In providing a dried insulation layer (46) upon conductor (12), a covering fluid layer (46) is initially provided by a process and apparatus in which the layer is formed from composite material (24) comprising magnetically permeable particles homogeneously mixed with a fluid carrier. The fluid layer is formed by passing the conductor through a reservoir of the material and then vertically through a die (26, 64). The reservoir of fluid applies pressure atthe die orifice and this pressure is adjustable to vary the rate at which the material passes through the orifice and thus varies the diameter. This pressure is controllable by adjusting the height of the reservoir above a die orifice or by adjusting gas pressure acting downwardly upon the material. The diameter of the layer is advantageously measured after it is dried and variation in measured diameter from that desired effects a change in pressure to alter the diameter towards that desired. The diameter of the layer may, however, be measured with the layer in fluid form.