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



nature of the discbarge tends to augment the forces which 

 cause the nodes. It is only necessary to consider that the heat 

 generated in the discharge between the molecules forming a 

 compression tends at once to augment the effect of rarefaction 

 usually produced by the elastic nature of the gas. Hence the 

 nodes are sustained in an automatic manner arising from the 

 very nature of the discharge. 



Again it is much more difficult to decompose the discharge 

 by the rotating mirror when the strata are close together (or as 

 we should say, when the frequency of oscillation is greater) ; 

 "as the battery surface is still further increased (i. e. current 

 working coil) these diagonal lines (image of an unsteady dis- 

 charge) appear more and more crowded together, until at last 

 they blend into unbroken fiocculent strise."* Or the mirror 

 was not revolving sufficiently fast to decompose them. 



Iielmholtz showed that the surface of a wave in a tube is 

 not plane ; we have seen that in the case of the strata it is con- 

 cave to the positive terminal. The dark spaces are only rela- 

 tively dark.f 



It has been often stated that the strata are but accidental in 

 the discharge ; and in reality they only occur for a very 

 limited range of pressures. Geissler tubes may be lit up by 

 rubbing them with a cloth, but in this case strata do not seem 

 to be produced. Such tubes may also be used as detectors for 

 Hertz waves, showing that an oscillatory discharge will light 

 them up. 



With regard to the fact stated by Trowbridge (loc. cit.), that 

 the flaming discharge prevented him from observing this dis- 

 continuity of the current in a Geissler tube, (he makes no 

 mention of the discharge being stratified,) we may refer to a 

 paper by Sir D. Salomons;}: in which he states that a weak dis- 

 charge is the most favorable for studying the strata : in this 

 case there would be no flaming discharge. 



We have tried the effect produced by using a machine in 

 connection with a varying number of Ley den jars. While the 

 distance between the strata does not present any regular 

 change, which may be accounted for by the different intensi- 

 ties, etc., of the spark, one characteristic effect was produced. 

 With one large jar the strata were about 0*5 cm apart: with 

 four jars we were able to produce strata nearly twice as far 

 apart by carefully regulating the charge, etc. This result is 

 of importance as showing the effect of capacity. However, we 

 do not insist on this fact, as it was not always produced on re- 

 peating the experiment. 



* Spottiswoode, loc. cit. 



f de la Rue and Miiller, Nature, 1883, p. 383. 



% Proc. Roy. Soc, 1894, p. 229. 



