(100 D.V.) and the resonant vibration in the recorder induced by it. This latter had been previously tuned to give exactly 100 double vibrations in a second. A light aluminium stylus attached to the tuning-fork traced a sinuous line on a falling plate of smoked glass. The top of the vibrating recorder was so adjusted as to make successive dots during its vibration, simultaneously with the tuning-fork tracings. It will be seen from the record (fig. 65) that, corresponding to the crest of each tuning-fork wave and slightly to its right, we have a dot. The record given represents a period of fourteen one-hundredths of a second, there being fourteen crests made by the tuning-fork
Fig. 65.—Simultaneous record of vibrating-recorder and 100 D.V. tuning-fork exciter.
time-marker, and exactly coincident with these are the fourteen dots made by the vibrating recorder. The interval between any two dots, therefore, is an accurate measurement of one-hundredth part of a second. If the plate be moving at a uniform rate, the interval between these dots will be uniform. But the accuracy of the time-measurements in the curve is independent of the rate of movement of the plate, fur we calculate not by the distance but by the number of the dots. In the present figure the record was made on a plate which had been released and during its fall was acquiring increasing speed. The tuning-fork waves are thus gradually broadening out, and in exact correspondence to this the intervals between the dots are lengthening. When the phonograph motor which lets down the plate is just released, there is a short interval during which both that and the dependent plate are
