030 Mr. H. G. J. Moseley and Dr. K. Fajans on 



product which can be both separated from its parent and 

 brought to an instrument for the measurement of its radiation, 

 provided the time interval is not large compared with the 

 period of the product. 



For the separation, advantage was taken * of the fact 

 that the residue after the expulsion of an a particle is posi- 

 tively charged, and will therefore, if driven by recoil into 

 the air, be carried to a negatively charged plate. By using 

 the face of a rotating disk as the negatively charged plate, 

 the separated product was quickly brought to an ionization 

 vessel for the measurement of its radiation. 



The period was deduced from the loss of activity of the 

 plate during the time taken to pass from the one to the other 

 of two similar ionization vessels. 



The rotating disk used was built for running at high 

 speeds, and was originally used by Schuster and Hemsalech 

 for photographing sparks. Details of the construction will 

 be found in their paper t. It could conveniently be run at a 

 peripheral speed of 160 metres per second. 



The disk was turned by an electric motor, and no difficulty 

 was experienced in keeping constant the speed, which was 

 read from a speed counter attached to the hub of the disk. 

 When working with the product thorium A (0*14 sec.) the 

 speed was reduced by introducing a pair of intermediate 

 wheels between motor and disk. 



Preliminary experiments showed that there were no serious 

 difficulties involved in the use of the method. It was ac- 

 cordingly applied to the investigation of the supposed 

 anomalies J in the decay of the recoil product from the active 

 deposit of actinium. 



It was next intended to search for products of short life 

 associated with the emanations of actinium and thorium, the 

 existence of which had been rendered probable by the work 

 of Geiger and Marsden on double scintillations §. 



At this stage Geiger || by a different and ingenious method 

 proved the existence of the new actinium product, and formed 

 an estimate of its period. 



We are much indebted to Dr. Geiger for his kindness in 

 leaving to us the further investigation of the period of this 

 product. 



The following apparatus was used for investigating the 

 products which follow actinium and thorium emanations. 



* Halm and Meitner, Ber. d. D. Phys. Gesel. vol. xi. p. 55 (1909). 



t PMl, Trans. A. vol. cxciii. p. 189 (1899). 



\ Blanquies, Le Radium, vol. vi. p. 230 (1909). 



§ Phi/s. Zeit. vol. xi. p. 7 (1910). 



j| Phil. Mag-, vol. xxii. p. i>01 (1911). 



