1876.] 



Mr. W. Crookes on the Radiometer. 



313 



have the black outside, and consequently more facing the side of the bulb, 

 greatly increases its sensitiveness. 



The above experiments show that shape has even a stronger influence 

 than colour. A convex bright surface is strongly repelled, whilst a con- 

 cave black surface is not only not repelled by radiation but is actually 

 attracted. 



I have also tried carefully shaped cups of gold, aluminium, and other 

 metals, as well as cones of the same materials. I will briefly describe the 

 behaviour of a few typical radiometers made with metal cups, which I 

 have the honour of exhibiting to the Society. 



No. 1035. A two-disk, cup-shaped radiometer, facing opposite ways ; 

 both sides bright. The disks are 14-5 millims. diameter, and their radius 

 of curvature is 14 millims. 



Exposed to a standard candle 3-5 inches off, the fly rotates continu- 

 ously at the rate of one revolution in 3*37 seconds. A screen placed in 

 front of the concave side so as to let the light shine only on the convex 

 surface, repels the latter, causing continuous rotation at the rate of one 

 revolution in 7' 5 seconds. When the convex side is screened off so as 

 to let the hght shine only on the concave side, continuous rotation is 

 produced at the rate of one revolution in 6*95 seconds, the concave side 

 being attracted. 



These experiments show that the repulsive action of radiation on the 

 convex side is about equal to the attractive action of radiation on the 

 concave side, and that the double speed with which the fly moves when 

 no screen is interposed is the sum of the attractive and repulsive actions. 



No. 1037. A two-disk, cup-shaped aluminium radiometer, as above, 

 lampblacked on the concave surfaces. 



In this instrument the action of light is reversed, rotation taking place, 

 the bright convex side being repelled, and the black concave attracted. 



That this attraction is not apparent only, is proved by shading off the 

 sides one after the other. "When the light shines only on the bright convex 

 side no movement is produced, but when it shines on the black concave 

 side, this is attracted, producing rotation. 



No. 1038. A cup-shaped radiometer similar to the above, but having 

 the convex surfaces black and the concave bright. 



Light shining on this instrument causes it to rotate rapidly, the convex 

 black being repelled. No movement is produced on letting the light shine 

 on the bright concave surface, but good rotation is produced when only 

 the black convex surface is illuminated. 



No. 1039. A cup-shaped radiometer like the above, but blacked on 

 both sides. 



With this a candle causes rapid rotation, the convex side being repelled. 

 On shading off the light from the concave side the rotation continues, but 

 much more slowly ; on shading off the convex side the concave is strongly 

 attracted, causing rotation. 



