423 
1910-11.] A Modified Form of Atwood’s Machine. 
spindle partly of metal and partly of non-conducting material. Before the 
plan described above was adopted, the effect was tried of coating half of one 
end of the spindle with a very thin layer of hard varnish. This answered 
the purpose sufficiently well for one or two experiments, but the varnish 
soon cracked in places, giving rise to confused records on the chronograph. 
Something more permanent is required. 
When the apparatus is to be used as an Atwood machine for deter- 
mining the acceleration due to gravity, an inking chronograph is not at all 
essential ; in fact, the accuracy of the time measurement obtained with the 
simpler forms, in which a smoked plate travels or a smoked drum revolves 
in front of a vibrating tuning fork, would reach the order of l/500th second. 
Most laboratories now possess such a chronograph in their equipment, and 
most students are called upon to use it at some stage of their laboratory 
experience ; and as equation (2), containing the dynamics of the method, is 
extremely simple, there is no reason why any junior student should be un- 
able to apply it : the extra knowledge of experimental work required is 
but small, while the resulting gain in accuracy is great. 
University College, Dundee, 
March 1, 1911. 
{Issued separately May 15, 1911.) 
