460 



ON PHYSICAL LINES OF FORCE. 





n 



* A K 



We shall always mark by an arrow-head the direction in which we must 

 look in order to see the vortices rotating in the 

 direction of the hands of a watch. The arrow-head 

 will then indicate the northward direction in the 

 magnetic field, that is, the direction in which that 

 end of a magnet which points to the north would 

 set itself in the field. 



Now let A be the end of a magnet which 

 points north. Since it repels the north ends of 

 other magnets, the lines of force will be directed 

 from A outwards in all directions. On the north 

 side the line AD will be in the same direction with 

 the lines of the magnetic field, and the velocity of 

 the vortices will be increased. On the south side 

 the line AC will be in the opposite direction, and 

 the velocity of the vortices will be diminished, so 

 that the lines of force are more powerful on the 

 north side of A than on the south side. 



We have seen that the mechanical effect of the 

 vortices is to produce a tension along their axes, 

 so that the resultant effect on A will be to pull 



it more powerfully towards D than towards C '; that is, A will tend to move 

 to the north. 



Let B in fig. 2 represent a south pole. The lines of force belonging to B 

 will tend towards B, and we shall find that the lines of force are rendered 

 stronger towards E than towards F, so that the effect in this case is to urge B 

 towards the south. 



It appears therefore that, on the hypothesis of molecular vortices, our first 

 term gives a mechanical explanation of the force acting on a north or south 

 pole in the magnetic field. 



We now proceed to examine the second term, 



Here a' + ^ + y* is the square of the intensity at any part of the field, and 

 is the magnetic inductive capacity at the same place. Any body therefore 



