THE PRESSURE UPON THE POLES OP THE ELECTKIC ARC. 
121 
From a comparison of tl)e anode and cathode pressure curves we see that it 
depends upon the arc-lengths whether one or the other is the greater, but for long 
arcs there is little doubt that the anode pressure preponderates. 
The fact tliat for direct current the total pressure is greater for short arcs than for 
long arcs is in agreement with the observation upon alternating current arcs quoted 
on p. 115. 
Variation of Total Pressure 
with Current for Constant A ro-lenyths. 
Fig. 13 shows graphically the observations contained in Table IV., Series B, 
single-arc method, 3‘5 mm. arc-length, and fig. 14 depicts the results of Table VII., 
Series C, double-arc method, 3 mm. and 6 mm. arc-length. The graphs ai'e 
1 - 5 - 
i-o- 
aJ 
0 - 5 - 
V 
u 
rf 
in 
tn 
<v 
0- 
o 
Series B 
o Anode 
• Cathode 
3-5 non. 
•- O 
• O 
o 
•o 
<p 
o 
o 
-1-1-1-“T-'-T” 
^ I'O 
Amperes 
Fig. 13. From Table IV. 
not straight lines but appreciably convex to the current axis, indicating that 
the total pressure increases rather more rapidly than with the first power of the 
current, not, however, as rapidly as the second power. I he curves do not pass 
through the origin but cut the current axis at about 3 amperes. The approximately 
linear rate of variation is shown for other lengths of arc in fig. 15 for the anode, 
and in fig. 16 for the cathode, both single- and double-arc methods. For reasons 
which will appear in Part II., special attention has been given to arc-lengths of 6 mm. 
We defer further discussion of these curves until convexion-current effects have been 
eliminated from the total pressure. 
