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The introduction of ihe Type-TT-25 Single Convectron Unit marks an important advance in the technique of determining and correcting deviations from a vertical reference position wholly by electronic means. This tube consists of a fine wire sensing element supported in a gaseous atmosphere within a glass envelope. Connections to the sensing element are made to external metal caps at both ends of the tube. Two Type TT-25 tubes are required for the instaliation. These are mounted so that their axes form an angle of 90 degrees and they are connected electrically to form a bridge circuit as iilustrated in the drawing on the reverse side of this sheet. The filaments carry sufficient current to heat them to an average temperature of about 400° C. The heat leaves the filaments by (1) metal conduction {2) gas conduction {3) radiation and (4) gas convection. The percentage of heat leaving by the first three means is comparatively small, while that leaving by convection is large. Gas convection cur- rents rise along the vertical and so pass the filament at whatever angle it happens to be relative to the vertical. The relative rate at which each filament loses its heat to these con- vection cu