Conductivity of the ^Ether. 381 



moreover, that on increasing the electromotive force the 

 resistance of the sparks in air decreased. By quickly drawing 

 apart the terminals of my large battery I can produce a 

 flaming discharge in air of about three feet in length. Eighi 

 has also observed the same phenomenon with sparks from an 

 electrical machine. We see that no increase in resistance 

 results. I then placed the secondary spark-gap in a receiver 

 and studied the resistance offered by rarefied air at the point 

 when long ribbon-like white disruptive discharges can be 

 obtained. This point is at about 100 millim. pressure. 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 of one quarter of an inch in air ; the latter 

 have a resistance of from two to three ohms. On measuring 

 by the above 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 secondary 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. The additional spark was also obtained 

 in hydrogen gas generated by electrolysis at atmospheric 

 pressure, and no appreciable difference in resistance between 

 this gas and air was noticed. The length of spark which 

 could be obtained with a given voltage was somewhat more 

 in hydrogen than in air. It was interesting, in the next 

 place, to determine by this method whether differences in the 

 material of the spark-gaps made any difference in the resist- 

 ances observed in the case of disruptive discharges *. I accord- 

 ingly employed terminals of platinum, iron, aluminium, brass, 

 cadmium, and zinc, and could perceive no difference. More- 

 over, any difference of resistance between spheres and between 

 pointed terminals, or between a point and a plane, seemed to 

 be inappreciable. With powerful discharges such differences, 

 if they exist, apparently disappear. The additional spark 

 was next placed in a heated flame. It is well known that 

 the spark-length can be thus greatly increased. On photo- 

 graphing a spark in an additional gap the resistance appeared 

 to be slightly increased in the flame ; doubling the length of 

 this spark, however, made no change in the resistance that 

 was encountered in the heated medium. The phenomenon 

 was exactly analogous to that observed in the receiver ex- 

 hausted to 100 millim. I was interested to observe whether 



* Righi, Nuovo Cimento (2) xvi. p. 97 (1876) ; De La Rue and Hugo 

 Muller, Phil. Trans, clxix. pt, i. p. 93 (1878). 

 Phil. Mag. S. 5. Vol. 43. No. 264. May 1897. 2 G 



