Your New Servants: The Electronic "Watchmen" 129 



tive purchaser "Green" . . . "Firm" . . . "Just right!" . . . 

 "Dead ripe half price." 



Checking Moving Objects 



One of the most fascinating electronic instruments in use 

 today is the stroboscope. Let us imagine we are watching the 

 process as an engineer checks the revolutions per minute of 

 a whirling machine. He will bring his stroboscope, which in 

 appearance is simply a metal box with a strong flashlight 

 in one end of it, a dial on top, and a meter he can read. He 

 turns his "flashlight" on the machine and a strong glow of 

 light illuminates a whirling wheel. It seems to us to be a 

 steady light, but in reality it is flickering off and on so rapidly 

 that we cannot detect the flicker. "Watch the wheel," he 

 says, and we watch it. He turns his dial. A strange thing 

 happens. He has not touched the machine; we can hear it still 

 running at breakneck speed. But there, before our eyes, the 

 wheel slows down, wavers, and stops! Seeing is no longer 

 believing! Our ears tell us the wheel is still revolving. Our 

 eyes tell us it is standing still. The engineer reads his meter. 

 "Fifty-five hundred revolutions per minute," he says. He snaps 

 off the light of the stroboscope, and again the wheel revolves 

 before our eyes. 



How is this possible? Because he has changed the fre- 

 quency of the flickering of the light in his stroboscope until it 

 is going off and on at exactly the speed at which the wheel 

 was revolving; and, therefore, each time the light illuminated 

 the wheel, it caught it in exactly the same position. 



For a simple, quick, and accurate method of determining 

 the speed of a revolving wheel, the stroboscope is invaluable. 

 Likewise, it may be used to detect any "wobble" in the wheel 

 which may develop at a given speed, so that this fault can 

 be corrected before a breakdown occurs. It can also be used 



