314 



Dr. S. Guggenheimer on the Spark-discharge. 



so far as I am aware, have not yet been taken into conside- 

 ration in tbe various attempts that have been made to explain 

 the phenomena of retardation and of the spark-discharge in 

 general. These two points are — (1) The fact discovered by 

 Geitel* and by C. T. R. Wilson f, that the air always con- 

 tains ions, and that there is a continuous production of ions 

 connected naturally with the re- combination of these ions. 

 (2) The principle first established by Prof. J. J. Thomson %, 

 and also brought forward and strengthened by experiments 

 executed in tbe Cavendish Laboratory by Prof. Townsend §, 

 viz., the principle of tbe production of new ions by the col- 

 lisions of negatively charged corpuscles moving under the 

 influence of strong electromotive forces with the molecules 

 of the gas. 



A careful comparison of the results of experiments under- 

 taken from different standpoints^ shows that the electromotive 

 force per cm. which is required to give to the negative ions 

 a velocity such that they can produce new ions by collisions 

 with the molecules of the gas, is very nearly the same as the 

 electromotive force required to produce spark-discharge in 

 the gas at the same pressure and with the electrodes at the 

 distance of 1 cm. 



Prof. Thomson illustrates this, in the paper mentioned 

 above, by a small table taken from a paper by Skinner (Phil. 

 Mag. [5] 1. 1900). Here X (the potential-gradient per cm. 

 in the positive column) means the above described minimum 

 E.M.F., and p is the pressure. The table is as follows: — 



V- 



mm. 

 06 



X/cm. in Yolts. 



X 



27 



i 

 45 



10 



40 



40 



15 



56 



38 



Liebig (Phil. Hag. [5] xxiv. p. 106) found the value 

 required for a spark-discharge at a distance of 1 cm. in air 



* H. Geitel, Phys. Zeitschrifi, ii. p. 116 (1900) ; J. Elster & H. Geitel, 

 ibid. p. 560 (1901). 



t C. T. R. Wilson, Proc. Roy. Soc. lxviii. p. 151 (1901). 



t -T. J. Thomson. Phil. Mag. [51 1. p. 278 (1900) ; ibid. [61 i. p. 361 

 (1901). 



§ J. S. Townsend, 'Nature/ August 1900; Phil. Mag. February and 

 June, 1901. 



