THE THERMODYNAMICS OF ELAvSTIC BODIEvS. 



BY C. M. BROOM ALIv.* 



The elasticit}' of a body ma}' in a general way be defined 

 as the ability it possesses of regaining its original shape or 

 volume after a deformation or change of volume caused by an 

 applied force. The measure of this elasticity, its coefficient 

 of elasticity, is specifically defined as the ratio of the unit 

 stress to the unit strain. That is, other things being equal, 

 that body which requires the greater force to produce a given 

 deformation or change of volume, has the greater coefficient 

 of elasticity, or in general terms is spoken of as the more 

 elastic. The elastic force of bodies depends upon various cir- 

 cumstances, as for instance, the material of the body, the 

 manner of application of the force, etc. Thus there are the 

 coefficient of bulk elasticity, the coefficient of rigidity and 

 Young's modulus. The former two are the coefficients of 

 pure strain, while Young's modulus is a quantity depending 

 upon both bulk elasticity and rigidity. The latter is the coef- 

 ficient most commonly met with, being applicable to the usual 

 cases of bodies under compression and tension. 



With ordinary solid bodies and under ordinary circum- 

 stances it is assumed that the coefficient of elasticity of a 

 body is independent of temperature. This, however, is far 

 from being true. It is found that the elastic force of a bod^^ 

 is always some function of its temperature. In some cases 

 the elastic force increases as the temperature rises, and in 

 other cases the elastic force is lessened by an increase of tem- 

 perature. In other words, we may class all bodies either as 



( I ) Those whose coefficient of elasticity is a direct func- 

 tion of the temperature, and 



(2) Those whose coefficient of elasticity is an inverse 

 function of the temperature. 



The cause of this difference is not fully known. It depends 



* SniniiKirv of talk l)eloiv tlu' Plivsioal v'^fction ol' the Instilnlc 



