122 Dr. E. Bouty on the Magnetization of Steel by Currents 



I have effected only relative measurements of /jl ; but the 

 physicists who have studied the question have found its values 

 so variable from one steel or soft iron to another, and even 

 for one and the same sample, in physical circumstances ap- 

 parently so closely similar, that there is but little interest in 

 realizing absolute measurements as long as the laws which 

 govern this variability are not entirely known to us. 



The greater part of these researches were effected at the 

 Lycee of Rheims ; they were finished in the laboratory of M. 

 Jamin. Permit me here to return thanks to that able master 

 for the kindness with which he has welcomed me and for the 

 counsels which he has not ceased to lavish upon me. 



I. The Permanent Magnetization of thin Needles tempered 



hard. 



The quantity of magnetism acquired temporarily or per- 

 manently by a steel needle submitted to the action of a cur- 

 rent depends on the intensity of the magnetizing force em- 

 ployed. It is important first of all to define the latter with 

 precision. 



In order to magnetize it, the needle is placed in the axis of 

 a helix very long relatively to the needle, and sufficiently 

 wide. Under these conditions the action exerted by the 

 current is the same at all the points of the needle, and pro- 

 portional to the intensity of the current and to a coefficient 

 which depends on the number and width of the turns ; but if 

 we limit ourselves to employing always the same helix and to 

 relative measurements, we can take the measure of the in- 

 tensity of the current for that of the action exerted upon the 

 needle — that is to say, of the magnetizing force. 



The relative measurement of the intensity of the currents 

 was performed by means of a sort of tangent- compass of very 

 simple construction. A small helix AB (fig. 1), excited by 



Bar. 1. 



a— 



the current is adjusted so that its axis, sensibly perpendicular 

 to the magnetic meridian, passes through the centre o of a 

 small magnetized needle a b, furnished with a mirror and 

 suspended by a cocoon-fibre. The tangent of the deflection 

 produced is proportional to the current-intensity*. Great 



* Suppose the needle a b and the helix A B indefinitely small in com- 

 parison with the distance Ao = r. The helix can he replaced by a magnet 

 of which the moment M is approximately equal to the product Si of the 

 sum of the surfaces embraced by each turn separately multiplied by the 



