the gaskets 92 are spiral wound gaskets having inner keeper rings 94 and outer keeper rings 96 whilst the gaskets 92 are electrically conductive they are separated by the non conductive spacer ring 90 which in this case is made of non porous alumina ceramic the spacer ring 90 and gaskets 92 provide complete sealing against leakage of fluorine and the spacer ring 90 provides electrical insulation of the anode 30 from the skirt 20 however the existence of the holes 76 may provide a potential path for fluorine leakage and in the interests of complete safety an auxiliary closure member 100 is provided which is itself sealed to the closure member 72 by means of a further spiral wound gasket 102 a recess 104 is provided in the auxiliary closure member 100 to accommodate the heads of the screws 74 the auxiliary closure member 100 the closure member 72 the spacer ring 90 gaskets 92 and 102 are all held together by means of threaded studs 108 and nuts 110 the studs are insulated from the closure member 72 and auxiliary closure member 100 by insulating sleeves 112 in this case of mylar trade name plastics material extending along the length thereof and the nuts 110 are insulated from the auxiliary closure member top face 114 by insulating washers 116 in this case of phenolic material such as tufnol trade name for example the spacer ring.