26 J. K. Moore — Electrical Discharge from the point 



of the radiometers is shown in detail in fig. 2. The movable 

 vane consists of an H-shaped piece of clear mica. The two 

 vertical members are lampblacked on opposite sides. The 

 radiometer vane is provided with a mirror for reading angular 

 deflections and a small magnet to give it directive force ; the 

 whole system is supported by a single cocoon fiber attached to 

 a hook at the upper end of the suspension tube. The tube (e) 

 leads to a mercury pump. 



The radiometers (R, R) are employed for the purpose of 

 measuring the pressure in the regions (A) and (B) respectively 

 of the tube, and in this way serve as means for indicating 

 whether or not, during electrical discharge, the gaseous pres- 

 sures in these regions become different. 



That the deflecting couple acting on a given radiometer vane 

 (when a constant light source is used) may be taken as an in- 

 dication of the pressure (or molecular density) of the gas within 

 the containing vessel, has been shown experimentally by 

 Crookes* and is a direct consequence of the beautiful theory 

 of the radiometer as given by Reynoldsf. Without considering 

 the theory of the radiometer at this point, the relation existing 

 between the gaseous pressure within the radiometer vessel and 

 the deflecting couple acting on the radiometer vane, produced 

 by a constant light source, is clearly shown by fig. 3, which is 

 taken from Crookes's paper just cited. 



* On Repulsion resulting from Radiation, Phil. Trans., Roy. *Soc. vol. cLxx, Pt. 

 I, p. 87. 



f Dimensional Properties of Matter in the Gaseous State, Phil. Trans. Roy. 

 Soc, vol. clxx, Pt. II, p. 727. 



