262 PROCEEDINGS OF THE AMERICAN ACADEMY. 



in length and when it was one inch in length. I found, moreover, on 

 increasing the electromotive force, that the resistance of the sparks in air 

 decreased. By quickly drawing apart the terminals of the large storage 

 battery of ten thousand cells, a flaming arc discharge can be produced 

 in air of nearly three feet in length. Rhigi has shown also that sparks 

 from an electrical machine or Leyden jars can be greatly increased in 

 air by quickly drawing apart the spark terminals. We thus see that 

 very little resistance is encountered by more than quadru2:)ling the length 

 of discharge. 



I next placed the additional spark gap in a receiver connected with an 

 ordinary air pump, and studied the resistance offered by rarefied air at 

 the point when long ribbon-like white disruptive discharges can be ob- 

 tained. This point is about 100 cm. pressure in the receiver. The 

 resistance of such discharges of about six inches in length in a receiver 

 containing air at this pressure is two or three ohms more than sparks 

 one quarter of an inch in air. The latter offer a resistance of from 

 two to three ohms. On measuring by the damping method the resistance 

 of sparks of different lengths in the receiver at this pressure, no difference 

 in resistance could be perceived between a spark of six inches in length 

 and one of three inches in length. The method would have detected a 

 difference of half an ohm. 



The additional spark gap was next placed in a chamber of air which 

 was compressed to four atmospheres. This amount of compression 

 made no difference in the resistance to the disruptive discharges. It 

 would be interesting to push this research to the amount of compression 

 reached by Professor Dewar in the case of liquid oxygen. He has ob- 

 tained a dielectric constant for liquid oxygen of 1.45. When this dielec- 

 tric, however, is broken down by a disruptive spark, I am inclined to 

 believe that it would show little more resistance than air under the same 

 circumstances. 



The additional spark gap was next placed in hydrogen gas generated 

 at atmospheric pressure by electrolysis. No appreciable difference at 

 this pressure was noticed between the resistance offered by this gas and 

 air at the same pressure. The length of spai-k which could be obtained 

 with a given voltage was somewhat more in hydrogen than in air. It has 

 been shown by Professor T. W. Richards and myself that the resistance 

 of gases at low pressures diminishes with the increase of electromotive 

 force.* I was interested to test this question by the employment of the 



* Am. Journal of Science, April, 1897. 



