﻿464 Dr. Norman Campbell on 



the parallel electrodes, B being connected to the apparatus 

 for measuring the current, while A was connected to a source 

 of potential. The a, rays emitted by the polonium plate P 



Ear. 1. 



M or /In leaf 



Current 



O l 2 



SOILE OF CtflS. 



issued through the hole in A, covered with a thin foil of the 

 material under investigation. The dimensions were arranged 

 so that all a rays which passed through A and were not greatly 

 scattered struck B. 



The electrodes were contained in an air- tight chamber, 

 and the pressure of the dry air in it measured by a McLeod 

 gauge. Pressures throughout this paper are expressed in 

 millimetres of mercury reduced to 15° C. The current was 

 measured by the method involving the use of high resistances 

 described in a former paper *. 



4. The results of the measurements are given in Tables 

 I.-IV. For Table I. A was covered with gold and B with 

 aluminium ; for Table II. the position of these materials was 

 reversed ; for Tables III. and IV. both A and B were 

 covered with the same material, aluminium and gold respec- 

 tively. The first row of figures gives the pressure of the air 

 at which the experiments were made ; the subsequent rows 

 the values of -the current for potential differences between A 

 and B given in the first column. The columns marked 

 " sum " contain the arithmetical sum (or algebraical dif- 

 ference) of the currents measured with the electric field in 

 opposite directions ; the columns marked " diff." give the 

 arithmetical difference (or algebraical sum) of the same 

 currents. The unit of current is about 2 X 10~ 12 ampere. The 

 figures in different columns are not strictly comparable ; 

 allowance has to be made for the decay of the polonium 

 and for slight alterations in its position when the apparatus 



* Phil. Mag. Aug. 1911, p. 301. 



