Prof. E. Bouty's Studies on Magnetism. 83 



and applied by Coulomb — the torsion method, and that of oscil- 

 lations. The first is applied especially to bars ; the second ex- 

 tends also to small needles, but loses much of its precision when 

 the magnets are of very small dimensions. 



Divers physicists, among others Wiedemann *, have calculated 

 the magnetic moments of a magnetized needle from the deviation 

 produced by it upon a very short magnetized needle furnished with 

 a mirror and movable about a vertical axis. This process is pe- 

 culiarly suitable when the magnetic moments of one and the 

 same needle variously magnetized are to be compared, provided 

 that the magnetic moments to be compared are not too feeble. 



In this case the needle A, whose magnetic moment we wish 

 to compare, is fixed in a horizontal groove, the direction of which 

 is sensibly perpendicular to the magnetic meridian. The dis- 

 tance from the centre of the needle A to the centre of needle B 

 should be sufficiently great relatively to the length of A to avoid 

 a notable alteration of the results by a slight displacement of the 

 poles of A in consequence of a more or less considerable magne- 

 tization. The azimuth of the line of the centres of A and B can 

 be chosen arbitrarily ; and if the needle A receive the same posi- 

 tion in all the experiments, the deviations observed will be pro- 

 portional to the moment it possesses. If the needle is regular, 

 it may be returned to its groove with its ends reversed, and the 

 mean of the deviations be taken. 



I have frequently employed this process of measurement in 

 the study of magnetization by currents; but mostly, as in the 

 study of the breaking of the current, I have had to measure the 

 ratio of the magnetic moments of needles of very small dimen- 

 sions, and different in length and diameter. I have then made 

 use of a special arrangement, which I will describe in detail. 



Principle of the method. — Suppose a rigid support, movable 

 about a vertical axis. Fix upon this support (1st) a horizontal 

 needle the magnetic moment M of which is known, (2nd) the 

 needle whose magnetic moment x we 

 wish to determine. The two needles 

 are placed one above the other in such 

 wise that their axes form a right 

 angle, and at a sufficient distance 

 for their reciprocal action not to alter 

 the distribution of the magnetism in 

 either of them. 



The system thus formed takes, 

 under the influence of the earth's 

 magnetism, a determinate position 

 of equilibrium, such that the mag- 



* Galvanismus. 

 G2 



