278 ELEMENTARY LESSONS ON [CHAP. v. 



ever be. Their potential energy will have run down to 

 its lowest minimum, their mutual potential being a neg- 

 ative maximum, and their coefficient of mutual potential 

 M, having its greatest possible value. Two circiiits, 

 then, are urged so that their coefficient of mutual potential 

 M shall have the greatest possible value. This justifies 

 Maxwell's Rule (Art. 193), because M represents the 

 number of lines of force mutually intercepted by both 

 circuits. And since in this position each circuit induces 

 as many lines of magnetic force as possible through the 

 other, the coefficient of mutual potential M is also called 

 the coefficient of mutual induction. 



NOTE ON MAGNETIC AND ELECTRO- 

 MAGNETIC UNITS. 



321. Magnetic Units. All magnetic quantities, strength of 

 poles, intensity of magnetisation, etc. , are expressed in terms of 

 special units derived from the fundamental units of length, mass, 

 and time, explained in the Note on Fundamental and Derived 

 Units (Art. 254). Most of the following units have been directly 

 explained in the preceding Lesson, or in Lesson XL; the others 

 follow from them. 



Unit Strength of Magnetic Pole. The unit magnetic pole is 

 one of such a strength, that when placed at a distance of 

 one centimetre from a similar pole of equal strength, 

 repels it with a force of one dyne (Art. 125). 

 Magnetic Potential. Magnetic potential being measured by 

 work done in moving a unit magnetic pole against the 

 magnetic forces, the unit of magnetic potential will be 

 measured by the unit of work, the erg. 



Unit Difference of Magnetic Potential.- Unit difference of 

 magnetic potential exists between two points when it 

 requires the expenditure of one erg of work to bring a 

 (N. -seeking) unit magnetic pole from one point to the 

 other against the magnetic forces. 

 Intensity of Magnetic Field \$> measured by ihejbrce it exerts 



upon a unit magnetic pole : hence, 



Unit Intensity of Field is that intensity of field which acts 

 on a unit (N. -seeking) pole with a force of one dyne. 



