SECT. XXXIV. INDUCTION. 337 



the more powerfully the less the distance, but the induced force 

 is always exactly equal to the force which produces it. When 

 the north end of a magnet is brought near to, and in the line 

 with, an unmagnetised iron bar, the bar acquires all the proper- 

 ties of a perfect magnet ; the end next the north pole of the 

 magnet becomes a south pole, while the remote end becomes a 

 north pole. Exactly the reverse takes place when the south end 

 is presented to the bar, so that each pole of a magnet induces the 

 opposite polarity in the adjacent end of the bar, and the same 

 polarity in the remote extremity ; consequently the nearest ex- 

 tremity of the bar is attracted, and the farther repelled ; but as 

 the action is greater on the adjacent than on the distant part, the 

 resulting force is that of attraction. By induction the iron bar 

 not only acquires polarity* but the power of inducing paramag- 

 netism in a third body ; and although all these properties vanish 

 from the iron as soon as the magnet is removed, a lasting increase 

 of intensity is generally imparted to the magnet itself by the re- 

 action of the temporary paramagnetism of the iron; Iron acquires 

 the inductive force more rapidly than steel, yet it loses it as 

 quickly on the removal of the magnet, whereas the steel is im- 

 pressed with a lasting polarity. 



A certain time is requisite for induction, and it may be accele- 

 rated by anything that excites a vibratory motion in the particles 

 of the steel ; such as the smart stroke of a hammer, or heat suc- 

 ceeded by sudden cold. A steel bar may be converted into a 

 magnet by the transmission of an electric discharge through it ; 

 and as its efficacy is the same in whatever direction the elec- 

 tricity passes, the effect arises from its mechanical operation ex- 

 citing a vibration among the particles of the steel. It has been 

 observed that the particles of iron easily resume their neutral 

 state after induction, while those of steel resist the restoration of 

 equilibrium, or a return to the neutral state : it is therefore evi- 

 dent that any cause which removes or diminishes the resistance 

 of the particles will tend to destroy the paramagnetism of the 

 steel ; consequently the same mechanical means which develop 

 the power will also destroy it. On that account a steel bar may 

 lose its paramagnetism by any mechanical concussion, such as by 

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

 to redness, which makes the steel soft. The circumstances which 

 determine whether it shall gain or lose are its position with 



Q 



