EFFECT OF TIME. 



301 



uniformity in the material and slight inaccuracies in 

 taking the measurements. 



Owing to the fact that the part of the bar which 

 partakes of the local extension assumes a more or less 

 conical form, the reduction in area is necessarily greater 

 than the extension of the locally-extended portion calcu- 

 lated on the final length, and therefore all the more so 

 when taken on the original length. 



158. Influence of Time on Plastic Strain. When a 

 stress beyond the elastic limit is applied to a tension bar, 

 it appears reasonable to suppose that the semi-plastic 

 strain cannot take place at once, because the particles 

 require time to move into their new positions as 

 they follow the flow of the metal. This fact is often 

 apparent in tension tests when it is desired to take 

 measurements of the length of the specimen at stated 

 intervals of load ; with a ductile metal a considerable 

 time must be allowed to elapse at the higher loads before 

 the beam of the machine comes sufficiently to rest to 

 allow the extension to be measured. If two similar bars 



Loads. 



A Loaded at 5-4 tons per square inch per minute. 

 B 1-8 

 C 0-66 



FIG. 153. PROFESSOR BARR'S TESTS ON CHARCOAL IRON WIRE. 



are tested, the measurements in one case being taken as 

 soon as the load is applied, and, in the other, a con- 

 siderable time being allowed to elapse, it will be found 



