MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
117 
ROTATION OP THE CASE OE THE RADIOMETER. 
442. Granting the existence of pressure acting between the fly and the case of the 
radiometer, it follows that one being held fast the other must move. Usually, the 
bulb is fixed and the fly rotates, but if the conditions are changed and the fly 
fixed while the globe was free to move, the latter must rotate. On March 30, 1870, 
I brought before the Royal Society an experiment in which this was shown to 
be the case. I quote the following opening sentence from the short note of this 
experiment which was published at the time*:—“ During the discussion which followed 
the reading of Professor Reynolds’s and Dr. Schuster’s papers at the last meeting of 
the Royal Society, I mentioned an experiment bearing on the observations of Dr. 
Schuster. I have since tried this in a modified form, and as the results are very 
decided, and appear calculated to throw light on many disputed points in the theory 
of these obscure actions, I venture to bring a description of the experiment, and to 
show the apparatus at work before the Society.” 
The experiment was as follows : A radiometer was employed, the fly of which 
carried a magnetised needle. The radiometer was floated in water, and four candles were 
brought near. The fly immediately rotated, carrying round the magnetic needle. A 
powerful magnet was held over it, when the rotation of the fly was arrested, and the 
glass envelope rotated in the opposite direction to that in which the fly had been 
moving, the rotation keeping up as long as the candles were burning. 
443. I have tried numerous forms of radiometer in the endeavour to ascertain the 
best kind for showing this rotation of the envelope. Turbines with pith or with metal 
flies, and arranged to rotate when floated on hot or cold water, did not give good 
results; neither did any variety of radiometers with metallic cup flies. By far the 
most sensitive form is one in which the fly carries four silver-flake mica disks, lamp- 
blacked on both sides. Over the glass cap carrying the fly, another fly stilly works, 
carrying four other disks made of clear mica. A reference to fig. 4, par. 392, will 
explain the arrangement, it being understood that the instrument now described is 
four-vaned instead of two-vaned, and the opaque mica disks are blacked on both sides 
instead of on one side. The arms carrying the clear mica screens also carry a magnetic 
needle. By tapping the instrument the screens can be brought close to the black 
vanes on either side. When exposed to light, rapid rotation is produced, the exposed 
side of the lampblacked vanes being repelled. By altering the position of the screens, 
so as to obscure either one or the other black surface, rotation can be produced in 
either direction. By increasing the distance between the screens and the blacked mica, 
the rotation is made slower; and by putting the screens midway between the vanes 
rotation ceases. Reference to par. 392 shows that the present results entirely confirm 
those there given. Shots are sealed in the lower part of the case to act as ballast and 
keep it upright when in water. 
* Proc. Roy. Soc., No. 168, March 30, 1876. 
