350 LAW OF MAGNETIC INTENSITY. [SECT. xxx. 



sion of an electric discharge through it : and, as its efficacy is 

 the same in whatever direction the electricity passes, the 

 magnetism arises from its mechanical operation exciting a 

 vibration among the particles of the steel. It has been 

 observed that the particles of iron easily resume their 

 neutral state after induction, but that those of steel resist the 

 restoration of magnetic equilibrium, or a return to the neutral 

 state : it is therefore evident, that any cause which removes 

 or diminishes the resistance of the particles will tend to 

 destroy the magnetism of the steel ; consequently, the 

 same mechanical means which develope magnetism will 

 also destroy it. On that account a steel bar may lose its 

 magnetism by any mechanical concussion, such as by falling 

 on a hard substance, a blow with a hammer, and heating 

 to redness, which reduces the steel to a state of softness. 

 The circumstances which determine whether it shall gain 

 or lose, are its position with respect to the magnetic equator, 

 and the higher or lower intensity of its previous magnetic 

 state. 



Polarity of one kind only cannot exist in any portion of 

 iron or steel ; in whatever manner the intensities of the two 

 kinds of polarity may be diffused through a magnet, they 

 exactly balance or compensate one another. The northern 

 polarity is confined to one half of a magnet, and the southern 

 to the other, and they are generally concentrated in or near 

 the extremities of the bar. When a magnet is broken across 

 its middle, each fragment is at once converted into a perfect 

 magnet ; the part which originally had a north pole acquires 

 a south pole at the fractured end : the part that originally 

 had a south pole gets a north pole ; and, as far as mechanical 

 division can be carried, it is found that each fragment, how- 

 ever small, is a perfect magnet. 



A comparison of the number of vibrations accomplished 

 by the same needle, during the same time, at different dis- 

 tances from a magnet, gives the law of magnetic intensity, 

 which follows the inverse ratio of the squares of the dis- 



