THE PRESSURE UPON THE POLES OF THE ELECTRIC ARC. 
1 19 
Variation of Total Pressure -with Arc-length for Constant Current. 
This relationship is ilhistrated by fig. lO, 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 sliort arcs. 
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. 11 suggests a minimum value at an 
arc-length of about 3 mm. for small currents, whereas there is vei-y little indication of 
this in fig. 10. Two further sets of observations were made with the double-arc 
method to check this point, and tig. 12, from Table VIII., confirms the accviracy 
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 
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