528 Prof. De la Rive on the Action of Magnetism upon 



solely under the influence of the pole of the electromagnet. In 

 order to examine the effect of this alteration, the two jars, 

 one containing the brass rod and the other a soft-iron rod as 

 before, were introduced into the circuit one after the other, one 

 being placed on each polar surface of the electromagnet, and 

 both being filled with dry air rarefied to various degrees. Ac- 

 cording to this arrangement of the jars, the air in each was tra- 

 versed by the same discharge. The following are the results 

 obtained : — 



Pressure. 



Number of turns in 15 seconds. 



Iron rod. 



Brass rod. 



Ring 



Ring 



Ring 



Ring 





positive. 



negative. 



positive. 



negative. 



millims. 











20 



20 



12 



10 



9 



18 



20 



12 



11 



10 



16 



24 



13 



12 



11 



14 



28 



14 



14 



13 



12 



32 



14 



16 



15 



10 



38 



16 



20 



19 



8 



46 



19 



21 



22 



7 



49 



22 



22 - 



26 



6 



56 



27 



24 



30 



5 



59 



48 



26 



34 



4 





70 



... 



40 



In another set of experiments the jar with the brass rod gave 

 25 turns in 15 seconds at a pressure of 8 millims., whichever 

 way the current was sent, — whence we may conclude that the 

 rate of rotation is sensibly the same at this pressure when 

 the ring is negative as when it is positive. The same thing 

 may be said of the higher pressures also, at which the differ- 

 ence, although always on the same side, is too small to be of 

 importance. At lower pressures the rate of rotation is greater 

 when the ring is negative than when it is positive, contrary to 

 what takes place with the jar containing the iron rod ; and the 

 difference becomes greater as the pressure diminishes. 



It may be observed that the absolute rate of rotation is always 

 greatest in the jar with the iron rod, in consequence of the mag- 

 netic pole, which in this case is at the end of the iron rod, being 

 much nearer to the electric jet. Moreover the rate of rotation 

 in the jar with the brass rod increases when the iron rod in the 

 other jar is not magnetized and there is consequently no 

 rotation in this jar, notwithstanding that the same discharge 

 passes through it as before. These conditions are easily fulfilled 



