The Stroboscope

 Edgerton Observing a Synchronous Motor1

Although many people associate the stroboscope with Edgerton, it was actually invented in 1832.  The term "stroboscope" comes from Greek for "whirling watcher".  The "whirling watcher" was simply a disk with slots at regular intervals.  As an observer looked at moving subject through the slots in the spinning disk, he could see successive stages of the subject's motion.

When Edgerton began his electrical engineering studies at MIT in 1926, his research was focused on the stability of synchronous motors, observing the changes in angular displacement of the rotors as a result of disturbances to the system.  As the story goes, Edgerton noticed that flashes from the mercury arc lamps he was using made the rotor visible without blur.  This gave him the idea to build an electronic stroboscope which could be used to visually observe the changes in the motors angular displacement.

While the rotor appears as a blur to the naked eye, the stroboscope allows one to observe changes in the rotor's speed.  Say a marking is placed on the rotor.  If the light flashes at the same rate as the motor's rotation (i.e. the number of flashes per minute is equal to the motor's revolutions per minute), then the rotor will make exactly one complete turn for each flash of light.  To an observer the rotor will appear stationary:

If the frequency of the flashes is less than the motor's rotation rate, then the light will flash before the rotor can make a complete revolution, and it will appear as though the mark on the rotor is slowly rotating backwards:

Similarly, if the frequency of the light flashes is greater than the motor's rotation rate, the mark will appear to turn forwards:

Thus, a stroboscope set to flash at the motor's fundamental frequency can be used to visually observe when the rotor's angular displacement has changed, and the stroboscope can be used to slow down periodic motion enough that the human eye can perceive it.

Using this principle, Edgerton was also able to create high speed motion pictures of his motors in action.  An ordinary motion picture camera takes 24 frames per second.  It moves the film through the camera frame by frame and opens a shutter to expose each frame.  The speed of this camera is limited by the speed of the shutter mechanism.  Edgerton was able take motion pictures at 6,000 frames per second by using a stroboscope instead of a continuous light source, and pulling the film through the camera at 100 feet per second.

Edgerton's electronic stroboscope had several advantages over the mechanical stroboscope invented a century earlier.  This stroboscope increased the illumination of the object, allowed flashes of shorter duration at controllable intervals, and facilitated several observers at once.

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1 Edgerton, Harold E. and James R. Killian, Jr. Moments Of Vision: The Stroboscopic Revolution in Photography MIT Press: Cambridge, 1979.