THE PRESSURE UPON THE POLES OF THE ELECTRIC ARC. 



Ill 



readings of the torsion head, T, when the current was on and off measures the couple 

 acting upon the suspended copper rod if the constants of the suspension are known. 



The movable parts of the apparatus were completely enclosed in a box, B, with a 

 glass top to prevent disturbance from air currents in the room, and appropriate 

 windows and holes were made in it to enable observations to be made. The torsion 



Fig. 2. The V-grip. 



fibre was enclosed in a vertical tube. A lens focussed an image of the arc upon a 

 screen to facilitate the measurement of the arc-length. 

 The observed couple is due to : 



(1) A pressure upon the poles due to forces within the arc, including the effect 



of convexion currents, electrostatic effects, &c. These will be treated as a 

 whole in the first instance, and called the total pressure ; 



(2) The interaction between electric currents in the suspended part of the circuit 



and the earth's magnetic field ; 



(3) Interaction between electric currents in the suspended part of the circuit 



and the currents in the rest of the circuit, called briefly the electromagnetic 

 effect. 



SERIES A. ALTERNATING CURRENT. 



Method 1. The current was conducted to the rod, E, through the mercury 

 trough, M. 



Alternating current was used because it at once eliminated the couple due to the 

 action of the earth's magnetic field. The couple due to the electromagnetic effect 

 was estimated by experiments described on p. 124. The values given in the first row 

 of Table I. represent the total pressures upon the pole after allowing for this couple ; 

 the arc length was 3 '5 mm. throughout. 



TABLE I. Alternating Current. 



R 2 



