THE PEAL SPEED TESTER.
which the wheel is travelling being known, the length of the arc shows the duration of exposure. For example, fix a small piece of tinfoil to the outer edge of, say, the front wheel of a bicycle, and place it in the sunshine against something very dark, so that the tinfoil may reflect as much light as possible. Focus the wheel exactly 3 in. diameter, near the top of the ,-plate (a circle that size may be drawn in pen cil on the focussing screen as a guide) ; set the shutter at its lowest speed, and cause the wheel to revolve rapidly. See how ninny complete revolutions it makes in ten seconds. If possible, it should be made to go round ten times exactly ; but, supposing it only goes eight, then it will make one revolution in !} sec. Release the For accurately testing and timing shut ters there is nothing simpler than the Peal device shown by Fig. 122. An ordin ary deal box A is divided clown the centre E ; in the front is a cardboard dial K, and running smoothly from back to front is a knitting needle B, to which a wooden hand c is attached, having at its extremity a silver bead D. The dial is divided into 100
parts, and the hand is made to describe a circle twice in a second. To regulate the speed, the needle passes through a cork IT, on either side of which is fastened a piece of card F, accurately balanced. The rotating power is a length of worsted or silk cord i, wound a few times round the needle ; to one end is fastened a 4-oz. weight s, and a smaller one to the other, the cords being kept apart by a drum a. The time occupied in passing between two points is noted, and the fans altered to give more or less resistance till this is exactly fifty seconds, during which time the hand should have made 100 revolu tions.