[satterly] 



CROOKES RADIOMETER 



77 



up to 6 R.P.M. beyond that the R.P.M. seem to increase at a greater 

 rate than the number of candles. 



The Radiometer was studied at an early date by Schuster^ and 

 Osborne Reynolds^, both of Manchester. They criticised the 

 theory Crookes had put forward to explain its action and the con- 

 troversy was quite keen. They tried to elucidate the actions going 

 on within the instrument, to find out how much the motion is due of 

 to external forces, how much due to internal forces, and the exact 

 parts played by the vessel, the remaining gas and the surface of 

 the vanes. In one experiment the radiometer was floated in water. 

 A stream of radiation was allowed to act upon it and the vanes 

 prevented from rotation by holding a bar magnet above the instru- 

 ment (an iron wire had been tied to two of the arms). They found 

 that the instrument rotated in the opposite direction to that in 

 which the vanes would have gone if they had been free. They con- 

 cluded that the action and reaction are wholly internal or in other words 

 that no external force acts on the light mill. 



Reynolds took up the question as to the relative parts played 

 by the friction at the pivot and the friction by the residual gas. If 

 the pivot friction were the only friction present the speed would go 

 on increasing. But experiment shows that the speed gets steady, 

 this indicates that the remaining air exercises retardation, and Rey- 

 nolds, following Maxwell, showed that for the moderately low vacua 

 used the friction was proportional to the speed even up to the high 

 speeds obtained in the radiometer. 



2Phil. Trans. Roy. Soc, Lond., Vol. 166, 1876, p. 715. 

 sphil. Trans. Roy. Soc, Lond., Vol. 166, 1876, p. 725. 



