18 Domestic Science 



them so that when the instrument is held by the hinge 

 between the thumb and forefinger and twirled slightly, 

 the points slide easily on the surface of the tumbler, 

 yet still remain in contact with the glass. Remove 

 the calipers and determine the distance between the 

 points with a scale. Make two more measurements of 

 the internal diameter of the tumbler at its mouth, 

 arranging the jaws of the calipers along a different 

 diameter in each case. Find the mean internal dia- 

 meter from your results. 



Next determine similarly the mean internal dia- 

 meters (a) midway between the top and bottom of 

 the tumbler, (6) at the bottom of the tumbler. 



Exercise for Student. 



Devise two distinct methods of finding the thickness of the 

 glass of the tumbler by means of the calipers. Carry out both 

 methods and compare the results obtained. 



Greater accuracy of measurement may be attained 

 by the use of calipers provided with a scale, and 

 capable of more delicate adjustment by means of a 

 screw-thread ; for greater ease of reading, the scale 

 is provided with a second small sliding-scale similar 

 to that described in the next paragraph. 



14. Some difficulty will probably have been ex- 

 perienced in estimating tenths of a millimetre by eye 

 as suggested in Experiment 1. Where such accuracy 

 in reading a finely divided scale is required, a small 

 subsidiary scale, known as a " vernier ", is used in 

 order to reh'eve the strain upon the eyes involved in 

 closely examining small scale-divisions. A simple form 

 of vernier is illustrated in Fig. 6. AB represents the 

 vernier, which is attached to a scale showing centi- 

 metres and millimetres in such a way that it slides 



