324 FRAGMENTS OF SCIENCE. 



their centres, and are suspended from the point a, so that 

 both magnets swing in the same horizontal plane. It 

 is so arranged that the poles N' s' are opposite to the 

 central or neutral point of the helix, so that when a 

 current is sent through the latter, the magnets, as 

 before explained, are unaffected. Here then we have 

 an excited helix which itself has no action upon the 

 magnets, and we are thus enabled to examine the 

 action of a body placed within the helix and excited 

 by it, undisturbed by the influence of the latter. The 

 helix being 12 inches hig'h, a cylinder of soft iron 6 

 inches long, suspended from a string and passing over 

 a pulley, can be raised or lowered within the helix. 

 When it is so far sunk that its lower end rests upon 

 the table, the upper end finds itself between the poles 

 N' s' of the astatic system. The iron cylinder is thus 

 converted into a strong magnet, attracting one of the 

 poles, and repelling the other, and consequently deflect- 

 ing the entire astatic system. When the cylinder is 

 raised so that the upper end is at the level of the top 

 of the helix, its lower end comes between the poles 

 N' s' ; and a deflection opposed in direction to the 

 former one is the immediate consequence. To render 

 these deflections more easily visible, a mirror m is 

 attached to the system of magnets ; a beam of light 

 thrown upon the mirror being reflected and projected 

 as a bright disk against the wall. The distance of 

 this image from the mirror being considerable, and its 

 angular motion double that of the latter, a very slight 

 motion of the magnet is sufficient to produce a dis- 

 placement of the image through several yards. 



This then is the principle of the beautiful apparatus 1 



1 Devised by Prof. W. Weber, and constructed by M. Leyser, 

 of Leipzig. 



