792 Discharge of Electricity through rarefied Gases. 



the surface of the anode P, it was interesting to compare the 

 results of these experiments with the results given above. 

 The ratio R proved to be strictly proportional to the pressure 

 in the bulb, but at a given pressure P was much smaller 

 (about one-third) than in the ease of positive ions producing 

 electrons by collision. This is not at all surprising if we 

 bear in mind that the free path of electrons moving with 

 large velocities is much longer than that of the positive ions. 



It may be mentioned here that the method and apparatus 

 described in this paper proved to be useful also for the study 

 of some questions in connexion with ionization by collision, 

 and for the determination of the free path of electrons and 

 positive ions in rarified gases. 



6. In the experiments described in the previous chapter 

 the energy of the positive ions was acquired by their 

 fall through a potential difference not exceeding 2000 v. 

 Attempts were also made, however, to carry out the same 

 experiments by applying much greater electric forces be- 

 tween the heated strip and the plate P. The difficulties in 

 such experiments mainly arise from the fact that the strip 

 and the third electrode have to be brought to different 

 potentials, and the potential difference between them must 

 be kept steady and comparatively small. All attempts to 

 use an induction-coil for this purpose were unsuccessful. 

 The writer hopes to continue these experiments by means of 

 a motor-generator or other source of large and sufficiently 

 steady electromotive force. 



Summary. 



I. It is shown that the initial discharge of electricity 

 through vacuum-tubes is not brought about by the impact of 

 positive ions against the surface of the cathode. . 



II. A method is described by means of which the stream 

 of ions or electrons emitted from a hot wire in a vacuum- 

 tube may be isolated from the ions produced by collision 

 within the tube. 



III. It is shown that positive ions impinging upon the 

 cathode with velocities corresponding to a fall through a 

 potential difference up to 2000 volts, are unable to liberate 

 electrons from the surface of the cathode. 



In conclusion, I wish to express my thanks to Prof. W. L. 

 Bragg for placing the necessary facilities at my disposal and 

 for the interest he has taken in the work. 



The Physical Laboratory, 



Victoria University, Manchester. 



October 1920. 



