on the Polarity of the Diamagnetic Force, 167 



magnet are observed by means of a distant scale and telescope, 

 according to the method applied to the magnetometer of Gauss. 

 The rectangle da, did, fig. 2, is the outline of a copper damper, 

 which, owing to the currents induced in it by the motion of the 

 magnet, soon brings the latter to rest, and thus expedites ex- 

 periment. 



It is well known that one end of a magnet attracts, while the 

 other end repels the same pole of a magnetic needle ; and that 

 between both there is a neutral point which neither attracts nor 

 repels. The same is the case with the helices HE, H'E' ; so that 

 when a current is sent through them, if the astatic magnet be 

 exactly opposite the neutral point, it is unaffected by the helices. 

 This is scarcely attainable in practice ; a slight residual action 

 remains which draws the magnets against the helices ; but this 

 is very easily neutralized by disposing an external portion of 

 the circuit so as to act upon the magnets in a direction opposed 

 to that of the residual action. Here then we have a pair of 

 spirals which, when excited, do not act upon the magnets, 

 and which therefore permit us to examine the pure action 

 of any body capable of magnetic excitement placed within 

 them. In the experiments to be described, it was always 

 arranged that the current flowed in opposite directions through 

 the two spirals ; so that if the bodies within them were polar, 

 the two upper ends of these bodies should be poles of opposite 

 names, and consequently the two lower ends opposed also. 

 Supposing now our two cylinders to occupy the central position 

 indicated in fig. 2 : even if the cylinders became polar through 

 the action of the surrounding current, the magnets, being oppo- 

 site to the neutral points of the cylinders, would experience no 

 action from the latter. But suppose the wheel W' to be so 

 turned that the two cylinders are brought into the position 

 shown in fig. 1, the upper end o of ojo and the lower end n of 

 mn will act simultaneously upon the suspended magnets. For 

 the sake of illustration, let us suppose the ends o and n to be 

 both north poles, and that the section, fig. 4, is taken when the 

 bars are in the position shown in fig. 1. The right-hand pole 

 will attract S' and repel N, which attraction and repulsion sum 

 themselves together to produce a deflection of the system of 

 magnets. On the other hand, the left-hand pole n, being also 

 north, will attract S and repel N', which two effects also sum 

 themselves to produce a deflection in the same direction as the 

 former two. Hence, not only is the action of terrestrial mag- 

 netism annulled by this arrangement, but the moving force due 

 to the reciprocal action of the magnets and the bodies within 

 the helices is increased fourfold. By turning the wheel in the 

 other direction, we bring the cylinders into the position shown in 



