392 



ELECTRICAL MACHINERY 



But is is found that a tube filled with ionized* mercury 

 vapor (the air having been exhausted from the tube) , having 

 mercury for one terminal and some such metal as iron for 

 the other terminal permits of the passage of current in one 

 direction only. If an alternating voltage is impressed on 

 the two terminals of the tube, current will flow through the 

 tube only when the mercury terminal is negative and the 

 iron terminal positive. In the tube itself, the current can 

 flow from the iron to the mercury but cannot flow from the 

 mercury to the iron. 



FIG. 258. A Tube Filled with Ionized Mercury Vapor Acts like a 



Rectifying Valve. 



The Mercury Tube Acts Like a Check Valve. If a mer- 

 cury tube is kept filled with ionized mercury vapor it will 

 therefore act as a rectifying valve; the current which flows 

 through it will be pulsating as shown in Fig. 258. Although 



* The term ionized is used to designate a gas in which the normal 

 atom has been somewhat changed; a minute quantity of negative 

 electricity (called an electron} has been freed from the atom and can 

 move away from it. A gas in which this change has occurred conducts 

 an electric current quite freely and it is said to be ionized; a gas in its 

 normal condition conducts very poorly, in fact it is a good insulator. 

 A gas may be ionized by the passage through it of an electric spark, 

 by the influence of radium, etc. 



