graphite the entrance passage 20 has a small chamber 27 communicating with a tube 28 which is connected to an unshown waste gas disposal system having a gas pressure slightly less than the pressure of the reactive gases in the tube 11 in this manner the chamber 27 and the tube 28 act as an entrance sealing means by preventing any of the reactive gases from escaping to atmosphere through the working clearance between the filament 18 and the upper portion of the entrance passage 20 alternatively the tube 28 may supply an innocuous gas for instance argon or nitrogen or some other gas appropriate to the coating process under a pressure slightly greater than that in the tube 11 whereby the tube 28 and chamber 27 again act as an entrance sealing means with either embodiment the respective entrance sealing means isolates the reactive gases within the tube 11 from the surrounding atmosphere it will be noted from fig3 that the entrance passage 20 is formed as two misaligned portions 201 and 202 of which the axes intersect in the region of the chamber 27 each portion 201 202 has an internal diameter that will provides a close sliding clearance for the filament 18 of which the diameter is typically between 20 u03bcm and 200 u03bcm the upper portion 201 is displaced laterally typically 1 mm or less from a centre line.