MAGNETO-OPTIC PHENOMENA OF IRON, NICKEL, AND COBALT. 
125 
so that for iron 
Also 
(1 — x)! {I x) = (-7500) e ‘<'-^’ 21 ') 
= (-6887) — I (-2973). 
— 2tM/i = — 2t (cos a + t sin a) 'Rmh 
= (sin a. — L cos a) Wl (47r/X) 
= - 477 {(3-21) + t(2-36)} h/X. 
In Drude’s experiments the values ol h/X lie between '065 and ‘332. 
In Lobach’s experiments the values of h/X lie between ‘042 and ‘167. 
Hence, in the two sets of experiments, the greatest value of the modulus of is 
about -1838, and therefore the greatest value of the modulus of is about 
‘1035 ; this corresponds to the thinnest film, for the thicker films the modulus is very 
much smaller. 
Hence if we neglect the fourth term in the numerator and the second term in the 
denominator, and put 
F/E = 
{(1 -a‘)/(l + x)} 
1 — e 
-201/i ) 
S 
+ t. 
we have an approximate formula whose error, lor the very thinnest film considered, 
will not exceed about 10 per cent., and is very much smaller for the large majority ol 
the experiments. 
Putting in numerical values, this becomes (for iron, and sodium light), 
F/E = 
-2 - 2t< 
cXCoyoe‘'''Pv -e 
(3-21) 
In the experiments the film is generally magnetised as strongly us possible, but 
there is no direct wav of ascertainina’ the intensity of magnetisation attained. Thus 
Jq is to a certain extent indeterminate. According to Ewing the maximum intensity 
of magnetisation for some specimens ol iron is about 1730 C.G.S. units. I shall 
therefore assume yo = 1730 ; also I take Cq as determined by log^^g ( ^ 0 ) = 11 8623. 
In one of Lobach’s experiments the thickness ol the film is given by 
/«,= 82 X 10 "1 centim. 
The light used is sodium light, so that X = 5896 X 10”® centim.; and the rotation 
{0) in the direction of the magnetising current is observed to be r62 degrees. 
