COSMIC RADIATION — JOHNSON 



205 



FiGDRH 3. — Regions of the sky, repre- 

 sented in white, illuminated by rays of 

 rigidity corresponding to 10-billion-volt 

 positive electrons. 



7. THE PROBLEM OF THE REAL EARTH 



The only difference between the simplified problem and the prob- 

 lem of the real earth is in regard to the numerical relationship 

 between any value of the rigidity and the angle at which it cuts off, 

 for any latitude. In the case of the real earth St0rmer, and Lemaitre 

 and Vallarta have determined these angles by solution of the mathe- 

 matical equations of motion of the rays in the field of the magnetic 

 doublet of the earth. The results 

 are illustrated in figure 3. The 

 areas of the sky, represented in 

 white, give the angles from which 

 rays of the rigidity of a 10-billion- 

 volt positive electron can reach the 

 observer at the various latitudes. 

 On the Equator the bending force 

 of the field is greatest, and rays of 

 this rigidity almost miss the earth 

 completely. There is but a small 

 white area on the western horizon. 

 To reach the earth from all direc- 

 tions at the Equator, rays must 

 have the rigidity of a GO-billion- 

 volt electron. In northern Mexico, 

 on the other hand, the 10-billion-volt electrons can come in from all 

 directions. Positive rays of low rigidity appear first on the western 

 horizon, negatives on the eastern horizon. 



8. THE EXPERIMENTAL MEASUREMENTS OF LATITUDE-INTENSITY 



VARIATIONS 



The first indication of an effect of the earth's magnetic field in 

 altering the cosmic ray intensity distribution was found in 1928 by 

 Clay. From his measurements the intensity seemed to be lower in 

 equatorial latitudes. Compton's world survey begun in 1932, has 

 placed the result on a firm experimental basis and has shown that 

 the intensity depends upon geomagnetic and not upon geographic 

 latitude. The lower intensity at the Equator is thus caused by the 

 magnetic field and not by some other systematically varying quan- 

 tity such as temperature. Very careful studies of these variations 

 by Millikan and his associates have resulted in accurate data, par- 

 ticularly in the case of automatically recorded sea-level measure- 

 ments. In all three cases electroscopes have been used, and the re- 

 sults pertain to the average of the effects from all directions. Some 

 directionally selective measurements, ideally more favorable for the 

 analysis, have been made with coincidence counters mounted on 

 shipboard by Auger and Leprince-Ringuet. 



