412 H. V. Gill — Electric Discharge in Geissler Tubes. 



agrees with the statement by Mach that the velocity of the 

 wave is greater near the spark than at some distance from it 

 (see above).* 



Many other interesting developments might be made, but 

 enough has been said to show the reality of the air waves, and 

 to give some idea of their dimensions. Whether this result 

 could be used to determine exactly the period of oscillation of 

 a spark is a point outside the scope of the present article. In 

 Kundt's experiment the period of the note is calculated from 

 the formula v = n\ where v — the velocity of sound, and X 

 the wave length, i.e., twice the distance between two consecu- 

 tive heaps of powder. In experiments with sparks the waves 

 are not propagated with the velocity of ordinary sound waves ; 

 again, the intensity of the spark has so considerable an effect 

 on the distance between the heaps that measurements derived 

 from these distances would not be of much value. An induc- 

 tion coil produces similar results, but not nearly so well marked 

 as does the Yoss machine. 



Having thus established that electric sparks can produce, in 

 air under ordinary conditions of pressure, the effects among 

 which we place the stratification in Geissler tubes, let us examine 

 the result produced when the spark takes place in a vacuum. 



As gases at low pressures — within certain limits — are good 

 conductors of electricity, it is not quite so easy to produce 

 intense sparks in a vacuum as in the air. This difficulty is got 

 over by arranging the apparatus as in Fig. 6. When A and B, 



the terminals of the machine, are at a suitable distance apart, 

 sparks take place simultaneously at C and D. Of course we 

 could simply charge a Ley den jar and then discharge it, but 

 this operation is troublesome. The pressure of the gas was 

 about 10 mm . A few sparks were passed : the powder arranged 

 itself exactly as in our former experiments. Fig. 2, Plate IY, 



*Cf. Lord Rayleigh, "Sound." The air waves (sound) produced by very violent 

 causes, such as explosions, travel much faster than "ordinary " sound waves. 



