624 Mr. G. Moody Hobbs on the Relation between P.D. 



should remain constant until the electrodes approach so near 

 that the discharge is effected by the metal ions themselves. 

 To take for example the case where the pressure of the gas 

 was 15 cm., as the distance between the electrodes is 

 decreased the sparking potential falls in a straight line to 

 350 volts, reaching this value when the sparking distance is 

 57X (31'6yLt). It then remains absolutely constant until the 

 distance has reached 5\(3//,), when the potential drops 

 rapidly to zero. An examination of the curve for 15 cm. 

 pressure in fig. 4 will show that the results obtained are in 

 perfect accord with this theory. 



(3) Effect of Change in Pressure on the Curves. — It will be 

 noticed that the curves for the various pressures reach the 

 minimum potential at distances inversely proportional to the 

 pressure. This agrees with Paschen's law which has been 

 beautifully verified by Carr, who showed that for a given 

 potential, e. g. the minimum potential, the discharge was only 

 dependent upon the mass of the gas per unit surface between 

 the electrodes. Taking Carres value of 4*98 mm. for the 

 critical pressure at 1 mm. distance, it is easy to calculate at 

 what points the curves should reach the minimum potential 

 for the various pressures. The following values were 

 obtained : — 







Distance in 



Distance in 



Spark-Potential. 



Critical Pressure. 



Wave-lengths, 



Wave-lengths, 







calculated. 



- observed . 



350 kil. 



75 cm. 



13-2 



125 



350 



40 „ 



25 



26 



350 



25 ., 



40 



35 



350 



15 „ 



66-6 



58 



350 



1 „ 



1000 





The discrepancies in the results for the lower pressures are 

 undoubtedly due to the distortion produced in the field by the 

 spherical electrode, a fact which would seem to indicate a 

 limiting distance of 15 jll, within which the field between the 

 electrodes is practically uniform. The minimum spark- 

 potential of 350 volts agrees with the value obtained by 

 Carr and others. 



Observations at lower potentials than 150 volts were seldom 

 taken, for the reason that the portion of the curve from this 

 point to the origin has been well explored in an investigation 

 in this laboratory by Professor Kinsley* in connexion with 

 a coherer problem, the apparatus used being susceptible of 

 a higher order of accuracy at these minute distances. 

 * Kinsley, Phil. Mao-. [6] vol. ix. p. 692 (1905). 



