ioo ACCURACY OF ATWOOD'S MACHINE 



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 gr, 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 results here appended are not to be taken as giving the 

 limits of accuracy of the method, for, unfortunately, the 

 construction of the friction rollers in the apparatus used is not 

 quite satisfactory, with the result that friction is somewhat 

 variable and in need of constant evaluation : with a more 

 carefully constructed apparatus friction would be much 

 smaller, and its constancy more assured. 



DESCRIPTION OF APPARATUS 



A six-spoked aluminium wheel, cut with a V-groove in 

 the rim, and mounted on a steel spindle, runs on four aluminium 

 friction rollers. These rollers have conical sockets and are 

 mounted on points screwed into a brass plate, so that if the 

 steel spindle were completely homogeneous and were laid on 

 the rollers, electrical contact would be complete from one side 

 of the apparatus to the other during one whole revolution of 

 the wheel. At one end of the spindle, however, a semi- 

 cylindrical portion of the steel is removed and replaced by an 

 identical piece of hard ebonite, the whole being then turned 

 true in the lathe ; it follows that at a certain point during 

 each revolution the ebonite is in contact with both friction 

 rollers on which it rests, and at that moment electrical contact 

 is no longer possible between the two sides of the apparatus. 

 Hence, by connecting with a chronograph, a single record is 



