The paper outlines the merits of a number of high-permeability magnetic alloys under a variety of conditions likely to be met in instrument practice. The properties found with d.c., a.c. and combined a.c./d.c. operation are indicated, and recommendations are made for the best alloy to be employed in particular circumstances. Information is given on the spread of the magnetic characteristics of Mumetal spiral cores produced on a commercial scale.A Section of the paper is devoted to a.c. permeability and losses of spiral cores, rings and particular assemblies of laminations, and the adverse effect of joints in the magnetic circuits is clearly demonstrated.A method of predicting harmonic distortion in small transformers and chokes is described briefly, and curves of distortion-coefficient variation with induction for a number of nickel-iron and silicon-iron alloys are given. Non-uniform flux distribution has a marked effect on harmonic generation, and this is studied in detail; a number of anomalies are examined and practical recommendations are made.Magneto-striction and the variation of magnetic properties with temperature are treated, together with a brief comparison of permeability and relative costs of a number of magnetic materials.The paper includes curves which will be of reference value to designers.