8 Bumstead — Heating Effects produced by Rontgen Bays. 



per second is equal to the heat lost per second through the two 

 surfaces by radiation and convection. (The possibility of any 

 appreciable loss by conduction through the ebonite support of 

 the strips will be considered later.) The heat lost through 

 any surface is proportional to its emissivity and, for such small 

 temperature differences, to its excess of temperature above its 

 surroundings. The lead and zinc strips had the same surface — 

 aluminium leaf — and jt is natural to assume that the emissivity 

 is the same in both cases ; this assumption, however, will be 

 justified experimentally. Moreover, it will appear from the- 

 oretical considerations that, in the steady state, the difference 

 in temperature between the front and back surfaces of either 

 metal is a small fraction of the excess of its temperature above 

 its surroundings ; and this too will be confirmed experimentally. 

 It follows therefore, that, very approximately, half the heat 

 generated in either strip is lost through each of its two sur- 

 faces, and that the total heat generated per second will be pro- 

 portional to the temperature of either surface in the steady state, 

 and hence to the repulsion of the radiometer vane exposed to 

 that surface. 



The first step in a series of experiments was to test the bal- 

 ance of the radiometer by exposing the two lead strips simul- 

 taneously to the Rontgen rays. The balance was usually 

 found to be fairly good and could be to some extent adjusted 

 by moving the bulb horizontally before the window ; the stand 

 carrying the bulb could be moved by a horizontal screw for 

 this purpose. In two of the series of experiments (quoted 

 below) it was found impossible to get a good balance by mov- 

 ing the bulb ; in these cases the lack of balance was determined 

 and applied as a correction. The wheel was then clicked 

 round to the second position, in which one vane of the radi- 

 ometer was opposite to a lead strip and the other opj)osite to a, 

 zinc strip. This was a somewhat delicate operation, as each 

 click caused a violent disturbance of the radiometer (partly 

 magnetic and partly thermal) and it was necessary to get it 

 under control by means of a subsidiary magnet before making 

 another click ; otherwise the vanes might have become entan- 

 gled in the moving wheel and the suspension broken. After 

 the shift from one position to another, several hours had to elapse 

 before the radiometer was again fit for use. Several expo- 

 sures to Rontgen rays were then made in which the zinc and 

 lead strips were exposed separately and both together. The 

 wheel was then moved to the third position, in which the zinc 

 and lead strips were in reversed order, and similar observations 

 taken. It may be said at once that the reversed position of 

 the lead and zinc did not alter the character of the results. 



The general nature of the results may be most readily set 



