22 On the Stria seen in the Electrical Discharge in vacuo. 



that I could stop the discharge entirely by placing the poles of 

 two magnets opposite each other and in a particular direction 

 with reference to the line of discharge. The two platinum wires 

 in this tube are inserted perpendicularly to the axis^ and pene- 

 trate half an inch into the tube. If, now, the tube be placed 

 horizontally, the platinum wires uppermost and the observer 

 looking along the tube from the positive to the negative wire, 

 the north pole of a magnet should be placed on the right-hand 

 side, and as nearly as can be to the platinum, i. e. touching, or 

 nearly so, the glass tube, while the south pole of another magnet 

 is similarly placed on the opposite or left-handed side. The dis- 

 charges cease entirely, and there is no conduction through the 

 tube, showing that it is not a case of dark discharge, but an 

 arresting of the passage of the discharge through the attenu- 

 ated medium. If the tube be turned one-fourth round its 

 axis in either direction, the discharges reappear, as they also do 

 if the magnets be reversed. The experiment requires care, and 

 it seems to be necessary to make it with a power of coil just suf- 

 ficient to enable the discharge to traverse the distance between 

 the platinum terminals and with a certain magnetic power : if 

 made with a powerful electro-magnet, the discharge is stopped, 

 whichever side the poles be placed with reference to the platinum 

 wires, but the discharge is only made intermittent when the 

 platinum wires are placed in a line joining the poles of the 

 magnet. In the above-mentioned tube, the portion of one end 

 of the glass around one of the platinum wires was covered with 

 a dark deposit of platinum, from the terminal at this end having 

 been constantly used as the negative pole ; when this was made 

 positive instead of the clean end, the effect of arresting the cur- 

 rent could not be produced : this circumstance, and the delicacy 

 of the conditions necessary to ensure success in this experiment, 

 lead me to think that the stoppage of the discharge is due to 

 the diversion in direction of the electric line of force produced 

 by the magnet ; the discharge is, so to speak, blown out by the 

 magnet : but I hesitate to express a positive opinion on this 

 point. 



When magnets are made to approach the negative platinum, 

 there is no stoppage of the discharge, but attractions and repul- 

 sions are observed on the blue flame, which have been described 

 by Mr. Gassiot, and which I need not therefore repeat. 



I tried the effect of magnets on the spark from Ruhmkorff^s 

 coil in air, but could not arrest them, though they appeared to 

 pass with somewhat greater difficulty and longer intennissions j 

 the difference was, however, very slight. 



June 4, 1858. 



