The paper gives an elementary and popular account of some atomic and molecular concepts which are necessary for the formation of a model that will give a rational description of certain behaviour of dielectrics. It opens by explaining that the dielectric constant of many materials is found to depend on the radius of the molecule, and that the phenomenon of permittivity can be included in the inverse-square law of force between charges if the molecules can be supposed to exhibit an effect equivalent to that of electrostatic induction. An outline is then given of the Bohr atom and the electronic theory of valency, and this provides a mechanism equivalent to induction: It also provides a mechanism whereby it is to be expected that certain molecules possess a permanent electric moment, and moreover direct experiment shows that these molecules are electric dipoles. It then attempts a very graphic description of the manner in which dipoles must produce heat when the dielectric is in an alternating electric field. It closes with a brief description of certain experiments which have been made and designed to show that molecular dipoles are a cause of dielectric losses, and a brief discussion of whether they are the main cause of the losses experienced in the insulating materials commonly used in electrotechnology. The paper is intended to be read in conjunction with Dr. H. Fröhlich's paper entitled “Theory of Dielectric Constant and Energy Loss in Solids and Liquids,”† and has been written to stimulate the interest of electrical engineers in certain molecular concepts which it seems possible may soon become necessary to use freely on practical everyday problems of electrotechnical construction.