through Exhausted Tubes without Electrodes. 



333 



confined to the glow-discharge, but is also operative when the 

 discharge passes, entirely through the gas. A square tube 

 ABCD (fig. 6) is placed outside the primary EFGH, the 

 lower part of the discharge-tube CD being situated between 

 the poles L M of an electromagnet. By altering the length of 

 spark of the Wimshurst machine, the electromotive intensity 



Fisr. 6. 











E _ _-p 





iol 



H L \ \ C 









acting on the secondary can be so adjusted that no discharge 

 passes round the tube ABCD when the magnet is off, whilst 

 a bright discharge occurs as long as the magnet is on. The 

 two effects of the magnet on the discharge, viz. the stoppage 

 of the discharge across the lines of magnetic force, and its 

 acceleration along them, may be prettily illustrated by placing 

 in this experiment an exhausted bulb 1ST inside the primary ; 

 then the spark-length can be adjusted so that when the 

 magnet is off the discharge passes in the bulb, and not in the 

 square tube, while when the magnet is on the discharge 

 passes in the square tube, and not in the bulb. 



The experiments on the effect of the magnetic field on the 

 discharge were tried with air, carbonic acid, and oxygen, but 

 I could not detect any difference in the behaviour of the 

 gases. 



The explanation of the longitudinal effect of magnetic force 

 seems more obscure than that of the transverse effect ; it is 

 possible, however, that both may be due to the same cause, 

 for if the feeble discharge which we suppose precedes the 

 main discharge branches away at all from the line of main 

 discharge, the action of the magnetic force when it is along 

 the discharge will tend to bring these branches into the main 

 line of discharge ; and thus there will be a greater supply of 

 dissociated molecules along the main line of discharge, and 

 therefore an easier path for the subsequent discharges when 

 the magnetic force is acting than when it is absent. 



It is perhaps not necessary to assume that the mechanical 

 action of the magnetic force is on a small discharge preceding 



