THE PRESSURE UPON THE POLES OF THE ELECTRIC ARC. 



119 



Variation of Total Pressure, with Arc-length for Constant Current. 



This relationship is illustrated by fig. 10, from Table V., for Series B, and by 

 fig. 11, from Table VII., for Series C. Neither set is corrected for convexion- 

 currents. 



The Cathode. The two diagrams agree in showing a rapid drop in the total 

 pressure as the arc-length is increased from very small values ; for long arcs the 

 total pressure approaches a constant value which is usually reached at about 10 mm. 

 More weight is attached to the curves of Series B as the experimental method was 

 not so difficult. For very small arcs the curve appears to be asymptotic to the 

 pressure axis, indicating very high values of the reaction for very short arcs. 



Series 



Series B 

 Cathode 



bumps. 



5amps. 



ib iains 

 A re- length 

 4 amps. 



-0-5 



Fig. 10. From Table V. 



3<im.ps. 



2 amps. 



The Anode. Fig. 10 differs appreciably from fig. .11 in that the latter shows 

 at first a pronounced fall of total pressure with increasing arc-length, while the 

 former indicates a more constant value. Fig. 1 1 suggests a minimum value at an 

 arc-length of about 3 mm. for small currents, whereas there is very little indication of 

 this in fig. 10. Two further sets of observations were made with the double-arc 

 method to check this point, and fig. 12, from Table VIII., confirms the accuracy 

 of fig. 10 as far as the constancy of the pressure beyond 3 mm. is concerned, but 

 it also shows that the drop in the value observed for short arcs in Series C is 



8 2 



