390 Mr Bevan, The Influence on Light, etc. 



and this, when P, <x and X are in electromagnetic units, 



, \PsinS- 



a 



C 

 Now X = — where G is the Hall coefficient, so the change of 



phase is finally, where P is in volts and a- in ohms, 



PC 



= 4sinS-^.10- 10 . 

 cr 



P 



Now — is the current through unit area in amperes, so if 



G is the current through unit breadth of the metal strip 



G = P 



t a ' 

 The change of phase is therefore 



sin^ k I I GC,.. w f . ., 



—. -, — To A / ~i9 nrr — ■ 10 -10 of a wave-length. 



And this effect is again too small to be measurable, since the 

 plate to transmit any light must be very thin, and so the current 

 G cannot be made large. 



To conclude, we have shown that the effect of the current 

 in the metal is to cause an alteration in the constants of the metal 

 but this alteration is too small for the effect to be measurable 

 (taking 10~ 2 of a wave-length as the minimum change of phase 

 which can be detected). The alteration also of the plane of 

 polarisation, or rather the major axis of the ellipse of polarisation, 

 of reflected light is similarly too small to be detected, so that as 

 far as any measurable effects are concerned, the metal's optical 

 properties would not be altered by the current. 



The cases considered have been worked out with certain 

 approximations which will not be valid for some metals, silver, 

 sodium, etc., but it is easy to see that in these cases the changes 

 will still be of the same order of magnitude as in the cases 

 considered. In all cases the thickness of metal through which 

 light can be passed being only a few wave-lengths, there is no 

 possibility of a small difference of velocity producing a large 

 enough change of phase to admit of measurement. 



