32 



Mr. J. J. E. Durack on Lenard Hays. 



To obtain the velocity from these measurements we have 

 the formula given by J. J. Thomson*, 



t? = H 



d* + $ 

 28 



where 



v is the velocity of the ions ; 



H is the magnetic field producing the deflexion S; 



d is the distance travelled from the neck o£ the camera to 

 the plate; 



ejm is the ratio of the charge on an ion to its mass, which 

 has been measured by several experimenters and found 

 to be approximately 10 7 in electromagnetic units. 



The magnetic field was varied so that the deflexion was 

 nearly the same in all cases, it was measured with a small 

 coil of known dimensions and a ballistic galvanometer. 



The velocities as determined in this way varied from 5.10 9 cm. 

 per second for the high discharge potentials to 3*2 10 9 for the 

 lower ; none of the velocities measured were smaller than this, 

 but probably higher velocities could be obtained with good 

 discharge-tube insulation. 



The photographs varied very much in character, being very 

 faint for the higher velocities and most intense just before the 

 Lenard rays disappeared altogether. At this point double 

 images were obtained, the one deflected, due to the Lenard 

 rays, the other undeflected, due to Kontgen rays or to the 

 lighting up of the aluminium window. 



§ 2. To test further the variation of the energy with dis- 

 charge-tube pressure, the apparatus shown in fig. 2 was used 

 to measure the ionization produced by the rays. 



Fijr. 2. 



c 



W 



\ 



To £ltCr/?OM£T£/f 



v To Csiis 



To Earth 



An aluminium cylinder with a flat end CC, also of alu- 

 minium, fitted into the observation-tube and could be moved 

 along it ; the end was 1 mm. thick and had a hole 1 mm. in 



* J. J. Thomson, Phil. Mag. Oct. 1897. 



