968 Mr. Beer and Prof. Tyndall on Manometric 



Table II. 



Oathod 

 Hole 



e = 0'9 cm. 

 = 015 cm. 



diam. 



Cathode = - 9 cm. 

 Hole - 0-11 cm. 



Arc-length Current in 

 in mm. amperes. 



7-0 13-5 



diam. 



Arc-length 

 in mm. 



Current in 

 amperes. 



13-5 



Pressure 



in dynes 



per sq. cm. 



6-9 



Pressure 



in dynes 



per sq.cm. 



24-7 









7-0 



17-0 



132 



1-3 



17'0 



18-6 



5-0 



18-1 



5-7 









60 



19-5 



31-8 



For this reason it was not found possible to obtain any 

 definite relationship between arc-length raid pressure or 

 between current and pressure, and so to rule out evolution 

 of gas as the cause of the pressures, as was possible for the 

 anode. But the experiments at least afford no evidence of 

 regular variation of pressure with arc-length ; and as the 

 temperature is lower and the current more concentrated at 

 the cathode than at the anode, there is now still more reason 

 than in the case of the anode to suppose that we are dealing 

 with the effects of space-charge near the electrode and not 

 with those of gas evolution. 



It is clear that with the large holes used by Dewar he 

 did not observe the positive cathode pressure of a perfectly 

 central arc,, but was dealing with negative pressures similar 

 to those shown in curves V. Comparison of these with the 

 anode results under similar conditions show that in agree- 

 ment with him the cathode negative pressures are smaller 

 than the anode positive pressures, though the difference in 

 magnitude is perhaps not so great as his remarks suggest. 

 He did not state, however, what range of current he was 

 using, and the fact that the luminous patch takes up a more 

 or less central position on the anode more readily than on 

 the cathode may well have led to an under estimate by him 

 of the cathode pressure. 



Anode and Cathode together. 



Lastly, some attempts were made to measure the total 

 difference of pressure between the electrodes by connecting 

 the two carbons to opposite sides of the gauge at the same 

 time. The purpose of these measurements was partly to 

 check the previous results, but mainly to obtain readings 

 from which the effects of convection were eliminated. But 

 the difficulties of hissing and of obtaining centrality of arc 



