ON THE ACCURACY ATTAINABLE WITH A 

 MODIFIED FORM OF ATWOOD'S MACHINE 



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 bestowing 

 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 compli- 

 cated apparatus specially designed for reaching an accuracy of 

 O'l 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 experi- 

 mental 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 



