40 SEC. 3. MEASUREMENT. 



SET OF MEASURING INSTRUMENTS CONTRIBUTED BY SIR J. WHIT- 

 WORTH & Co., MANCHESTER. 



210. Box of Standard Lengths, of end measure, 1 inch to 

 .12 inches. 



These are standard lengths of end measure, and are used for adjusting the 

 measuring instrument to any required length, and for any other purpose 

 where a standard of end measure is required. 



211. Measuring Instrument, graduated to measure one 

 millionth of an inch. 



The power of measurement is one of the most important elements in 

 practical mechanics, and can only be obtained in the highest degree by 

 means of the true plane. 



The value of a machine, when made of the best material, depends on truth 

 of surface, and the proper difference of size between the external and internal 

 diameter of the various parts that have to work together ; when experience 

 has determined this, accurate measurement comes into play, and enables 

 these exact differences of size to be carried out. 



One of the principal uses of the measuring instrument is to ensure extreme 

 accuracy in the manufacture of difference gauges. The principle is that of 

 employing the sense of touch instead of sight. 



In order to measure end measure with the utmost accuracy a thin flat 

 piece or bar is introduced, having its two sides made true planes ; this piece 

 is called the gravity piece, and is placed between the end of the piece to be 

 measured (which is also made a true plane) and one of the end surfaces of 

 the machine, so as just to allow it to fall by its gravity ; by bringing the 

 planes into closer contact, even the one millionth of an inch, the gravity 

 piece will be suspended friction overcoming gravity. 



212. Measuring Machine, graduated to measure one ten 

 thousandth of an inch. 



This instrument was designed by Sir Joseph Whitworth for use in the 

 workshops where high-class work is carried on. 



It is graduated to read to one ten thousandth of an inch, but one forty 

 thousandth of an inch can readily be measured by it. 



The principle of this instrument is the same as that of the one previously 

 described. 



213. Difference Gauges, i.e. gauges differing in size one from 

 the other by very minute quantities ; they are essential to enable 

 a workman to work to extreme accuracy. 



To illustrate the importance of small differences in size the internal gauge 

 here exhibited is '5, while the two external gauges or solid cylinders are 

 respectively '5 of an inch and '4998 of an inch ; the latter is one five thousandth 

 of an inch larger than the former ; the one fits tightly, the other appears not to 

 fit at all. 



Without the aid of difference gauges it would have been impossible to 

 have produced rifle barrels so exactly similar that the shooting of one is not 

 distinguishable from another as regards their relative accuracy. 



214. Sample of Gauges, external and internal, in general 

 use. 



Gauges, one inch external and internal, as made for use in the workshop. 

 These gauge?, which are in general use, are made from -j-^th of an inch to 

 6 inches in diameter. 



