Analogue computers are useful devices that perform calculations of mathematical, physical and technical problems through the interaction of continuously varying physical quantities such as voltages. An analog computer is classified either in accordance with its use (general- or specific-purpose) or based on its construction (hydraulic, mechanical, or electronic). General-purpose implies programmability and adaptability to different applications or the ability to solve many kinds of problems. Most electronic analog computers were general-purpose systems, either real-time analog computers in which the results were obtained without any significant time-scale changes, or high-speed repetitive operation computers. Analogue computers also specialize in solving differential calculus and other mathematical problems. Many computer theorists around the world refer to these unique analogue computers as real computers, due to the fact that they work with real numbers. Because of all these qualities, today many industries use these analogue computers as data processing systems.