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this at twice the frequency of the supply. The usual 50-cycle supply therefore produces 100 reversals of the current per second. At each reversal the current falls momentarily to zero and the brilliance of the lamp falls also. Although our eyes cannot normally detect this rapid flickering, we can use it to observe the speed of a gramophone turntable.
Most gramophone dealers stock stroboscopic discs, consisting of a card, about the size of a gramophone label, and printed with black and white spokes or bars. When you rotate a disc of this kind, the white spaces between bars are seen most clearly when the light is momentarily most brilliant. While the light is momentarily less brilliant, the white spaces are less easily seen. So if the white spaces are always in the same position each time the light is at its greatest brilliance, the bars and spaces appear to stand still. Since a 50-cycle supply makes a lamp reach peak brilliance 100 times every second, 100 bars must pass a given point in that time.
You can calculate the number of bars required on a stroboscope from the equation:
No. of black bars = Light flickers per second x 60 Revolutions of disc per minute
There must always be a whole number of bars on the disc and consequently you cannot always provide a stroboscope for exactly the speed required. For example, using the equation to determine the number of bars for a record running at 78 r.p.m. :
100 x 60
78
= 76 92
The best you can do is to provide 77 bars. Then you can calculate the speed at which the 77 bars will appear stationary. Thus:
77 _ 100 x 60
Revolutions of disc per minute
and so the speed is 77-92 r.p.m., which is quite close enough to 78 r.p.m.
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