510 Mr. H. P. Walmsley on the Distribution of the 



each. A glass bulb containing tightly-packed cotton-wool 

 connected the T-piece to a reservoir in which radium emana- 

 tion, mixed with air, was stored over mercury. The other 

 ends of the cylinders were closed by ebonite stoppers through 

 which passed short brass rods. The electrodes were made 

 so that they could be slipped into small holes bored in these 

 rods. It was found that the apparatus could be made quite 

 air-tight by attaching a rubber ring to the ebonite stoppers 

 and employing a little stopcock grease. The chambers were 

 insulated on supports of paraffin wax. 



To produce the electric field, the inner electrode of one 

 chamber was earthed and the chamber itself connected to 

 the negative pole of a battery of small accumulators, the 

 positive pole of which was earthed. In the second chamber 

 the field was reversed. Thus in one chamber the rod was 

 the anode, and in the other it formed the cathode of the 

 electric fields. By means of a reversing key K, either rod 

 could be charged negatively to the case. The metal T-piece 

 was kept permanently earthed. 



An experiment was performed as follows : — Both chambers 

 were first evacuated by means of an air-pump, through the 

 side tube C. On opening the tap T, the emanation mixed 

 with air, expanded into the chambers. The active gas was 

 dried by passing over phosphorus pentoxide and active 

 deposit, and dust nuclei were removed by the cotton-wool in 

 the bulb. The pressure was adjusted to atmospheric by 

 raising the level of the mercury in the reservoir, and then 

 the tap T was closed. After the electrodes had been exposed 

 to the emanation for the desired time, the emanation was 

 removed from the chambers by a current of dried air which 

 entered by the tap C and carried it through the side 

 tube D, which communicated with the open air outside the 

 laboratory. The electric field was always put on before 

 the emanation entered the chambers, and was not removed 

 until all the emanation had been expelled at the close of an 

 exposure. 



The active deposit on the central rods was then measured 

 in an ordinary a-ray electroscope : that on the earthed rod 

 is referred to as the anode activity ^ and that on the nega- 

 tively charged rod as the cathode activity. To correct for 

 any slight difference in the volumes of the chambers A and 

 B, all readings were duplicated by using the electrodes in 

 A and B alternately, as anodes. 



