96 ELEMENTARY LESSONS ON [CHAP. n. 



(by the law of inverse squares) be proportional to 

 ,' . 2 ; and the attraction will be proportional to ' 



Hence the bit of iron a b will experience a pair of forces, 

 turning it into a certain direction, and also a total force 

 drawing it bodily toward A. Only those bodies are 

 attracted by magnets in which magnetism can thus be 

 induced ; and they are attracted only because of the 

 magnetism induced in them. 



We mentioned, Art. 83, that a magnet needle floating 

 freely on a bit of cork on the surface of a liquid, is acted 

 upon by forces that give it a certain direction, but that, 

 unlike the last case, it does not tend to rush as a whole 

 either to the north or to the south. It experiences a 

 rotation, because the attraction and repulsion of the 

 magnetic poles of the earth act in a certain direction ; 

 but since the magnetic poles of the earth are at a dis- 

 tance enormously great as compared with the length 

 from one pole of the floating magnet to the other, we 

 may say that, for all practical purposes, the poles of the 

 magnet are at the same distance from the N. pole of the 

 earth. The attracting force on the N. -pointing pole of 

 the needle is therefore practically no greater than the 

 repelling force acting on the S.- pointing pole, hence 

 there is no motion of translation given to the floating 

 needle as a whole. 



118. Measurement of Magnetic Forces. The 

 truth of the law of inverse squares can be demonstrated 

 by measuring the attraction between two magnet poles 

 at known distances. But this implies that we have 

 some means of measuring accurately the amount of the 

 magnetic forces of attraction or repulsion. Magnetic 

 force may be measured in any one of the four following 

 ways : ( i ) by balancing it against the torsion of an 

 elastic thread ; (2) by observing the time of swing of 

 a magnetic needle oscillating under the influence of the 

 force ; (3) by observing the deflection it produces upon a 



