516 Intelligence and Miscellaneous Articles. 



The measurement of the magnetic moment by the deflection of 

 the declinometer requires in strictness that the position of the poles 

 be known, or at any rate that the experiments be made at two 

 different distances so as to eliminate the factor of correction ; but 

 with very narrow cylinders, Coulomb's rules furnish a sufficient 

 approximation for the degree of exactitude I had in view. Finally, 

 the field F of the magnetizing coil is determined by the intensity I 

 of the current, and the number n of turns of the wire for unit 

 length, 



E=4 7 rn 1 I. 



The quotient of the magnetic moment M of the cylinder by its 

 volume V gives the mean intensity of magnetization A, and we 

 may denote as the mean coefficient of magnetization, 



A M 

 J ™ 



F 47mjV 



On the other hand, if S is the section of the cylinder, A the 

 intensity of magnetization in the median region, F the action of 

 the magnetism induced in this region, p the number of turns of 

 the small coil, E, the resistance of the circuit of which it forms 

 part, and Q the discharge induced by the reversal of the magneti- 

 zation, we have 



QR=2pS(47rA 1 -F 1 ) = 8pirS('A-^\ 



Putting 



we shall obtain by experiment two mean coefficients / and f x which 

 have slightly different significations. 



Experiment shows in the first place that we have always/ 1 >/. 

 These two quantities, which are very different for short cylinders, 

 approximate more and more, and this ratio tends to unity as the 

 length of the cylinder increases. At the same time, the greatest 

 values of the coefficients / and / correspond to fields which are 

 gradually weaker. Lastly, the values of / and of / furnished by 

 very long cylinders are equal to the coefficient 7c given by closed 

 rings. 



With the iron which I used, when the ratio of the length to the 

 diameter varied from 40 to 500 or 600, the maximum value of the 

 coefficients / or / varied from 25 to 190, or from 40 to 220, while 

 the corresponding fields diminished from 20 to 25 C.Gr.S. units to 3 

 units ; the same wires used as rings gave for the maximum of the 

 coefficient Jc, a value of about 200 with a field of 3 units. The 

 agreement of these results is sufficient, if we allow for the difficulty 

 of obtaining specimens which are identical. 



The method of cylinders, provided their length is at least 500 



