322 Prof. Bragg and Mr. Kleeman on 



screen instantly cleared all the space between itself and the 

 gauze B of ions, because the gauze was at a high potential 

 and the screen was earthed. No more ions were made in this 

 space until the screen was withdrawn. When this was done, 

 the space just below the gauze and above where the screen 

 had been was momentarily empty of ions, and though the 

 continued passage of the u rays through it soon refilled it, 

 yet there was a short time during which the ions in the space 

 were below normal value, and therefore the drift upwards 

 into the ionization-chamber was small. If this drift existed, 

 it was certainly to be avoided, and we therefore placed below 

 the gauze B a parallel gauze C about three millimetres 

 away, which was put to earth. No ions could cross the 

 powerful field which existed between the two gauzes. The 

 second gauze cut off a number of a rays which would other- 

 wise have entered the chamber, but there were still quite 

 enough. With this new arrangement the irregularity 

 described above has disappeared completely. Another 

 irregularity has also disappeared at the same time ; it was 

 probably due to the same cause, and removed in the same 

 way. It occurred when one or other of the bundles of rays 

 was just out of ranpre of the ionization-chamber, and took the 

 form of a small leak which seemed to prelude the arrival of 

 the real rays ; it formed a slight projection on the ionization 

 curve just above the true and large increase which represented 

 the entrance of the rays of the bundle into the ionization- 

 chamber, as the radium was pushed higher and ' nearer. 

 It often blurred the corners of the ionization curves, and 

 made it more difficult to determine the exact ranges of the 

 rays. 



The electrometer used in these experiments is of the 

 DoJezalek pattern. It has been been frequently calibrated, 

 either by charging it and causing it to share its charge with 

 an auxiliary condenser, or by measuring a given radium leak, 

 first, when the electrometer was in parallel with the condenser, 

 and again when it was not. The deflexion for a given charge 

 is wonderfully constant over a wide range of voltages of the 

 needle ; though the deflexion for a given potential varies 

 considerably. It is not found satisfactory to maintain the 

 conductivity of the quartz fibre by dipping it in a solution of 

 calcium chloride ; in dry weather it does not conduct. It is 

 better to recharge the needle to 250 volts every few hours, 

 if it is used for so long ; the needle may then leak con- 

 siderably without alteration of the deflexion for a given 

 charge. 



