DEPARTMENT OF TERRESTRIAL MAGNETISM. 283 



As before, the diurnal variation of the penetrating radiation appears too 

 small to make its nature evident with certainty. 



It is naturally not an easy matter to secure data on the annual variation 

 in the case of ocean observations; nevertheless, in the various sections of the 

 cruise, the Carnegie has covered the same range of latitude several times, and 

 so it is possible to obtain some indication of the annual variation by meaning 

 observations for a week or more taken at different times in the various lati- 

 tudes. Thus, in the case of latitude 54° S., as a result of a four-point curve, 

 the potential gradient was found to show a strong minimmn in summer 

 (January), and although the data for this latitude did not extend beyond 

 March, the curve indicated an approximation to a maximum about Aprik 

 The air-earth current-density showed a minimum in January and an indica- 

 tion of an approach to a maximum in the winter. The conductivity gave 

 indications of a principal minimum in January, interrupted, however, by a 

 weaker maximum about the same time. The negative ionic density was 

 practically constant from December to March, but the positive ionic density 

 showed a sharp change from a minimum in the middle of January to a maxi- 

 mum in the middle of February. 



Conclusions mth regard to the annual variation must necessarily be tenta- 

 tive at this stage, but it is hoped that they will become stronger and stronger 

 as the data for their elucidation increases. 



On the Detection of Single a Particles. W. F. G. Swann. 



A method of detecting single a particles was first devised by Rutherford. 

 In this method the a particle was caused to ionize air which was under such 

 electric strain that it was just on the point of breaking down. The current 

 passing between the electrodes during the passage of an a particle into this 

 critical region was, under these conditions, very much greater than the 

 saturation current which could be produced by the ionization of the a particle 

 alone. 



In the present work, the aim has been to increase the stability and sensi- 

 tivity of the apparatus to such a degree that the saturation current due to the 

 direct ionization produced by the a particle may be detected. If this can be 

 accomplished readily and conveniently, a number of uses may be made of the 

 method. Thus, for example, it becomes possible to form an estimate of the 

 radium-emanation content of the atmosphere by counting the a particles 

 emitted by the air in a closed vessel. 



The general principle of the method is as follows : A vessel of about 30 liters 

 capacity forms an ionization chamber in which the a particles produce their 

 ionization. The central member of the ionization chamber is connected to 

 the fiber of a unifilar electroscope. The wall of the chamber is supplied with 

 a potential of about 500 volts, and the central member is protected from it by 

 a guard-ring. 



The system connected to the fiber is joined to the case of the electroscope 

 through a resistance of the order of 1,011 ohms, constructed by depositing 

 platinum upon a quartz fiber. 



The plates of the unifilar electroscope, instead of being connected to the 

 two ends of a battery whose mid-point is connected to the electroscope case, are 

 connected to the two ends of a megohm. The megohm is connected across the 

 battery and its mid-point is joined to the case of the electroscope. In this 

 way it results that fluctuations in the e. m. f. of the battery affect both plates 

 in equal and opposite directions, so that the electroscope is very much more 

 stable than if the megohm were not employed. A sensitivity as high as 1,000 



