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IV. llie Pressure upon the Poles of the Electric Arc. 
By W. G. Duffield, D.Sc., Professor of Physics, Captain, Royal Air Force, 
Thos. H. Burnham, B.Sc., and A. H. Davis, B.Sc., Post-Graduate Students 
in Physics, University College, Reading. 
Communicated hy Prof. O. W. Richardson, F.R.S. 
Received February 3,—Read February 27, 1919. 
Suspending one pole of a carbon arc and keeping the other pole fixed it was found 
that there was an apparent repulsion between them. There is in fact a pressure 
upon each electrode which tends to separate them. The first part of this paper is 
devoted to the experimental methods of estimating this pressure, the second to a 
discussion of its origin. 
PART L—EXPERIMENTAL. 
Three series of observations have been made, the original observations and 
preliminary series by Duffield in 1912,* the second series in conjunction with 
Burnham, and the third in conjunction with Davis. In spite of the verv small 
forces examined the three series agree within reasonable limits. The general form 
of the apparatus (fig. l) was the same in each series, though there were important 
differences in the dispositions of the carbons in different sets of experiments. 
A stirrup was suspended by a torsion fibre, or sometimes by two fibres, F, as in 
the illustration, in this was placed &, copper rod, E, to whose extremity was fixed at 
right angles a short carbon rod, C, which was balanced by a counterpoise, W, at the 
other end. The arc was formed between this carbon rod and another, D, fixed either 
as shown in the figure or in some other manner to be described later. 
In its zero position the copper rod swung freely between two stops, S, placed close to 
one end. The sensitivity of the suspension and the long period of swing necessitated 
some simple means for bringing the rod back to the zero position, and the V-grip 
device illustrated in fig. 2 was ultimately adopted in place of the stops, S ; the adjust¬ 
ment was made by twisting the torsion head until on turning down the V-grip the 
suspended wire remained stationary ; this control also enabled the arc-length to be 
maintained nearly constant during an experiment. Tbe difference between the 
* A paper entitled “ The Pressure upon the Poles of a Carbon Arc,” was read in title at the British 
Association Meeting, Australia, 1914. 
VOL. CCXX.—A 574. R [Published December 22, 1919. 
