170 Tangential Stress due to Light. 



Two circular glass disks, each 2*75 sq. cm. area, were fixed 

 at the ends of a horizontal light glass rod 5 cm. long, the 

 disks being perpendicular to the rod and fixed to it at their 

 highest points. One of the disks was lampblacked, and the 

 other silvered. The rod was placed in a light wire cradle 

 and suspended by a fine quartz fibre about 25 cm. long in a 

 brass case with glazed sides. On the cradle was a mirror by 

 which deflexions could be observed with a telescope on a 

 millimetre-scale 1/8 metres distant. The moment of inertia 

 of the system was 2'35 gm. cm. 2 and the time of vibration 

 was 146 seconds. A deflexion of 1 scale-division therefore 

 corresponded to a tangential force on a disk of about one 

 two-millionth of a dyne — more exactly *483 X 10~ 6 dyne. 



The air was pumped from the case till the pressure was 

 less than 1 cm. of mercury. At this pressure the irregularity 

 of the disturbances due to the residual gas is very greatly 

 reduced. A parallel beam of light from a Nernst lamp was 

 then directed so as to be incident obliquely on the lamp- 

 blacked disk. From the arrangement of the disks it is 

 obvious that a uniformly distributed normal force would 

 have no moment tending to twist the system, while a tangential 

 force would have a moment and would twist it. In all 

 cases the disk moved away from the source of light. The 

 deflexion was a maximum when the incidence was not very 

 far from 45°, and fell off on each side of the maximum value. 



As there are various sources of error not yet removed, we 

 have not made a complete series of measurements but have 

 only made sure that the effect is of the order to be expected 

 from the theory, by finding the deflexion for an angle of 45°. 



The beam from the Nernst lamp when incident at 45° 

 turned the rod through 16*5 scale-divisions. Assuming total 

 absorption, the tangential force should be ^E sin 20 x area of 

 disk = iEx2-75. 



Equating to the value of the force given by the deflexion, 

 viz., 0-483 x 10" 6 x 16-5, we have E = 5'8 x lO" 6 . 



The same beam was then directed on to a small lamp- 

 blacked silver disk of known heat capacity, through a glass 

 plate of thickness equal to that of the side of the case. The 

 initial rise of temperature per second was measured by 

 a thermo junction of constantan wire soldered to the disk. 

 The energy-density of the stream was thus found to be 

 E = 6'5xl0- 6 . 



The agreement of the two values is quite as close as could 

 ,be expected in so rough a determination. 



When the beam was directed on to the silver disk at the 



