in Crystalline and Non-crystalline Substances. 179 



Definition. — The total magnetic force at any point is the force 

 which the north pole of a unit bar-magnet would experience from 

 all magnets which exert any sensible action on it, if it produced 

 no inductive action on any magnet or other body. Or, 



The total magnetic force at any point is the quotient obtained 

 by dividing the force experienced by either pole, placed at that 

 point, of an infinitely thin bar, uniformly and longitudinally 

 magnetized to a finite degree of intensity, by the infinitely small 

 numerical measure of the magnetic strength of the bar ; and its 

 direction is that of the force experienced by the north pole of 

 the bar. 



Definition. — Any space at every point of which there is a finite 

 magnetic force is called "a field of magnetic force;" or, magnetic 

 being understood, simply "a field of force;" or, sometimes, "a 

 magnetic field." 



Definition.— K " line of force " is a line di-awn through a mag- 

 netic field in the direction of the force at each point through 

 which it passes ; or a line touched at each point of itself by the 

 direction of the magnetic force. 



Definition. — A ""uniform field of magnetic force " is a space 

 throughout which the lines of force are parallel straight hues, 

 and the intensity of the force is vmiform. 



Definition. — A substance magnetized so that the intensity and 

 direction of magnetization at each point are represented by the 

 diagonal of a parallelogram, of which the sides represent the 

 intensities and directions at the same point in two other distri- 

 butions, is said to possess a distribution of magnetism which is 

 the resultant of these two superimposed, one on the other. 



It is demonstrated by Poisson, that the force at any point due 

 to a resultant distribution of magnetism is the resultant of the 

 forces that would be produced at the same point if the compo- 

 nent distributions existed separately. 



Aocioms of Magnetic Force. 



I. All mechanical action which a magnet experiences in virtue 

 of its magnetism is due to other magnets*. 



II. The action between any two magnets is mutual. 



III. The whole action experienced by any magnet is the me- 

 chanical resultant of the actions which it would experience from 

 all the magnets in its neighbourhood, if each acted on it as if the 

 others were removed, the distributions of magnetism in the two 

 remaining unaltered. 



* This principle appears, from liis discovery that the phsenomena of ter- 

 restrial ma{,'iieti«rn an- productil by the earth acting as a great maguet, to 

 have beeu tir»t rccoguised by Gilbert. 



N2 



