﻿Ionization by Alpha Hays. ■ 169 



known to J per cent.* The smallest ratio was compared with 

 a barometer gauge, and the absolute values thus known to 

 1 per cent. Pressures from 1-3 mm. were read to '0025 mm., 

 from 3-6 to *012 mm., from 6-20 to "025 mm. The values 

 are given in millimetres of mercury reduced to 15° C. 



The currents were measured by the high resistance method 

 as before. Owing to the wide range which had to be 

 measured two resistances in the ratio of 5*281 : 1 were em- 

 ployed. Currents less than 2 units (1 unit = 2*4 x 10 -11 amp., 

 to within 5 per cent.) were measured to "0005 unit ; those 

 greater to "003 unit. 



Differences of potential were measured to 02 volt by means 

 of a Weston laboratory standard voltmeter, said by its makers 

 to be accurate in absolute measure to 1 in 1000. 



8. Two complete series of measurements were taken, one 

 with the electrodes covered with gold, the other with 

 aluminium. In each series 6 or 7 values of y were chosen ; 

 for each of these values the relation between i and p was 

 determined over the widest possible range of p. This range 

 was limited on the lower side by V — 30, the lowest potential 

 difference for which the experiments in a vacuum had shown 

 the delta-ray current to be saturated ; on the higher side by 

 the values of V and p at which, according to the figures 

 given by Townsend, ionization by the positive ions was likely 

 to become noticeable. Since the potential could only be 

 varied conveniently in steps of about 2 volts, readings were 

 taken on each side of the value of V determined by the 

 measured pressure and the selected value of y, aud the correct 

 value found by interpolation. In every case two measure- 

 ments were taken, one with the field from A to B and one 

 with the field in the reverse direction ; the arithmetic sum 

 of the two currents is represented by the value of i. The 

 experiments described above had shown that by this pro- 

 cedure a true value for the current due to the ionization 

 caused by the alpha rays would be obtained, and the necessity 

 of a knowledge of that part of the current which is unaffected 

 by the field eliminated. 



9. n f is the total number of ions made per second in the 

 gas by the delta rays at a pressure of: 1 mm. In order to 

 determine this quantity measurements had to be made at 

 pressures very much higher than any of those used in the 

 other experiments, for it was only at pressures greater than 

 100 mm. that a. sufficient approach to a saturation current 



* The numbers given are only approximate, and represent what the 

 ratios were intended to be when the gauge was made. Thai the values 

 of y given in the table are net whole numbers is due to the difference 



between intention and realization. 



