102 ON THE ACCURACY ATTAINABLE WITH 



obtained of the distance fallen through during each revolution 

 of the wheel (being the distance equal to the effective circum- 

 ference of the wheel and string), and of the time taken to 

 describe that distance. A sensitive relay is interposed between 

 the revolving wheel and the chronograph, so that a very small 

 current may be used, and no sparking occurs during motion. 

 In the experiments given in this paper a three-pen motor-driven 

 chronograph was used in conjunction with a clock beating 

 half-seconds, and times were easily obtained to T ^^th of a 

 second. The third pen served to record the actual moment 

 at which motion began. Its magnet is connected in parallel 

 with a small electro-magnet held in an adjustable stand. 

 When an experiment is about to be made, the pan with the 

 smaller load, to the bottom of which is soldered a small iron 

 disc, is brought down to this release magnet, and, if necessary, 

 its height is adjusted until the point of no contact in the wheel 

 revolution is just reached. Paper is inserted between the 

 magnet and the pan to ensure immediate release ; the current 

 is then broken, setting the system in motion and recording the 

 initial point of the experiment on pen No. 3 ; every complete 

 revolution of the wheel is recorded on pen No. 2, while the 

 half seconds are marked by pen No. 1. 



The string used was a strong silk fishing line, fitting well 

 into the groove and continued beneath the pans to form an 

 endless loop. The effective fall for one pulley revolution was 

 determined by attaching a 10-metre tape to one of the pans 

 and reading against a fixed point the distances covered 

 between successive markings of the chronograph while the 

 pulley was slowly rotated. The mean of fourteen separate 

 measurements gave an effective fall of 38-92 cm., and conse- 

 quently a mean pulley radius p=6T94 cm. The constancy of 

 these individual measurements showed that the string did not 

 slip appreciably in the V-groove. 



