SummaryAll instruments designed to give a record of some fluctuating quantity as a function of the time, both those employed to give “instantaneous” values of the quantity measured and also those which totalise or integrate it during a given time, are liable to sufler from error due to the inertia of their moving parts and consequent “lag”. In each class instances are given of instruments which are free from the error, but the Robinson cup anemometer—in which the authors are more particularly interested—is not one of those. As nothing was known definitely as to the effects of lag in this case, an investigation was undertaken, using a specially designed apparatus analogous to the cup anemometer, and, for greater accuracy of measurement, currents of water instead of air.A description of this apparatus is given; and the remainder of the paper is occupied with the mathematical theory and the results of experiment. The most striking of these is the fact that the instrumental error is always positive; that is to say, that when the current fluctuates the instrument registers too great a flow.The authors desire to record their thanks to Professor MacGregor for the facilities he has provided, and for the kind interest he has taken in their work.Part of the special apparatus described was obtained by a grant from the Tait Memorial Fund.
