434 Mr. J. Henry on the Deflexion hy Maynetic 



deflexion was so small that irregularities in it could not be 

 detected very well. 



Experiments. 



Experiments were next made to find how the deflexion of 

 the discharge by magnetic force changed with — 



(1) The pressure of the gas in the discharge- tube; 



(2) The mean difference of potential between the electrodes 



of the discharge; 



(3) The mean current in the discharge. 



The method adopted in these experiments was to find the 

 deflexions produced in the same gas by a constant magnetic 

 force, first for different pressures of the gas while the mean 

 current in the discharge was kept constant ; and, second, for 

 different strengths of current in the discharge, as measured 

 by the galvanometer, while the pressure of the gas was kept 

 constant. The mean current carried by the discharge through 

 the gas was controlled by a variable water-resistance, con- 

 nected in series with the discharge-tube. Thus by increasing 

 the water- resistance in circuit when the pressure of the gas in 

 the discharge-tube fell, the mean current through it was kept 

 constant. 



As the difference of potential between the electrodes de- 

 pended for any gas on its pressure in the discharge- tube, so long 

 as the distance between the electrodes remained unchanged, 

 it could not be varied independently of the pressure ; but, as 

 we shall see, this did not matter. 



Details of Experiments. 



The length of the discharge in these experiments was 

 8*8 cms., and the gases most used were air and carbonic acid 

 gas, especially the latter, owing to the large deflexions pro- 

 duced in it. In these gases when the pressure was less than 

 2 cms. of mercury, the discharge had a " fuzzy " appearance; 

 and for pressures less than 1 cm. the photographs obtained 

 were too indefinite to be of any use. The highest pressure 

 for which the discharge would pass properly between the 

 electrodes was about 5*5 cms. 



Results. 



The following figures are taken from photographs of the 

 deflected discharge obtained during these experiments. 



Figs. 8 and 9 show the effect of change of pressure in air 

 and carbonic acid gas respectively, while the mean current in 

 the discharge, as measured by the galvanometer, was kept 



