Wellisch — Experiments on Active Deposit of Radium. 299 



potential of —160 volts was applied to the case, and an earthed 

 rod was introduced for one minute and then withdrawn, the 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 ; after 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 KaA 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 (f) of the activity (G-) in 

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

 tain fraction (Jc) of the neutral deposit particles formed during 

 the exposure (1 minute). Suppose a fraction^? of the 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 fG 

 together with the fraction (k) of all the deposit particles formed 

 in one minute. In the third experiment (4-160) the rod will 

 receive the same as in the first experiment together with pQ. 



If a, b, 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 also the 

 correction for decay from the maximum value, we will have the 

 following relations : 



fG + k (1 — p)Q = m.a ( — 160 volts) 



fG + kQ —m.b ( volts) 



fG -\- pQ + k (1 - p)Q = m.c ( -f 160 volts) 



We deduce 



pQ =z m.(c — a) 

 b — a 



c — a 

 Q f(o-a) 



G a-(l-p)b 



(11) 



The assumptions made in obtaining the above relations may 

 at first sight appear doubtful in their applicability ; still they 

 appear to be entirely justified by experience. The separation 

 of Q into pQ positive and (1— ^>)Q neutral has been justified 

 repeatedly by the author in numerous experiments. The 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, No. 226.— October, 1914. 

 21 



