The concept of point elasticity is applicable where change in price and the resulting change in quantity are infinite or small. Though, where change in price and consequent hunger in demand is substantial, concept of arc elasticity is the pertinent concept. Arc elasticity is a measure of the average of responsiveness of quantity demanded to a considerable change in the price. Or we can say, the measure of price elasticity of demand between two finite points on a demand curve is called arc activity. For instance, the measure of elasticity between points J and K (Fig. below) is: the measure of arc elasticity. Movement from point J to K along the demand curve D) demonstrates a fall in price from 25$ to 10$ so that AP = 25 - 10 = 15. Consequent increase in demand, AQ = 30 - 50 = - 20. The arc elasticity between point J and K and (moving from J to K) can be attained by substituting these values in the elasticity formula.
EP = (-δQ/δP). (P/Q) = (-20/15)(25/30) = 1.11
Which means that a one percent decrease in price of commodity X results in a 1.11 percent increase in demand for it.
Figure: Measuring Arc Elasticity