This book was originally written as an informal mimeographed text for one of the so-called " Out-of-Hour " courses at Bell Telephone Laboratories. The bulk of the material was prepared in 1938 and 1939 and was given in course form to my colleagues there in the winters of 1939—40 and 1940-41. During the war, however, the text has also been supplied as a reference work to a considerable number of other laboratories engaged in war research. The demand for the text on this basis was unexpectedly heavy and quickly exhausted the original supply of mimeographed copies. It has consequently been decided to make the text more widely available through regular channels of publication.