Prof. J. J. Thomson on Cathode Rays. 



309 



occupied by the plates, the distance between the coils was 

 equal to the radius of either. The mean value of the magnetic 

 force over the length I w T as determined in the following way: 

 a narrow coil C whose length was /, connected with a ballistic 

 galvanometer, was placed between the coils ; the plane of the 

 windings of C was parallel to the planes of the coils; the cross 

 section of the coil was a rectangle 5 cm. by 1 cm. A given 

 current was sent through the outer coils and the kick 

 a of the galvanometer observed when this current was 

 reversed. The coil C was then placed at the centre of two 

 very large coils, so as to be in a field of uniform magnetic 

 force : the current through the large coils was reversed 

 and the kick j3 of the galvanometer again observed ; by com- 

 paring a and /3 we can get the mean value of the magnetic 

 force over a length / ; this was found to be 



60 Xi, 



where i is the current flowing through the coils. 



A series of experiments was made to see if the electrostatic 

 deflexion was proportional to the electric intensity between the 

 plates ; this was found to be the case. In the following 

 experiments the current through the coils was adjusted so that 

 the electrostatic deflexion was the same as the magnetic : — 



Gas. 



Air 



Air 



Air 



Hydrogen ... 

 Carbonic acid 



Air 



Air 



8/110 

 9-5/110 

 13/110 



9/110 

 11/110 



6/110 



7/110 



II. 



5-5 

 5-4 

 66 

 6-3 



F. 



1-5 X10 10 

 1-5 Xl0 10 

 1-5 X10 10 



1-5x10 



10 



6-9 I 1-5x10 



3-6 



1-8 xl0 l 

 1x10 



10 



rn/e. 



V. 



l-3xl0 -7 



2-8 xlO 9 



1-lXlO"-' 



2-8 X10 9 



1-2X10 -7 



2-3 XlO 9 



rsxio -7 



2-5 XlO 9 



l-5xlO~ 7 



2-2 xlO 9 



r3xio -7 



3-6 XlO 9 



1-1x10-7 



2-8 XlO 9 



The cathode in the first five experiments was aluminium, 

 in the last two experiments it was made of platinum ; in the 

 last experiment Sir William Crookes's method of getting rid 

 of the mercury vapour by inserting tubes of pounded sulphur, 

 sulphur iodide, and copper filings between the bulb and the 

 pump was adopted. In the calculation of mje and v no allow- 

 ance has been made for the magnetic force due to the (.toil in 



Phil. Mag. S. 5. Vol. U. No. 269. Oct. 1897. Z 



