Dr. E. Bouty on the Magnetization of Steel by Currents. 123 



care must be taken, in order to eliminate all action extraneous 

 to the helix, to recurve the two electrodes which convey the 

 current to it the one upon the other, and very close, so that 

 their action upon an exterior point shall be very sensibly nil. 

 A commutator permits the reversal of the current in the helix, 

 and thus the elimination, by a second measurement of the 

 deflection in the opposite direction, of the error resulting 

 from the imperfection of the adjustment. 



With respect to the measurements of magnetic moments, 

 they were usually effected by the method which I have pre- 

 viously described for very small magnetized needles*. 



I have investigated, in the first place, the permanent 

 magnetism of thin needles tempered very hard. These 

 needles have a length at least equal to, and generally above, 

 fifty times their diameter. It is easy to verify that the 

 moments y acquired permanently by these needles submitted 

 to the same magnetizing force are represented by the formula 



y = m(x — cT) ; 



and consequently the method indicated in the Introduction is 

 applicable to such a series of needles f. It would therefore 

 suffice to measure the corresponding values of x and y for a 

 great number of needles of the same diameter and different 

 lengths, and to make all the observations cooperate for the 

 determination of m and d. 



But here the method presents an inconvenience : it is in 

 fact very difficult to communicate to a great number of 

 needles a truly identical degree of hardening; and if this 

 condition is not realized, one of the two determinations, that 



intensity of the current. If now a be the deflection of the needle a b, 



fj. its magnetic moment, we have, after Gauss, 



2M / x_2S/z. 

 tan a = -^r — f^i. 



If the lengths of a b and A B are no longer negligible in comparison 

 with r, the coefficient of i is much more complicated, but tan a remains 

 • proportional to i. / 



* Annates de VEcole Normale, 2 e serie, t. iii. p. 12. This method, ab- 

 solutely faultless in the case of very smpJl needles, sometimes becomes a 

 little faulty when one wishes to measure the moment of very long ones 

 very feebly magnetized. This is in consequence of its not being per- 

 missible in the latter case to suppose no reciprocal influence between the 

 directing bar and the needle. But this cause of error, when present, is 

 betrayed by the non-accordance of measures 1 and 3 ; and the correspond- 

 ing determinations are rejected. 



t Equation (1) represents a right line, on the condition that x and y 

 be regarded as current coordinates. The simplest way of proving the 

 applicability of this formula consists in verifying that the characteristic 

 points of the various needles really fall in a straight line. 



