Magnetic Field on the Discharge through a Gas. 251 



been noticed by Birkeland *. It was found when the field at 

 the cathode reached a certain critical value that the difference 

 o£ potential between the terminals suddenly fell to less than 

 one-tenth of its previous value ; the appearance of the 

 discharge also showed important alterations. 



In mv experiments an electromagnet capable of producing 

 a strong local transverse field was used. It was then found 

 that the nature of the effects was different at low pressures 

 according as the cathode or other parts of the tube were 

 placed in the field. 



"With the cathode in the field the following results were 

 obtained :— At a pressure of 1 mm. the effect of the field was 

 to decrease the current, but as the pressure was lowered this 

 effect became less and less, until at a certain pressure no 

 effect at all was produced on the current or the potential- 

 difference between the terminals, although the discharge 

 itself was distorted. As the pressure was still further 

 lowered the current was increased by putting on the mag- 

 netic field. 



Observations were made in the following" manner : — 

 The discharge being produced by a large battery of small 

 storage-cells, the current-strength was measured by a gal- 

 vanometer, while the electrodes were connected to the 

 terminals of a Kelvin electrostatic voltmeter ; the current 

 in the magnet was measured at the same time by an ammeter. 

 The magnet being off, the current through the galvanometer 

 was arranged so as to give a convenient deflexion, which was 

 the same throughout a series of observations, the magnet 

 was put on, and the current in the galvanometer again noted. 



Fig. 1. 

 so 



40 

 30 

 20 

 10 





 10 

 20 

 30 

 40 

 50 

 60 

 70 



































































A 





















































C 















2 



4 



6 



8 



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)0 22 



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PREi 



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y 



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111 



The results of one set of readings are shown graphically 

 ■fig. 1 (B). The abscissas represent pressures, 223 being 

 * Comptes Rendus, cxxvi. p. 536 (1898). 

 S2 



