266 Dr. Tyudall on the Laws of Magnetism. 



the attracted mass from the magnet. The importance of this 

 condition appears to have been ovei'looked by the discoverers of 

 the law. It has been generally assumed, that if the defects inci- 

 dental to the modes of experiment hitherto adopted could be 

 avoided, the same law would pronounce itself in the case of im- 

 mediate contact. Were this the case, our two first propositions 

 would be identical, the solution of the one would necessarily 

 imply the solution of the other ; it ^nll be shown, however, that 

 the laws in both cases are entirely diflPerent. 



Two principal causes are assigned by physicists as- giving birth 

 to the discrepancies alluded to — the incompleteness of contact, 

 and the peculiar shape of the mass of iron attracted. The question 

 naturally occurs, cannot these causes of divergence be removed ? 

 1 . To annul, as far as possible, the disturbances arising from 

 mere form, a number of regularly-shaped masses of good soft 

 iron were procured ; they included cubes, cylinders, and spheres 

 of various diameters. It is easy to see the practical difficulty of 

 experimenting with cubes and cylinders ; indeed, to render such 

 experiments pure, conditions are required which it is almost im- 

 possible to fulhll. Conceive the cube suspended by a wire 

 attached to the middle of one of its faces, and laid with its op- 

 posite face Hat upon the polished end of the magnet ; let the wire 

 ascend vertically, pass over a pulley, and be attached to a scale- 

 pan at the other side ; on this scale-pan let weights be laid until 

 the cube is separated from the magnet ; the weight which effects 

 the separation expresses the sustaining power. That the experi- 

 ment, however, shall be faultless, it is necessary that all parts of 

 the surface of the cube should give way at the same time, other- 

 wise the mass will hold on by its edges and corners, and thus 

 totally vitiate the experiment. To effect this, it would be neces- 

 sary, first, that the production of the wire should go exactly 

 through the centre of gravity of the cube ; and secondly, that all 

 portions of the surface should be equally in contact, or that any 

 deviation from the one condition should be compensated by a de- 

 viation from the other. The difficulty of complying with these 

 requirements has compelled me to abandon both cubes and cy- 

 linders, and to resort to a body with which the smallness of the 

 sui'face in contact reduces the irregularity hence arising to a 

 minimum ; a body of symmetrical shape, and which is able to 

 accommodate itself to tlie slight divergences of the wire. That 

 body is the sphere. 



2. The magnet used was that formerly applied by my friend 



Professor Knoblauch and myself in an investigation ' On the 



Magneto-optic Properties of Crystals*.^ A.s then used, it consisted 



of two soft iron cylinders set upright in a glass case, and united 



* Phil. Mag., March 1850 and July 1830. 



