416 Proceedings of the Boyal Society of Edinburgh. [Sess. 
XXVI.— On the Accuracy attainable with a Modified Form of 
Atwood’s Machine.* By John P. Dalton, M.A., B.Sc., Carnegie 
Research Fellow. Communicated by Professor W. Peddie. 
(MS. received March 6, 1911. Read same date.) 
1. Introduction. 
A careful determination of g by means of the ordinary type of Atwood’s 
machine does not, as a rule, lead the average student in a Physical 
Laboratory to a better result than 930 or 940 cm/sec 2 . From the point 
of view of successful teaching, it is somewhat unfortunate that, after bestow- 
ing reasonable care and attention to his work, a student should be unable 
to obtain a result approximating satisfactorily to what he knows to be the 
correct figure. Not unnaturally he takes it for granted that the actual 
numerical result obtained from his experimental labours is quite immaterial 
as long as the processes involved are clearly comprehended, and to him 
Experimental Physics is anything but an exact science. On the other 
hand, to set before the ordinary student a complicated apparatus specially 
designed for reaching an accuracy of 04 per cent, would be proceeding to 
the other extreme, and one could hardly expect much benefit to be 
derived from its use. But even the student who has already had some 
experimental training, and who has realised that quantitative relationship 
is just as important as qualitative, could not do any better in this case, for 
the defects are inherent to the method usually followed of timing the fall 
through a distance of 150 or 200 cm. with a metronome or stop watch: 
in his interests, at any rate, a more accurate procedure should be adopted. 
The purpose of the present paper is to show how the usual type of 
Atwood machine may very readily be modified so as to give synchronous 
chronographical records of both time and distance at various points of the 
fall. The apparatus was, in fact, devised in the course of a research upon 
the wind pressure law and the efficiency of air-drags, for the purpose of 
obtaining accurate time-distance curves for the fall of a parachute. Some 
results of this research will soon appear; but as the calibration of the 
apparatus showed that it could be used with tolerable accuracy for the 
determination of g, it seemed desirable to publish a description of it, along 
with a few examples of the degree of accuracy that may be reached with it. 
* The expenses of this research were met by a grant made by the Carnegie Trust. 
