320 Prof. Bragg and Mr. Kleeman on 



but with this arrangement the remaining moisture of the 

 air seemed to do more harm in liberating emanation than 

 the current did good by carrying it away. We now pass the 

 current of dry air through the vessel from one end to the 

 other ; but the radium film probably feels little draught, 

 being protected by the set of vertical metal tubes which 

 stand over it (see the previous papers). When this is done 

 the leak which the electrometer shows, the radium being out 

 of range, is very small compared with the principal leak and 

 is fairly constant. In the experiments described later it 

 amounted usually to one or two millimetres in ten seconds, 

 whilst the principal leak might be as much as sixteen centi- 

 metres in the same time. 



This residual leak is due to various causes. One of them 

 is the leak which takes place from the metal of the ionization- 

 chamber, and the wire which connects it with the electrometer, 

 to the earthed metal case which screens the chamber and its 

 connexions from external influence. This current is supplied 

 by the ions which are continually formed by various agents 

 in the air within the earthed casing, and is driven by the Volta 

 potential-differences between the metals of the connexions 

 and the casing. It can be materially altered by changing 

 the material of the casing, or part of it, e. g. exchanging zinc 

 for copper. If a speck of radium is placed within the casing 

 so as to increase the supply of ions the effect is surprising. 

 As an illustration we allowed the electrometer connexions 

 to end in a brass wire 15 cm. long, which projected horizon- 

 tally in the open air, the ionization apparatus being dis- 

 connected. When a large earthed copper tube containing 

 the radium speck was placed so that the brass wire was along 

 its axis, the electrometer showed a large deflexion, whereas, 

 when the copper was displaced by zinc, the deflexion was 

 also large, but in the opposite direction. 



The whole of the apparatus including the keys which connect 

 the electrometer to earth or the ionization-chamber is enclosed 

 in earthed metal casing. The keys are worked by electro- 

 magnets which are fixed externally to the casing, and operate 

 on iron armatures within. We have found it necessary to be 

 careful as to the contacts of these keys. Want of perfect 

 contact causes incomplete charging or discharging of the 

 electrometer quadrants, shown by kicks and irregular move- 

 ments of the light- spot when the keys are used. We 

 have obtained good results by making the contacts of 

 platinum surfaces, so arranged as to rub each other at the 

 make or break. This does not introduce any want of 

 definition into the beginning or ending of a leak, since both 



