device for viewing a revolving or oscillating object by making the object appear to be at rest. In its simplest form, a stroboscope is a revolving disk with one or more slits or holes in its periphery through which the observer looks. By changing the speed of the disk it can be synchronized with the moving object, so that each time the slit passes before the eye of the observer, the moving object is in the same position. The simplest way of obtaining synchronization is to revolve a disk with a single slit or hole at precisely the same speed as the revolution or oscillation of the object; the same effect would be obtained using a disk with three evenly spaced holes running at exactly one-third the speed of the object. The observer, looking through the stroboscope disk, sees the object in exactly the same position each time the slit passes his or her eye, and, because of the persistence of vision, the individual glimpses blend into a single image that appears to be stationary, just as the individual frames of a motion picture are blended when the film is projected. If the stroboscope disk is not in exact synchronism with the moving object, the moving object will appear to move slowly in the direction of its actual motion if the disk is revolving at a slower rate than the object, and slowly in the opposite direction if the disk is turning faster.

Modern stroboscopes are generally equipped with a light that gives short flashes at the same rate that the moving object is revolving or oscillating. The neon or other vapor lamp (see ELECTRIC LIGHTING,; NEON LAMP,) used in such stroboscopes is flashed on and off by either mechanical means or an alternating current of variable frequency.

The stroboscope is useful in engineering. Such devices can be used to study vibration, wear, and distortion of moving parts while a machine is actually running. Stroboscopes can also be used to adjust the running speed of a machine to the exact rate desired; a simple example of this type is the stroboscopic disk that is used to regulate the speed of phonograph turntables. This is a paper disk that is printed with a pattern of lines and placed on the turntable. The lines are so spaced that, if the turntable is revolving at standard speed, they appear stationary when viewed by the light of a lamp flashing at the regular 6-cycle frequency of the AC power line.

High-speed gas discharge stroboscopic lamps, developed (about 1931) by Harold Eugene Edgerton and his associates at Massachusetts Institute of Technology, have made possible both still and motion picture photography of swiftly moving objects. These lamps are capable of producing flashes whose duration is as short as .000001 second, and more than 10,000 exposures per second can be made on motion picture film.