430 Mr. J. Henry on the Deflexion by Magnetic 



tube attached ; a pair of coils for producing a magnetic field ; 

 an induction-coil for producing the discharge, a galvanometer 

 for measuring it, and an electrostatic voltmeter for giving the 

 difference of potential between the electrodes of the discharge ; 

 and a camera for photographing the discharge. 



The vacuum-tube is shown in fig. 1. It was found that in 

 a narrow tube the direction of the discharge for high pres- 

 sures of the gas was very unsteady, owing to the walls of the 

 tube becoming electrified ; so for this reason, and also to 

 allow room for the discharge to be deflected without coming 

 too close to the glass, a wide tube had to be used. An ordi- 

 nary bell-jar, fitted as shown in fig. 1, made a very convenient 



Fig 1. 





a and b are the terminals: c,c the ebonite plate; dthe ebonite plug; e the 

 tube leading- to the air-pump; id sealing-wax. Diameter of bell-jar 

 9*4 cms. 



form. The ebonite plug, d, through which the three-way 

 glass tube passes, w r as made air-tight in the neck of the jar 

 with sealing-wax ; the glass tube contained the electrode, 6, 

 and also allowed of gas being admitted to, or pumped out of, 

 the bell-jar by the side tube, e. The electrode was fixed in 

 the tube with the sealing-wax, ic; but by softening the wax 

 it could be easily shifted in or out, so as to change the length 

 of the discharge. 



The bottom of the jar was closed by pressing a plate of 

 ebonite (e) turned plane, against the ground glass rim of the 

 jar. The joint was at first made air-tight with sealing-wax, 

 but grease was found to answer the purpose quite as well, 

 and it was much easier to work with. 



The electrode, a, passed through the centre of the ebonite 

 plate, and was secured with sealing-wax. The tube e com- 

 municated with a mercury manometer, the air-pump, and the 

 gas-supplying apparatus, through an arrangement of three- way 

 tubes with stopcock?. 



