378 



HON. R. J. STRUT! ON THE LEAST POTENTIAL DIFFERENCE 



positive column remains it is clear that the negative glow is not constricted, and 

 consequently it only requires 340 volts to produce it. The greater the length of the 

 positive column the greater the corresponding potential difference ; so that the 

 striking potential will be the least possible when the pressure is low enough to make 

 the negative glow occupy the whole space between the plates, but not low enough to 

 make it require more space. 



Although Mr. PEACE found the minimum striking potential to vary very little with 

 the distance between the plates, and consequently very little with the pressure, yet 

 the variation with the pressure was much greater than that observed by WARBURG 

 for the cathode fall. WARBURG gives a table showing that a tenfold diminution in 

 the pressure does not alter the cathode fall by 1 volt, i.e., by per cent. 



Mr. PEACE gives data showing a rise of 67 volts (20 per cent.) in the minimum 

 striking potential. 



It will thus be seen that, though there are theoretical reasons for thinking that the 

 minimum striking potential should be equal to the cathode fall, the experimental 

 evidence hitherto produced is scarcely sufficient to establish this relation. My 

 experiments have been made with a view to determining the relation. The results 

 will be discussed after the experimental arrangements have been described. 



Description oj Experimental Arrangements. 



It was necessary to design the apparatus so as to require as little of the gas as 

 possible, since it was intended to make some experiments on helium as well as on the 

 common gases. The two brass plates of 1^ inches diameter used as electrodes were 

 embedded quite flush in ebonite plates 3 inches in diameter. The object of this was 

 to prevent any tendency to sparking from the backs or edges of the brass discs. 

 Three small ebonite distance pieces were placed between the brass plates, and the 

 whole arrangement fastened together by means of three screws passing through the 

 ebonite. Electrical connection to the brass plates was made by means of wires 

 screwed into them, passing out axially through the ebonite. 



The ebonite distance pieces were carefully measured before the apparatus was put 

 together by means of a micrometer screw gauge. The lengths in millims. were : 



No. 1 



ro- 

 lo- 



755 

 755 



No. 2 



ro- 

 lo- 



755 

 752 



No. 3 



ro- 

 lo- 



757 

 756 



Mean 0755 mm. = 0'0297 inch. 



The brass plates being large compared with the distance between them, the 

 electromotive intensity between them is sensibly uniform. 



In order to be able to introduce various gases between the brass plates, and to vary 

 the pressure, it was necessary to enclose them in an air-tight chamber, 



