316 Dr. S. Guggenheimer on the Spark-discharge. 



of the intensity of the maximum current observed by Mr. 

 Kreusler; thus let I (intensity) be 10~ 10 ampere. Then a 

 simple calculation by means of the formula 



I = qe 

 gives 



q = '2x 10 9 approximately, 



where q equals the number of ions per unit-volume. Com- 

 paring this value with Loschmidt's number, we see that only 

 about the 10 -11 part of the molecules become ionized. If 

 no external ionizing agents are acting, then the transforma- 

 tion of the 20 ions contained in unit volume according to 

 C. T. R. Wilson (if I is greater then, of course, there is a 

 correspondingly greater number of ions) into about 2 . 10 9 

 ions must take place by collisions. The time which is neces- 

 sary for this transformation is what Prof. Warburg calls the 

 period of retardation. It is obvious that this time is con- 

 siderably shortened if we produce by radiation a new set of 

 ions whose number is large compared with the number of 

 ions originally present, which new ions are also put in motion, 

 thus producing still more new ions by collisions with the 

 molecules of the gas. 



It will be easily seen that the above considerations explain 

 fairly well Prof. Jaumann's results. 



The following is a new definition of Maxwell's " electric 

 strength ; ' of a gas based on this view of the nature of the 

 spark-discharge: — "The electric strength of a gas at a pres- 

 sure p is defined as the electric intensity required to give 

 to the negative ions a velocity sufficient to enable them to 

 produce other ions by collisions with the molecules of the gas." 

 According to this view, the determinations of the spark- 

 potential under the action of radiation made by Prof. Warburg 

 and his pupils are to be regarded as giving the normal spark- 

 potential, as Prof. Warburg maintains. It follows also that 

 when working without radiation one ought to obtain the same 

 normal potential, provided sufficient time is allowed for the 

 electric intensity to act. 



I hope soon to be able to publish further experimental 

 results in support of these views. 



The spark itself appears to amount practically to a short 

 circuit between the electrodes. In addition to the production 

 of ions by collisions, the following causes help to explain 

 this : — 



1. The ionization resulting from the high temperature of 

 the spark. 



2. The presence of hot metal vapour. 



