the Method of its Transmission. 101 



§ 4. Velocity of Carriers of Thorium Excited Activity. 



For experiments on thorium emanation, a thick layer of 

 thoria was placed in a shallow copper vessel inside an ebonite 

 box 11 cms. square and 3 cms. deep, which was tightly waxed 

 down to a metal base. The thoria was completely covered 

 with two layers o£ filter-paper, which cut off' most of the 

 direct radiation, but readily allowed the emanation to pass 

 through. The apparatus was rendered air-tight by a metal 

 lid, dipping into a mercury trough round the top of the 

 ebonite box. At the beginning of an experiment a square 

 sheet of aluminium foil was placed over the paper covering 

 the thoria, a zinc plate on top of the ebonite box, and the lid 

 placed in position. This was done as quickly as possible, 

 and the alternating electric field was then applied. 



The emanation rapidly diffused through the paper and thin 

 aluminium foil, and distributed itself between the plates in 

 .the electric field. After an interval, varying in the experi- 

 ments from 20 to 90 minutes, the aluminium and zinc plates 

 were removed and their radioactivity tested in the usual way 

 with the Dolezalek electrometer. The ratio of the excited radio- 

 activity on the two exposed plates was thus determined. 

 This ratio was found to be independent of the time the plates 

 were left before testing, as the radioactivity on each plate 

 decays at the same rate. 



The amount of thoria used in these experiments varied 

 from 25 to 100 grammes. The amount of excited activity 

 in a given time varied with the amount of thoria, but the 

 ratio of the activity in the two plates was unaltered. For a 

 given voltage and time of alternation the value of p was 

 slightly greater when the lower plate was negative. This is 

 due to the unequal distribution of the emanation between the 

 plates ; for on account of the time taken in diffusion, the ema- 

 nation is more concentrated near the surface of the thoria. 

 The mean of the values of p with top plate negative and lower 

 plate negative was taken as the true value. 



In the early experiments a two-part commutator driven by 

 a motor was used. In the later experiments for a more rapid 

 rate of alternation a four-part commutator was employed. 



With this arrangement of apparatus a large number o£ 

 experiments were made in order to test the truth of the general 

 theory. Comparisons of the velocity of the carrier have 

 been made over a wide range of period of alternation and of 

 voltage, and for different distances between the plates. The 

 results obtained were in general agreement with the theory 

 put forward. When the voltage was kept constant, the value 



