THE MECHANICAL ACTION OF LIGHT. 



77 



must put on the fibre to lift up the T l~$ of a grain weight, I can tell 

 how many degrees of torsion are required to lift up any other weight ; 

 and conversely, putting an unknown weight or pressure on the pith, 

 I can find its equivalent in grains by seeing how much torsion it is 

 equal to. Tims, if T ^-g- of a grain requires 10,000 of torsion, -fa of 

 a grain would require 20,000 ; and conversely, a weight which re- 

 quired 5,000 torsion would weigh -g-Jg of a grain. Once knowing 

 the torsion equivalent of y^j- of a grain, the ratio of the known to 

 the unknown weights is given by the degrees of torsion. 



Having thus explained the working of the torsion balance I will 

 proceed to the actual experiment. On the central mirror I throw a 

 ray from the electric light, and the beam reflected on a particular 

 spot of the ceiling will represent zero. The graduated circle <Iof the 

 instrument also stands at zero, and the counter which I fasten on at 

 the end L stands at O. The position of the spot of light reflected 

 from the little concave mirror being noted, the torsion balance enables 

 me to estimate the pressure or weight of a beam of light to a sur- 

 prising degree of exactness. I lift up my little iron weight by means 

 of a magnet (for working in a vacuum I am restricted in the means 

 of manipulating), and drop it in the centre of the pith : it knocks the 

 scale-pan down, as if I had placed a pound weight upon an ordinary 

 balance, and the index-ray of light has flown far from the zero-point 

 on the ceiling. I now put torsion on the fibre to bring the beam again 

 into equilibrium. The index-ray is moving slowly back again. At 

 last it is at zero, and on looking at the circle and counter I see that I 

 have had to make 27 complete revolutions and 301, or 27x360 + 

 301 = 1 0,021, before the force of torsion would balance the -^ 

 of a grain. 



I now remove the weight from the pith-pan of my balance, and 

 liberate the glass thread from torsion by twisting it back again. Now 

 the spot of light on the ceiling is at zero, and the counter and index 

 are again at O. 



Having thus obtained the value of the jfo of a grain in torsion 

 degrees, I will get the same for the radiation from a candle. I place 

 a lighted candle exactly 6 inches from the blackened surface, and on 

 removing the screen the pith scale-pan falls down, and the index-ray 

 again flies across the ceiling. I now turn the torsion handle, and in 

 much less time than in the former case the ray is brought back to 

 zero. On looking at the counter I find it registers four revolutions, 

 and the index points to 188, making altogether 360 x 4 + 188 = 1628, 

 through which the torsion fibre has to be twisted to balance the light 

 of the candle. 



It is an easy calculation to convert this into parts of a grain weight ; 

 10,021 torsion degrees representing 0.01 grain, 1628 torsion degrees 

 represent 0.001624 grain. 



10,021 : 0.01 grain:: 1628 : 0.001624 grain. 



