6io MOTONORI M ATS U YAM A 



around a knife-edge at its center which rested on a horizontal plane. 

 The whole equipment was kept in a thermos box, the temperature 

 inside of which could be kept low and constant by using freezing 

 mixture. The torsional stress was applied by putting weights on 

 the scale pan hanging from the limb of the circular disk through 

 the bottom of the box. The amount of torsion was observed by 

 means of a telescope through a window in one side of the box. 



In the first set of experiments, the test piece was cut out from 

 artificial ice and consisted of parallel crystals with their optic axes 

 transverse to the longer dimension of the bar. The first bar was 

 13 . 15 cm. long and i . 73 cm. in mean diameter. 



The deformation was found to depend upon the rate of increase 

 of force, especially near and beyond the elastic limit. In the first 

 experiment the force was increased by adding two pieces of weights, 

 each weighing 4.40 gm., every five minutes. The total time 

 duration for this observation was 3 . 5 hours and meanwhile the 

 temperature was constant at — 7?oC. The observed relation 

 between the weight and deformation is shown in Figure i . 



From this curve one can easily see that a rather marked change 

 of yielding is recognizable at the point B, up to which the strain 

 is more or less proportional to the stress. This feature is none 

 but the characteristic of an elastic curve^ and we may say that ice 

 behaves like an elastic solid. 



Later it was learned that the point B was not so well defined 

 in some cases as in the present one. It is for further study to see 

 when this point will be sharply shown and when not. 



MODULUS OF RIGIDITY 



Rigidity is the resistance of a material to shearing force and is 

 given by the ratio of the amount of shear to the applied force. 

 If a circular rod of length / and radius a is twisted through an 

 angle $ by an external couple PL, the modulus of rigidity n will 

 be givetf by 



L 



' Given in every book on elasticity. 



2 Poynting and Thomson, Properties of Matter, p. 79. 



