FORMS OF TEST PIECES. 297 



Similarly, omitting two, three, and so on, always 

 retaining the point of fracture, the extensions become : 



Extension. 

 Per cent. 



Inch No. 1 to inch No. 10, inclusive 30*6 



2 10, 31-9 



3 10, 33-4 



4 10, 35-1 



5 10, 37-0 



6 10, 38-5 



7 10, 41-2 



8 10, 45-0 



8 9, 55-0 



From which it will be seen that, for a test bar of a given 

 material and having a given cross section, the extension 

 may vary from 30 to 55 per cent., according to the length 

 of the bar and the length upon which the extension is 

 measured. So that it is not only necessary to report 

 on the percentage extension, but also to mention the 

 measured length. The extension, without the length 

 on which it has been measured, is of little use. 

 It is, unfortunately, not always possible to measure 

 the extension on any desired length, because many speci- 

 mens, especially these cut out of larger masses of the 

 material, are necessarily restricted in their length. In 

 ordinary commercial testing, extensions are measured on 

 lengths varying from lin. to lOin. in the more usual cases. 



It has been suggested that the measurement of exten- 

 sion should be confined to the parallel part of the bar and 

 not include that part which partakes of the local con- 

 traction. 



As applied to the case already quoted, the result 

 of doing this would be : 



Extension. 

 Per cent. 



Percentage extension on inches Nos. 1 19'0 



land 2 19-5 



1,2, and 3... 20'0 



1,2, 3, and 4 21'0 



1,2, 3, 4, and 5 22'5 



1, 2, 3, 4, 5, and 6 23'5 



1,2, 3, 4, 5, 6, and 10 23'7 



