Mr. J. P. Gassiot on the Voltaic Discharge in Vacuo. 77 



discharge became visible, and the leaves of both electroscopes par- 

 tially collapsed ; the vacuum-tube was then placed as before, between 

 the armatures of the electro-magnet, and immediately the magnet 

 was excited, the luminous discharge disappeared, and the leaves of 

 the electroscopes diverged to their original maximum extent, thus 

 proving the disruption to be complete. 



If the smaller tube B is placed across both poles of the magnet, 

 the luminous discharge at its centre assumes the appearance of being 

 nearly separated into two parts, each part showing a tendency to 

 rotate round the pole of the magnet on which it is placed, the one 

 in an opposite direction to the other. I endeavoured to obtain a 

 disruption of the battery discharge when in this state, and possibly 

 with a more powerful electro-magnet this experiment would succeed ; 

 but although I reduced the intensity of the battery discharge and 

 increased the power of my electro-magnet, I could not in this manner 

 obtain an actual discontinuity of the battery discharge ; but when the 

 same vacuum-tube was placed in a longitudinal or equatorial position 

 between the poles, or even approached them within three or four 

 inches in that direction, an immediate interruption of the dis- 

 charge took place. 



When both vacuum-tubes are placed in the battery circuit, the 

 interruption can be shown in a very striking manner : the general 

 arrangement of the apparatus represented in the figure shows how 

 this experiment is made. A is fixed on a wooden support. One 

 wire (b) is attached to the copper terminal of the battery, the other 

 wire (a) being connected to one of the wires in B, which is held by 

 the hand, the other wire (6') being connected with the zinc terminal 

 of the battery, gold-leaf electroscopes being placed as before. In 

 this manner all the apparatus is fixed except B, which being held by 

 the hand, and the connecting wires being flexible, can be placed in 

 any required position. 



As long as the vacuums are at a sufficient distance from the 

 action of the magnet, the luminous discharge is visible in both, and 

 the leaves of the electroscopes partially collapse ; but immediately the 

 discharge in B is placed in the position described in the previous 

 experiment, between the poles of the magnet, the discharges in both 

 vacua instantly disappear, and the leaves of the electroscopes di- 

 verge to their original maximum. 



The actual position of what is termed the magnetical field, around 

 and between the poles of a magnet, has been generally delineated by 

 means of iron filings placed between the poles on a sheet of paper. 

 Assuming the lines in which these particles arrange themselves to 

 represent the direction of the power of the magnet, or the magnetic 

 field, they also explain the actual position through which the vacuum- 

 tube should be placed to obtain the preceding result, and in this 

 manner to show by experiment that a voltaic discharge which has 

 sufficient intensity to pass through a space of upwards of 6 inches in 

 attenuated carbonic acid gas is not only interrupted, but absolutely 

 and entirely arrested by magnetic force. 



Postscript. — In repeating the experiments with Dr. Tyndall in my 



