friction fitted over it and a hex nut portion of the upper body 7 to discourage disassembly of the body the button and grommet have bores in them aligned with the terminals 53 to permit passage of the wires welded to the head 47 and terminals 53 the constructions of the parts other than the cell 3 are well known to or easily determined by those skilled in the art a 5 yttria zirconia powder in an acrylic binder is lubricated with a fatty acid such as palmitic acid in an ethanol vehicle the powder has an average particle size of less than one micron the powder is dried in air and uniaxially pressed at a pressure of 2 000 to 15 000 pounds preferably 3 000 to 4 000 pounds into a thimble compact having a mirror outer surface a tapered mandrel forms a central tapered bore in the compact the compact is bored with a diamond drill to form a uniform cylindrical bore having a central point at its lower end remaining from the bore formed by the mandrel drilling the bore rather than machining the exterior of the thimble compact as is generally done reduces the labor required the compact is then dipped in an alcohol slurry of stabilized zirconia powder and spray dried stabilized zirconia granules to deposit a coating about fifty microns thick a preferred composition of the slurry is 4 5 mole percent yttria stabilized zirconia with acrylic binder 48 00 g spray dry granules 200 250 mesh 3 0 mole percent yttria stabilized zirconia 24 83 g powder lt 1 u03bc particles y 2 o 3 1 5 u03bc particles 0 77 g al 2 o 3 1 10 u03bc particles 6 40 g.