Discontinuously working elements (on-off controls) are widely used in automatic control systems. From an engineering point of view they are attractive because they are nearly always Simpler, more rugged, and cheaper to build than continuous controls. But prediction of their effects in the controlled system is sometimes so complicated that engineers have avoided discontinuous control where it would have been preferable to continuous control. Originally published in 1953. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.