974 SCHOOL SCIENCE AND MATHEMATICS

A CLASSROOM DEMONSTRATION WITH A STROBOSCOPE.

By JACOB W. MOELK,

Proviso Township High School, Maywood, III.

Why do the spokes of wheels always seem to be runningbackward in the movies? What is happening in an electriclight bulb through which an alternating current is passing ?

These and other questions can be cleared up quite easilyand in an interesting manner by the use of the stroboscope.The accompanying diagram (see Fig. 1 of the precedingarticle) shows how very simple the device is. The blackstrips, about a half inch wide and two inches long, wereinked on a disc of white cardboard one foot in diameter.The disc is mounted on the shaft of a rotator and an elec-

tric light is placed so that the card is strongly illuminated byit. The lamp should be shaded from the eyes of the class.As the speed of rotation slowly increases, the grayish blurchanges to a series of black lines which seem to be rotatingin the opposite direction to that of the disc. As the speedis increased still further, the rotation of the lines seemsto slow up and stop, but with still greater speed theyacquire velocity in the direction of rotation of the disc.This is explained by the fact that the filament of the lightis glowing brightly one hundred twenty times every sec-ond (in a sixty cycle current) and is relatively dark a likenumber of times every second. If the disc is going fastenough so that a given black strip has time to come upduring a period of no illumination and take the same posi-tion formerly held by the preceding black strip while therewas light, then during the next fraction of a second whenthere is light on the disc there will be no apparent shiftingof the strips. If the light is turned off and the disc is il-luminated by daylight, the whirling, no matter how fast,will produce only a blur.

Simple and interesting problems can be given by the in-structor in connection with the demonstration. When thedisc is illuminated by the lamp and is rotating at a speedsufficient to produce no apparent motion of the strips :The number of alterations of the current per minute=

the number of black strips X the number of rotations ofthe disc per minute.