Alan Herries Wilson was a widely respected scientist and industrialist. Born in 1906, Wilson won a Cambridge scholarship at the age of 16. Just 20 years later, he was Fellow of the Royal Society. In 1963, Wilson became chairman of Glaxo, leading the company to great success by boldly engaging with the European markets. In this 1939 text, Wilson shows his aptitude for scientific thought and writing, providing a fluent and informative introduction to the electron theory of metals. His aim was to make clear the physical principles on which the theories of the 1930s were based, with chapters addressing the fundamental elements of electron theory and the structure, conductivity and thermal potential of metals and semi-conductors. Over 70 years after its initial publication, this book will continue to appeal to all readers with an admiration of Sir Alan Wilson, or an interest in the history of electron theory itself.