278 ADVANCED ELECTRICITY AND MAGNETISM. 



An ion is an atom of a gas from which a negatively charged 

 electron has been detached leaving the remainder of the atom 

 positively charged, or an atom to which an extra electron has 

 become attached thus giving the atom a negative charge. For 

 example the canal rays in a Crooks' tube consist of positively 

 charged atoms of the residual gas and the a-rays which are given 

 off by a radio-active substance such as uranium are positively 

 charged atoms. 



138. lonization by the electric field. According to the kinetic 

 theory of gases, a molecule of a gas travels on the average a 

 certain distance between successive collisions with neighboring 

 molecules. This distance is called the mean free path of the 

 molecule. The mean free path of an electron in a gas is about 

 41/2 times as great as the mean free path of the molecule of the 

 gas,* because of the very small size and great velocity of the 

 electron, whereas the mean free path of an ion is equal to or even 

 less than the mean free path of the molecule of the gas. When 

 a gas is subjected to an electric field by being placed between two 

 oppositely charged metal plates, a certain amount of energy is 

 imparted by the electric field to the electrons between successive 

 collisions, and a much smaller amount of energy is imparted 

 to the ion between successive collisions (because of their shorter 

 mean free path) . If the energy imparted to an electron between 

 successive collisions exceeds a certain value, the electron is able 

 to ionize an atom of the gas at its next collision, and when an 

 atom of the gas is thus ionized a new electron and a new ion are 

 produced. Similarly if the energy imparted to an ion between 

 successive collisions exceeds a certain value, the ion is able to 

 ionize an atom of the gas at its next collision thus producing a 

 new ion and a new electron. Experiment shows that an electron 

 must fall freely through a difference of potential of about 30 volts 



* The discussion of the electron theory in this chapter is necessarily very brief. 

 The student who is interested in the subject should read Sir J. J. Thomson's Con- 

 ductivity of Electricity through Gases, and Sir Ernest Rutherford's Radio-active 

 Tra nsformations . 



