SM120     Hypsometer     circa 1950     Heat

  This instrument is used to fix the upper point of 100 deg C on the thermometer scale.

  The boiling point of water varies with the amount of dissolved solids, the shape of the containing vessel and the atmospheric pressure.

  The hypsometer eliminates inaccuracies due to the above causes by taking the temperature of steam near the surface of boiling water.   The pressure of the steam is the same as the atmospheric pressure and this is verified on a manometer at the side of the instrument.

  The rest of the apparatus is a copper vessel in which water is boiled.   The steam rises in an inner cylinder which surrounds the thermometer stem, then descends inside a concentric outer cylinder and out through an escape tube.

  The thermometer is kept a small distance from the water, level with the manometer, the water being heated until steam issues steadily from the escape tube.   The atmospheric pressure is measured and a correction of 1 deg C is added to the boiling point for every 27 mm of mercury above an atmospheric pressure of 760 mm.