An electric arc is formed when a current passes between two conductors through a non-conducting medium like air. Although the phenomenon was discovered during early electrical experiments and utilised widely in lighting by the end of the nineteenth century, its problems were not fully understood. First published in 1902, this book represents one of the first systematic investigations of the electric arc, and the best-known work of suffragist and electrical engineer Hertha Ayrton (1854-1923). It includes a chapter on the history of the discovery, over a hundred illustrations and tables, and Ayrton's explanation of the enduring problem of arc instability. As a result of her research, she went on to patent anti-aircraft lights and new arc-lamp technology. She later became the first female recipient of the Royal Society's Hughes Medal. Remaining relevant to students of electrical engineering and the history of science, this book shares her insights and expertise.