224 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



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

 electron, whereas the mean free path of a simple or compound 

 ion is equal to or even less than the mean free path of a 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 simple or compound ions 

 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 

 the atoms of the gas when it collides with them, producing at 

 each collision a new electron and a simple ion. Similarly, if 

 the energy imparted to an ion between successive collisions ex- 

 ceeds a certain value, the ion is able to ionize the atoms of the 

 gas when it collides with them, producing at each collision a 

 new ion and an electron. Thus, the electron must fall freely 

 through a certain difference of potential (about 30 volts) in order 

 to receive enough energy to ionize air molecules, and a positive 

 ion must fall freely through a certain difference of potential (about 

 440 volts) in order to receive enough energy to ionize air molecules. 



127. The electric spark in a gas. When a gas is subjected to 

 an electric field of which the intensity is sufficient to cause both * 

 the electrons and the positive ions to ionize the gas, an extremely 

 rapid increase in the number of electrons and ions takes place, 

 and the result is the production of an electric spark. The mean 

 free path of the positive ions in a gas is inversely proportional to 

 the pressure of the gas so that the electric strength of a gas 



* When the intensity of an electric field is sufficient to cause only the electrons to 

 ionize the gas, then all of the electrons which are present in the gas flock towards the 

 positive electrode forming new ions and new electrons on the way, and when they 

 reach the positive electrode the action ceases except for the occasional formation of 

 an electron by outside influences. When the electric field is sufficiently intense to 

 cause electrons and positive ions both to produce ionization, then new ions and elec- 

 trons are formed everywhere between the electrodes and the number of free ions and 

 electrons increases indefinitely. It is a well-known fact that an electric field must 

 continue to act for an appreciable time before a spark is produced. 



