168 Prof. Tyndall on the Nature of the Force by which 



of a magnetic body continues to be attracted after it has assumed 

 the equatorial position, while the total mass of a diamagnetic 

 body continues to be repelled after it has taken up the axial 

 one. 



III. On the Uistriuution of the Magnetic Porce 



BETWEEN TWO FlAT VoLES. 



In experiments where a uniform distribution of the magnetic 

 force is desirable, flat poles, or magnetized surfaces, have been 

 recommended. It has long been known that the force proceeds 

 with great energy from the edges of such poles : the increase 

 of force from the centre to the edge has been made the subject 

 of a special investigation by M. von Koike "i". The central portion 

 of the magnetic field, or space between two such magnetized 

 surfaces, has hitherto been regarded as almost perfectly uniform, 

 and indeed for all ordinary experiments the uniformity is suffi- 

 cient. But, when we examine the field carefully, we find that 

 the uniformity is not perfect. Substituting, for the sake of 

 convenience, the edge of a pole for a point, I studied the phse- 

 nomena of rotation described in the last section, in a great 

 number of instances, by comparing the deportment of an elon- 

 gated body, suspended in the centre of the space between two 

 fiat poles, with its deportment when suspended between the top 

 or the bottom edges. Having found that the fibre of wood, in 

 masses where form had no influence, always set equatorial, I 

 proposed to set this tendency to contend with an elongation of 

 the mass in a direction at right angles to the fibre. For this 

 purpose thirty-oue little wooden bars were carefully prepared 

 and examined, the length of each bar being about twice its width, 

 and the fibre coinciding with the latter dimension. The bars 

 were suspended from an extremely fine fibre of cocoon silk, and 

 in the centre of the magnetic field each one of them set its 

 length axial and consequently its fibre equatorial. Between the 

 top and bottom edges, on the contrary, each piece set its longest 

 dimension equatorial, and, consequently, the fibre axial. 



For some time I referred the axial setting of the mass, in the 

 centre of the field, to the directive action of the fibre, though, 

 knowing the extreme feebleness of this directive action, I was 

 surprised to find it able to accomplish what the experiments 

 exhibited. The thovight suggested itself, however, of suspending 

 the bars with the fibre vertical, in which position the latter could 

 have no directive influence. Here also, to my surprise, the 

 directive action, though slightly weakened, was the same as 

 before : in the centre of the field the bars took up the axial 

 position. Bars of sulphur, wax, salt of hartshorn, and other 

 * Poggendorflf's Annalen, vol, Ixxxi. p. 321. 



