255 



vibrated, rising almost to the critical sparlv potential, and falling almost to 

 zero. In this case the mean position was recorded. 



It may be observed from the above data that when the form of dis- 

 charge was not changed by the magnetic field, there was no change in 

 the current or the potential, and that when the form of discharge was 

 changed there was an increase in the potential, and often a decrease in the 

 current. The photographs of series A correspond to speed 1, B to speed 4, 

 C to speed 6, and D to speed IG. 



LONGITUDINAL ilAGNETIC FIELD. 



After taking the above data the magnet v.'as turned through an angle 

 of 90°, and four series of photographs taken of the discharge parallel with 

 the lines of force. These are as follows : 



E — silent glow discharge same as A. 



F — brush discharge same as B. 



G — spark discharge same as C. 



H — spark discharge same as I). 



Distance between points, 17. SS mm. 



Of these photographs, none show a change of form except those of 

 series H. In this case the rich spark was sometimes scattered, and some- 

 times transformed to a wide violet brush at the positive point when the 

 magnet Avas excited. In the first case it generally consisted of a visible 

 undeflected central thread, with spiral thread encircling it like the threads 

 of a tapering screw, the larger diameter of the spiral being at the positive 

 I)oint, and all merging together at the negative terminal. Sometimes, how- 

 ever, the central thread was al)sent and only the spiral showed. The sense 

 of the rotation of the spiral was the same as that of the lialo of luminous 

 gases about the .spark of an induction coil in a longitudinal magnetic field. 

 In degree of deflection it was mucli less. In the case of the discharge 

 Ktudied here, the spiral was only a few millimeters in diameter in a 

 magnetic field of G,400 gausses, while the halo about the spark of an iudu^-- 

 tion coil showed a spiral of four or five centimeters in diameter in a field 

 of about 1.000 lines per square centimeter. Photographs ?, and 5 show 

 the point discharge when the ])ositive ions move in the same direction as 

 the lines of force, while in Nos. 2 and (> tlie magnetic field is in tin; opiiosite 

 direction to that of the discharge. Unless there was a change of form of 

 discharge, no change of potential nor of current occurred when the mag- 

 nets were excited. Some changes (if iKilcnlial with transforination of form 

 of discharge are ap follow^; 



