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An absolute calibration of a set of counters has been obtained by determining their efficiencies and effective volumes. From observations recorded with this calibrated set, the number of ionizing cosmic rays is 0.80±{}0.028 per cm² per minute from unit solid angle at the vertical at sea level. The number of such rays from all directions as obtained by integration over an angular distribution curve is 1.48±{}0.055 cm^-2 min.^-1. This combined with Millikan's ionization chamber measurements gives an average specific ionization of 100±{}3.7 ion pairs per cm path of a ray. Comparison with previous measurements is made.
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Street et al. (1934) studied this question.
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