The necessity for greater attention to these problems has been forced upon engineers by the advent of high voltages and larger and more costly units. Modern scientific research has thrown much light on the electrical behaviour of dielectrics, and much scattered data has been published dealing with the properties of insulating materials. This information, however, has not been available heretofore in a co-related form whereby it can be used as a fundamental basis for the practical insulation of electrical apparatus. In this treatise the authors have endeavoured to set forth the underlying principles and methods whereby the design of insulation can be carried out with precision, and have embodied the results of many years of practical experience in connection with insulating problems.