BARS 



113 



definite relation between X and fj,, and consequently no universal 

 value of cr. We may note that in an absolutely incompressible 

 medium we should have 



* = oo, E=%n, <r = i (13) 



The following table gives the results of a few determinations 

 by Everett (1867). The second column gives the volume- 

 density in grammes per cubic centimetre. The next three 

 columns give the respective elastic constants, in dynes per 

 square centimetre. These are followed in the last column by 

 the corresponding values of a. The last two rows illustrate the 

 fact that the elastic constants may vary appreciably in different 

 specimens of nominally the same substance. 



For technical purposes the elastic constants E, K, fj, are 

 often expressed in gravitation measure, e.g. in grammes per 

 square centimetre. The corresponding numbers in the above 

 table are then divided by g. Another mode of specification, 

 employed by Young, is in terms of the length of a bar of the 

 particular substance, whose weight per unit area of cross-section 

 would be equal to the modulus in question when expressed in 

 gravitation measure; this is called the "length-modulus." Thus 

 if L be the length-modulus of extension of a bar free to contract 

 laterally we have 



E= ffP L (14) 



Taking g 981, the above table gives, in the case of steel, 

 L = 278 x 10 6 centimetres. 



