198 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 193 5 



depend upon the ability of a cosmic ray to ionize, or split apart, the 

 electrical charges of the atoms of matter through which they pass. 



If the air in a chamber is supersaturated with moisture the 

 charged atomic fragments, or ions, act as centers of condensation 

 for droplets of water and these may be photographed. Plate 1, 

 figure 1, shows such a photograph of the ion trail left in the wake 

 of a cosmic ray. 



Another simple device for detecting ionizing radiations is the 

 Geiger-Mueller counter, represented in figure 1. A metallic cylinder 

 is placed in a glass tube, within which the air pressure is reduced 

 to about a tenth of the normal atmosphere, and a fine wire is 

 stretched along its axis. In operation the cylinder is charged nega- 

 tively to 1,500 volts and, if a ray produces as much as a single ion 

 within the cylinder, this is swept by the electric field toward the wire. 

 As it approaches, the acceleration increases until the energy gained 

 between encounters with atoms of the air causes it to ionize these at 



each successive collision. 



Each new ion becomes 

 "If — ~ an ionizing agent and an 

 avalanche resembling an 

 electric spark is started. 

 /O'^O/im^ \ Xhe flow of electric 



charge becomes great 

 enough to be detected 

 directly or it can be am- 

 plified and recorded on 



FiGDRE 1. — Diagrammatic sketcli of the Geiger-Mueller •+ ui ^ ^r ' A 



counter aud its electrical connections. *^ SUltaDie eiCCtriC QCVlCe. 



Wlien a small counter 

 of this kind, 4 centimeters long and 1 centimeter in diameter, is con- 

 nected to an amplifier and loudspeaker, about 20 clicks are heard in 

 a minute. A radioactive substance placed near the tube can increase 

 the count to several hundred a minute, each count indicating the pas- 

 sage of one ionizing ray. The radioactive radiations are well known 

 and will not come into the discussion further. The subject of interest 

 pertains to the 20 rays per minute indicated when all radioactive 

 materials are removed. 



These rays have been known to exist for 35 or 40 years but only 

 in recent times have differences been recognized between them and 

 the radioactive radiations. 



One such difference of particular significance in the investiga- 

 tions is the ability of the residual radiation to excite avalanche dis- 

 charges in several tubes at once. If two counters are connected 

 in an appropriate electrical circuit the simultaneous discharges can 

 be automatically selected from the others and recorded separately. 

 In the case of twoi large counters whose individual discharge rates 



/SOO Vo/fs 



