TROWBRIDGE. — ELECTRIC DISCHARGES. 635 



travelled slowly toward the cathode. These rings seemed to be formed 

 primarily from the gases which came out of the electrode. In all cases 

 the bulge at the centre of the striae disappeared when the water resist- 

 ance was greatfy increased or a spark gap was inserted in the exterior 

 circuit. 'The diminution in the density of the positive carriers in the 

 central line of discharge seemed to bring out the diverging effect of the 

 magnetic field on the cathode ions shown by the luminous effect of 

 the collisions in a ring form. This ring form is more strongly developed 

 near the cathode. 



Figure 3 is a photograph of two of these rings taken in perspective. 

 The walls of the tube distort the appearance, but the ring character of 

 the striae, I believe, is shown. 



The table on page 636 shows the effect of the magnetic field. with 

 varying pressure and varying currents- The lines of magnetic force are, 

 in every case, directed along the line of discharge. 



In all cases there was an increase in the cathode light when the mag- 

 netic field was excited. If the increase or diminution of the light in the 

 tube due to the magnetic field was plotted as an ordinate and the pres- 

 sure as the abscissa of a curve, it was seen that at 1 cm. pressure the 

 light rose to a maximum with a current of 6 milliamperes, then fell very 

 slowly with increasing current. At 3 mm. pressure light increased with 

 comparatively weak currents. At 1 mm. pressure light decreased, the or- 

 dinate representing the strength of light becoming negative, the curve 

 crossing the axis of X. There seemed to be several critical points. 



4. The tube in this experiment was 36 cm. long, 4 cm. wide. The 

 rod of iron, forming one terminal, was provided at its end with an alu- 

 minum disc diameter. The coil described in Experiment 1 formed with 

 the rod of iron the electro-magnet. When the aluminum disc formed 

 the cathode, at pressures below the stratification point, between 2 cm. 

 and .5 mm., the violet light on the cathode enlarged from a spot, so 

 as to cover apparently uniformly the entire rim of the disc, while 

 luminous streams emerged from the edges of the disc. The effect of 

 the magnetic field was to drive the discharge away from the centre 

 of the magnetic field. 



5. When the disc was made the anode, the effect of the field was just 

 the opposite. The discharge was drawn to the centre of the field. 



6. A revolving mirror was so arranged that an image of the aluminum 

 disc was thrown into a camera, the tube was covered with black paper so 

 that the light only of the cathode was reflected into the camera. The 

 photograph (Figure 4) shows a unipolar rotation. This increased in 



