1918.] On the Pressure of Light. 429 
18 e.m. 
‘083 mm. } 
Weight of the silvered va = ‘105 gm. 
Weight of the lampblacked-vane = -128 gm. / 
Length of the arm = 2cm. j 
Weight of B =/ Spm. 
The pressure within the bell-jar is reduced to about 1 to 2 
ems. of mercury. It is extremely important that the joints 
should be all air-tight, for the slightest leakage of air may 
produce disastrous effects. After pumping out we allowed the 
apparatus to stand for 3 days in order to be sure that it was 
quite air-tight. The vanes should be placed symmetrically 
Theory of the Apparatus :— 
The equation of motion of the vanes is given by— 
PO | do 
p peel Sd PS (i) 
ae ae tte 
where (1) moment of inertia of the system about the fibre, & 
"Viscosity factor, » is the torsional coefficient, = angle of rota- 
tion, L moment of the force of pressure about the axis of 
rotation (i.e. the fibre). 
k 
The solution is (0-4 Be ae oe (nt + a) (ii) 
B 
where PA all (iii). 
i ae, 
ste After a sufficiently large time the deflection should become 
d ady if the disturbing causes are entirely absent. Le 
€note this steady deflection. 
L=pl, where p=total pressure (or thrust) on the 
di m the 
te of rotation. The light should be concentrated on the 
“nite of the disc. Leta be the steady deflection. Then 
po 
gre Ee 
pe . The constant » is obtained from observations of the free 
Nod of oscillation of the system. 
From (ii) w. Poe, F 
(ii) we see that Fear +o 
