ROTATION OF LIGHT IN BISULPHIDE OF CARBON. 
351 
Thus if n be the whole number of windings on the helix, the difference of potential 
from L to M corresponding to the unit current is 
Ann—Ann 
A« 2 3 —A+ 
( 1 
L 1 \ 
12 ewq 
+A.LB 
r MA.MB/ 
A« 2 5 —A«y 
/LB 3 + LA.LB + LA 2 
MA 2 + MA.MB + MB 2 ' 
80 a 2 rq 
{ LA 3 . LB 3 
1 MB 3 .MA 3 
In the present case 
Aa 2 =2‘065 (inches), Aa 1 =l , 094, 
from which we get 
Aaf-Aa* Aap-Aap 
12 ci^ ’ 80 a % a x 
« 2 a 1 ='971, 
•4632. 
In the remainder of the calculation we have to distinguish the two tubes. For the 
first 
LA=MB=10‘800 inches 
LB=MA=20‘790 inches; 
and for the second 
LA=MB= 9’887 inches 
LB=MA= 19*877 inches. 
Hence for the first tube we have 
Amr(l- -00573 + ‘00006)= 4wrX ‘99433 ;* 
and for the second 
4mr(l-‘00655 +*00008)=4mrX *99353, 
the correction for finite length thus somewhat exceeding one-half per cent. 
16 . We have now obtained the difference of potential at the ends of the column of 
CS 2 due to the passage through the helix of unit current. It yet remains to describe 
the means adopted for the measurement of the actual current in absolute measure. 
In a former paper “ On the Electro-chemical Equivalent of Silver, and on the Absolute 
Electromotive Force of Clark Cells,” f it was shown how the E. M.F. of a Clark cell 
was obtained by comparison with the difference of potentials at the extremities of a wire 
of known resistance, due to the passage of a current known either directly from its 
effect upon a current measuring apparatus, or indirectly through the deposition of 
silver. For the purposes of the present investigation this process was reversed, the 
* In the Preliminary Note the reducing factor for this tube was given as '99449. The alteration is 
due to the use of more precise data in place of some quite rough measurements in round numbers on 
which, by an oversight, the first calculation was founded, 
t Phil. Trans., 1884, Part II., §§ 35, 36, 38. 
2 z 2 
