FORCE OF THE WESTON NORMAL CELL IN SEMI-ABSOLUTE VOLTS. 
159 
The large and the small coils were arranged in parallel, currents were passed through 
in opposite directions and adjusted until the balance point for equal magnetic forces 
at the centre could be determined by interpolation from the observations on the 
deflections of the small magnetic system. Readings were taken with the small coils 
in the four possible positions (viz., at either side of the dynamometer and then 
reversed in each case) for each of the four positions of the fixed coils (as enumerated 
in the account of the deflection measurements), making sixteen positions in all. In 
this way the effects of small asymmetries in the coils were eliminated and the mean 
equivalent value of the constants could be calculated. It is pointed out later that 
four positions are sufficient to determine the mean to a high degree of approximation. 
Further data concerning the arrangements, the observations, and the avoidance of 
temperature variations in the coils under the necessary experimental conditions, are 
given below. It is first necessary to determine the expression for calculating the 
constants of the small coils from such measurements. 
(b) The Theory of the Method. 
Fig. 2 represents the two sets of coils arranged co-axially with their centres at a 
distance x apart, a , d, a and S have the same significance as before, and at o the 
u y 
i CL 
U U 
ex : 
Qi* d 
rfi - M 
v- 
n n' 
FIG. 2 
magnetic system of strength m is represented as being deflected through an angle 0 
which should of course be in a plane at right angles to the paper. When no current 
is flowing, and also when a perfect balance is obtained, 6 = 
A 
