﻿Ionization by Positive Rays. 789 



observations must necessarily be interrupted for some time 7 

 and since it is proposed so to rearrange the apparatus that 

 direct observation of the speed of the particles can be made 

 by the use of a magnetic field, it appears better to ^ive 

 the results already attained than to wait for a complete 

 elucidation of this matter which, I feel sure, will not affect 

 the main conclusions. 



A limit to the values of Y which could be investigated 

 was set by the occurrence of a discharge between S and A. 

 If this discharge had been regular it would probably not 

 have made measurements impossible ; but, as might be 

 expected in so high a vacuum, the discharge appeared as a 

 flickering green phosphorescence on the glass walls, while the 

 current through the tube varied wildly. Work at higher 

 potentials will probably be possible only if the continual 

 evaporation of gas from the walls of the vessel is obviated by 

 the methods employed in the making of X-ray bulbs. The 

 highest reading of V for which any satisfactory reading was 

 obtained was 54,000 volts (equivalent to a spark-gap of 

 2*7 cm.) ; the corresponding value of P was 0*5 ; it is not 

 shown in fig. 2. 



7. No very great interest attaches to the values of R 

 plotted in fig. 4. The readings obtained are somewhat 

 irregular, but it must be remembered that an error of 

 1 per cent, in i x or i z means an error not less than 10 per 

 cent, in the value of R. The greatest value of R obtained 

 was about 0*1, agreeing with the value assigned by Fiicht- 

 bauer. R does not decrease very rapidly with the energy 

 of the primary rays until that energy is less than 1000 volts. 

 For V = 400 I could detect no difference between iy and i 3 ; 

 R was certainly less than 0*01. When V was greater than 

 30,000 volts, the readings for i { became so irregular that no 

 satisfactory values for R could be obtained ; but the curve 

 has been extrapolated beyond this limit for reasons which 

 will be noted immediately. 



8. Fig. 3 shows that the curve for P approaches the axis 

 of zero ionization asymptotically. There is, therefore, no 

 definite ionization potential such as is found for cathode 

 rays. This conclusion agrees with that of v. Bahr and 

 Franck for the ionization of a gas ; but Avhile they find an 

 appreciable ionization of a gas for potentials as low as 

 10 volts, I have been unable to detect any sign of ionization 

 below 40 volts. For 80 volts P is about 0*0005 ; for 

 40 volts it is certainly less than 0*0001. The difference in 

 this respect between a metal and a gas may be due only to 

 the possibility of detecting smaller ionization in the case of 



