390 



Reports on Special Researches 



RADIOACTIVE CONTENT OF THE ATMOSPHERE. 



In former work on the ocean, and in much of the work which has been done on land, 

 attempts have been made to obtain relative estimates of the amount of active material in 

 the atmosphere by the method devised by Elster and Geitel. This method suffers from 

 various disadvantages in respect of the indefiniteness of the meaning to be attached to its 

 results,' and even under the most favorable conditions it can not be said to afford a very 

 satisfactory method of estimating the radioactive content. 



Undoubtedly one of the best methods of determining the radium-emanation content 

 of the atmosphere is that involving the absorption of the emanation from the air by char- 

 coal, with a subsequent determination of the amount absorbed. The time required for 

 this operation, however, and the nature of the apparatus necessary, is such as to render the 

 method impracticable for use aboard ship. The method at present employed on the 

 Carnegie consists in drawing air between two concentric cylinders, the central one of which 

 is charged negatively to such a high potential that all of the active carriers entering the con- 

 centric cylinders are brought to the central system. The saturation current produced in 

 an ionization chamber by the active deposit collected in a given time is measured. This, 

 combined with a knowledge of the air flow during the collection of the deposit, enables 

 the amount of active material per cubic meter of au- to be estimated, 

 if one assumes a knowledge of the nature of the deposit, which latter 

 can be obtained from the form of the decay curve. The general prin- 

 ciple of this method has been used, in one form or another, by several 

 investigators on land, and the chief modifications introduced in the 

 present apparatus have been made with the object of rendering the 

 results more susceptible of accurate theoretical interpretation and 

 of increasing the sensitivity of the apparatus and its adaptability for 

 use at sea. 



The collecting apparatus, as at present employed, is shown in its 

 essential featm-es in Figure 24. It comprises a copper cylinder A, 

 64 cm. long and about 20 cm. in diameter, with an anemometer at one 

 end and a fan at the other. The central system consists of an insu- 

 lated wooden cylinder B, 12 cm. long and 12 cm. in diameter, sup- 

 ported by a rod passing through its axis and insulated from it by 

 sulphur, S. The surface of the wooden cylinder is covered with 

 copper foil, held on by rubber bands, and it is on this foil that the deposit is collected. 

 Earthed metal-caps CC, attached to the central rod, cover the top and bottom of the 

 central cylinder without touching it, and insure that the negative charge, and consequently 

 the active deposit, are confined to the copper foil. 



A large air-current is necessary if a large amount of deposit is to be obtained, and in 

 order to secure saturation with a reasonably low potential on the central cylinder, it is 

 necessary that the latter shall be large. A large cylinder, when afterwards introduced into 

 the ionization chamber, so as to form the central system there, would, however, on account 

 of the large capacity, reduce the sensitivity in the ionization-chamber measurements. 

 For this reason, the central system of the ionization chamber is formed of a thin rod, and the 

 foil, after removal from the inner cylinder of the collecting apparatus, is bent over and 

 made to line the walls of the ioiiization chamber, with the active surface facing inwards. 

 In this way, the foil does not contribute to the capacity of the system. The height of the 

 ionization chamber is about twice that of the foil cylinder, so that the latter only covers the 

 middle portion of the wall of the chamber, and in this way it is insured that none of the a 

 particles strike the top or botto m of the chamber. Thus, although the range of some of the 



•See C. W. Hewlett. Terr. Mag., vol. 19, pp. 146-148, 1914; also W. F. G. Swann, Terr. Mag., vol. 19, p. 91, 1914; also 

 Terr. Mag., vol. 19, pp. 176-182, 1914; also Terr. Mag. vol. 20, pp. 18-22, and pp. 30-43, 1915. 



Fig. 24. — Diagram of 

 Collecting System of 

 Radioactive- Content 

 Apparatus. 



