WITH MAGNETISM AND ELECTRICITY. 169 



paths these fluids can only be set in motion by first overcoming 

 a resistance proportional to their velocity. The generation of 

 pure diamagnetism (unmixed with magnetism) would further 

 necessitate the assumption, that the molecules which contain 

 the above paths or canals are not capable of being rotated ; for 

 were the contrary the case, rotatory molecular cuiTents might 

 be generated, of such a strength that a portion of their intensity 

 during the rotation might be regarded as constant, and hence, 

 according to Ampere, would produce the magnetic state as a 

 consequence. Conformably to this assumption, the diamagnet- 

 ism or electro-diamagnetism of a body can be determined from 

 the magnetizing or electro-magnetizing force exerted upon it. 



Determination of the Diamagnetism or Electro-diamagnetism of 

 a Body from the Magnetizing or Electro-magnetizing Force 

 exerted upon it* 



The magnetizing or electro-magnetizing force expressed by 

 X* exerts upon a circle of the radius r, electromotive forces 

 whose integral value, for the time during which this circle is 

 moved out of a position perpendicular to the direction of the 

 magnetizing force into^ a position parallel with it, according to 



* Every magnetizing force may be compared with terrestrial magnetism, 

 and reduced to the same unit of measure. The electro-magnetizing force of 

 a cylindrical spiral, through which a current passes of the intensity i, in ac- 

 cordance with the fundamental laws of electro-magnetism, is expressed by 



- . -, where w signifies the number of coils, r the radius, and a the length 



^ aa-\-rr 



of the axis. This value is true, in the first place, for the magnetizing force in 

 the middle of the cylinder, and approximates to that due to every other point 

 of the interior space of the cylinder, excepting those which lie near its end, the 

 more closely as tl)e spiral increases in length and diminishes in radius. When, 

 therefore, a bar of bismuth is situated in the centre of such a spiral, nearly equal 

 electro-magnetizing forces are exerted upon all its particles ; and hence it can 

 be moved to or fro between certain limits within the spiral without any per- 

 ceptible change of these forces. Hence such a spiral is particularly suited to 

 experiments in which it is required that the diamagnetism shall remain un- 

 changed. The above expression gives the electro-magnetizing force referred 

 to the same unit as that of magnetizing forces (namely, to the absolute unit used 

 in the determination of terrestrial magnetism), where i denotes the strength of 

 the bar-magnetism, the action of which is equal to tlie action of the current cir- 

 culating round a unit of surface. 



