MEASUREMENT OF COUPLES. 



in vibration, than for an identical wire which has been at rest for the 

 same time. 



The viscosity of glass is much greater than that of silver, of iron, 

 or of aluminum ; that of zinc is exceptionally large, and only a very 

 small number of oscillations can be counted with this metal. 



711. The following table gives the values of the coefficients 

 (j> and //. for the ordinary metals. The gramme is taken as unit of 

 force, the centimetre as unit of length. 



Some other useful coefficients have been added. The tenacity 

 of a wire is the greatest weight it can support without breaking ; this 

 weight is evidently independent of the length of the wire, and 

 proportional to the section. The coefficient of tenacity T, is the 

 tenacity of a wire the section of which is equal to a square 

 centimetre. 



When a wire is stretched by a weight, it experiences an elongation 

 \ proportional to its length /, to the stretching weight, and inversely 

 as the section ; we may therefore write 



x- /p 



ES'. 



The coefficient E, which is called the first modulus of elasticity, or 

 Young's modulus, would represent the weight necessary to double 

 the length of a wire of unit section. The various coefficients //., </>, 

 T, and E have the same dimensions ; they represent the quotient 

 of a force by a surface. We shall have their values in C.G.S. units 

 if we multiply the numbers of the table by the value of g, expressed 

 in centimetres that is to say, by 981. 



* Alloy of platinum and silver, consisting of 2 silver and I of platinum, 

 t Coulomb's experiments led to the number 744. io 5 . Memoires de f Academic 

 pour 1784. 



