34 MM. Julius Elster and Hans Geitel on the 



lowest order produced in the film between the jets is green 

 of the third order at normal incidence : the thickness of the 

 film is, roughly speaking, 2^/2 x 6000 x 10~ 8 centims., that is 

 0*000075 centims. It appears that the highest minimum 

 electromotive force to produce union is about 0'75 volt. This 

 gives a difference of potential in volts per centimetre of 10,000. 

 Now Sir W. Thomson's experiments show that, to produce a 

 spark between brass plates nearly 100 times further apart 

 than the jets in the case we are considering, an electromotive 

 force at the rate of about 80,000 volts per centimetre is re- 

 quired. De La Rue and Muller have lately shown that the 

 substitution of saturated aqueous vapour for dry air between 

 the plates does not make any great difference. It is, however, 

 conceivable that there is diminished pressure between the jets. 

 Crowthorne, Wokingham, June 1885. 



V. Observations on the Electrical Processes in Thunder- 

 clouds. By Julius Elstek and Hans Geitel*. 



IN an investigation, with which we have been some time 

 engaged, on the production of electricity by the friction 

 of finely divided liquids against solids of various temperatures, 

 we had occasion to observe the exceeding sensitiveness to 

 electrical induction of disintegrating jets of liquid as well as 

 of all solid and liquid particles suspended in a current of air. 

 This action is the chief source of error to be contended against 

 in such experiments ; and, as we shall have occasion to show 

 in a subsequent paper, it makes it very difficult to settle the 

 question whether electricity is produced when water-spray 

 rubs against cold bodies. The phenomenon has long been 

 known for jets of liquid, and has already had a practical appli- 

 cation in Thomson's drop-collector and the water-induction 

 machine. It was an obvious idea to consider that inductive 

 actions of a similar kind are also at work in the processes 

 which take place in a thunder-cloud — that is, just to regard 

 the latter as a self-acting duplicator. 



The principle of this idea will be best seen from an experi- 

 ment which can be made by the simplest means. 



A cylindrical metal tube, A, open at both ends (about 

 50 ccntim. long and 8 centim. in diameter), is fixed vertically 

 to a lateral insulated holder. A small vessel, B, which is also 

 cylindrical, can be inserted from the top, being held by an 

 insulated handle. This latter vessel is closed below, and is 



* Translated from a separate impression from Wiedemann's Annalen, 

 No. 5 (1885), communicated by the Authors. 



