Measurement of Length 



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freely along the edge of the latter. It consists of a 

 scale 0'9 cm. long, divided into ten equal parts. 

 Suppose we wish to know the reading on the large 

 scale at a given point, such as C, to the nearest tenth 

 of a millimetre. The zero mark of the vernier is ad- 

 justed so as to coincide with G as in the figure. The 

 reading is evidently between 18'1 and 18'2 cm. 

 To obtain a measurement to 0*01 cm., examine the 

 graduations of vernier and scale and note where one 

 graduation mark on the vernier is as nearly as possible 

 in the same straight line as one of the marks on the 

 scale. In the figure this occurs at the 7th vernier 



Fig. 6. 



division at point D. This shows that 0'07 cm. must 

 be added to 18- 1 cm. to obtain the reading at G to 

 0*01 cm. This reading is consequently 18-17 cm. 



That this is the case is rendered evident by the 

 following considerations. Each vernier division is equal 

 to 0'9 mm., while each of the small divisions of the scale 

 is TO mm. Hence the 6th vernier division is O'Ol cm. 

 from the first scale-division to the left of Z), the 5th is 

 0'02 cm. from the 2nd scale-division to the left, and 

 so on. Finally, the zero mark of the vernier at G is 

 (V07 cm. from the 7th division on the scale to the left 

 of D, i.e. from the division which shows 18' 1 cm., and 



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