472 SCIENCE PROGRESS 



are absent, or that decrease of strain is facilitated by super- 

 posed motion in the wire. 



The fact that the magnitude of the deformation in a body- 

 stressed beyond the elastic limit depends upon the time should 

 be of importance to those who require to know what will be 

 the strain under given conditions ; yet it seems at present to 

 be little enough appreciated. One cannot expect to be able 

 to predict the value of any physical quantity unless all the 

 conditions which may affect its value are clearly specified. Yet 

 it is common to find diagrams purporting to give the magnitude 

 of the strain in a body subjected to stress, even when the latter 

 exceeds the elastic limit, without any reference being made to 

 that other factor upon which, as we have seen, the strain largely 

 depends — viz. the time of application of the stress. Since the 

 strain is a function of the time in the region beyond the elastic 

 limit, it is clear that no definite meaning can be attached to a 

 curve representing stress against strain unless the duration of 

 the stress is also recorded. In the case of stresses less than the 

 elastic limit, there is, of course, no necessity to specify the time 

 of application. The initial immediate deformation is the only 

 one which occurs, does not increase with time, and has a per- 

 fectly definite value for every value of the stress. The strain is 

 a function of the stress only, and the curve mentioned above, 

 in so far as it refers to the region below the elastic limit, can be 

 rightly interpreted without reference to the question of time at 

 all. But when this limit is passed (under which circumstances 

 the strain may have, as time goes on, practically any value for 

 any one particular stress), it is absolutely necessary that the 

 duration of the stress, as well as the stress itself, should be 

 known before the magnitude of the strain can have any useful 

 meaning. A series of curves showing strain against stress, and 

 also specifying how long after the application of the stress the 

 determination of the resulting strain was made, would certainly 

 be useful. The curves shown in fig. 2 are not experimental 

 curves, but probably indicate roughly what might actually be 

 determined. In this diagram strain is plotted vertically and 

 stress horizontally. Let us suppose it to represent the increases 

 in length in a wire brought about by applying various forces, 

 and further, that a unit of length horizontally represents unit 

 force, and unit length vertically unit increase in length in the 

 wire If op is the elastic limit, then if a force om less than 



