MEASUREMENT OF QUANTITY OF MATTER 15 



is from a to b, the vernier standing at b, between 5 and 6 of 

 the scale. Looking along the scale, it is found that division 

 line 7 of the vernier and 1 2 of the scale more closely coincide 



than any others. Since each vernier division is T ^ less than 

 a scale division, the distance between 5 and b is -^ of a scale 

 division that is, the whole distance is 5 plus T % or 5*7. 

 With smaller divisions greater accuracy is obtained. 



Samples of verniers, for practice in reading, will be found 

 on the barometer, cathetometer, and spectroscope. On the 

 last it is used for reading very small angles. 



14. The Micrometer Screw and Spherometer. A small 

 distance may be very accurately measured by means of screws 

 carefully constructed, so that a given length of the stem con- 

 tains a suitable number of threads. This micrometer screw 

 works within a corresponding hollow screw. 



If the screw has 10 threads in a centimetre, and the 

 hollow screw is fixed, one complete turn of the screw will 

 cause it to advance ! centimetre. If the head of the screw 

 takes the shape of a large graduated circle, containing, for 

 example, 200 divisions, then a turn through one of these 

 divisions will cause the screw to advance ^^ of '1 centimetre, 

 or -0005 centimetre. 



In the Whitworth measuring machine a distance may be 

 read to one-millionth of an inch, by a modification of this 

 process. 



This method of measurement is illustrated in the use of 

 the spherometer (fig. 3). A three-branched piece of metal 

 stands on three fixed equidistant legs, and a micrometer screw 

 with a large divided circular head moves through the centre, 

 and is read by the aid of a fixed upright scale. The points 

 of the legs and moveable screw are of hard steel, and they are 

 adjusted on a true plane, so that all four are in the same plane 



