128 CORRELATION OF PHYSICAL FORCES. 



another direction, it will not alter their position, but will itself 

 be definitely deflected. Should the conditions of force and 

 stability be intermediate, both the breeze and the vanes will be 

 slightly deflected ; or, if there be no breeze, and the spindles 

 be all moved in any direction, preserving their parallel rela- 

 tion, they will themselves create a breeze. Thus it is with 

 the molecules of hard iron or steel in permanent magnets : 

 they are polarised with greater difficulty than those of soft 

 iron, but, when so polarised, they cannot be affected by a 

 feeble current of electricity. Again, if the magnets be moved, 

 they themselves originate a current of electricity ; and, lastly, 

 the magnetic polarity and the electric current may be both 

 mutually affected, if the degrees of mobility and stability be 

 intermediate 



The above instance will, of course, be taken only as an 

 approximation, and not as binding me to any closer analogy 

 than is generally expected of a mechanical illustration. It is 

 difficult to convey by words a definite idea of the dual or 

 antithetic character of force involved in the term polarity. 

 The illustration I have employed may, I hope, somewhat aid 

 in elucidating the manner in which magnetism acts on the 

 other dynamic forces ; i.e., definitely directing them, but not 

 initiating them, except while in motion. 



Magnets being moved in the direction of lines, joining 

 their poles, produce electrical currents in such neighbouring 

 bodies as are conductors of electricity, in directions transverse 

 to the line of motion ; and if the direction of motion or the 

 position of the magnetic poles be reversed, the current of 

 electricity flows in a reverse direction. So if the magnet be 

 stationary, conducting bodies moved across any of the lines of 

 magnetic force (i.e. lines in the direction of which the mutual 

 action of the poles of the magnet would place minute portions 

 of iron) have currents of electricity developed in them, the 

 direction of which is dependent upon that of the motion of 

 the substance with reference to the magnetic poles. Thus, as 

 bodies affected by an electrical current are definitely moved 

 by a magnet in proximity to them, so conversely bodies 

 moved near a magnet have an electrical current developed in 

 them. Magnetism can, then, through the medium of electri- 



