478 Mr. G. Stead on the 



From a study of the characteristics of helium it. has been 

 -concluded that the highest pressure of helium which is 

 practicable in valves of the design used is about 1 mm. 

 Valve No. R2A was designed to contain helium at a pressure 

 of about 0*6 mm. This pressure is considerably higher than 

 would be practicable with any other gas except hydrogen, 

 and for this reason it was thought at first that contamination 

 by electrode gas would be less noticeable than in nitrogen. 

 This did not prove to be the case, owing to the fact that 

 positive ions appear in a valve containing helium when the 

 difference of potential between the anode and the negative 

 «nd of the filament amounts to about 21 volts, whereas the 

 corresponding difference of potential required to produce 

 positive ions in electrode gas is about 15 volts *, so that a 

 small admixture of electrode gas is able to cause a large 

 alteration in the characteristics of a helium valve. This is 

 brought out clearly in the contour characteristics, parti- 

 cularly in the case of the grid current contours. It will be 

 seen from fig. 8 (PL VIII.) that the grid curves rise nearly 

 vertically until a potential of about 21 volts is reached, when 

 they suddenly bend over and become nearly horizontal. 

 This is due to the formation of positive ions, whereby the 

 negative space charge is partially neutralized, so that a given 

 grid voltage is able to produce a much larger current than 

 previously, i. <?., a vertical line corresponding to any particular 

 grid potential would at this point begin to cut across the 

 grid contours in rapid succession. Now if the helium con- 

 tains a small percentage of electrode gas, these horizontal 

 portions of the grid contours occur at about 15 volts instead 

 of 21 volts, and, in consequence, the characteristics of the 

 valve are very much modified. 



Many attempts were made to produce satisfactory helium 

 valves without bombarding the electrodes, the process 

 adopted being to heat the electrodes as much as possible by 

 radiation from the hot filament. It was found that, even 

 when the filament was run at such a high temperature as to 

 cause serious damage to itself, the removal of electrode gas 

 from the anode was very incomplete, and the form of the 

 valve characteristics was controlled mainly by the electrode 

 gas and very little by the helium. Ultimately it was found 

 best to subject the electrodes to heavy electron bombardment 

 during the evacuation, just as in the case of hard valves, 

 and then introduce the necessary quantity of helium, which 

 had been purified by standing for some time in contact with 

 charcoal cooled by liquid air. 



* Stead & Gossling, Phil. Mag. xl. p. 413 (1920). 



