296 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
blacked on the concave surfaces, as shown in fig. 23, A, B, and C. On exposure to 
the standard candle, 3 inches off, the shallowest cups. A, rotated with a speed of 16 ‘6 
revolutions a minute, the intermediate cups, B, made 54 revolutions a minute, and the 
deepest cups, C, made 60 revolutions a minute. 
These experiments corroborate those tried with the aluminium cylinders (313), and 
prove conclusively that the deeper the curvature the stronger the repulsion. 
319; It will be observed that the cups are blacked on the concave side, and 
bright On the other. Had the vanes been flat disks, the candle would have strongly 
repelled the black side. The very slight curvature of the shallowest disks, A, is, 
however, sufficient to overcome the repelling action of radiation on the black surface, 
and to leave a considerable balance in favour of the polished side. 
It now became of interest to know what would be the effect of light on cups 
blacked on different sides. The following experiments were therefore tried with the 
pair of gold cups of intermediate curvature, B, fig. 23, and also, for the sake of com- 
parison, on similar pairs of thin aluminium cups. For convenience of comparison 
I put the results in a tabular form. 
Gold Gups. 
Both sides bright 
Standard candle 3 inches off. 
. Hot shade. 
Cooling. 
Speed of rotation. 
Direction of 
rotation. 
Direction of 
rotation. 
Direction of 
rotation. 
18 revs, a minute 
54 „ 
60 „ 
67 „ 
26 „ 
54 „ H 
75 „ „ 
86 „ 
— — ( 
^ ( 
m ^ 
Concave blacked 
^ ^ 
5gg>. ■ — (f 
Convex blacked 
Both sides blacked 
Aluminium Cups. 
Both sides bright . 
Eli 
m — > J 
Concave blacked 
El 
EEl 
^ ^ ^ 
Convex blacked 
Both sides blacked 
320. These four sets of aluminium cups were permanently mounted in bulbs, and 
were exhausted together as described at par. 314. They were sealed off at the point 
of maximum sensitiveness, and tested with a standard candle 3 '5 inches off, the 
whole being enclosed in a space lined with black velvet. To avoid lengthened descrip- 
tion, I give the results in the form of diagrams. Three experiments were tried 
with each radiometer : — a, when the light shone on both cups simultaneously ; b, 
when the concave side was screened off ; and c, when the convex side was screened off. 
They were then tested with a hot ring at the top, and another at the equator ; and 
finally the direction of motion on cooling was observed. 
321. To save repetition, I will here state that in all four cases, the hot ring 
applied to the upper part of the bulb gave negative rotation, and equatorially 
applied gave 'positive rotation. The direction of movement during cooling was in 
each case positive (301). With the cups blacked on both sides, the revolutions, both 
