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



Current. 



d. 



Difference. 



Observed. 



Calculated. 



1809 

 24-99 

 30-31 

 39-32 

 4704 

 55-07 



00 



millim. 



12-37 



11-86 



1012 



8-86 



7-32 



719 



5-50 



millim. 



12-37 



11-32 



10-52 



916 



799 



6-78 



000 

 +0-54 

 -0-40 

 -0-30 

 -0-67 

 +0-41 



It is seen that the poles approach the extremities in propor- 

 tion as the intensity of the magnetizing current is augmented. 

 The numbers in the third column were calculated by the for- 

 mula d— 15' 1 ( 1— ttja ) I an d we see that they very well re- 

 present the facts of experiment, considering the errors of which 

 the measurement of d is susceptible ; in the vicinity of satura- 

 tion, however, the decrease of d is slower than would be indi- 

 cated by the formula, and the values of d asymptotically ap- 

 proach 5*5 millims., which is characteristic for saturated needles 

 of this diameter. 



We know that, when the diameter of the needles changes, 

 the limit of d is proportional to the diameter* ; therefore the 

 general formula for the distance of the poles is 



d=2aD(l—px), (3) 



where a and p are constants. The constant a depends on the 

 degree of tempering, as will be seen in the sequel. Here the 

 value is a = 13*65 ; it represents in millimetres the limit towards 



which the distance. »■ for a needle of 1 millim. diameter tends 



when the force which produces the magnetization tends to- 

 wards zero. The value of the coefficient p depends on the 

 unit by means of which the magnetizing force is expressed. 

 The limit of d for x = go is 



S=2aD; (4) 



and the absolute value of a is 4*973 millims. f 



In brief, and under the restriction that all the needles em- 



* See Annates de VEcole Normale, 2 e ser. t. iii. p. 40. 



t Experiments made on thicker bars by the methods which will be in- 

 dicated further on have furnished results accordant with these. Thus bars 

 10 millims. in diameter, steeped at a red heat, gave for a the value 1375 

 millims., nearly identical with the preceding'; and yet the value of ex, re- 

 mained equal to 8 — doubtless because saturation could not be attained, 

 notwithstanding the employment of the current of twenty-five good Bun- 

 sen elements. 



