330 H. A. Rowland—Absolute Unit of Electrical Resistance. 
Substituting the limits for z, 7 and a, we find 
R+V7X?2+R?2 
r+V7X2472 one r+ a2 + 7? 
H,=xX log, 
R? r 1 R* - 
=—1} Me a cake | 
1 Rs 
H,=— =a z (20X*+4-7X?R?+ 2R*) — 
X!(X'-R)}? 
— (20X'4 7X4 ant) —_B’_ (n+ 1a? R*+ 2B") 
X*(X? +r’)? : se* (a? +R?) 
r 
4 yd or 
The needle consisted of two parallel lamina of steel of length, 
l, and a distance, W, from each other. As the correction for 
length is small, we may assume that the magnetism of each 
lamina is concentrated in two points at a distance n / from each 
other, where n is a quantity to be determined. 
ence 
where cos # Ta tana wat seeing that the needle hangs par- 
nt)” A 
allel to the coils. In short thick magnets, the polar distance 
is about 3 J and the value of n will be about 4 For all other 
magnets it will be between this and unity. In the present case 
n= nearly. ‘ 
As all the terms after the first are very minute, this approx!- 
mation is sufficient, and will at least give us an idea of the 
amount of this source of error. 
Induction Coils. 
The induction coils were in the shape of two parallel coils of 
nearly equal size and of nearly square section. 
Let A and a be the mean radii of the coils. Let 5 be the 
Was distance apart of the coils. 
t 
2n/ Aa 
young 
V(A+a)?403° 
Supposing the coils concentrated at their center of section we 
know that 
M,=42V Ka} (S—e) F(e)—2K(6) } 
where F(c) and E(c) are elliptic integrals. 
