556 Mr. J. J. E. Durack on the Specific Ionization 



The first column gives the pressure in mm., the second column 

 the average P.D. during the leak between PP and pp in volts, 

 the third column the current when PP was connected to the 

 positive pole of the battery, and the fourth the current when 

 the field was reversed. 



The positive current is taken in the direction PP to pp, so 

 that pp receives a positive charge, the unit of current here, 

 as in all the tables and curves, is 10 -14 amp. 



It will be seen from fig. 2 that at a pressure of 19 mm. the 

 gas is practically saturated with a P.D. of 40 volts. 



Care must be taken, not only that the E.M.F. is sufficient 

 to saturate the gas. but also that it is not too great, otherwise 

 the negative ions generated by the Becquerel rays may, as has 

 been shown by Townsend (loc. cit. Phil. Mag. vol. i.), themselves 

 generate other ions ; calculation from Townsend's numbers 

 shows that the effect would begin to be evident in my experi- 

 ments when V = 45 j>, where V is the P.D. in volts and p is 

 the pressure in mm. In all the tables V is less than 45^>. 



As all the observations could not be carried out in a single 

 day, and the radium had to be kept dry to prevent it from 

 deteriorating in strength, the box B was removed at the end 

 of each day and kept in a drying-apparatus, stops were made 

 for B to fit into so that it could be replaced in the same 

 position on the following day, to make sure that this was so 

 the last readings of the previous day were repeated each 



Table II. 



Pressure. 



P.D. 



i 

 Current. 



Lower Plate + . Lower Plate — . 



205 



15-7 



12-0 



93 



73 



60 to 100 

 100 



60 to 100 

 20 to 100 



+ 18-4 

 + 12-9 

 + 8-6 

 + 6-0 

 + 3-5 



-28-8 

 -230 

 -18-3 

 -14-7 

 -12-7 



The first series of observations for the determination of a. 

 is given in Table II. and plotted in fig. 3 ; when these five 

 points were determined the box B was accidentally knocked, 

 and the exposed surface of the radium thereby considerably 

 altered, as was seen afterwards on removing the thin aluminium 

 leaf which covered B when the observations given in Tables II. 

 and III. were taken. 



