Mr. J. J. E. Durack on Lenard Bays. 33 



diameter bored in its centre to face the window W. A plate 

 of aluminium, p />. was placed inside parallel to the end and 

 4 mm. from it ; this plate was connected to an electrometer 

 and the connecting-wire screened from electrostatic effects 

 with a metal tube : this tube was kept at a constant potential 

 equal to the initial potential of the plate from which the 

 leak was measured. The leaking system was supported by 

 this tube with rods of sealing-wax, rr, and the outer earthed 

 cylinder supported the shield also with sealing-wax rods r' r', 

 so that the insulation of the leaking plate was good. An 

 initial potential-difference of 440 volts was put on between the 

 plate and the cylinder so that a field of 1100 volts per cm. was 

 applied to the gas; this was considered sufficient to saturate it*. 



When necessary an auxiliary capacity was placed in 

 parallel with the quadrants of the electrometer^to decrease 

 the fall of potential while the rays were on. 



Some difficulty was experienced in getting constant read- 

 ings, even when the pressure of the discharge-tube was kept 

 the same : this was thought to be due to the interrupter, but 

 different forms of mercury-break made practically no dif- 

 ference. Finally, consistent readings were obtained by cutting- 

 down the current in the primary circuit and allowing the 

 interrupter to work easily. The E.M.F. of the primary circuit 

 was 8 volts ; an additional ohm resistance was added to the 

 \ ohm of the primary, making the total resistance of the 

 circuit 1*5 ohms. 



Some of the Lenard rays proceeding from the window 

 enter the aperture in the end of the cylinder, and make 

 the gas a conductor. By this means only the ions produced 

 between the plate and the end of the cylinder are collected, 

 and we are able to find the ionization at different distances 

 from the window. 



With this apparatus it was found that the conductivity 

 increased greatly with decreasing discharge-tube pressure, 

 being a minimum for the low pressures (corresponding to high 

 Lenard-ray velocities), and increasing steadily to a maximum 

 at the point where the Lenard rays begin to disappear : here 

 the conductivity began to decrease and quickly fell to zero. 



This shows that the increasing energy, as shown by the 

 photographs, was due to increasing Lenard-ray current, or, 

 perhaps, to increasing ionizing power. More experiments will 

 be described to settle this point later. 



* The E.M.F. required to saturate the gas of course depends on the 

 number of ions formed in 1 c.c. of the gas, i. e. on the intensity of the 

 Lenard ravs. 



Phil. 'Mag- S. 6. Vol. 4. No. 19. July. 1902. D 



