THE PRESSUEE UPON THE POLES OF THE ELECTRIC ARC. 



129 



TABLE X. The Pressure upon the Poles of an Arc, 6 mm. long, carrying 10 amperes. 



The values for the observed pressures were taken from the diagrams named in column - by drawing 

 a line through the points of observation and noting where it met the vertical axis, and this point taken 

 as the origin. When the graph was not straight the tangent was drawn to touch the curves at 10 amperes 

 and similarly treated. 



* Calculated for arc-length of 7 mm. 



Previous Investigation. 



In 1882 DEWARJ measured the hydrostatic pressure within the arc by using 

 hollow carbons connected to delicate water manometers and found that " during the 

 maintenance of the steady arc the manometer connected with the positive pole 

 exhibited a fixed increase of pressure corresponding to 1 to 2 mm. of vertical 

 water pressure in different experiments and under varied conditions. The mano- 

 meter connected with tne negative pole shows no increase of pressure, but rather, 

 on the average, a diminution." 



DEWAR gives no data respecting current and arc-length, but we may note that the 

 hydrostatic pressure near the anode is about one hundred times the total pressure 

 upon the poles measured in the present research. 



DEWAR suggests that the phenomenon is consistent with " the well-defined 

 boundary of the heated gases acting as if it had a small surface tension," and he 

 suggests as a possible cause ' ' the motions of the gas particles under the conditions 

 of transit of material from pole to pole or a succession of disruptive discharges." 



t DEWAR, ' Roy. Soc. Proc.,' xxxiii., 262, 1882. 



