182 WEBER ON THE CONNEXION OF DIAMAGNETISM 



give a brief description of the experiments by which the theory 

 is established. 



1. Electro-diamagnetism, and Measurement of the Moment of an 

 Electro- diamagnet. 



The most convenient arrangement of an electro-diamagnetic 

 apparatus of measurement, for the observation of diamagnetic 

 polarity, consists in a galvanic spiral which is set vertical and 

 symmetrical between the two poles of a magnetic needle bent 

 into the horseshoe form. A (see Plate I. fig. 2) represents the 

 transverse section of the spiral, which lies symmetrically be- 

 tween the poles N and S of the bent magnetic needle NBS. 

 This magnetic needle is held by the clamp DE, in the centre 

 of which, C, the suspending fibre is fixed. Figs. 3 and 4 

 represent two side views of the instrument. It is of advantage 

 to give the spiral a considerable length, say from 400 to 500 

 millimetres ; this renders it easier to regulate the suspension of 

 the needle so that it shall swing in the horizontal plane which 

 bisects the length of the needle (spiral? J. T.), no moment of 

 rotation being here exerted upon the latter when the current 

 passes through the spiral. If, however, a small moment of ro- 

 tation should exist, this is easily compensated by a multiplier 

 M (fig. 4) consisting of a few coils, through which the same 

 current is conducted and brought within a suitable distance of 

 the magnetic needle. For the observation of the latter it is 

 necessary to furnish it wath a reflector P (fig. 3), and to observe 

 therein, by means of a telescope, the image of a distant scale. 

 The magnetic needle is further surrounded by a damper QlQl 

 (fig. 3). The bismuth bar aa (figs. 3 and 4) is suspended ver- 

 tically from a fibre within the spiral ; it can be raised or sunk, 

 so that either its under or its upper end shall lie between the 

 poles of the magnetic needle, as represented in figs. 3 and 4. 

 The observations are made most conveniently by means of an 

 arrangement of pulleys or levers, which permits the observer 

 himself, while standing beside his telescope, to raise or sink the 

 bar of bismuth. When the circuit is established and the mag- 

 netic needle perfectly at rest, the bismuth bar is raised and the 

 consequent small motion of the needle is observed. As soon as 

 the needle has attained its maximum elongation the bismuth 



