36 PRESSURE OF LIGHT 



contact with the disc. But if any convective action 

 still remained it was sought to eliminate it by 

 directing the beam first on to the front face and 

 then on to the back face of the disc, and taking the 

 difference of the two displacements. For the con- 

 vection currents produced would depend on the 

 rise in temperature of the disc, and also on the 

 difference of temperature of the two sides. The 

 rise in temperature would be very considerable, 

 but with the very thin discs used the difference 

 of temperature of the two sides would be negligibly 

 small. The convective action would, therefore, be 

 due practically to the rise in temperature of the 

 disc, and would be the same in magnitude and 

 direction on whichever side the beam fell. Its 

 effect would depend on the " lie " of the disc ; and 

 if the disc could be set absolutely vertical, it 

 would probably vanish, for the air would simply 

 stream up equally against two vertical sides. But 

 this absolutely vertical arrangement is practically 

 unattainable. If the light presses the disc back 

 a distance P, and if convection moves it C, when 

 the light falls on the front face we observe P + C, 

 and when the light falls on the back face we 

 observe P + C, so that the difference between the 

 two positions of the disc is 2P, and C is eliminated. 

 The radiometer action depends on the small 

 difference of temperature of the two sides of the 

 disc. This difference is greater with the thicker 



