COSMIC RADIATION 



By P. M. S. Blackett, M. A., F. R. S. 

 Professor of Physics, Birkbeck College, University of London 



The study of cosmic radiation is rather a curious one. It is related 

 ultimately both to astronomy, to geophysics, and to physics. The 

 subject started about 1900 with the discovery by C. T. R. Wilson, and 

 by Elster and Geitel, that the air in a closed vessel had a slight residual 

 conductivity. The apparatus used for these early experiments con- 

 sisted of an ionization chamber. A simple form of this apparatus con- 

 sists in a metal box in which is suspended an insulated wire carrying 

 a gold leaf; when charged electrically, the movement of this leaf records 

 the electrical conductivity of the gas in the box. With such a simple 

 apparatus as this it was found that there was a residual conductivity 

 of the air, which could not be explained by the effects of the known 

 radioactivity of the earth's crust, and which was probably due to the 

 presence of some very penetrating radiation. In fact, C. T. R. Wilson 

 himself, in 1901, speculated as to whether this residual ionization might 

 not be due to some radiation coming from sources outside the atmos- 

 phere, either electromagnetic radiation, like X-rays, or corpuscular 

 rays, like cathode rays, but of enormously greater penetrating power. 

 Since that time at least 1,000 researches have been made on the subject 

 of cosmic rays and a great many facts have been found out. We know 

 now that this residual ionization is, in fact, due to atomic particles of 

 enormous penetrating power coming into the earth's atmosphere from 

 some sources outside the solar system, but exactly what these particles 

 are, or where they come from, or how they were formed, or when, we 

 still do not know. 



In this first lecture I am going to describe mainly the researches on 

 cosmic rays which have been made with the ionization chamber as the 

 instrument for their detection. There are, in addition, two other 

 instruments by which these rays are detected and measured, and these 

 will form the subject of the two succeeding lectures. 



Soon after the earliest experiments ionization chambers were taken 

 to different places on the earth to find out whether this residual ioni- 

 zation varied from place to place. Then a great series of experiments 

 began in which ionization chambers were taken up mountains, in 

 balloons, lowered down to the depths of the sea and carried in aero- 



1 The first of three lectures delivered before the Royal Society of Arts. Reprinted by permission from the 

 Journal of the Royal Society of Arts, vol. 85, No. 4421, August 13, 1937. 



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