282 Prof. W. C. Unwin on Measuring-Instruments 



which shows a tolerably close correspondence with the results 

 obtained by the writer. A more complete discussion of 

 Peirce's results is reserved until the relation between " No. 3 " 

 and S* and S^ shall be more exactly known. The latter 

 standards would appear to be the more trustworthy, since 

 they are based on various independent determinations ; while 

 " No. 3 " is based on an indirect comparison with metre 

 " No. 49," a standard concerning the exact length of which 

 there seems to be some little doubt. 



XXXI. Measuring -Instruments used in Mechanical lasting. 

 By Prof. W. C. Unwin, F.R.S* 



THE determination of the exact distance between two fine 

 marks on a standard of length is an operation of some 

 difficulty, as is well known to physicists. But that operation 

 is free from many of the difficulties which attend the measure- 

 ments which have to be made in the engineering laboratory. 

 Among these the determination of the modulus of elasticity 

 (Young's modulus) of a bar by measuring its change of length 

 by stress is one of the most important. Now the bars sub- 

 jected to test are usually, in the part which can be measured, 

 not more than 10 inches in length ; and the whole elastic ex- 

 tension of such a bar is generally only about 0*007 inch. It 

 is obvious, therefore, that measurements must be made with 

 considerable accuracy and refinement to be of any value. 

 But the bar cannot be placed in a position convenient for 

 measurement; and the attachments to the testing-machine are 

 more or less in the way of the measuring-apparatus to be 

 applied. The bar itself is a somewhat rough bar, the form of 

 which must not be interfered with to facilitate the measure- 

 ments. Then also bars of very different forms have to be 

 tested, fiat and round, of various widths and diameters; and 

 the measuring-apparatus must be applicable to all these with 

 equal readiness. Last, but not least, the work of an engi- 

 neering laboratory is pressing, and measurements must be 

 carried out with rapidity. 



In some cases, two diamond scratches have been made on 

 the bar, and the distance between these measured by two 

 micrometer-microscopes. Apparatus of this kind is awkward 

 to apply on the testing-machine, and tedious to adjust and 

 read. 



A cathetometer has been used. But then two adjustments 



* Communicated by the Physical Society : read January 22, 1887. 



