Field on the Discharge through a Gas. 375 



If n curve be plotted showing the relation between terminal 

 voltage and pressure of the gas in the discharge-tube, then, 

 as the latter i- diminished, starting from a few mms., the 

 relation between the two is a linear one. As soon, however, 

 as striae appear at the cathode end of the positive column the 

 voltage decreases less quickly than before ; when the whole 

 length of the positive column is striated, the voltage reaches its 

 minimum value. If the pressure is further decreased, the 

 voltage required to maintain the discharge rapidly increases. 

 This observation was confirmed on tubes up to 50 cms. in 

 length, of diameters ranging from 3-25 mms., and on air, 

 carbon dioxide, and hydrogen. The last gas was irregular. 



It appears, therefore, that the positive column striates 

 because, by that means, the discharge is enabled to pass most 

 readily. After it is completely striated, the magnetic field 

 causes the current to pass more easily. 



The field always causes more striae to appear. Measure- 

 ments were made on the longer tubes to see if it caused an 

 alteration in the distance between the striaa when they were 

 so far away from the cathode as to be unaffected by the 

 field directly. The results obtained with a coil as source 

 of current were too irregular to admit of conclusions being 

 drawn. 



That the positive column is differently affected in different 

 gases may be inferred from the following experiment. A 

 long tube filled with air giving striae at a suitable pressure 

 was used. If a current was then sent through the electro- 

 magnet and gradually increased, from each of the original 

 striations a smaller one emerged on the side remote from the 

 anode : with a further increase of field, this smaller one 

 eventually coalesced with the original stria next nearest the 

 cathode, and this could be repeated several times by con- 

 tinuouslv increasino- the field. Below is a set of observations. 



Current in magnet in 

 amperes. 



Appearance of stria-. 





 1-4 



l •(•,:, 



24 



•JV 



5-8 



6-6 



7-3 



Fairly clear. 



Very distinct double, one small one appearing 



exactly midway between two large ones. 

 No doubling. 

 Clear. 

 Doubb. 

 Very clear. 

 Very double. 

 Nbl so double. 



