BRIDGMAN. — MEASUREMENT OF HYDROSTATIC PRESSURES. 331 



the linear law, but are increasingly deflected at the higher pressures. 

 The change was not large, but perfectly distinct, about 6 per cent less 

 weight being required to produce a given deflection at 650 lbs. than 

 initially. This is rather surprising in a substance like this spring 

 steel, which ordinarily follows the linear law to the elastic limit. The 



TABLE I. 



Relation between Load and Deflection of Springs without Device 



FOR Avoiding Hysteresis. 



effect is evidently due here to the change of geometrical shape, the 

 springs becoming so much flatter under the higher stresses that the 

 geometrical configuration as such has a lower elastic constant, although 

 the elastic constant of the material itself is unaltered. It is customary 

 in the mathematical treatment of the bending of thin rods or plates to 

 assume that the deflection remains proportional to the stress up to the 

 elastic limit. This experiment shows that this approximation may 

 become invalid at considerably less than the elastic limit. 



A fourth result was that the gauge does show some hysteresis, a 

 result which was not expected in. view of the second result. For as a 

 general rule, hysteresis and elastic after-effects, while not directly 

 related, occur together, both being evidence of some molecular in- 

 stability. Table I., giving the difference between the reading under 

 increasing and decreasing pressure, shows the usual magnitude of 

 the effect. The first column in the table gives the total load at 

 each step. The effect is much less than that due to departure from 

 linearity mentioned above, so that here we have a hysteresis loop 

 of the unusual shape shown in Figure 4. The lag in similar cases is 



