THE PRESSURE UPON THE POLES OF THE ELECTRIC ARC. 



123 



The Separation of the Pressure, from the Electromagnetic. Effects. 



The evidence contained in the preceding sections proves heyond dispute the 

 existence of a pressure upon the poles of the arc, and shows the general nature of its 

 variation with the current strength. The experiment now to be described 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 maybe due to the 



1 



Fig. 17. 



couple, V, occasioned by the earth's field, or to the electromagnetic couple. K. 

 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. 

 Tn the above arrangements the deflexions were as follows : 



TABLK TX. 



The value of V is given by e or ^(a + c) or ^(b + d), 

 E -z(b-d), b-e, e-d, 

 P ,, a-b, d-c, 



