produced by the Corpuscles given out by Radium, 553 



so-called " a. " or non-deflectable rays, and hence the con- 

 ductivity produced in the gas between PP and pp was due 

 to the deflectable rays; this was verified by preliminary 

 experiments on the action of a magnetic field on the rate of 

 leak. 



It has been shown by many experimenters that a metal 

 plate struck by Rontgen or cathode-rays gives out negatively 

 charo-ed ions ; moreover, that the rate at which these ions 

 are given out is greater for the heavy metals than for the 

 light ones, this effect is very small when Rontgen rays fall on 

 an aluminium plate. 



For this reason the plate pp has been made as described 

 above ; there will be very little absorption of the Becquerel 

 rays by the aluminium plate on the face of pp, and conse- 

 quently very little tendency to give out negative ions, 

 practically all the absorption takes place in the lead, but this 

 will not cause any error in the measured leak due to the cor- 

 puscles themselves unless these ions are given out with 

 sufficient velocity to carry them back through the aluminium 

 plate ; from the experiments of Sagnac on the secondary 

 ionization produced at the surface of a metal by Rontgen 

 rays it is probable that they would not have sufficient velocity 

 to do this. 



The opposite surfaces of PP and pp being both of the 

 same metal, there should be no error due to a contact P.D. 

 effect. 



The electrometer used in these experiments was of the 

 Dolezalek type, the insulation of the quadrants being greatly 

 improved and a gilt mica needle used instead of the paper 

 needle of Dolezalek. The needle was kept charged to a con- 

 stant potential by dipping the suspending quartz fibre into a 

 solution of CaCl 2 and connecting the needle through the fibre 

 to a battery *. 



The ordinary formula for the sensibility of a quadrant 

 electrometer, the needle being kept at constant potential F 3 , is 



where 6 is the deflexion produced by a P.D. (V x — V 2 ) be- 

 tween the two pairs of quadrants, t is the torsion of the 

 suspending fibre per unit angular displacement, and q u is fehe 

 coefficient of capacity of a pair of quadrants, all other con- 

 ductors in the neighbourhood being kept at zero potential. 



* Dolezalek, Zeits. fur Instrument. 1V(\ 1901, 



