Magnetism on Gases traversed by Electric Discharges. 221 



the case of carbonic acid, and 40° in the case of hydrogen. We 

 see by this that the electric conductivity of the rarefied gas con- 

 tained in the axial tube is notably augmented by the magneti- 

 zing. At the same time we ascertain that the effect is not the 

 same in the different gases : it is maximum in hydrogen, mini- 

 mum in air ; the order of the three gases is the same here as in 

 the experiments with the transverse discharge. The less the 

 pressure, the more marked is the effect ; at J millim. pressure 

 we have seen the deflection of the galvanometer pass, in conse- 

 quence of the magnetizing, from 15° to 26° with air, to 30° with 

 carbonic acid, and to 38° with hydrogen : it is seen that the in- 

 tensity of the current was more than doubled in hydrogen. 



2nd position. The negative electrode is in the immediate vici- 

 nity of one of the magnetic poles. The modification induced in 

 this case in the appearance of the electric jet is much more re- 

 markable than in the preceding. In proportion as the pressure 

 diminishes below 2 millims., the luminous envelope which sur- 

 rounded the negative electrode lengthens more and more, inva- 

 ding the dark space; at the lowest pressures we have been able 

 to attain, the negative part of the jet formed at last a very elon- 

 gated frustum of a cone, filling the whole interval between the 

 two electrodes, the positive part having been driven back into the 

 interior of the soft-iron cylinder. In this position of the tube 

 the increase observed in the conductivity of the rarefied gas is a 

 little less than that obtained in the first position. By slowly 

 moving the tube, it may be made to take all the positions be- 

 tween the two on which we have particularly dwelt. Starting 

 from the second position, we see the negative luminous cone 

 shorten more and more, then give place to the positive dart, 

 which advances as far as behind the negative electrode. The 

 direction of the magnetization has no influence, either on the 

 increase of conductivity or on the appearance of the electric jet. 



M. de la Rive had already, in his study on the same subject 

 (cited at the beginning of our memoir), described a special case 

 in which the action of magnetism appeared to him to increase the 

 electric conductivity of the gas instead of diminishing it : it was 

 where the discharge was transmitted through a spiral tube placed 

 in a peculiar manner between the poles of the electromagnet ; 

 but he did not dwell on it, reserving the study of it for a future 

 memoir*. 



IV. The Action of Magnetism on the Electric Jet when this is ro- 

 tated continuously round the pole of the Electromagnet. 



The rotatory movement of the jet may either be performed in 

 a plane perpendicular to the axis of the electromagnet which 

 * Archives des Sci. Phys. et Nat. Dec. 1866, vol. xxvii. p. 296. 



