Circuit breakers, relays, terminals, microphones, current collectors and commutators, all operate with electric contacts. Industry is making extensive use of contacts with ever-increasing demands on efficiency. The need of up to date reliable theories as well as of formulas and tables for applications is becoming pressing. This book is an attempt to meet this need. As is the case in many other branches of technology today, problems which arise in the field" of electric contacts involve insight in various other disciplines of physics, including parts which have not yet developed to such an extent that they. are treated in elementary text books. Con- sidering the lack of introductions to some topics of this kind it is felt that orienting chapters, for instance, on the tunnel effect, the theory of the arc, the structure of carbon, and the band theory of electric conduction in solids might be valuable for many readers. In order not to burden the main text with such chapters they have been presented as appendices. Several chapters have been devoted to the theory offriction and wear.