THE PEESSURE UPON THE POLES OF THE ELECTRIC APtC. 
I 2.‘5 
The Separation of the Pressure from the Electromagnetic Effects. 
The evidence contained in tlie preceding sections proves beyond dispute the 
existence of a pressure upon the poles of tlie arc, and sliows tlie general nature of its 
variation with the current strength. The experiment now to he descrilied provides 
confirmatory evidence that the electromagnetic effects of the earth’s field and of the 
rest of the circuit have been eliminated, and evaluates these effects. 
The current was led to the carbon through a mercury cup as in fig. 1, and the 
carbon fixed axially at the end of the swinging arm. The dispositions of the poles 
in the different experiments are depicted in fig. 17.* The deflexion may Vie due to the 
Fig. 17. 
couple, V, occasioned by the earth’s field, or to the electromagnetic couple, E, 
occasioned by the rest of the circuit, or to the total pressure upon the poles, P, in 
which category, as before, we include for the present the influence of air currents, &c. 
In the above arrangements the deflexions were as follows ;— 
Table IX. 
8 amperes, 3 mm. 
9 amperes, 5 mm. 
Arrangement. 
Couple due to 
Torsion. 
Torsion. 
1 
Set A. 
Set B. 
a 
P + E + V 
o 
290 
o 
312 
« 
- 40 
1 h 
E + V 
235 
290 
- 87 
r 
-P-E+V 
90 
60 
-323 
d 
-E + V 
120 
104 
-275 
1 e 
V 
175 
126 
- 188 
The value of V is given by e or |-(a + c) or ^{b + d), 
E ,, „ ^{b-d), h-e, e-d, 
P ,, ,, a — b, d — c. 
