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665 HANDBOOK OF PROJECTION FOR
Note : — There probably will be a slight error, due to discarding fractions, but it should only total the fraction of 1/1000 of an inch.
Another, less accurate method, is to measure the distance from the mark to each end of the paper, and see how many times the shorter is contained into the longer, which tells you how the period of rest compares to the period of movement. It may be "five to one," "six to one," etc. Still another method is to set a carpenter's compass to cover the distance from the mark to the end of the paper on the shorter end, and then "step" it along the longer end, which will give you the number of times longer one is than the other. But, after all, the degree method is the only right way.
The other paper method is to cut a circle of paper the diameter of the flywheel, and on it, before cutting out the center, while the compass center mark still is there, lay out two lines across the paper, at right angles to each other. These are lines A A and B B, figure 230c. Their purpose will be made clear presently.
Now cut out the center of the paper, if it is necessary, on account of the end of the shaft or flywheel hub protruding beyond the plane of the rim of the wheel, thus preventing the paper from lying flat on the face of the rim of the wheel. Next attach the paper (which must be light cardboard or heavy, tough writing paper) to the rim of the wheel by means of a few drops of paste or shellac, so that it lies perfectly flat against the rim of the wheel.
Now locate points A and B, figure 230b, of sprocket movement, as before described, making marks A B, figure 230b, on the edge of the paper disc.
Remove the disc from the wheel, attach it to a clean sheet of writing paper by means of two or three small drops ot paste, and, using lines A A and B B, locate their crossing point, which will be the center of the disc.
Now you need a "protractor," which is a half circle of brass, with a center mark and the degrees of half a circle marked on its rim. You may secure one for a small price at any store which deals in draughtsmen's supplies. Usually stationery stores have them.
Lay the protractor with its center mark exactly at the center of the disc, and you have but to read the number of degrees between marks A B, figure 230b, to know the number of degrees during which the sprocket, hence the film, is in movement, and that subtracted from 360 gives the degrees in its period of non-movement, or rest.