54 



PRESSURE OF LIGHT 



the discs being perpendicular to the rod. They 

 were hung by a quartz fibre in a glazed case as 

 shown in fig. 22. Attached to the rod was a 

 mirror by which the reflexion of a scale could be 

 viewed in a telescope and so the position of the 

 rod could be determined. 



The case was then exhausted till the pressure 

 was I or 2 cm. of mercury, and a horizontal beam 

 of light from a Nernst lamp was directed on to 



C 



FIG. 23. 



the black disc at 45 to its normal as shown in 

 plan in fig. 23. 



The disc was pushed and the rod was turned 

 round in the direction of the arrow. The energy 

 in the beam was measured by directing it on to a 

 blackened silver plate of known weight and by 

 observing its rate of rise of temperature. The 

 torque corresponding to a given deflection of the 

 rod was found in the usual way and so the actual 

 torque exerted was measured and could be com- 

 pared with that which the momentum in the beam 

 should exert as calculated from its energy. 



