THE PEESSURE UPON THE POLES OF THE PXECTRIC ARC. 
in 
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 
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 eftect 
of convexion currents, electrostatic eftects, &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. 
Amperes 
1 
3-5 j 4-3 
1 
5-0 
5-2 
6-5 
7-0 
8-0 
9-0 
9-5 
1 
lO'O ! 12-0 
Total pressm-es in dynes. 
Row 1 
Method 1 
1 
! 
i 0 
! 
1 
1 
0 
; 0 
0-27 
0-40 
0-60 
0-87 
0-94 
1-02 
1-07 
Row 2 
Method 2 
• 
1 -0 
• 
1-7 
Row 3 
Method 3 
• 
0-76 
Row 4 
Method 4 
0-32 
' 
0-59 
1-0 
R 2 
