1912-13.] Error caused by “Lag” in a Recording Instrument. 103 
IX. — The Error caused by “Lag” in a Recording Instrument: An 
Experimental Study. By J, R. Milne, D.Sc., and H. Levy, 
M.A., B.Sc., Carnegie Scholar. 
(Read February 17, 1913. MS. received February 18, 1913.) 
Introduction. 
Some time ago one of us was engaged in experimental work in which 
it was necessary to measure the total flow of air past a certain instrument 
exposed out of doors.* The attempt to do this by means of a Robinson 
cup anemometer raised the question of the accuracy of such an instrument 
in a gusty wind. Owing to the inertia of the moving parts the apparatus 
revolves too slowly at the beginning of a gust, and too fast at the con- 
clusion of a gust ; and it is hardly to be expected that the two errors will 
exactly compensate each other. All instruments designed to record some 
rapidly fluctuating quantity are liable to suffer from the effects of “ lag,” 
and much ingenuity has been exercised in minimising or overcoming the 
difficulty. If, for example, the differential equation of motion be known, it 
may be possible to subsequently correct the record of the apparatus.]* That 
many cases, however, are insusceptible of such treatment is evident for a 
variety of reasons ; a case in point is furnished by Professor Chrystal’s 
investigation of “ The Theory of the Leaking Microbarograph,” J in which 
it is shown that the instrument record seriously distorts even ideally simple 
atmospheric disturbances, and also that those which are actually observed 
are of a most complicated nature. In the case of other instruments it has 
been found possible to so arrange the design that the effects of lag are 
almost or altogether absent. Modern oscillographs furnish a striking 
example of this,§ as to a less degree do certain types of seismographs. || Now, 
no doubt an instrument might be constructed to record rapidly fluctuating 
* “ On Atmospheric Cooling : An Experimental Investigation,” J. R. Milne, D.Sc., Jour. 
Scot. Met. Soc. (3), vol. xvi., No. xxix. p. 9, 1912. 
t In regard to this see, e.g., Prof. Hopkinson’s tract on Vibrations of Systems having One 
Degree of Freedom , chap. ii. 
t Proc. Roy. Soc. Edin., vol. xxviii., part vi., p. 437, 1908. 
§ See papers by A. Blondel, Gomptes Rendus , vol. cxvi. pp. 502 and 748, 1893 ; “Oscillo- 
graphs,” W. Duddell, Brit. Ass. (Toronto), 1897 ; and Electrician , vol. xxxix. p. 636. 
|| For an interesting account of such instruments see The Physics of Earthquake Pheno- 
mena, by C. G. Knott, D.Sc., chaps, iv. and v. 
