Dr. E. Bouty on the Magnetization of Steel by Currents. 131 



would be directly produced by the magnetic force x' . Expe- 

 riment has verified this anticipation in a satisfactory manner 

 when we consider the smallness of the displacements to be 

 measured. The following are the numbers belon ginff to needles 

 0*553 millim. in diameter: — 



Current x. 



d 

 ( I st passage). 



d 



(5th passage). 



Difference. 



Observed. 



Calculated. 



33-29 



47-88 



millim. 

 10073 



7-869 



millim. 

 9-730 

 7570 



+ 0-343 

 +0-299 



+0-309 

 + 0-444 



II. Temporary Magnetization of thin Needles tempered hard. 



In order to study the temporary magnetization of the 

 needles, the magnetizing helix is placed in a groove perpendi- 

 cular to the magnetic meridian, and of which the prolongation 

 meets the centre of the galvanometer-needle which served us 

 in the preceding section. The deflection produced by the helix 

 alone is proportional to the intensity of the current, as already 

 explained*. When a needle is introduced into the helix, the 

 deflection is increased by a quantity which may serve as a 

 measure for the temporary magnetic moment of the needle, 

 but only under certain conditions, which we will state pre- 

 cisely. 



The deflection produced by the needle whose temporary 

 moment is M is given by the formula of Gauss, 



tana =-Tyr> C 1 ) 



provided that the length of the needle may be neglected in 

 comparison with the distance r of its centre from the galvano- 

 meter-needle. If it be otherwise, Gauss's formula must be 

 completed by multiplying the second member by a factor 

 whose first term is unity, and the succeeding terms of the 



order of the powers 2, 4, 6, . . . of -. The values of tan a. 



remain proportional to M for one and the same needle, or 

 needles of the same length, but cease to be so for needles of 

 different lengths whose centres coincide. 



* To render the present experiments comparable with those of the prece- 

 ding section, the ratio of the deflections a, and a! must be determined which 

 are impressed upon the needle by one and the same current traversing the 

 galvanometric helix of the preceding section and the large magnetizing 

 helix, and the new measures of the intensity must be multiplied by the 



,. u,' 

 ratio — 



K2 



