transmitted through a Metal of a Current in the Metal. 387 



units, v being 3.10 10 , so a = Q ; X is in these units the same 



as in the electromagnetic system. If P'is in volts P' = 3,1 2 P, 

 P being in electrostatic units ; if cr" is in ohms cr' = 10V", so that 

 the change of phase is, when the quantities are in the ordinary 

 practical units, dropping the dashes, 



3.10 9 ^?PsinScos^. 

 The Hall coefficient C is related to X by the equation 



cr 



The change of phase is therefore 



3 AGr . 

 -r— „ — — P sm S- cos S- ; 

 47r 2 cr 2 



P being in volts, a in ohms, and C in electromagnetic units as 

 given by Hall. 



P 



Now — is the current per unit area of the metal in amperes. 



If we have a metallic mirror 1 centimetre broad and t centi- 

 metres thick with a current G flowing through it, then 



P = G 



cr t ' 

 G being in amperes. 



The change of phase is therefore 



-. — - — — G sin ^ cos S\ 



4tt 2 ta- 

 li we take the case of Bismuth which has a large Hall 

 coefficient, viz. 8-58. 10~ 6 , and for which a = 1-4.10 -4 , we get the 

 change of phase 



^f ^GlO- 2 sin^cosS- 3 



IT 2 t 



which as A is less than 1, and r is 2.10 -15 for yellow light, is 

 a quantity too small for any measurement since G cannot be 

 made very large for a thin film of metal. 



For light polarised perpendicularly to the plane of incidence, 

 this plane containing the direction of the current, we have the 

 ratio of the reflected wave's amplitude to that of the incident 

 wave 



an — v 



an + v' 

 VOL. XI. pt. v. 28 



