THE AMPERIAN CURRENTS. 703 



between the first bar and the second, between this and 

 a third, and so on. 



A piece of iron in which the molecular currents flow 

 in parallel and like directions, and which thus acquires 

 the property of attracting other pieces of iron, is called 

 magnetic. Every other piece of iron brought in contact 

 with one which is magnetic becomes also magnetic, but 

 the number of pieces which may be thus attracted and 

 supported, in the manner of fig. 354, very soon reaches 

 a limit. The reason is that although the Amperian 

 currents in soft iron are easily rendered parallel by the 

 influence of a strong current, such as passes through 

 the spiral or a magnetic piece of iron, yet they quite as 

 easily return to their original irregular positions and 

 always tend to do so ; hence it requires a very strong 

 influence to produce perfect parallelism of all molecular 

 currents and to maintain it, that is, to make the iron 

 perfectly magnetic. This strong influence is exerted 

 upon the first bar by the current in the spiral, if it is 

 sufficiently intense ; the ' magnetisation ' of this bar may 

 therefore possibly be complete. But this bar cannot 

 exert the same influence upon the second, because only 

 a very small portion of the second bar is in close 

 vicinity to the first, while the remainder is at some 

 distance from it, and the influence is weakened by 

 distance. For the same reason the parallelism in the 

 third piece is still less complete than that in the second; 

 hence its magnetism is still less perfect, and so on. 



The facility with which in soft iron the Amperian 

 currents return to their original irregular positions ex- 

 plains why soft iron cannot become permanently mag- 

 netic ; it assumes the magnetic state only temporarily 



