1G8 JAMES CLEUK MAXWELL 



of force and pressure at right angles to them. It 

 this last idea, as wo shall see shortly, of which Max- 

 well made use 



" I proi>ose now'* [ho writes (" On Physical Lines of Forro," 

 Phil. Mug., vol. xxi.)] " to examine magnetic phenomena from 

 a mechanical point of view, and to determine what tensions 

 in, or motions of, a medium are capable of producing tho 

 mechanical phenomena observed. If l>y the same hypothesis 

 we can connect tho phenomena of magnetic attraction with 

 electro-magnetic phenomena, and with those of induced cur- 

 rents, we shall have found a theory which, if not true, can 

 only be proved to be erroneous by experiments, which will 

 greatly enlarge our knowledge of thi.s part of physics." 



Lord Kelvin had in 1847 given a mechanical 

 representation of electric, magnetic and galvanic forces 

 by means of the displacements of an elastic solid in a 

 state of strain. The angular displacement at each 

 point of the solid was taken as proportional to tho 

 magnetic force, and from this the relation between 

 the various other electric quantities and the motion 

 of the solid was developed. But Lord Kelvin did not 

 attempt to explain tho origin of tho observed forces 

 by tho efVects due to these strains, but merely made 

 use of the mathematical analogy to assist the imagi- 

 nation in the study of both. 



Maxwell considered magnetic action as existing 

 in the form of pressure or tension, or more goner- 

 ally, of some stress in some medium. The existence 

 of a medium capable of exerting force on material 

 bodies and of withstanding considerable stress, both 

 pressure and tension, is thus a fundamental hypothesis 

 with him; this medium is to be capable of motion, 



