The Discharge of Electricity from Points. 



235 



also served as the plane to receive the discharge ; the upper 

 face of this plate was platinized. To the top of the glass 

 cylinder was cemented a brass plate D. This plate supported 

 the mechanism for holding and adjusting the plate, and was 

 insulated from it by the ebonite ring C. The plate D was 

 raised to the same potential as the point, and thus served as 

 a guard-ring ; any leakage over the glass came from the 

 plate D and did not pass through the galvanometer used for 

 detecting a current through the gas. A silver wire P served 

 as the discharging point. It was fixed in a brass rod Q ; on 

 this rod was cut a screw-thread of one millimetre pitch which 

 worked in the brass supports F and F'. This rod could be 

 turned from outside the apparatus by the part A, and the 

 distance from the plane was measured on the scale S, divided 

 into centimetres, and the wheel W whose circumference was 

 divided into ten parts. This gave a direct reading to one- 

 tenth of a millimetre, and the distance could easily be adjusted 

 to '002 cm. The reading for zero distance was obtained 

 from the point at which there was electric contact between 

 the wire and the plane. Wire gauze, connected to the plate 

 Z, was fitted round the inside of the lower part of the glass 

 to prevent distortion of the electric field in the neighbour- 

 hood of the point due to unknown charges on the surface of 

 the glass. 



The arrangement of the apparatns is shown in fig. 2. It 



is the same as that used for determining the Sparking Poten- 

 tials of concentric cylinders, except that there is an additional 

 conductor leading from the leyden-jars to the plate D. 

 M is a Wimshurst machine driven by a small motor, V is an 

 electrostatic voltmeter, Gr is a mirror galvanometer capable 

 of detecting a current of 10~ 8 ampere. 



The gas used was air. It was taken from the room and 

 dried and freed from dust in the same manner as that used 

 with cylinders. 



3. In taking the readings the pressure was adjusted and 



