PRESSURE OF LIGHT 



absorbed is aE. This absorbed momentum must 

 be neutralized by the window CD pressing against 

 the gas more than the window AB. Or the 

 pressure of the gas close to CD exceeds that close 

 to AB by aE. 



What is needed, then, 

 is the measurement of 

 E and of a on the one 

 s* hand, and the measure- 

 ment of the difference 

 of pressure / of the gas 

 at the two ends of the 

 chamber on the other 

 hand, and we should ex- 

 pect to find that/ = aE. 

 To measure E the 

 same plan was followed 

 as in the experiment 

 described on p. 38, the 

 beam falling on a small 

 calorimeter in which the 

 rate of rise of tempera- 

 ture was observed. 

 To determine a, the fraction of E absorbed by 

 the gas, a thermo-electric junction of platinum 

 and constantan was placed in front of the window 

 AB and another similar junction was placed 

 beyond the window CD. The second junction 

 was less heated than the first, and from the two 



A 



FIG. 29. 



