On first acquaintance the electric arc discharge appears to be both visually attractive and a relatively simple phenomena to understand. To those of us engaged in prolonged study of this discharge, it remains a constantly exciting phenomena but we become only too aware of its complex nature and the difficulties in interpreting its bulk properties. This is particu larly true when the arc exists in a practical device and is subjected therefore to extreme conditions. In recent years the possibilities for the beginning of a fuller understanding of the complexities of the arc has arisen out of the...