504 OX PHYSICAL LINES OF FORCE. 



Electric currents are known to produce effects of transference in the direc- 

 tion of the current. They transfer the electrical state from one body to another, 

 and .they transfer the elements of electrolytes in opposite directions, but they 

 do not* cause the plane of polarization of light to rotate when the light tra- 

 verses the axis of the current. 



On the other hand, the magnetic state is not characterized by any strictly 

 longitudinal phenomenon. The north and south poles differ only in their names, 

 and these names might be exchanged without altering the statement of any 

 magnetic phenomenon ; whereas the positive and negative poles of a battery are 

 completely distinguished by the different elements of water which are evolved 

 there. The magnetic state, however, is characterized by a well-marked rotatory 

 phenomenon discovered by Faraday f the rotation of the plane of polarized light 

 when transmitted along the lines of magnetic force. 



When a transparent diamagnetic substance has a ray of plane-polarized light 

 passed through it, and if lines of magnetic force are then produced in the 

 substance by the action of a magnet or of an electric current, the plane of 

 polarization of the transmitted light is found to be changed, and to be turned 

 through an angle depending on the intensity of the magnetizing force within 

 the substance. 



The direction of this rotation in diamagnetic substances is the same as that 

 in which positive electricity must circulate round the substance in order to 

 produce the actual magnetizing force within it ; or if we suppose the horizontal 

 part of terrestrial magnetism to be the magnetizing force acting on the sub- 

 stance, the plane of polarization would be turned in the direction of the earth's 

 true rotation, that is, from west upwards to east. 



In paramagnetic substances, M. VerdetJ has found that the plane of polari- 

 zation is turned in the opposite direction, that is, in the direction in which 

 negative electricity would flow if the magnetization were effected by a helix 

 surrounding the substance. 



In both cases the absolute direction of the rotation is the same, whether 

 the light passes from north to south or from south to north, a fact which dis- 

 tinguishes this phenomenon from the rotation produced by quartz, turpentine, &c., 



* Faraday, Experimental Researches, 951954, and 22162220. 



t Ibid., Series xix. 



J Camples Rendus, Vol. XLIII. p. 529; Vol. iuv. p. 1209. 



