stationary tempera- 

 ture. When this takes place the zinc has time to overtake the 

 lead, hoth temperatures become the same, and the radiometer 

 returns to zero. When the lamp is turned off and the window 

 begins to cool, the lead cools more rapidly than the zinc and 

 hence there is a similar deflection in the opposite direction 

 which again lasts only so long as the temperature of the win- 

 dow is. variable. 



Although this explanation is reasonable and, in fact, inevi- 

 table, the matter here discussed touches so vitally the main con- 

 clusion of this investigation that it was desirable to test in a 

 decisive manner the behavior of the instrument when the 

 strips were heated by ordinary radiation. For this purpose, a 

 glass window was substituted for the one of aluminium and 

 '(everything else being left exactly as before) a beam of light 

 was admitted from an eight candle-power incandescent lamp, 

 125 cm from the window. The result of such an experiment is 

 shown graphically in tig. 5, where, as before, the abscissae 

 represent time in minutes; the ordinates, deflections in centime- 

 ters ; a positive deflection means repulsion by the zinc, a nega- 

 tive deflection, repulsion by lead. From m to 7 m both strips 

 were exposed to the light and the curve clearly shows the tem- 

 porary deflection in favor of the lead, returning to zero as the 

 zinc overtakes the lead ; from 7 m to 14: m the window was cov- 

 ered by the lead screen so that both strips cooled, the zinc lag- 

 ging behind as before and so giving a similar temporary deflec- 

 tion in the opposite direction. From 14 m to 22 m , the zinc strip 

 was exposed while the lead was shielded, and from 29 m to 36 m 

 the lead alone was exposed. It will be observed that the 

 behavior is perfectly in accord with the above explanation of the 

 heating effect, and that the deflections, when the lead and zinc 

 are separately illuminated, are equal. The contrast with the 

 effects of the Rontgen rays is sufficiently marked. I have 

 exposed both strips at once to weak Rontgen rays (giving a 

 deflection of about 2 cm ) for 30 minutes without observing any 

 tendency of the radiometer to return to zero. 



2. At first sight, there appears to be a possibility that (on 

 account of different heat-conductivities, etc.) one of the metals 

 might lose more heat through the front surface and the other 

 through the rear surface, thus giving rise to different surface 

 temperatures, even though the quantities of heat generated 

 were the same in both cases. A number of considerations 

 show that this cannot account for the effects observed. It is 

 easy to show that, when all the heat enters through the front 

 surface, the ratio of the temperature of the front face to that 

 of the rear face will be (when the steady state is attained) 



I. , , M 



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