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



netic axis of the needle M (fig. 1) makes, with the plane of the 

 magnetic meridian N S, an angle a determined by the equation 



;r=Mtan« (1) 



If the moment x is sufficiently small in proportion to M, the 

 angle a can be determined by PoggendorfFs optical method*. 

 For that purpose the support of the needles carries a small ver- 

 tical silvered mirror, in which, with the aid of a telescope, the 

 image of a horizontal graduated scale is observed, which is placed 

 beneath and very near the objective, and perpendicular to the 

 optic axis of the telescope. 



The moment M of the directing needle may be made as small 

 as we please. It follows that it is possible by this method to 

 measure the magnetic moment of very small needles, comparable 

 in their dimensions to particles of file-dust. I have been able 

 to effect measurements relative to needles of 2 millims. length 

 and 0*2 millim. diameter. 



When we wish only to compare with one another the magnetic 

 moments x, x l of several minute needles, it is not necessary to 

 know the moment M of the directing needle ; we have, indeed, 

 designating by a, a' the deviations they produce : — 



x' __ tan a! 

 x ~~ tan a ' 

 and, on account of the minuteness of the angles a and a!, 

 x' tan a' a! tan 2a' n 1 



tan a a tan 2a n 



(2) 



where n and n' are the two readings made upon the scale, the 

 zero of which is supposed to be in the plane of the magnetic 

 meridian. 



Apparatus. — The support of the needles is merely a small 

 stick, thin and light, of sealing-wax, hard and not readily fusible. 

 The directing needle A B (fig. 2) is attached to the lower extre- 

 mity of the wax; and a very narrow glass tube T passes through 

 the latter towards its upper part, in such a way that its axis is 

 horizontal and perpendicular to the axis of the directing needle. 

 The system is suspended by a nipper or hook of copper to a single 

 cocoon-thread, and enclosed in a wooden box blackened on the 

 inside, and closed in front by a plane glass. The cocoon-thread 

 passes through an aperture in the top of the box, and is grasped 

 by a brass nipper. And a disk of red copper is placed under and 

 very near the directing needle, the oscillations of which it deadens. 



* " Methodeder Spiegelablesung " (Pogg. Ann.vo\.\'n. 1826), employed 

 by Gauss (Gottingische gelehrte Ariz. 1833, Nos. 205-207), to whom the 

 invention of it is most frequently attributed. 



