Discharge in a Magnetic Field. 253 



If both poles are made N., or S., and the tube placed 

 between them, the bands in the tube will cross at the centre 

 and appear as in fig. 11, the dotted lines representing bands 

 to avoid confusion. 



Fig. 11. 



Let us now consider what takes place when the same tube 

 is placed between the poles N. and S. with an increased 

 current. The bands now stretch nearly across the tube. In 

 the strongest portion of the field there stand out very bright 

 and well defined the magnetic lines of force filling the tube 

 in a three-dimensional form. The effect is very striking and 

 unmistakable. Besides, the whole tube is filled with a faint 



light. 



Fig. 12 is a general diagram showing what takes place ; 

 and fig. 13 illustrates the tube in greater detail. The pheno- 

 menon is beautifully shown in the globular tube, fig. 14. 



Fur. 12. 



j^\ 



Fiar. 13, 



C^) )))))))))) ) ([Qj 



)))))))))))) 



Of course, what appear to be the lines of force are not really 

 the lines made visible but the effects due to the form of the 

 field ; since if an effect is due to the field at any given point and 

 varies with the strength of the field, the result will take upon 

 itself the form of the lines of force. It is known that the re- 

 sistance to the passage of the current is less with the lines of 

 force than across them, which effect may have some influence 

 in producing the phenomenon. The bright lines are nothing 

 more than bright bands very closely packed ; and the form is 

 due to self-induction. 



