i) 
1 
On 
vibrated, rising almost to the critical spark 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 16. 
LONGITUDINAL MAGNETIC FIELD. 
After taking the above data the magnet was 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 D. 
Distance bet cen points, 17.88 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 was 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 
point, and all merging together at the negative terminal. Sometimes, how- 
ever, the central thread was absent and only the spiral showed. 'The sense 
of the rotation of the spiral was the same as that of the halo of luminous 
gases about the spark of an induction coil in a longitudinal magnetic field. 
In degree of deflection it was much less. In the case of the discharge 
studied here, the spiral was only a few millimeters in diameter in a 
magnetic fie!d of 6,400 gausses, while the halo about the spark of an induc- 
tion coil showed a spiral of four or five centimeters in diameter in a field 
of about 1,000 lines per square centimeter. Photographs 8 and 5 show 
the point discharge when the positive ions move in the same direction as 
the lines of force, while in Nos. 2 and 6 the magnetic field is in the opposite 
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 of potential with transformation of forn) 
of discharge are as follows: 
