90 Prof. E. Bouty's Studies on Magnetism. 



produced in the derivation, and are inverse on the closing, direct 

 on the opening of the circuit. 



"We should add that, from our present point of view, a very 

 feeble coil is equivalent to a conductor of enormous capacity. 



2. On the Magnetization of Steel. 



A steel needle, recently tempered, is transported from infinity 

 into the interior of a spiral animated by a current, and then ex- 

 tracted from the spiral and transported to infinity in the oppo- 

 site direction. This needle is attracted into the spiral; and 

 during its introduction the work absorbed by the magnetization 

 of the steel is added to the work of the attractive forces developed 

 between the spiral and the needle. These two effects in the 

 same direction produce in the wire outside the coil an induced 

 current opposite in direction to the principal current. When the 

 needle is withdrawn from the coil, the work restored by the loss 

 of the temporary magnetization is added to the negative work of 

 the attractions — whence a direct induced current outside the 

 coil*. 



The considerations unfolded in the preceding article concern- 

 ing the extra currents apply also to the induced currents. It is 

 probable that these currents are without effect within the coil 

 from which they emanate. In all cases, if the needle is intro- 

 duced or extracted very slowly, the intensity of the induced cur- 

 rents is very feeble ; and in this case, at least, their magnetic 

 effect within the coil may be neglected. We have therefore good 

 ground for admitting that the magnetism carried away by a 

 needle which is passed once to the spiral is due solely to the 

 action of the principal current. 



I. The circuit comprises only a pile with a constant current 

 and the coil within which the magnetizing takes place. 



(1) If the needle be introduced and extracted slowly, and the 

 permanent magnetic moment which it has carried away be mea- 

 sured, we find that repetition of the passing of the needle aug- 

 ments the residual moment. It tends, through the repetition, 

 towards a limit A ; and the magnetic moment y, after x passages, 

 is sufficiently well represented by the empiric formula 



y= A ~l> ■ ■ 0) 



* This direct current is equal in quantity to the inverse current ; whence 

 this proposition : — The work absorbed by the production of permanent 

 magnetization is equal to the excess of the work of the attractive forces 

 during the extraction of the magnet from the spiral above the work of the 

 same forces during its introduction. The permanent magnetizationhas 

 therefore a mechanical origin, and derives nothing from the current. 



