304 BELL SYSTEM TECHNICAL JOURNAL 



charge is e), but to say more would be premature. The basis for 

 supposing it equal to the mass of the electron is the feeling that there 

 ought not to be any other fundamental masses in Nature than we knew 

 already, together with certain suggestions from the quantum-mechan- 

 ical theories of Dirac. The estimation of the mass may be bettered, 

 if it is possible to observe collisions between positive and negative 

 electrons with the expansion-chamber and to trace the paths of the 

 colliding particles; there are reports that this has already been done 

 with some success. 



The action of cosmic rays being something which we cannot intensify 

 nor control, it is doubly fortunate that another agent has already been 

 discovered which is capable of generating positive electrons; for these 

 particles have been observed, by several people in several different 

 schools, leaping out of sheets of metal bombarded by "hard" or high- 

 frequency gamma-rays. At the first observations, the bombarding 

 radiation was a mixture of gamma-rays with neutrons, and it was not 

 unnatural to suppose that so novel a result must be due to the action 

 of the novel kind of corpuscle. Perhaps in those experiments the 

 neutron did participate in the effect; but it has now been found — by 

 Anderson and Neddermeyer in Pasadena, by Meitner and Philipp in 

 Berlin — that gamma-rays suffice. Those employed so far are chiefly, 

 if not altogether, the radiation from thorium C" consisting of photons 

 of energy 2.6 millions of electron-volts. 



A theory quite extraordinary, indeed by all prior concepts revolu- 

 tionary, has been propounded by Blackett and Occhialini: it is the 

 idea that the photon converts itself into a pair of electrons, positive 

 and negative respectively. The net charge of the universe is not 

 altered by such a process, since the two created charges balance one 

 another; neither is the total mass of the universe, for the masses of 

 the two electrons (including the kinetic energy wherewith they are 

 endowed) are equal altogether to that of the vanished photon. For 

 this theory it may be said, in the first place, that positive electrons 

 frequently appear jointly with negatives, one particle of each kind 

 springing forth from a single point: Anderson and Neddermeyer have 

 observed no fewer than 22 of such cases. Moreover if the theory is 

 true, the total kinetic energy of the two particles of such a pair — and 

 a fortiori the kinetic energy of the positive electron by itself — must lie 

 below a certain upper limit, which is computed by deducting a million 

 electron-volts from the energy of the responsible photon; for this is 

 the amount of energy which by Einstein's relation (which will figure 

 prominently in the latter part of this article) must be used in building 

 the electrons by themselves. Thus if in these experiments with 



