A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



579 



If we differentiate the third of these equations with respect to y, and 

 the second with respect to z, and subtract, J and disappear, and by remem- 

 bering the equations (B) of magnetic force, the results may be written 



* 



d* 



(69). 



(95) If we assume that a, /3, y are functions of Ix-j-my + nz- Vt = w, the 

 first equation becomes 



d'a 



or 



, a a ,,,MO. 



"Ptt^^V-V^ 



1 



(70), 



F= 



(71). 



The other equations give the same value for V, so that the wave is propa- 

 gated in either direction with a velocity V. 



This wave consists entirely of magnetic disturbances, the direction of mag- 

 netization being in the plane of the wave. No magnetic disturbance whose 

 direction of magnetization is not in the plane of the wave can be propagated 

 as a plane wave at all. 



Hence magnetic disturbances propagated through the electromagnetic field 

 agree with light in this, that the disturbance at any point is transverse to 

 the direction of propagation, and such waves may have all the properties of 

 polarized light. 



(96) The only medium in which experiments have been made to determine 

 the value of k is air, in which /*=!, and therefore, by equation (46), 



V=v (72). 



By the electromagnetic experiments of MM. Weber and Kohlrausch *, 

 v = 310,740,000 metres per second 



* Leipzig Transactions, Vol. v. (1857), p. 260, or PoggendoriFs Annalen, Aug. 1856, p. 10. 



732 



