including an Examindtidn of the Magnetic Field, 341 



^1. The little test-sphere can also be made available for ex- 

 amining the change brought about in the magnetic field by the 

 introduction of a small bar of iron^ as in the experiment of 

 M. Pliicker quoted by Mr. Faraday*. Removing the helix from 

 the magnetic field, the little sphere was at liberty to traverse it 

 from wall to wall. When the magnet was excited, the sphere 

 passed slowly on to the pole to which it was nearest and came to 

 rest against it. When forcibly brought into the centre of the 

 magnetic field, after a moment's apparent hesitation it passed to 

 one pole or the other with a certain speed ; but when a bar of 

 iron was brought underneath while it was central, this speed was 

 considerably increased. Over the centre of the bar there was a 

 position of unstable equilibrium for the sphere, from which it 

 passed right or left, as the case might be, with greatly increased 

 velocity. The distribution of 



the force appears in this case «>^f\ '^*'^' i^HM^ -^^^ "'"^j' • ■ 



to have undergone a change ^s,^^ ; '^"^^ f* '^^^^' 'i"^ l)'^3j>oibiii 



represented by the line gef iirii)>H4p imm bxiu jh ill v^ib J^ 



in the diagram. From the ;.. .X^mu .> \ ^cn.y■2^vii%^ 



centre towards the poles ^ ''^^ 



the tension steepens sud- 

 denly, the quicker reces- 

 sion of a bismuth bar to- -^ J^ ^'^^ 

 wards the equator, as ob 



'Dili ijfo'i: 



served by M. Pliicker, being isirar^s*iq.i3i54i ifitMiie/r <il4w iitBiiK 



the natural consequence. ' " ■'' '""'"' "^ • <>* fcdto^a&iq 



22. Assuming the law i^itn inomi'ioqal) aiflT 



of action for a small magnetic sphere to be that' it proceeds from 

 weaker to stronger places of force, we find that the passage of an 

 electric current in the manner described so modifies the ' field/ 

 that the positions of its two diagonals are of unequal values as 

 regards the distribution of the force, the portion of the field in- 

 tersected by the diagonal which bisects 1 and 3, fig. 4, being 

 weaker than the portion intersected by the diagonal which bisects 

 2 and 4. But here the believer in diamagnetic polarity may 

 enter his protest against the use which we have made of the 

 assumption. " I grant you,'' he may urge, '' that in a simple 

 magnetic field, consisting of the space before and around a single 

 pole, what you assume is correct, that a magnetic sphere will 

 pass from weaker to stronger places of action ; but for a field 

 into which several distinct poles throw their forces, the law by 

 no means sufficiently expresses the state of things. If we place 

 together two poles of equal strengths but of opposite qualities 

 close to a mass of iron, it is an experimental fact that there is 

 almost no attraction ; and if they operate upon a mass of bis- 

 muth^ there is no repulsion. Why ? Do the magnetic rays^ to 

 V. .; J :.. * Phil. Mag., S, 3. vol. xxxvii. p. 104. 



