Transparent Bodies by the action of Magnetism. 491 



Consequently, if we conceive a system of circular conductors 

 constituting a helix whose dimensions do not exceed those of the 

 space where the magnetic action is constant, a rotation of 90 

 degrees round an axis perpendicular to the direction of the mag- 

 netic action will develope an induced current proportional to the 

 magnetic action. If the movement is made very rapidly, the 

 iuduced current can be measured without difficulty by the gal- 

 vanometer, and tbe magnetic action will thus be determined*. 



I constructed, therefore, a small bobbin capable of turning 

 about one of its diameters ; and in order to give to the phaeno- 

 mena all possible intensity, the dimensions of wire around the 

 bobbin were such that its resistance was almost equal to that of 

 the wire of the galvanometer, of which we shall afterwards 

 speakf. To this end, 23 metres of copper wire covered with 

 silk, 05 millim. in diameter, were rolled so as to form a bobbin 

 28 millims. in external diameter, 12 millims. in internal diameter, 

 and 15 millims. high. This bobbin was mounted upon a copper 

 support, represented in fig. 3, which was fixed upon the electro- 

 magnet in the middle between its two branches. By means of 

 the button B, the bobbin C could be turned 90 degrees round 

 the dotted line FG ; and in consequence of the arrangement of 

 the apparatus, the axis of rotation was perpendicular to the line 

 of the poles, i. e. to the direction of the magnetic action. The 

 transparent bodies submitted to experiment were placed on the 

 piece of copper L, fixed on the support above the bobbin. The 

 stem A could be raised or lowered by means of a rackwork regu- 

 lated by the screw D, the transparent body or the bobbin could 

 be brought at pleasure to the same point. The stem A could 

 be retained in a fixed position by the screw H. 



If the dimensions of the bobbin are given, by varying the 

 diameter of the wire, we vary in inverse ratio the number of 

 layers which can be arranged in the height and length of wire 

 of which each layer is formed. Calling then d the diameter, we 

 should have 



h 



n =d> 



* M. Weber has founded upon the same principles a very remarkable 

 method for comparing the two components of terrestrial magnetic action, 

 in onler to determine tbe value of the inclination. Mr. Faraday made use 

 of analogous processes in studying tbe distribution of tbe magnetic forces 

 round a magnet. 



t This condition is easily deduced from tbe law of Obm. In fact, let L 

 be the resistance of the galvanometer, X that of a layer of bobbin wires 

 contained in tbe same plane, » tbe number of these layus, of tbe electro- 

 motor force, developed in one layer ; tbe induced current would lie expressed 



by -^r- 

 3 nA+L 



