Abstract: Providing an electrical conductor with an insulation layer having a layer of dielectric carrier with magnetically permeable particles dispersed within the carrier, in which homogeneity is maintained in a fluid mixture of a fluid carrier and the particles, the conductor is moved vertically through a die means to coat it with a fluid layer of the mixture, the fluid carrier is dried and thickness of the dried insulation layer is controlled. This control is achieved by monitoring the thickness or diameter of the dried layer and, if a variation exists between the monitored and required values, the rate of fluid flow through the die means is adjusted to adjust the monitored values towards that required.