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D320-26 · Miscellaneous
1970
Object Overview
This Station hair hygrometer was manufactured in October 1970 by the Riga Gidrometpribor plant, which produced instruments for the hydrometeorological service of the USSR. The instrument belongs to the standard Soviet station type known by the designation M-19. It was part of the set of instruments installed in the psychrometric shelter at weather stations.
The hygrometer works on the principle that degreased human hair changes its length with the relative humidity of the air. This principle was first applied by the Swiss naturalist in 1783. The hair hygrometer was not considered a precision instrument and its readings required corrections. The main instrument for measuring humidity was therefore the , while the hair hygrometer was used in winter, when psychrometric observations were not carried out at temperatures below −5 °C. Corrections for the hygrometer were determined in autumn by comparing its readings with those of the psychrometer over a period of one and a half to two months before the onset of frost.
hygroscopicity
aluminium, enamel, hair, steel
The instrument is mounted on a black-painted metal frame. The frame consists of two vertical profiled uprights, joined at the top by a crossbar and at the bottom by a solid base block. At the ends of the upper crossbar are lugs with elongated slots for fixing the hygrometer to a stand inside the shelter. One of the lugs bears the stamped marking 7051.
The sensing element, a single degreased hair, is stretched vertically along the axis of the frame. Its upper end is fixed to the screw of an adjuster mounted in the centre of the upper crossbar. The adjuster has a knurled nut and a locknut. By turning the screw, the attachment point of the hair can be shifted and the pointer set against the scale, which is how the instrument is calibrated against reference readings. The locknut secures the chosen position. The lower end of the hair is attached to a small bracket that sits on an axle in the base block. A long, thin steel pointer is fixed to the same axle, and a small weight attached to the bracket keeps the hair under constant tension. When humidity falls, the hair contracts, lifts the weight, and the pointer moves to the left. When humidity rises, the hair lengthens, the weight descends, and the pointer moves to the right. Because of the way the hair is attached, Soviet meteorological literature referred to this type of instrument as a "lekalny" (template-type) hair hygrometer.
The scale is applied to an arc-shaped metal plate with a light cream enamel finish, fixed to the uprights of the frame with two screws. It is graduated from 0 to 100% relative humidity, numbered every 10% and divided into 1% increments. The divisions become progressively narrower from 0 towards 100: the elongation of the hair is non-linear, and the scale follows this non-linearity. The centre of the plate carries the factory emblem, a circle with the monogram "з-д ГМП" (Gidrometpribor plant) between stylised wings, and the inscription "СДЕЛАНО В СССР" (Made in the USSR). The date "1970 год" (1970) appears in the lower left part of the scale.
The hygrometer has survived together with its original cardboard case, in which it was supplied from the factory.
This instrument is a characteristic example of Soviet meteorological equipment, in which a classic 18th-century principle survived almost unchanged into the second half of the 20th century. Its simple, easily repairable design, built for many years of service in field conditions, makes it a vivid testament to the everyday practice of station observations. Its completeness, including the original factory case, adds to its value as a museum and collector's piece.