at Low Pressures. 251 



Two small electrodes E and E' were fixed in the tube K by 

 means of sealing-wax at its upper end. These electrodes were 

 made of platinum wire about O'l mm. in diameter, enclosed in 

 fine glass tubes 0*5 mm. in external diameter. At EE' the ends 

 of the wires were left bare for two millimetres. The small glass 

 tubes were fixed to the inside of the tube K near its lower end 

 with sealing-wax at opposite ends of a diameter of its bore. The 

 bare wires at E and E' were fixed in this way 6"5 mms. apart and 

 extended to the centre of the discharge tube AB. By turning K 

 the angle between the line joining E to E' and the axis of the 

 tube AB could be made to have any desired value. 



The upper part of the tube K was fixed with wax into a brass 

 tube which carried a graduated circle WW 15 cms. in diameter. 

 A vernier reading to minutes of arc on this circle was held in a 

 fixed position relatively to the tube AB so that the angular 

 position of K and consequently of the line EE' could be read off 

 on it. 



The small glass tubes enclosing the wires E and E' were 

 embedded at their upper ends in sealing-wax, and the wires were 

 brought out through holes in the brass tube at TT. These wires 

 were connected through a reversing commutator C to an insulated 

 quadrant electrometer V. The electrometer and connecting wires 

 were enclosed in metallic screens S S S S all well soldered together 

 and connected to a ' good earth.' In this way all variations in 

 the electrometer readings due to outside influences were com- 

 pletely stopped. 



The electrodes E and E' were found to be very well insulated 

 from each other and from the screens. Either would hold a 

 charge for several minutes without appreciable loss as indicated 

 by the electrometer. The electrometer was extremely well in- 

 sulated in all parts. One pair of the quadrants was connected 

 to the case, which latter was insulated by paraffin blocks. The 

 quadrants were supported on ebonite legs coated with sulphur, 

 and the electrometer 'jar' was made of a brass plate coated with 

 a thin layer of sulphur on which another brass plate was placed. 

 All four quadrants were provided with adjustable supports so 

 that each could be separately levelled and raised or lowered. 

 The quadrants were all very carefully levelled and arranged 

 symmetrically about the needle, and when the needle was charged 

 no appreciable deflection of the needle occurred. The sensibility 

 of the instrument usually only fell off 2 or 3 per cent, in 48 

 hours. If the needle was charged up to 2000 volts a sensibility 

 of about 250 millimetre divisions (scale at one metre distance) 

 for one volt was obtained. When measuring the Hall effect a 

 sensibility of about 70 mms. per volt was generally used. 



The tube AB was connected to a Toepler pump and M c Leod 



19—2 



