16 JSumstead — Heating Effects 'produced ~by Rontgen Rays. 



and the difference will be less than this when the heat is gene- 

 rated throughout the interior of the metals. Now a superior 

 limit to the value of h may be readily obtained, from the rates 

 of cooling of the strips, by assuming infinite conductivity in the 

 metals so that the whole strip has at any instant the same tem- 

 perature as the surface. Under these circumstances, we have 



2 H dt 



"Where G is the specific heat, p the density and I the thickness 

 of the metal. Also 



so that 



dV 

 -dt= yY 



h=lyCpl 



With a given curve of cooling of the surface and finite conduc- 

 tivity, the value of h will be less than this ; because when the 

 surface has cooled to any temperature the interior will have a 

 higher temperature and thus not so much heat need be emit- 

 ted in a given time as when the conductivity is infinite. In 

 this way we find for the lead strip 



A< -007 



and for the zinc strip 



A< -002 



and taking h = 0*1 we find, in the worst case 



V 



-^< 1-004 



More direct experimental evidence upon this point was 

 obtained by reversing the position of the ebonite disc with 

 reference to the case and putting the vanes on the side of the 

 strips next the window instead of on the further side. The 

 effects observed were the same as in the other position. 



3. The possibility of unequal values of the emission coeffici- 

 ent for the aluminium-covered surfaces of the two metals is 

 negatived by the experiments with light in which the two 

 metals produced equal effects. 



4. If much heat were lost by conduction through the ebonite 

 support, or the copper wire by which the strips were earthed 

 (diameter, l/4 mm ), the central part of the zinc strip would be 

 cooler than the lead on account of its greater conductivity and 

 thickness. If this were the cause of the higher temperature 

 of the lead it would act also when the strips are warmed by 

 light, which is not the case. Experiments with light were 



