Your New Servants: The Electronic "Watchmen" 125 



they are merely flaws and have not become the cause of a 

 tragic failure that could cost the lives of men. Recently a mil- 

 lion-volt machine was used to "see" through eight-inch armor 

 plate, and a new one-hundred-million-volt "induction elec- 

 tron accelerator" is being developed; so the future possibili- 

 ties promise detection of flaws in even more massive castings. 



Airplanes are now being examined by X-ray units which 

 can see through five inches of steel, detecting any flaws. 

 Tomorrow, when this equipment is generally available for 

 commercial planes, it will save countless hours of checking 

 time and many lives. 



The X-ray diffraction camera is used to show not merely 

 the flaws in a material but every element which goes into 

 making that material. The slightest variation in a substance 

 can be detected as quickly as we could distinguish chocolate 

 from angel cake. 



The Photoelectric Tube 



One of the most interesting vacuum tubes in common use 

 today is the photoelectric tube. In this tube the filament is 

 coated with caesium or another metal which will emit elec- 

 trons when light falls on the metal surface. When light shines 

 on the tube, current flows; when something casts a shadow 

 on the tube, the current stops instantly. By using various color 

 filters with the tube, it can be made to respond to one color 

 and not to another. 



If the current from a photoelectric tube is fed to the grid 

 of such a triode as the thyratron, it can start and stop large 

 currents thus starting and stopping heavy machinery. It is 

 this tube which turns on a fountain as you bend to drink; 

 your head interrupts the current flow, a switch is released, 

 and the water is turned on. It is this tube which opens a heavy 

 door as you approach. Your shadow interrupts the light beam 



