An apparatus is described with which magnetostrictive changes of length in strip samples can be measured with accuracy under direct and alternating conditions at various flux densities.The factors controlling the magnetostrictive pattern in grain-oriented silicon-iron have been studied. It has been found that considerable reduction in magnetostriction can be obtained by heat treatment at a suitable temperature, the time at temperature and the rate of cooling both having an effect on the ultimate value of the magnetostriction. With material heat-treated under optimum conditions, cooling under an applied load and magnetic annealing give no subsequent reduction.Tensile stress applied along the direction of flux produces a marked decrease in magnetostriction, as does, to a smaller extent, compressive stress applied at right angles to the sheet. It is found that a compressive stress of only 3001bf/in2along the direction of flux is very deleterious, so that in transformer construction the use of wavy or undulating sheets, which on clamping would have such induced stress components, should be avoided. Bending and handling have little effect on the material, provided that it is restored to its original form and that the applied stresses have caused no permanent deformation.The importance of these measurements to the transformer engineer is discussed.