152 Mr. B. F. Earhart on the Sparking Distances 



the plates were separated a greater distance than the potential 

 seemed to require, and then gradually brought together. 



Results. — Table I. and the corresponding curve designated 

 I. (fig. 3) show the results from distances of *5 X sodium light to 

 185 X, i. e. from *3 micron to 109 micra. In this case the 

 dielectric was air under atmospheric pressure. The readings 

 were taken during the winter months, and it was not deemed 

 necessary to guard against water-vapour. The readings 

 given in the following tables were taken in the spring and 

 summer months, when the humidity is likely to be larger. 

 In these cases the air was passed through suitable drying- 

 tubes. 



Table I. — Air, Atmospheric Pressure. 



Distance in 



Potential in 



Distance in 



Potential in 



Wave-lengths. 



Volts. 



Wave-lengths. 



Yolts. 



•5 



38 



16-0 



360 



•7 



46 



175 



400 



•7 



52 



18-5 



408 



•8 



84 



200 



408 



1-0 



100 



220 



416 



1-0 



106 



220 



416 



12 



108 



28-5 



454 



1-3 



111 



31-5 



460 



15 



152 



39 



512 



2-0 



202 



520 



556 



2-5 



240 



64-0 



604 



2-5 



250 



770 



664 



3-0 



300 



78-5 



670 



30 



304 



990 



712 



4-0 



312 



1195 



820 



50 



336 



125-0 



864 



5 



344 



1640 



986 



8-5 



348 



1850 



1080 



13-5 



350 







140 



360 







From the results indicated in Curve I. (fig. 3) it will be 

 observed that from readings from *5 to 3 wave-lengths the po- 

 tential required for discharge varies directly with the distance. 

 Between .wave-lengths 3 and 4 the curve takes a sudden 

 bend, and proceeds in nearly a straight line, varying with 

 the distance, but according to a different law. If we accept 

 the hypothesis of an air-film surrounding the surfaces of the 

 bodies, the sudden variation in the form of the curve between 

 wave-lengths 3 and 4 would seem to be another argument in 

 its favour, and would indicate the thickness of such a film as 

 being 1*5 X sodium light, i. e. about '9 ft. 



It is of some interest to compare the portions of the curve 

 indicated with those given by previous observers. The obser- 



