ELECTRONS AND ATOMS 49 



along an electric wire or other conductor 

 which constitutes an electric current. These 

 minutest of all particles may be deflected out 

 of their straight course when they are in 

 motion by a charged surface placed parallel 

 to their flight, or by a magnet, and the course 

 of deflection is the same as that of a stone or 

 bullet flying through the air. From the 

 velocity and the deflection and other experi- 

 mental data the mass of the particle and the 

 value of the electrical charge upon it can be 

 calculated. The mass of the electron lies 

 between the ^^-th and ^^th of that of a 

 hydrogen atom. Since such electrons can 

 be obtained either by spontaneous disruption 

 of radio-active bodies, or, by means of electric- 

 energy, from ordinary stable forms of matter, 

 they are believed to be the constituents of 

 ordinary atoms from which such atoms are 

 built up. 



The alpha-particle differs from the beta- 

 particle in two chief ways, first, the alpha- 

 particle carries a positive charge of electricity, 

 as is shown by the fact that it is deflected in 

 the opposite direction to the beta-particle by 

 a magnet or an electrically charged body, 

 and secondly, by the fact that it is many 

 times larger, possessing twice the mass 

 approximately of a hydrogen atom. When 



