High Frequency Rays of Cosmic Origin. ROBERT ANDREWS MILLIKAN. (Copyright by Science Service, Inc.)--It was as early as 19o 3 that the British physicists, Rutherford and McLennan, noticed that the rate of leakage of an electric charge from an electroscope within an air-tight metal chamber could be reduced by enclosing the chamber within a completely encircling metal shield or box with "walls a centimetre or more thick. This meant that the loss of charge must be due to highly penetrating rays, like the gamma-rays, which pass through metal walls and ionize the gas inside.
This radiation was provisionally called "penetrating radiation" and at first attributed to radio-active materials in the earth. In I9Io and 19II it was found that it does not decrease with altitude as rapidly as it should, on this view. A significant report was made by Gockel, a German physicist, who took an enclosed electroscope up in a balloon to 13,ooo feet and found the "penetrating radiation" about as strong as at the surface, although Professor Eve, of McGill University, had calculated that it would fall to half its surface value at 250 feet.
In i912, 1913, and 1914, Hess and K, ohlhorster repeated the balloon measurements of Gockel, the latter going to 5.6 miles. They found the radiation decreasing a trifle for the first two miles and then increasing until at 5.6 miles it was eight times as great as at the surface.
This indicated that the penetrating rays come from out the earth. The war put a stop to further studies, but as soon as proper instruments could be built in the newly equipped Norman Bridge Laboratory of Physics, Millikan and Bowen sent up at Kelly Field in 1922 , near San Antonio, Texas, four little recording electroscopes in sounding balloons to almost twice the height formerly attained. The highest point was near ten miles. These instruments were built to hold 300 c.c. of air at I5O pounds pressure, and provided with recording barometer, thermometer, and electroscope, three different sets of moving photographic films and the necessary driving mechanism. The total weight was I8O grams.
Millikan and Bowen expected to find very large rates of discharge; nine-tenths of the atmosphere was beneath, and only onetenth was left to cut down the intensity of the rays from outside. The results were contrary, but it was proved that the penetrating radiation was greater at great altitudes than at the surface, but that the amount of the increase was not more than a fourth of that expected. In I924 Hess and Kohlhorster reduced their estimates to accord with the above measurements.
The origin of the rays was uncertain, with indications of a cosmic source. Millikan and Otis studied how penetrating are the rays.