186 Mr. J. E. II. Gordon on lite Effect of Variation of 



to the lengths of sparks between flat plates. The extreme 

 length of spark used in his experiments was 1*52 millim. 



In the experiments made by M. Masson *, the spark passed 

 either between two balls in the air, or between two similar 

 balls inside a globe in which a more or less complete vacuum 

 could be produced. The distances between the balls could be 

 varied; as well as the pressures. Within the limits of his ex- 

 periments, he found that the length of spark was inversely 

 proportional to the pressure. The greatest length of spark 

 which he used was 11*1 millims. 



In 1843 M. Knochenhauerj worked with a constant length 

 of spark of about j inch J, and measured the electric density re- 

 quired to produce a spark in air at various pressures. Within 

 the limits of his experiments he found that the ratio of the elec- 

 tric density required to produce a spark, to the pressure of the 

 air, increases sensibly as the pressure diminishes. Now it is a 

 simple deduction from Harris's law that the length of spark 

 is proportional to the electric density; and therefore Knochen- 

 hauer's results show that the law given by Harris and Masson 

 does not hold for all distances and pressures. 



Wiedemann and Ruhemann§ found a purely empirical for- 

 mula for variations in the lengths of sparks where the longest 

 spark was 9*95 millims. 



In the experiments described in this paper, an attempt has 

 been made to determine the ratio of the spark-length to the 

 pressure for distances ranging from 6 inches to 30 inches by 

 means of one and the same apparatus. The experiments also 

 differ from any former experiments with which the author is 

 acquainted, in the fact that an induction-coil was used as the 

 source of electricity instead of an electric machine. 



Description of the Apparatus || used. 



The Coil. — This was a particularly fine instrument, giving 

 a spark of 17 inches (42*5 centims.) in air at the ordinary 

 pressure. It was worked by 10 quart cells of Grove's battery 

 arranged in series. It was provided with a vibrator and with 

 a clock contact-breaker, either of which could be used. 



The Air-pump was of the ordinary Tait's construction, but 



* Annates de Chimie, 3 e serie, t. xxx. ; or Mascart, Electricite Statique, 

 t. ii. p. 94. 



t Pogg. Ann. lviii. p. 219 ; or Mascart, t. ii. p. 95. 



I lie does not state the length of spark he used, but gives the height 

 of his whole apparatus, and a drawing which, if it is to scale, shows that 

 the discharging balls were about f inch apart. 



§ Mascart, t. ii. p. 97, 



II The whole of the apparatus was made by Mr. Apps. 



