198 JAMES CLEHK MAXWELL 



the" same rate; and Maxwell showed that tho direc- 

 tion of this magnetic force also lies in the wave front, 

 and is always at right angles to the electric displace- 

 ment. In the ordinary theory of light the wave of 

 linear displacement is accompanied hy a wave of 

 periodic angular twist ahout a direction lying in 

 the wave front and perpendicular to the linear dis- 

 placement. 



In many respects, then, waves of electric dis- 

 placement resemble waves of light, and, indeed, as we 

 proceed we shall tind closer connections still. Hence 

 comes Maxwell's electro-magnetic theory of light. 



It is only in dielectric media that electric force is 

 propagated by wave motion. In conductors, although 

 the third and fourth of Maxwell's principles given on 

 page 1S5 still are true, the relation between the electric 

 force and the electric current di tiers from that which 

 holds in a dielectric. Hence tho equations satisfied 

 by the force are ditVerent. The laws of its propagation 

 resemble those of the conduction of heat rather than 

 those of the transmission of light. 



Again, light travels with ditVercnt velocities in 

 different transparent media. Tho velocity of_eloctric 

 waves, as has been stated, is equal to l/ x //ilv; but 

 in making this statement it is assumed that the 

 simple laws which hold whore there is no gross 

 matter or, rather, where air is the only dielectric 

 with which we are concerned hold also in solid or 

 liquid dielectrics. In a solid or a liquid, as in vacuo, 

 the waves are propagated by the other. \Vo assume, 

 as a first step towards a complete theory, that so far 

 as the electric waves are concerned' the sole c fleet 



