^01 Alternating Current Spark Potentials. 



gives values for I\ raay . advancing continuously from 190 

 electrostatic units at 0*1 cm. to 327 at 1*5 cm. Using 

 the amended formulae, however, 1 was surprised to find 

 that the values only varied from 187 to 192, that is, from 

 56 - l to 57*6 kilovolts. I was still more surprised to find 

 the remarkable agreement between the results obtained by 

 various careful experimenters, both with direct and alter- 

 nating pressures and with electrodes of various shapes. 

 These results seemed to me to be of great practical value, 

 and indicated that the electric strength of air was roughly 

 38 kilovolts per centimetre. The discrepancy between this 

 number and the numbers obtained with very small elec- 

 trodes was in my opinion probably due to certain unwarrant- 

 able assumptions, such as, assuming that the electrodes were 

 spheres instead of cylinders with small spherical ends. At 

 that period no data, which I could trust, were available in 

 connexion with very large electrodes. I knew, however, that 

 the electrostatic theory failed to account for the phenomena 

 •observed with very high frequency currents and with im- 

 pulsive rushes of electricity. 



Tn my paper to the Physical Society (1905) I pointed out 

 that if the electrodes were closer than half a centimetre, the 

 values of K max . calculated by the formulas were appreciably 

 greater than those found by experiment, when the electrodes 

 were at greater distances apart. I also pointed out that if we 

 make the assumption that R max . can be calculated by a formula 

 of the type {( V — e)/.v\f, then by giving e a suitable value, 

 the values of R ma x. down to a distance as small as one milli- 

 metre, but not smaller, would be much the same as the A'alues 

 got at greater distances. 1 laid no stress on this assumption, 

 however, and told the meeting that if it were abandoned 

 altogether the numbers obtained for the electric strength of 

 air which were given in the paper would not be appreciably 

 altered. This assumption is what the authors call "Russell's 

 Theory"; and the experimental results quoted on p. 710 

 •et seq., instead of disproving it, prove that there exists a phe- 

 nomenon which it would be sufficient to explain. My surmise 

 may be wrong, but it has led electricians to consider the 

 point, and several * ingenious, if not convincing, reasons 

 have been suggested to explain it. 



* E. A. Watson, Journ. of the Inst, of Elec. Eng. vol. xliii. p. 113. 

 In the discussion on this paper J. Lustgaiten publishes some valuable 

 results on sparking voltages which agree remarkably well with the 

 authors, seeing that he did not earth an electrode and they do. The 

 resonance phenomena he noticed also are most instructive and explain 

 some of the anomalous results obtained with largre electrodes. 



