Analog computer is a machine that measures physical quantities such as temperature, speed, or weight and converts the value into a number. For example, a needle pointer in analog computer displays continuously changeable aspects of the physical weight and the pointer of a weight machine displays a analog data.
The key to the electronic analogue computer was the operational amplifier: a stable, linear device developed initially for use in repeaters in US transcontinental telephony. Simple circuits using operational amplifiers can carry out, to a great degree of accuracy, the mathematical operations of integration, multiplication, subtraction, inversion, etc, and can thus be used to simulate systems modelled by differential equations. The other major component in analogue computers was the coefficient potentiometer, used to ensure the proper scaling of the electrical analogue variables.