The upper part of the wire 13 is enclosed in an insulating covering or rod 14, made of some material such as PTFE or silicone rubber which will withstand the operating temperature, and which in turn lies co-axially within a tubular support 15, by means of which a conical flow controller 16 is located below or within the co-axial inlet pipe 5 and above the lower, uninsulated, part of the wir