the a Particles of Radium. 321 



these times are determined by the breakings of ^ contacts, 

 and these are done by the electromagnets with great 

 suddenness, 



An arrangement which we find to be of great importance, 

 is the ion-trap which is placed under the gauze of the 

 ionization-chamber. The space below the gauze B, forming 

 the lower plate of the ionization-chamber A B, is of course 



^ to qje. 



A '— 



;g -> to "battery 



p . 



" w ^ bo' Earth 



Milk*** 



llllp_ 



of tubes . 

 Ra. on jilate. 



filled with the ions formed by the rays which cross it. When 

 the gauze C is not present, then although B is at a high 

 potential, some of these ions manage to cross it and add 

 themselves to those in the chamber AB, thus disturbing the 

 results to be measured. We were first aware of this effect 

 occurring in any marked degree when we began to use a 

 platinum dish on which had been evaporated a few drops of 

 water containing a speck of very pure radium bromide. We 

 were so very fortunate as to be presented by Mr. Soddy with 

 a milligramme or two of this material; by its use our experi- 

 ments were very greatly improved, as will be described a 

 little later. On the first occasion of its use, however, we were 

 puzzled by a rather curious result. The radium and the 

 tubes above it were rather close up to the gauze B. We were 

 measuring the various leaks as, millimetre by millimetre, we 

 pushed the radium still closer. Every now and then we drew 

 the aluminium screen over the radium so as to discover how 

 much of w^hat was observed was really due to the a rays. We 

 found that after the screen had been so used and withdrawn, 

 the proper leak was materially lessened, but recovered its old 

 value in a short time. When the screen was drawn over 

 again, the leak (which was then not due to a rays) fell 

 instantly to its usual very small value, and again, when the 

 screen was withdrawn, the proper leak was for a time too 

 small. We guessed therefore that some of the leak, with the 

 screen off, was due to ions made outside the chamber, which 

 afterwards drifted into it. The interposition of the aluminium 



