A coating is formed by chemical vapor 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 the deposition chamber. The entrance passage is formed from misaligned portions which press the filament into direct electrical contact with their walls. A tube communicates with a chamber between the ends of the passage and acts as a sealing means to prevent gas escaping from the deposition chamber through the entrance passage. 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.