In the preparation of this work the author has received much assistance from the classical works and lectures of prominent electrical engineers and the technical press, and desires to acknow ledge his indebtedness to these authorities for the many ideas which have enabled him to prepare the following chapters. He has also to express his thanks to His Majesty's Stationery Office for permission to use the table of logarithms and anti-logarithms given in the appendix. As it is almost impossible in the first edition of a book of this nature to detect every slip which may occur during its preparation the author requests his readers who may detect any error or ambiguity to kindly notify him of the same so as to make the book as reliable as possible.