A coating is formed by chemical vapour deposition an electrically heated filament which is passed through an end plate into a deposition chamber and leaves the deposition chamber through a similar end plate. The filament slides through an entrance passage into a first electrode chamber, around part of a wheel electrode into the deposition chamber. The passage of the filament around the wheel electrode provides adequate direct electrical contact. The end plate operates in exactly the same manner. As no mercury or a low-melting point eutectic alloy is used, no contaminants associated therewith are produced and the resultant coated filament is free of such contaminants.