THE PRESSURE UPON THE POLES OF THE ELECTRIC ARC. 



121 



From a comparison of the 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 that 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 ivit/i ('ur rent for Constant Arc-lengths. 



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 are 



1-5- 



i-o- 



ci 



T3 

 13 



0-5- 



g 

 ic 

 <u 



u 



Cu 



-0-4 



Series B 



o Anode 

 Cathode 



3-5 mm. 







o 







Ampere? 



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 tin- 

 current, not, however, as rapidly as the second power. The. 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. 



