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IX.—The Error caused by “Lag” in a Recording Instrument: An Experimental Study
Published online by Cambridge University Press: 15 September 2014
Summary
All 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.
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- Copyright © Royal Society of Edinburgh 1914
References
page 103 note * “On Atmospheric Cooling: An Experimental Investigation,” Milne, J. R. D.Sc., Jour. Scot. Met. Soc. (3), vol. xvi., No. xxix. p. 9, 1912.Google Scholar
page 103 note † In regard to this see, e.g., Prof. Hopkinson's tract on Vibrations of Systems having One Degree of Freedom, chap. ii.
page 103 note ‡ Proc. Roy. Soc. Edin., vol. xxviii., part vi., p. 437, 1908.
page 103 note § See papers by Blondel, A., Comptes Rendus, vol. cxvi. pp. 502 and 748, 1893Google Scholar; “Oscillographs,” Duddell, W., Brit. Ass. (Toronto), 1897Google Scholar; and Electrician, vol. xxxix, p. 636.
page 103 note ∥ For an interesting account of such instruments see The Physics of Earthquake Phenomena, by C. G. Knott, D.Sc., chaps, iv. and v.
page 104 note * The mathematical theory of the Grassot fluxmeter is to he found in the catalogue of the Cambridge Scientific Instrument Co.
page 104 note † As regards cases in which this no longer holds true, see, e.g., Ferry, E. F., Phys. Rev., vol. i, No. 5, p. 338Google Scholar; and for the contrary see, e.g., Investigations on the Theory of the Photographic Process, Sheppard and Mees, p. 18, 1907.