CHAP, v.] ELECTRICITY AND MAGNETISM. 265 



CHAPTER V. 



ELECTROMAGNETICS. 



LESSON XXV. -Theory of Magnetic Potential. 



31O. That branch of the science of electricity which 

 treats of the relation between electric currents and mag- 

 netism is termed Electromagnetics. In Art. 1 1 7 the 

 law of inverse squares as applied to magnets was explained, 

 and the definition of " unit magnetic pole " was given in 

 Art. 125. The student also learned to express the strength 

 of poles of magnets in terms of the unit pole, and to apply 

 the law to the measurement of magnetic forces. It is, 

 however, much more convenient, for the purpose of study, 

 to express the interaction of magnetic and electromagnetic 

 systems in terms not of " force " but of "potential"; 

 i.e. in terms of their power to do work. In Art. 237 

 the student was shown how the electric potential due 

 to a quantity of electricity may be evaluated in terms of 

 the work done in bringing up as a test charge a unit of 

 + electricity from an infinite distance. Magnetic 

 potential can be measured similarly by the ideal pro- 

 cess of bringing up a unit magnetic pole (N. -seeking) 

 from an infinite distance, and ascertaining the amount 

 of work done in the operation. Hence a large number 

 of the points proved in Lesson XX. concerning electric 

 potential will also hold true for magnetic potential. The 

 student may compare the following propositions with the 

 corresponding ones in Articles 237 to 243 : 



