MAGNETO-OPTIG PHENOMENA OP IRON, NICKEL, AND COBALT. 
iI7 
Here the agreement is very good, the indicated value of L' being about ; if 
this be so, we have for cobalt 
log (- C^) = Tb'0889, 
i.e., 
— Co= 1-227 X 10- 
The. Hall Effect. 
28. Before leaving this part of the subject it is worth while to investigate whether 
the ordinary Hall etfect is large enough to contribute, to any appreciable extent, to 
the phenomena we have been considering. 
If ^ be Hall’s constant, as usually defined, the equations into which it enters are 
of the type 
P = P' + ^ - yffv), 
where {olq, /Sq', Jq) is the magnetic force. Comparing this with the form that 
equations ( 11 ) would assume if ( 6 ^, 63 , 63 ) were zero, it appears that 
so that 
Now for iron. 
= HV, = - 
< 2 ^ o 
4c"a,' 
ItV 
r 
T>4^4ia 
9 ^ — — ^ -452^2 ^0 • 
g = 7850 X lO-^h 
and if we substitute the values of 11, a, X, corresponding to yellow light, we find 
i/ 2 = 
where log^o Q = 16-4466. 
But we see from § 24 that 773 = /3g. Coe‘‘h where log^o (— Cq) = IT-8623. Also 
= p./ 3 o', where g is the magnetic permeability of iron and is greater than unity. 
Hence the modulus of the fraction ^ logarithm less than 6-5843, so that 
the modulus itself is less than Thus it appears that the ordinary Hall 
effect is more than two hundred thousand times too small to account for the Kerr 
phenomena. 
But, in order that the coefffcients b^, 63 ) should he real, it is necessary that the 
imaginary parts of the complexes ( 17 ^, 773 , 773 ) should be supplied by {g^, ^ 3 , g^), that is 
to say by the Hall effect. Hence it must be concluded that the coefficient of the 
Hall effect is very mucli greater for excessively rapidly alternating currents than 
