NUCLEUS OF THE ATOM — ANDERSON 239 



and energies of the individual particles which were known to be 

 ejected from all material substance through which these rays passed. 

 A modified form of the well-known Wilson cloud chamber was in- 

 corporated into a powerful electromagnet. By means of the cloud 

 chamber it was possible to photograph not an individual electron 

 itself but an outline of the exact path followed by the electron in its 

 passage through the gas of the chamber. We might compare this 

 process to that of an observer in an airplane flying very high, who, 

 though unable to discern a ship, might see very clearly its wake in 

 the water. Through the aid of the magnetic field the speeds or ener- 

 gies of the individual electrons can be determined. Any electrically 

 charged particle, such as an electron, when between the pole pieces 

 of a magnet, moves not in a straight line but in a curved path. A 

 measurement of the radius of curvature then allows the speed to be 

 easily calculated. 



This experiment showed at once several striking results. Elec- 

 trons of prodigiously high speeds were observed, some of them hav- 

 ing more than 99 percent the velocity of light; but of far more im- 

 portance than this was the fact that many of the particles, from the 

 sense in which they curved in the magnetic field, were seen to contain 

 a positive electrical charge. This observation showed immediately 

 that the nucleus of the atom plays an important role in the absorp- 

 tion of the cosmic rays. It was an observation of a nuclear process, 

 entirely different but in a sense analogous to that made by Ruther- 

 ford in 1919, when he observed protons driven out of nitrogen nuclei. 



The results of these preliminary observations were reported in the 

 fall of 1931, and the spring and summer months were spent in 

 studying the properties of these particles of positive charge. A 

 plate of lead was inserted into the cloud chamber to act as a barrier 

 for the particles. A particle in passing through this lead plate would 

 give up a part of its energy and emerge from the plate with a lower 

 velocity than it possessed upon entering; hence the sharpness of the 

 curvature in the magnetic field of the particle before and after trav- 

 ersing the plate would show a difference, depending upon the amount 

 of energy lost in the plate. Measurements made on the track of a 

 particle before and after passing through a plate, together with 

 observations of the density of the track itself, give definite infor- 

 mation about the mass of the particle and tlie magnitude of the 

 electric charge it carries. 



In August 1932 a photograph was obtained which showed clearly 

 a particle of positive charge passing through the plate of lead and 

 emerging with a lower energy. The evidence presented by tliis 

 photograph was so clear-cut that after the negative film was re- 

 moved from the developing bath and before it was dry, we reached 

 the conclusion that this particle might represent a positive electron. 



