GENERAL ACCOUNT OF MAGNETIC ACTIONS 167 



ments go to show that it is impossible to obtain one kind of pole 

 alone on a magnet. 



The two poles are equal in strength that is, they 

 exert at equal distances equal forces respectively of attraction and 

 repulsion. This may be proved by bringing up a magnet to a 

 compass needle, the magnet being held vertically with its centre 

 on a level with the needle, Fig. 109, and its poles N and S equidistant 

 from the pole n. The two forces then have a vertical resultant, 

 as will be seen from Fig. 110, and the compass needle is not 

 deflected in the horizontal plane, but only tends to dip. 



FIG. 109. 



FIG. 110. 



Another proof of this equality is given by the fact that a bar 

 of steel weighs the same before and after magnetisation. In the 

 latter case it is acted on by the earth's magnetisation, but with 

 forces at the two ends equal and opposite, and therefore at most 

 only forming a directive couple. The forces being equal, we 

 conclude that the poles are equal. 



The magnetic action takes place equally well through 

 most media within the limits of our powers of observation. If, for 

 instance, a magnetised needle is suspended so as to set North and 

 South and a magnet is brought near it as in Fig. Ill, the needle is 



LJ 



FIG. 111. 



deflected with the North-seeking pole towards the East. The deflec- 

 tion is still the same if a screen of wood or cardboard or brass be 



