A tubular linear generator comprises an elongate translator, containing a series of permanent magnets and one or more annular coils (not shown) contained within and affixed to a ferromagnetic sleeve. Relative movement between the translator and armature causes the generation of electricity in the coils. The sleeve is contoured in terms of permeability around its circumference at one or both its ends, as shown at, in order to reduce cogging forces between the sleeve and the permanent magnets along the translator.