966 Mr. Beer and Prof. Tyndall on Manometrie 



If we omit the results for the 5-mm. hole, it will be seen 

 that the values of the product in each vertical column are 

 reasonably constant considering the difficulties of obtaining 

 reliable results for comparison. The increase in the value 

 of the product for the larger holes is also not unexpected, 

 because such a hole reduces appreciably the effective area 

 of the electrode and causes an increase in the current 

 density which is not allowed for. In our opinion, there- 

 fore, there is strong qualitative and quantitative evidence 

 for the view that the Dewar pressure at the anode is due 

 to a negative space-charge in a layer of gas very close to 

 the electrode itself. 



With regard to graph D, some of the points on it are 

 taken from those in curves III., selecting a value of arc- 

 length of 1*4 mm. ; the other points included are from 

 similar pressure arc-length results not plotted in curves III. 



It seems quite clear, then, that for a truly central arc, 

 as given by curve D, the current and anode pressure are 

 proportional. Under other conditions the readings are too 

 low, due to the fact that the true average pressure at the 

 luminous patch is not felt at the hole. At low currents 

 and large holes it was found, indeed, that for a non-central 

 arc the pressure was in some cases actually reversed. 

 This effect, which in all cases was very minute, is referred 

 to again below. 



Cathode pressure. 



Quantitative work at the cathode is almost impossible, 

 owing to the fact that the luminous patch is much smaller 

 and will rarely remain centred on a small hole in the 

 electrode long enough to obtain readings which are definite 

 except in sign. It is simple to obtain readings of a sort 

 with arcs which are not quite central on the cathode ; and in 

 order that they may be correlated with similar curves for 

 the anode they are given in curves V. 



For large currents in all these non-central cases the 

 pressures were negative. In the case of the large holes 

 the luminous patch was sometimes seen to be actually 

 playing right inside the hole at a point on its wall. In the 

 case of the small holes the spot was about half on and 

 half off the hole. With small currents the pressures were 

 often but not always reversed in sign, but w r ere so irregular 

 that it is impossible to include them in any graph. 



