io8 ACCURACY OF ATWOOD'S MACHINE 



by making a and X a minimum, and k as nearly equal to p as 

 possible. This means that the supporting pivots must be 

 thin and well lubricated, and that the mass of the pulley must 

 be concentrated in the rim, the spokes being as light as possible, 

 consistent with the load they have to bear. 



In conclusion, it might be well to draw attention to the 

 fact that the only modification in the usual type of ATWOOD 

 pulley necessary to adapt it to the foregoing method is the 

 very simple one of forming the wheel 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. Some- 

 thing more permanent is required. 



When the apparatus is to be used as an ATWOOD machine 

 for determining 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 

 1 /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 unable to apply it ; the extra knowledge of experi- 

 mental work required is but small, while the resulting gain in 

 accuracy is great. 



JOHN PATRICK DALTON 



