96 



Prof. E. Bouty's Studies on Magnetism. 



But it must be remarked that the intensity of the extra current 

 of the coil is superior to that of the principal current. The 

 direct extra current strongly charges the condenser, which must 

 consequently discharge itself after the extra current has ceased. 

 The coil thus receives, by reflection, the extra current which it 

 has produced, and is traversed by it in the opposite direction to 

 that of the principal current. The reflex effect is, for a given 

 condenser, the more intense the more powerful the coil em- 

 ployed ; and experiment shows that with a very feeble coil the 

 effect of the interruption vanishes. 



IV. Magnetization by induced currents. — We have studied only 

 the following case : — 



When the circuit includes two coils, the slow introduction of 

 a core of soft iron into one of them, or its extraction, is without 

 effect upon the magnetism of a needle placed in the other coil. 

 But if the core be introduced slowly and withdrawn suddenly, 

 the direct induced current augments the magnetic moment of 

 the needle placed in the second coil. Repetition of the same 

 operation causes the moment of the needle to tend towards a 

 limit, which it rapidly approaches. The formula 



y=A+B(l-«— ), (2) 



in which A, B, and a. are constants, appears very well adapted 

 to represent the magnetic moment y after x passages. The fol- 

 lowing Table permits the appreciation of the degree of accuracy 

 of the formula : — 



Table VII. 



Number of 



direct induced 



currents. 



Magnetic moment. 



Difference. 



Observed. 



Calculated. 





 1 

 2 

 3 

 4 

 8 

 16 



71-55 

 75-30 



78-55 

 80-50 

 8080 

 8105 

 8105 



7155 

 75-55 

 78-53 

 79-20 

 80-03 

 80-92 

 81-03 



000 

 -0 25 

 +0 03 



+0-70 

 +0-77 

 +013 

 +002 



A=71*55, B = 9-50, log e-«= 1*7626391. 



The differences between calculation and observation are a little 

 more considerable in this Table than in the preceding ones ; 

 but account must be taken of the difficulty of always withdraw- 

 ing the soft iron in identically the same manner, in order to pro- 

 duced induced currents of the same intensity. Regard being 



