in the Discharge at Low Pressures. 



185 



polished. By this means a cathode was obtained with no 

 projecting edges and the central portion from which the 

 discharges take place of aluminium, thus getting rid of 

 sputtering. This cathode was pushed into a glass tube, 

 which it just fitted, in which a bulb 2 or 3 cm. in diameter 

 was blown; the cathode was about 1 cm. from the bulb. The 

 anode was a brass tube closed at both ends by sheet metal ; 

 through the end nearest the cathode a hole 1 mm. in diam. 

 was bored, and in the other a fine hole pierced with a needle- 

 point. Fig. 3 shows the complete arrangement. This gave 



Fig. 3. 



Brass ball for 

 spark gap and 



Secondary discharge tube. 



a steady beam of rays up to P.D. of 30,000 to 40,000 volts*, 

 the spot of phosphorescence on the willemite screen being 

 sharp and about 0*5 mm. in diameter. 



( 'orrections. 



It is assumed in applying the cathode raj method that the 

 equipotential surfaces in the discharge are planes perpen- 

 dicular to the axis of the tube. At very low pressures this 

 is certainly not the case, the appearance of the discharge 

 varies from point to point across any section near the cathode 

 perpendicular to the axis, and at the lowest pressures used 

 here a well-defined pencil of cathode rays passed down the 

 centre of the tube. At the higher pressures when the 

 boundary of the negative glow and Crookes' dark space 

 extends evenly across the tube, the forces in the dark space 

 may be calculated, but at lower pressures the method gives 

 only a rough idea of the magnitude of the forces in the 

 discharge. For this reason it is useless to apply any cor- 

 rections to the formula already noted. 



Experimental Procedure. 



The apparatus was exhausted by a water-pump and 



mercury-pump, and finally completed with the aid of liquid 



air and charcoal. The discharges were started and allowed 



to run for a short time to reach a steady state. The pressure 



* A constriction in the tube between the cathode and the bulb 

 " hardens " the discharge considerably. 



