Radio-active Products of Short Life. 631 



The actinium emanation was derived from actinium purified 

 in this laboratory by Prof. Boltwood * from the Uoyal Sociey 

 residues ; the thorium emanation from a preparation of meso- 

 thorium. The actinium was wrapped in blotting-paper, 

 through which the emanation readily diffused ; the meso- 

 thorium was contained in a small open tube. Either was 

 fixed inside an ebonite box, 4x3x3 cm., open at one end, 

 the edges of which, covered with with velvet, pressed against 

 the rotating disk, thus preventing much emanation from 

 escaping. In front of the active material and parallel to the 

 disk was a plate of copper gauze. When the latter was 

 charged positively, the positively charged residue of the 

 emanation was deposited on the face of the disk. Since 

 electroscopes were affected by the vibration caused by the 

 disk when rotating at high speed, ionization boxes and an 

 electrometer were used for measuring the radiation. The 

 two ionization boxes were of similar size and shape. The 

 bottom of each was closed with goldbeater's-skin, which, 

 while impervious to emanation, readily allowed a particles 

 to pass through, since its stopping power was equivalent to 

 less than 2 cm. of air. ' The goldbeater's-skin was coated 

 on its inner side with aluminium leaf in metallic connexion 

 with the rest of the box. For measuring actinium A, the 

 ionization boxes were placed side by side, and the ebonite 

 box containing the actinium was close to them. In the case 

 of thorium A, the three boxes were 120° apart. In either 

 case the boxes were similarly situated with regard to the 

 axis of rotation of the disk, the bottoms of the ionization 

 boxes facing the disk at a distance of about half a centimetre. 



The shape of the bottom of an ionization box was that of 

 a truncated sector of the disk, while a brass plate of similar 

 shape, slightly smaller, was insulated inside the box, parallel 

 to the bottom. This plate could be connected to the electro- 

 meter by a wire, enclosed in a air-tight metal tube, to guard 

 against induction disturbance and the entrance of emanation. 

 The ionization boxes were kept charged to a potential of 

 400 volts, and the current which passed between the box and 

 insulated plate was measured by a compensation method, the 

 plate and connected quadrants of the electrometer being kept 

 during the course of a measurement at zero potential by the 

 use of an induction balance t- 



In the case of thorium A, the thorium and radium emana- 

 tions — derived from the impure mesothorium — escaping from 

 the ebonite box, caused some ionization in the electrometer ; 

 * Proc. Rov. Soc. A. vol. lxxxv. p. 77 (1911). 

 t Townsend, Phil. Mag. vol. vi. p. 003 (1903). 



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