Action of Magnetic Field on Discharge through a Gas. 371 



field was used, and the strength of the field at different 

 distances from the line joining the poles, and for different 

 currents, was found by means of a fiat coil and ballistic 

 galvanometer. 



A long cylindrical tube of 25 mms. diameter, provided with 

 disk electrodes, was pumped down until the Crookes dark 

 space was 3 cms. long, and the transverse field applied 

 successively at the cathode and at distances of 1, 2. 3. and 

 4 cms. away from it in the direction of the anode. The 

 observations showed that when equal decreases in potential 

 were produced, the field at the cathode surface was practically 

 the same for all positions of the magnet. We may there- 

 fore conclude that it is at, or near, the surface of the cathode 

 that the action arises. The field shortened the dark space 

 from 3 cms. to about 2 mms. 



If Almy's explanation is the correct one, the critical pres- 

 sure,, above which the field causes an increase and below 

 which a decrease in the terminal voltage, might be expected 

 to be more or less irregular and ill-defined. In any case, it 

 was thought worth while to study this pressure more carefully 

 with different transverse fields, and an observation ot interest 

 resulted. Starting from a pressure of 3-4 mms., the volts at 

 the terminals were measured with the magnet off and on at 

 the cathode. As the pressure is continually lowered, the 

 magnet being off, it is, of course, known that the voltage 

 decreases, reaches a minimum, and then rapidly rises. The 

 value of the critical pressure corresponding to minimum volts 

 depend^ on the diameter of the tube, the nature of the gas, 

 and perhaps also on the current. The last point I could not 



investigate with a coil. It was found that above tliis critical 

 © 



pressure the field caused an increase in the voltage at the 

 terminals, while below it caused a decrease. At pressures much 

 above 5 mms. very little effect was noticed. This result was 

 confirmed by experiments on tubes with lengths varying from 

 50 cms. to 10 cms., diameters from 2o mms. to 3 mms., and with 

 disk or wire electrodes. By using hydrogen, carbon dioxide, 

 and air in these tubes, the critical pressure could be shifted 

 from *3 mm. to about 1 mm. and the observation still held, 

 although with hvdiwen the results were not so regular as 

 with the other gases. 



This critical pressure is much more sharply marked in 

 narrow tubes. The series of observations given below r will 

 serve as an example of the results obtained. They were 

 taken with a tube 3 mms. in diameter, 1<> cms. long, filled with 

 air. and a field of about (i0<> lines per sq. cm. A fraction 

 onlv of the voltage is given. 



2 B 2 



