The whole history of electrical communication shows an advance from an almost intuitive art of using a few specialized devices to a well-ordered understanding of satisfactorily functioning systems—an advance, in fact, from practice to theory. Practice, in the sense of cleverness with devices, of a feel for things electrical, of accumulated experience unconsciously absorbed through the pores, is not a very practical foundation for a well-developed field of technology. Rather, the most practical and valuable tool an engineer can have in any field of work is a broad and quantitative understanding of the problem with which he is dealing: in other words, a general mathematical theory of the subject. In many cases, tremendous progress has been made without any such broad understanding. However, such understanding makes work easier, cheaper and more thorough. It helps the engineer in several ways.