Prof. Norton on Molecular Physics. 285 



force of the magnet at any point of the field is the resultant of 

 the special impulsive forces due to all the circular currents of 

 the magnet, and has the same direction as these currents in the 

 upper face of the magnet. (2) This resultant will have the 

 greatest value at the middle of the magnet rn, and decrease in 

 all directions from this point. (3) The curves of equal impul- 

 sive force correspond very closely to Faraday's " lines of force." 

 For it will be seen on a little reflection, that the force will, be- 

 yond a certain distance from the middle, if not from the very 

 middle, decrease most rapidly parallel to the surface of the mag- 

 net ; and that the curves of equal impulsive force, proceeding 

 from points on the ends of the magnet, will recede over the 

 middle of the magnet to a greater distance from it than half its 

 length. This is strikingly true of the curves that originate from 

 near the centre of each end; for at the very centre the force in 

 question vanishes entirely, and therefore the curves for that 

 point would be thrown to an infinite distance from the middle 

 of the magnet. 



In making a comparative estimate of the impulsive force of 

 the magnet in different parts of the field, it should be observed 

 that in receding from the magnet the force that results from any 

 one molecular current is the resultant of the opposing impulses 

 propagated from the oppositely directed currents on the nearer 

 and further sides of the molecule, and that this not only depends 

 upon the distance of the point from the molecule, but also upon 

 the obliquity of the line connecting the two, to the plane of 

 the circular current. For the same distance the resultant will 

 be greatest when the point lies in this plane, and zero when the 

 point is opposite the centre of the current. Bearing this in 

 mind, it may readily be seen that if a line u v be drawn parallel 

 to the axis of the magnet at any distance from it, the impulsive 

 force of the magnet along this line will decrease from the point 

 n, opposite the middle of the magnet, indefinitely in both di- 

 rections ; also that the force will decrease along a line, as if, 

 parallel to the end, and vanish at o in the prolongation of the 

 axis. 



To obtain a general expression for the intensity of the exter- 

 nal impulsive force of the magnet, let -p. ,« 

 abed, fig. 10, be a magnet, and let us 

 regard its effective action upon any mole- * j ^ I* 

 cule aty as the result of the joint action j //!">. J 

 of two sets of opposing currents, the one 

 lying in the upper face a b, and the other 

 in the lower face cd. Let nf= y, nr = z, 

 nr ! = z ! , nb=zu, na = v, ac~d, and m= coefficient of the impulsive 

 force of an individual current. Then for the action of r upon f we 



