220 MM. A. de la Rive and E. Sarasin on the Action of 



millims. in internal diameter, and 40 centims. long ; the jet had 

 a length of 20 centims. To the two extremities of this tube were 

 cemented the two electrodes, each formed of a brass rod ending 

 in a knob, likewise of brass. One of these electrodes extended 

 externally in a long brass stem ending at the extremity of the 

 opening in the soft iron, the other in a lead pipe very narrow 

 and flexible, which, passing through the opening in the second 

 soft iron, put the glass tube in communication with the mano- 

 meter and the air-pump. The effect of magnetism on the elec- 

 tric discharge thus arranged axially varies with the distance of 

 the poles of the magnet. With our apparatus, the most favour- 

 able case appeared to be that in which we had a fixed distance of 

 10 centims. between the two poles, which distance we maintained 

 throughout our experiments. The effect of magnetism on the 

 resistance of the rarefied gas contained in the axial tube, as well 

 as on the appearance of the electric jet, varies likewise with the 

 position occupied by the electrodes in relation to the magnetic 

 poles; the action of magnetism on the resistance reaches its 

 maximum when the knob serving as the negative electrode is in 

 the middle of the interval separating the magnetic poles. The 

 influence of magnetism on the axial discharge is very feeble at 

 pressures above 2 millims. Nevertheless, even in this case the 

 magnetization sensibly modifies the appearance of the jet : having 

 been discontinuous and striated, it becomes much more conti- 

 nuous; still it is not possible as yet to verify an appreciable change 

 in the resistance. From 2 millims. the effect is much more pro- 

 nounced ; and here we must distinguish two principal positions 

 of the electrodes in the tube. 



1st position. The negative electrode is placed in the centre of 

 the interval separating the magnetic poles. When the gas is 

 brought to a pressure below 2 millims., the moment we magne- 

 tize, a complete modification in the appearance of the discharge 

 is observed. The extremity of the positive jet lengthens into a 

 dart which applies itself to the side of the tube, approaching 

 more and more to the negative electrode, and ends, at the lowest 

 pressures we have been able to attain, by passing between the 

 tube and the negative knob to unite behind it with a blue sheath 

 which has replaced the negative aureole. The change produced 

 in the resistance is still more remarkable than the modification 

 in the appearance of the jet : indeed, in this case, contrary to 

 what would take place with the transverse discharge, the electric 

 conductivity of the gas traversed by the electric jet is increased 

 under the influence of magnetism. At 1 millim. pressure, for 

 example, while the galvanometer placed in the derived current 

 gave a deflection of 30° when the electromagnet was not in action, 

 it marked 35° after the magnetization in the case of air, 38° in 



