Barometers Realm

This Kew-pattern station barometer from the early 20th century, highly likely manufactured by Lambrecht, is the long-anticipated first standard barometer in my collection. Standard barometers, also known as a normal or reference barometers, is a highly precise instruments used at meteorological stations. The standard barometer, based on the Kew principle, features a scale designed to compensate for the rise and fall of mercury in the cistern, thereby eliminating the need to read the mercury level directly at the cistern. Kew barometers were also widely used in the United Kingdom’s merchant fleet, where a special gimbal mount was employed to neutralize the effects of the ship’s motion.

The Kew barometer presented here marks the 120th milestone item in my collection. Its tubular casing, made of brass, houses the barometric tube. At the lower end of the brass casing is an iron cistern, which is sealed except for a small open aperture that allows communication with the atmosphere. The mercury tube features a reduced internal diameter and is equipped with a Bunten air trap (Fig. 9) to prevent air intrusion.

At the upper part of the barometer, a scale in millimeters of mercury, ranging from 690 to 820 mm, is engraved on a silvered brass frame. A vernier scale, which can be adjusted using a brass rack-and-pinion screw located on the instrument's casing, slides within a cutout of the frame. Whole millimeters are read from the main scale against the lower edge of the vernier, while tenths of a millimeter are determined from the vernier by the coinciding lines. The scale area is covered by a tubular mineral glass casing, one side of which is frosted so that readings can be taken from the light reflected off it.

The barometer is equipped with a suspension ring at the top, allowing it to be hung vertically. To account for temperature correction, the casing incorporates a mercury thermometer, also known as an attached thermometer. A highly sensitive mercury thermometer with a centigrade scale is mounted on the casing, with its attenuated bulb in direct contact with the barometer's glass tube, ensuring it measures the same temperature as the contained mercury.

Mercury barometers where the cistern level cannot be adjusted during readings are known as fixed-cistern barometers. Kew barometers belong to this type. In such barometers, the tube extends below the mercury level in the cistern, the cross-sectional area of which is significantly larger than that of the tube. As a result, the rise and fall of mercury levels in the cistern are minor but not negligible compared to changes in the tube. The larger the cistern's cross-sectional area (within certain limits), the smaller the zero-point shift caused by changes in the mercury meniscus in the cistern.

To compensate for the changes in mercury levels in the cistern due to atmospheric pressure fluctuations, the scale divisions are shortened; that is, differences of 1 mm on the scale are spaced less than 1 mm apart. Given that, in ordinary barometer designs, mercury can fluctuate violently during a ship's motion, allowing air to enter the tube, Kew barometers are fitted with a device to hinder air entry and minimize mercury oscillations. This device, known as the Bunten* air trap, consists of a small glass funnel with a narrow downward-facing opening, fused inside the barometric tube. Any air entering the barometer tube is trapped by this funnel. Additionally, the lower part of the tube is constricted into a capillary, ensuring relatively slow mercury level changes in response to atmospheric pressure variations, while preventing rapid rises caused by the ship’s pitch or roll.

*Bunten was a specialist in the manufacture of glass instruments who had already gained a high reputation several years before the 1823 exhibition. He produced standard thermometers and other glass-based registering instruments. Notable examples include a psychometer, a de Saussure-type hygrometer, and a barometer. By 1827, he had added photometers and Daniel’s hygrometer to his range, as well as making significant improvements to Gay-Lussac’s portable barometer. By the time of the 1834 exhibition, he had introduced a thermetograph for measuring deep-sea temperatures, as well as marine and mountain barometers.