442 Prof. Maxwell and Mr. F. Jenkin on the Elementary 

 The dimensions of the unit of magnetization are therefore 



M^ 



— =— } the same as in the case of intensity of field. 

 I>T 



9. Coefficient of Magnetic Induction. — When certain bodies, 

 such as soft iron, &c, are placed in the magnetic field, they 

 become magnetized by "induction"; so that the intensity of 

 magnetization is (except when great) nearly proportional to the 

 intensity of the field. 



In diamagnetic bodies, such as bismuth, the direction of 

 magnetization is opposite to that of the field. In paramagnetic 

 bodies, such as iron, nickel, &c, the direction of magnetization 

 is the same as that of the field. 



The coefficient of magnetic induction is the ratio of the inten- 

 sity of magnetization to the intensity of the field, and is there- 

 fore a numerical quantity, positive for paramagnetic bodies, 

 negative for diamagnetic bodies. 



10. Magnetic Potentials and Equipotential Surfaces. — If we 

 take a very long magnet, and, keeping one pole well out of the 

 way, move the other pole from one point to another of the mag- 

 netic field, we shall find that the forces in the field do work on 

 the pole, or that they act as a resistance to its motion, according 

 as the motion is with or contrary to the force acting on the 

 pole. If the pole moves at right angles to the force, no work 

 is done. 



The magnetic potential at any point in a magnetic field is 

 measured by the work done by the magnetic forces on a unit 

 pole during its motion from an infinite distance from the mag- 

 net producing the field to the point in question, supposing the 

 unit pole to exercise no influence on the magnetic field in ques- 

 tion. The idea of potential as a mathematical quantity having 

 different values at different points of space, was brought into 

 form by Laplace*. The name of potential, and the application 

 to a great number of electric and magnetic investigations, were 

 introduced by George Green, in his Essay on Electricity (Not? 

 tingham, 1828). 



An equipotential surface in a magnetic field is a surface so 

 drawn that the potential of all its points shall be equal. By 

 drawing a series of equipotential surfaces corresponding to poten- 

 tials 1, 2, 3 w, we may map out any magnetic field so 



as to indicate its properties. 



The magnetic force at any point is perpendicular to the 

 equipotential surface at that point, and its intensity is the reci- 

 procal of the distance between one surface and the next at that 



* Mecanique Celeste, liv. iii. 



