THE PEESSURE UPON THE POLES OF THE ELECTRIC ARC. 
115 
The Influence of Length oj Arc upon the Pressure. Alternating Current. 
In the above experiments the arcs were all approximately 3'5 mm. long. 
The following experiment was undertaken to determine whether the pressure was 
increased or diminished by shortening the arc. The disposition was that shown in 
fig. 7, a current of 9 amperes ~ was used. The arcs were both horizontal. 
-'TJW—■ 
Fig. 7. 
When a < 6 pressure on a greater than pressure on h. 
a'> b ,, b ,, ,, 
Hence for alternating currents the pressure is greater for small arc lengths. 
II. Direct Current. Series B. 
Observer: Mr. T. H. Burnham. 
The disposition of method 1 (fig. l) was employed. By reversing the direction of 
the current the pressures upon the anode and cathode were separately determined. 
In order to eliminate the effect of the earth’s magnetic field upon the swinging arm, 
E, which now carries a current, the fixed pole was placed first on the west and then 
on the east (dotted position in fig. l), so that upon one occasion the sum of the 
pressure upon the pole and the earth s effect was measured, and upon the other then 
difference. A typical example is shown in fig. 8. The mean of the two curves 
thus obtained is free from the influence of the earth s magnetism. The values 
were then corrected as before for the electromagnetic effect due to the rest of the 
circuit. 
The results are given in Tables IV., V., and VI. Discussion of them is reseived 
until a further method of attack has been described. 
