PARAMAGNETIC AND DIAMAQNKTIC FORCES. 247 



undisturbed by the influence of the latter. The helix 

 being 12 inches high, 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 K' 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 deflecting 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 

 fco 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 displacement of the image through 

 several yards. 



This, then, is the principle of the beautiful apparatus* by 

 which the investigation was conducted. It is manifest 

 that if a second helix be placed between the poles SN with 

 a cylinder within it, the action upon the astatic magnet 

 may be exalted. This was the arrangement made use of 

 in the actual inquiry. Thus to intensify the feeble action, 

 which it is here our object so seek, we have in the first 

 place neutralized the action of the earth upon the magnets, 

 by placing them astatically. Secondly, by making use of 

 two cylinders, and permitting them to act simultaneously 

 on the four poles of the magnets, we have rendered the 

 deflecting force four times what it would be, if only a 

 single pole were used. Finally the whole apparatus was 

 enclosed in a suitable case which protected the magnets 

 from air-currents, and the deflections were read off 

 through a glass plate in the case, by means of a telescope 

 and scale placed at a considerable distance from the instru- 

 ment. 



A pair of bismuth cylinders was first examined. Sending 



* Devised by Prof. W. Weber, and constructed by M. Leyser, of 

 Leipsic. 



