Your New Servants: The Electronic "Watchmen" 117 



If we seal a piece of tungsten wire into a glass tube, and 

 draw all the air out of the tube, and heat the wire, electrons 

 will be emitted or evaporated from the wire. If we seal another 

 piece of metal, called a plate or anode, into the tube, and con- 

 nect the tube into an electric circuit so that there is a positive 

 charge on the plate side of the circuit, the electrons which 

 are emitted from the tungsten filament or cathode will jump 

 through the space between the cathode and plate, and current 

 will flow across empty space. 



If the plus charge on the plate is increased, more and more 

 electrons will be drawn to the plate and more and more cur- 

 rent will flow. If the charge on the plate is changed to a nega- 

 tive charge, or an excess of electrons, the current will not 

 flow. 



Thus the diode, or two-element vacuum tube, gives us a 

 one-way path for the flow of electric current. If we wish to 

 change alternating current into direct, or one-way, current, 

 we can put a diode in the circuit; and the current will flow 

 but one way. We say the current has been rectified, and we 

 call a tube used in this way a rectifier. The usefulness of the 

 rectifier tube is limited only by the amount of voltage it can 

 stand without breaking down. For a long time this was a 

 severe limitation; the first rectifiers could not stand more than 

 thirty volts! A three-element rectifier, called the thyratron, is 

 capable of handling tremendous voltages. 



What this one tube can mean to the power industry has 

 been graphically stated by Raymond F. Yates in his article 

 "The Coming Electric Age" in the Science Digest: 



"The use of the new tube known as the thyratron will 

 eventually save the industry many millions of dollars annu- 

 ally. 



"For reasons well known to technicians, it has been impos- 

 sible to transmit anything but high-voltage alternating cur- 

 rent. We simply do not know how to generate high-voltage 

 direct current, yet this would be the ideal current if we could 



