Figure 1.

Diagram of NBS thermometer calibration apparatus used during the early 1950s. The diagram shows, from the left, a precision manometer that is similar in many respects to the one used in all of the NBS/NIST gas thermometry measurements, a steam-point boiler, an oxygen-point apparatus, and a sulfur-point boiler. A—End standard gage blocks, intended to be wrung together to define the cell separation. B—Large-meniscus (~7.3 cm inside diameter) mercury cells. C—Mercury pump for the adjustment of the mercury levels in the manometer. D—Movable mercury lines with rotating joints. E—Fixed capacitor plates for mercury level sensing. KO, LO, MO—Components of a capacitance diaphragm gage used to separate the helium gas used in the manometer from the oxygen used in the oxygen-point apparatus. (Reprinted from reference [7].)