the Active Deposit of Radium. 433 



It is apparent from the above that to obtain D we must 



determine the ratio p ; in order to accomplish this it is 



necessary to measure the ionization currents resulting from 

 Q and G under similar conditions. Put briefly, the method 

 employed was to deposit certain ascertainable fractions of Q 

 and of G on a central rod introduced into the cylinder in the 

 manner described in Section 5. These deposits could then 

 be compared by measuring the ionization currents to which 

 they gave rise in a second cylinder ; it was easy to verify 

 from the decay curves thus obtained that these fractions 

 were deposited on the rod chiefly in the form of RaA. 



The experimental procedure was as follows : the emana- 

 tion mixed with air carefully dried and freed from dust, was 

 passed into the cylinder and allowed to remain for several 

 days. A potential of — 160 volts was applied to the case, 

 and ah earthed rod was introduced for 1 minute and then 

 withdrawn, the cylinder being closed as before with a rubber 

 stopper. About half an hour later the experiment was 

 repeated with rod and case both earthed ; alter another 

 interval of about half an hour the experiment was repeated 

 with earthed rod and +160 volts applied to the case. The 

 activities of the rods were measured about 25 minutes after 

 withdrawal, when the decay curve is practically flat ; on this 

 account no correction had to be made for the decay of RaA 

 during the exposure of the electrode. 



The activity deposited on the rod in the first experiment 

 ( — 160 volts) will be due to two causes : in the first place, 

 the rod will receive a certain fraction (/) of the activity (Gr) 

 in the gas at the time of introduction ; it will also receive a 

 certain fraction (k) of the neutral deposit particles formed 

 during the exposure (1 minute). Suppose a fraction p of 

 tlio deposit particles are positively charged when a potential 

 of 160 volts is applied ; this fraction p of course takes into 

 account a small amount of columnar recombination which is 

 independent of the amount of emanation in the vessel. 



In the second experiment (0 volts) the rod will receive /G 

 together with the fraction (IS) of all the deposit particles 

 formed in 1 minute. In the third experiment ( + 160) the 

 rod will receive the same as in the first experiment together 

 with pQ. 



If a, />, and c represent on an arbitrary scale the measured 



ionization currents due to the active deposit collected on 



these rods, and if m denotes a factor of proportionality 



between amount of deposit and ionization current, including 



Phil. Mag. S. 6. Vol. 28. No. 166. Oct. 1914. 2 F 



