VERDET’S CONSTANT IN ABSOLUTE UNITS. 
15 
If LM were infinite, it would be 4:mC, where n is the number of windings. 
When the length of the tube is finite, and equal to LM, 
we must subtract the following correction'*: — 
r^|^- 2 |La-LA-(L5-LB) + M5-MB-(M«-MA)}. . . . 
When LA is great compared with Aa , 
T TAT Aa 2 
La LA — % 
A Aa 4 & 
8 LA 3 + (5 " c ' 
( 21 ) 
( 22 ) 
or, retaining only the first term, we get 
C2(A) /I 11 1 \_Ct{A)f AB AB \ 
AB • \LA LB + MB MaJ~ AB \LA.LB + MA.MBj 
= C ^(A) (l A . LB + MA.Mb) ' * * ^ 23 ) 
The corrected value is then 
(V 1 -V M )={4™-(S(A))( Q i E A +HS i ra )}c . i . . . 
Thus the complete formula for a is 
a 1 , 1 \ 1 Hr 3 tan 8* 
47 ™ \LA . LB + MA.MB ) } 5) (A) 
(24) 
(25) 
The following quantities were taken by measurement : — 
X. 
Helix 
M 
A 
Trzbe 
B 
AB=26 - 34 centims. 
LA=18-95 „ 
BM=23-72 „ 
Distance from O to centre of helix =141-64 centims. ; whence 
OA = OB =r= {l41-64 2 +13'17 2 } \ 
r 3 =2878499, and log r 3 =6-4591660. 
* This formula is due to Professor Maxwell. 
