152 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



electrons, and perhaps it is fair to add that the younger they are, 

 within reason, the more they know about them. The older among 

 us are conscious that this omniscience of the younger is not wholly 

 confined to electrons. However that may be, it is certain that if you 

 shoot electrons at right angles to a uniform magnetic field, the elec- 

 trons will go round in circles — exact circles. The stronger the field 

 and the slower the electrons, the smaller will be the circles, and the 

 converse is true. The mathematical, electrical, and mechanical prin- 

 ciples are simple and certain. People are beginning to believe that 

 physics is becoming indefinite and hazy, because they attend to the 

 fascinating borderlands of speculative science and are forgetful of 

 the definite and eternal verities. If, on the other hand, electrons are 

 projected obliquely to a magnetic field then the electrons will each one 

 describe a helix or a path with the shape of a corkscrew. 



If electrons are shot earthward from the sun, they will travel 

 through space and become entrapped by the magnetic field of the 

 earth. They will spiral round the lines of force until they meet the 

 upper atmosphere in the regions surromiding either the north or 

 south magnetic poles. The speed of such electrons may be sufficient 

 by their collisions to ionize the molecules, that is, to knock other 

 electrons from them, thus leaving positively charged molecules, or 

 ions. The recombination of electrons with positive ions is attended 

 with radiation, as has been amply proved in laboratory experiments. 

 It is generally believed that electrons spiraling round in one direc- 

 tion arrive near the north magnetic pole and give rise by ionization 

 to the aurora borealis; while similar electrons spiraling round the 

 lines of magnetic force in the other sense proceed toward the south 

 magnetic pole and occasion the aurora australis. This result is well 

 confirmed by experiment. In spite of some effort, I have not yet been 

 able to discover whether major displays of northern and southern 

 lights occur together at the same time in both Arctic and Antarctic 

 regions. There are some theoretical reasons for expecting such coin- 

 cidence, and some of the major displays, such as that of February 4, 

 1872, have been seen in both northern and southern latitudes. 



There are some authorities who declare that light charged par- 

 ticles such as electrons would mutually repel one another on their 

 long journey from the sun, so that they would be scattered far afield 

 and in that case there should be no auroras at all ! Prof. S. Chap- 

 man states that there are positive, negative, and neutral particles all 

 coming from the sun. There is a very wide choice of possible pro- 

 jectiles — electrons, positrons, protons, neutrons, deutons, alpha par- 

 ticles, and cosmic rays, besides photons. Therefore it is not wise to 

 be too dogmatic as to the nature of the bombardment that arrives at 

 the earth's surface, but it is right to insist that only electrically 



