of the metallic deposit caused by the presence of rib 138 it is further anticipated that this electrical isolation can be accomplished by masking during a metallization process referring again to fig6 it can be seen that the parabolic surface portions 122 surrounding each of the cavities 170 220 are separated along a centerline of the body 102 by the ribs 130 132 134 136 138 140 142 144 146 specifically the parabolic surface portions 122 surrounding cavity 170 are divided along a centerline of the body 102 by ribs 130 132 the parabolic surface portions 122 surrounding cavity 180 are separated by ribs 132 134 the parabolic surface portions 122 surrounding cavity 190 are separated by ribs from 136 138 the parabolic surface portions 122 surrounding cavity 200 are separated by ribs 138 140 the parabolic surface portions 122 surrounding cavity 210 are separated by ribs 142 144 and the parabolic surface portions 122 surrounding cavity 220 are separated by ribs 144 146 the parabolic surface portions 122 surrounding adjacent cavities are not separated by a rib 130 146 and the metallic deposit 124 on these parabolic circuit portions 122 is therefore electrically continuous ribs 134 136 140 142 are not fully contained on the centerline of the body 102 but run from the centerline.