The Law of Demand 83 



tion, we are able, by means of the laws of demand for 

 the several commodities, to measure their respective 

 degrees of elasticity of demand. It will be recalled that, 

 in the form in which the laws of demand have been 

 presented in preceding pages, the variable x has been 

 taken to represent the relative change in the quantity 

 of the commodity, and the variable y, the corresponding 

 relative change in the price. The coefficient of the 



elasticity of demand, therefore, is equal to j- when x is 



zero. All that is needed to obtain the measure of the 

 degree of elasticity of demand is to differentiate y with 

 respect to x in the equation to the law of demand, 

 place x = zero, and then take the reciprocal of the result. 

 The method may be illustrated in case of the four 

 representative commodities, corn, hay, oats, and pota- 

 toes. The law of demand for corn see Figure 17 is 



y = .94- 1.0899* + .02391z*-.000234z 

 Therefore, ^ = - 1.0899 + 2(.02391)z-3(.000234)a;* 



When x = 0,| = -1.0899, | =-^=-.92 ' 



and consequently the coefficient of the elasticity of 

 demand for corn is .92. Since the law of demand for 



hay is 



y = 4.17 .946z .0077z 2 + .000385s 8 



- = .946 when x = zero, 

 dx 



and the coefficient of elasticity of demand is 1.06. 

 For similar reasons the degrees of elasticity of demand 



