494 Mr. A. B. Meservey on the Potentials required 



V 3 , are respectively the first, second, and third values of the 

 sparking potential ; R is the difference between V 2 and 

 the mean of Y l and V 3 , reckoned positive when V 2 is the 

 greater : P is the pressure in millimetres o£ mercury. 

 There was usually more or less variation in the value of the 

 sparking potential, so several readings were generally taken 

 for each of the qualities V^ V 2 , and Y 3 . The values given 

 in the table are in each case those that seemed most steady 

 and reliable. A table was also made out in which the first 

 reading obtained in each case was used, another in which the 

 second was used, and a third in which the last reading taken 

 was used. In these tables the values of R were sometimes 

 quantitatively different from those given in Table I., but 

 qualitatively they were the same, and in many cases differed 

 by only a volt or two. A few sets of readings were rejected 

 because "normal" conditions were not steady, apparently, 

 as there were large differences between V 1 and V 3 . Three 

 sets of readings are marked doubtful because of failure to 

 record at the time the point in the experiment at which 

 the spark chamber was opened, so that it was necessary to 

 depend on memory for this. The doubt, however, as to 

 the correctness of these readings is very slight indeed. The 

 experiments show that R was positive in all cases, that is, 

 that there w T as a rise in the sparking potential at all 

 pressures, both above and below the critical pressure, with 

 an average value of about 20 volts. The critical pressure 

 was about 1 mm. 



A further test was made by running the current w r hile the 

 spark chamber was closed. The initial sparking potential 

 was found either before or after closing the valves, experi- 

 ment bavins: shown that the closing of the valves made no 

 difference in it. The current was then run as before, but 

 with the valves closed, the gas was given an opportunity to 

 cool, and the sparking potential w r as again determined. 

 Finally, the valves were opened and a third determination 

 made, for comparison with the first. The results are shown 

 in Table II., in which the same notation is employed as in 

 Table I. In one case a small decrease in sparking potential 

 is recorded, in all the others an increase. In no case does 

 the sparking potential remain the same for V 2 as for V 1 

 and V 3 . 



Running the current with the valves open all the time 

 did not seem to have any appreciable effect on the sparking 

 potential. Two or three sets of readings were obtained 

 which showed an increase of two or four volts from the 

 average value before the current passed, to that afterward ; 



