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D324-26 · Miscellaneous
c.1915
Object Overview
The Edney Hair Hygrometer is a hair hygrometer manufactured around 1915 by the London firm Pastorelli & Rapkin Ltd., one of Britain's well-known manufacturers of scientific and meteorological instruments. The instrument bears serial number 510 and was designed to measure relative atmospheric humidity directly as a percentage.
The name "Edney" was a trade name used by Pastorelli & Rapkin for an extensive family of meteorological instruments. Products marketed under this name included various types of hygrometers, such as hair, paper, and psychrometric hygrometers, as well as self-recording hygrographs, thermographs, and combined recording instruments. In its advertising materials, the company described itself as the of Edney hygrometers. Thus, the inscription "Edney" on the dial identifies the instrument's trade name, while its actual manufacturer was Pastorelli & Rapkin Ltd.
hygroscopicity
brass, glass, hair, silvered brass
The hygrometer is housed in a substantial circular brass case, 140 mm in diameter and 40 mm deep, characteristic of British scientific instruments of the early twentieth century. Its construction combines functional simplicity with the restrained elegance typical of professional measuring equipment of the period.
The warm golden hue is due to a protective lacquer coating, possibly shellac-based. However, it is difficult to determine whether this coating is original.
The case is made entirely of brass and has a cylindrical form with a flat rear wall. Numerous circular ventilation holes, arranged in several rows, are distributed around the entire circumference of its cylindrical side. These openings allow ambient air to circulate freely through the interior, enabling the sensitive element to respond directly to changes in atmospheric humidity. Such an arrangement is particularly important for a hair hygrometer, whose accuracy and response time depend on unrestricted air exchange between the surroundings and the sensing element.
At the top of the case is a substantial brass suspension ring, attached to a small, profiled mounting boss. This allows the instrument to be freely suspended from a wall or a dedicated instrument panel while maintaining the vertical position required for accurate measurements.
The front of the case is fitted with a thin brass bezel retaining a flat mineral glass crystal. The bezel closely embraces the front edge of the case, providing a neat frame for the dial. The glass protects the pointer mechanism and scale from dust and accidental damage while allowing the readings to be observed clearly.
The dial is made of silver-plated brass and has a characteristic matte surface showing fine longitudinal machining marks. It is mounted within a similarly silver-plated annular reflector with a smoothly curved profile, forming a bright border around the measuring scale and reflecting light towards the dial.
The overall design follows a restrained, functional aesthetic. The absence of elaborate decoration is offset by the clarity of the layout, the contrasting inscriptions, and the elegant form of the pointer.
The upper central part of the dial bears the instrument's name:
"EDNEY"
HAIR HYGROMETER.
Below this appears the manufacturer's signature and address:
PASTORELLI & RAPKIN Lᵈ
46 HATTON GARDEN.
LONDON.
No. 510
The address, 46 Hatton Garden, London, was situated in one of the city's established centres of scientific and precision instrument manufacture and trade, where Pastorelli & Rapkin maintained its workshop and commercial premises.
The measuring scale occupies the lower portion of the dial and takes the form of a broad, shallow arc. It is graduated from 30 to 100 per cent relative humidity, with numbered markings at intervals of ten per cent and smaller intermediate divisions. Beneath the scale appears the explanatory inscription PERCENTAGE OF HUMIDITY.
The pointer is made of thin blued steel and is mounted on a pivot near the top of the dial. It has a distinctive elongated form, with its lower end, directed towards the measuring scale, shaped into a decorative forked tip resembling an arrowhead. The slender steel pointer allows precise readings, while the contrast between the dark metal and the silvered dial ensures excellent legibility.
Unlike conventional barometers, in which the pointer generally rotates around a central pivot, the pivot of this instrument is positioned considerably above the scale. This arrangement allows relatively small angular movements of the pointer to produce clearly visible displacements of its tip along the broad lower arc.
The operation of the Edney Hair Hygrometer is based on the hygroscopic properties of natural hair — its ability to change length in response to variations in the relative humidity of the surrounding air.
The sensing element consists of a bundle of long natural hairs, apparently horsehair, stretched between two attachment points inside the case. Using several hairs together provides a stronger and more mechanically stable sensing element while preserving the sensitivity required to detect changes in humidity. Before use, the hairs were generally subjected to special cleaning and degreasing treatments, since natural oils and contaminants can hinder the free absorption and release of moisture.
The instrument's internal mechanism is remarkably simple. The bundle of hairs is stretched between two lateral mounting points situated approximately along the same horizontal line. In the middle, it forms a shallow angle, the apex of which is connected to a slender transmission element linked to the instrument's pointer.
As relative humidity increases, the hairs absorb water vapour from the surrounding air and gradually lengthen. When humidity decreases, they release moisture and contract. Although the absolute change in length of each individual hair is small, the particular geometry of the tensioned bundle allows this change to be converted into a sufficiently large movement of the indicating mechanism.
The distinctive feature of this arrangement is that the sensing element forms a kind of mechanical tension system: both ends of the hair bundle are secured to opposite sides of the case, while its central section is connected to the indicating mechanism. As the length of the hairs changes, the position of the central point of the tensioned assembly shifts, causing the pointer to rotate about its upper pivot.
This arrangement eliminates the need for a complicated system of gears, levers, or additional transmission mechanisms. Small changes in the length of the sensing element are converted directly into angular movement of the pointer, accounting for the mechanical simplicity of the instrument.
Since hair responds to the relative humidity of the surrounding air, the hygrometer's scale indicates the percentage directly, without the need for additional calculations. However, like all mechanical hair hygrometers, the instrument exhibits a certain degree of response lag: its readings change gradually as the moisture content of the hairs approaches equilibrium with the humidity of the surrounding atmosphere.
It should also be remembered that the properties of the natural sensing element can change slowly over time. Contamination, ageing of the hairs, and prolonged exposure to excessively high or low humidity may affect the accuracy of the readings.
A small knurled brass adjustment knob is located on the right-hand side of the case and is connected to one of the attachment points of the hair sensing element.
It allows the tension of the hair bundle to be adjusted and the pointer to be correctly positioned relative to the measuring scale. Turning the knob changes the position of the lateral hair attachment point, thereby altering the geometry of the tensioned assembly and the corresponding position of the pointer.
This arrangement makes it possible to correct the instrument's readings periodically without removing the case or dismantling the mechanism. Calibration can be carried out by comparing its readings with those of a reference hygrometer or by exposing it to an environment of known relative humidity, then carefully turning the adjustment knob until the pointer indicates the corresponding value.
It should be noted that this adjustment is primarily intended to correct the reading at a particular reference point. It cannot compensate for changes in the characteristics of the hair sensing element itself and does not guarantee absolute accuracy throughout the entire measuring range.
The Edney Hair Hygrometer, manufactured by Pastorelli & Rapkin Ltd. around 1915, is a characteristic example of early twentieth-century British meteorological instrument making, combining mechanical simplicity with quality materials and careful craftsmanship.
Of particular interest is its exposed internal mechanism: a bundle of natural hairs connected directly to the indicating pointer, providing a clear illustration of one of the oldest principles of mechanical humidity measurement. The ventilated brass case, silver-plated dial, and elegant steel pointer give the instrument a character that is both professional and decorative.
This hygrometer is not only a functional measuring instrument but also a fascinating testimony to the development of mechanical methods of atmospheric observation, dating from an era when precise meteorological measurements still relied entirely on the physical properties of natural materials and the craftsmanship of skilled instrument makers.