THE PRESSURE UPON THE POLES OF THE ELECTRIC ARC. 



127 



The observation that convexion currents from the poles occasion a reduction of the 

 pressure between them is in accord with the experiments of DEWAR referred to 

 elsewhere. He states that " the effect of hot poles upon the registration of the 



I 



20 



Corivexion eifect 



O 



10 



Amperes 

 Fig. -1\. 



manometers was to produce a small negative pressure when the arc was stopped, due 

 to the passage of currents of hot air." 



The Elimination, of Convexion Kffects. 



The curve D in fig. 2,2 is the same curve as in fig. 21, but drawn to the appro- 

 priate scale, and shown below the current axis instead of above it. A is the 

 curve representing the observed total pressure upon the anode taken from 

 fig. 14. The corrected pressure is given by the curve A referred to D as datum 

 line, or by the curve B referred to the original axes. It will be seen that 

 correcting for convexion results in a rather more linear curve than that obtained 

 from direct observation, and that the curve now more nearly passes through the 

 origin. Remembering that the effects of convexion currents have been under- 

 estimated, and therefore that the convexion current curve should have been rather 

 lower, we see that it is probable that the corrected curve is linear, and that 

 the reaction varies directly with the current; it is unfortunate that information is 

 very difficult to obtain in the crucial part of the curve where the current is small ; 

 there is no evidence that the pressure when corrected for convexion ever becomes 

 negative. It is clear therefore that, without much error, we may take the origin of 

 the observed curve at the point at which the straight portion when produced back- 

 wards cuts the vertical or pressure axis. This provides us with the simplest means 



for correcting for convexion currents, and all previous diagrams should be so treated. 



T 2 



