258 Prof. Tyndall on the Nature of the Force by ivhich 



the body to be examined was suspended by a fibre which descended 

 through a sUt in the helix. The latter was placed between the 

 two flat poles of an electro-iuagnet, and could thus be caused to 

 act upon the bar within it, either alone or in combination with 

 the magnet. The disposition will be at once understood from 

 fig. 6, which gives a front view of the arrangement. 



Action of Magnet alone : Division of bars into Normal and 

 Abnormal. — A bar of soft iron suspended in the magnetic field 

 will set its longest dimension from pole to pole : this is the nor- 

 mal deportment of paramagnetic bodies. A bar of bismuth, 

 whose planes of principal cleavage are throughout parallel to its 

 length, suspended in the magnetic field with the said planes 

 vertical, will set its longest dimension at right angles to the line 

 joining the poles : this is the normal deportment of diamagnetic 

 bodies. We will therefore, for the sake of distinction, call the 

 former a normal paramagnetic bar, and the latter a normal diamag- 

 netic bar. 



A bar of compressed carbonate of iron dust, whose shortest 

 dimension coincides with the line of pressure, will, when sus- 

 pended in the magnetic field with the said line horizontal, set 

 its length equatorial. A bar of compressed bismuth dust, simi- 

 larly suspended, or a bar of bismuth whose principal planes of 

 crystallization are transverse to its length, wall set its length 

 axial in the magnetic field. We will call the former of these 

 an abnormal paramagnetic bar, and the latter an abnormal dia- 

 magnetic bar. 



Action of Current alone on normal and abnormal bars. — A nor- 

 mal paramagnetic bar was suspended in the helix above described; 

 when a current was sent tlu-ough the lattei-, the bar set its longest 

 horizontal dimension parallel to the axis of the helix, and con- 

 sequently perpendicular to the coils. 



An abnormal paramagnetic bar was suspended in the same 

 manner ; when a current was sent through the helix, the bar set 

 its longest dimension perpendicular to the axis of the helix, and 

 consequently parallel to the coils. 



A normal diamagnetic bar was delicately suspended in the same 

 helix ; on the passage of the current it acted precisely as the 

 abnormal magnetic bar ; setting its longest dimension perpen- 

 dicular to the axis of the helix and parallel to the coils* When 

 a fine fibre and sufficient power are made use of, this deportment 

 is obtained without difficulty. 



An abnormal diamagnetic bar was suspended as above ; on the 

 passage of the current it acted precisely as the normal magnetic 

 bar : it set its length parallel to the axis of the helix and perpen- 

 dicular to the coils. Here also, by fine manipulation, the result 

 is obtained with ease and certaintv. 



