WEIGHTS AND MEASURES 31 



from deformation." The physicist also uses the word 

 " strain," to represent the measure of the deformation 

 which a stress produces. This happens to make it 

 easier to remember the fundamental law of elasticity, 

 namely : The ratio of stress to strain is constant. 



In our definition for elasticity we see that the re- 

 covery when the deforming stress is removed may be 

 " entire or partial." If it is entire the body is perfectly 

 elastic. One of the best illustrations of perfect elas- 

 ticity is the hairspring of a watch, which may coil and 

 uncoil a million times without any permanent deforma- 

 tion. If, however, a body is deformed too far it does 

 not return to its original form, although it may still 

 show some ability to recover from the deforming stress. 

 This is imperfect elasticity. When the stress becomes 

 so large that the body does not recover entirely, the 

 " elastic limit" is said to have been reached, and for 

 greater stresses the alliterative law given above no 

 longer holds. For some still larger stress the body 

 breaks. This breaking stress measures the " ultimate 

 strength" of the body. 



With the quantitative applications of these prin- 

 ciples of elasticity the maker of spring scales must be 

 familiar. If no weights are applied to a spring scale 

 greater than that corresponding to the elastic limit, 

 the strain will always be proportioned to the weight. 

 If then we mark the positions of the pointer when the 

 scale pan is empty and when it carries N pounds, we 

 may divide the total deflection, or distance between 

 these two marks, into N equal spaces and number the 

 dividing lines correspondingly. The process is that 

 of calibration. Thereafter we may use it as a direct 



