Electrical prospecting is defined as the science and the art of determining the variations of the electrical constants (resistivity, magnetic permeability and the dielectric constant) of the earth's crust and of interpreting these variations in terms of geological structure. The most successful systems of prospecting are based on the study of resistivity variations. The basic assumption made is that in general changes of resistivity follow the bedding planes. Electrical methods of exploration may be divided into two classes, direct current methods and alternating current methods. In part II the fundamental theory of direct current method is discussed and a typical survey is described. Part III deals with the theory of alternating current methods, with particular reference to the optimum frequency to be used. It is shown that in general very low frequencies are desirable. Two alternating current surveys made by the Swedish American Prospecting Corporation are briefly described. In the conclusion (part IV) some of the difficulties of electrical prospecting are discussed. The depth to which investigations may be carried is limited. In the present stage of the art, it would take exceptionally favorable conditions to obtain reliable information much in excess of 2,000 feet. However, it is stated that improvements in methods of interpretation and in field technique should give electrical methods a definite field of usefulness in prospecting for oil.