Measuring Air Humidity

Early hygrometers used hair, whalebone or catgut - materials that stretch when subjected to humidity - to gauge the dampness of air. The rate of water evaporation from a wet surface is another way of determining the quantity of water vapour in the surrounding atmosphere. Humidity plays a large role in how we 'feel' temperature.

Early studies of dew point

Studied since the fifteenth century, dew-point is the temperature at which moisture in the air condenses as liquid. An investigation into this phenomena by the Grand Duke of Tuscany, Ferdinand II (1610-1670), led to the construction of a condensation hygrometer.

He noticed that during the hot summer months water formed on the outside of an iced glass depending on the temperature and wind. Curious about the nature of the atmosphere and the conditions that changed water vapour into liquid, Ferdinand II invented his instrument. Consisting of a cone-shaped vessel filled with ice, Ferdinand's crude hygrometer enabled him to conduct experiments on how temperature and air circulation affected water vapour in our atmosphere and the conditions that produced dew.

In 1751, Charles le Roy (1723-1789) attempted to chart dew-point by pouring cold water into a glass container, observing the formation of condensation and then measuring the water's temperature. The water was then decanted into a second glass and the process repeated.

Wet and dry hygrometers

Hair hygrometer — Image 1 A De Saussure-type hair hygrometer, c.1820 (Wh.3999).

Wet and dry bulb hygrometers, like the one above made by Fastre Aine in 1851, were more reliable than condensation hygrometers for measuring air humidity. First developed in 1755 by William Cullen (1710-1790) and Joseph Black (1728-1799), wet and dry bulb hygrometers measured humidity based on the evaporation of water.

Cullen and Black noticed that the level of a wet thermometer would drop slightly as it dried. With further studies, Cullen and Black observed how evaporation produced a coldness that could be measured. If one thermometer was kept in a wet muslin sleeve, indicating the temperature at which water evaporated, and a second thermometer measured normal air temperature, the difference provided a measurement of humidity. The rate of evaporation and the amount of cooling was determined by the amount of water vapour already in the air.

Domestic hair hygrometer — Image 2 Lambrecht's Hygienischer Rathgeber, made in Göttingen, Germany (Wh.5814).

Hair tension hygrometers

The Swiss savant Horace-Bénédict de Saussure (1740-1799) experimented with a variety of materials to test air-humidity and found that degreased human hair was a ready and reliable medium (image 1). Damp weather lengthened human hair whilst dry weather shortened it. The tension produced from the changing hair-length moved a pointer that indicated humidity along a marked scale.

While hair hygrometers were used for meteorological purposes, they were also produced for domestic settings. Lambrecht's domestic hygrometer from 1896 functioned as a medical aid, indicating the quality of air inside the home (Image 2).

In nineteenth-century miasma theory of disease, humidity and air-quality were closely associated with notions of health. An inscription on the reverse of the hygrometer suggests further reading to help educate home dwellers on the dangers of unhealthy airs. Recommended books included Dr Fleischer's Healthy Air and Dr Wurster's Temperature of the Human Skin and Its Relation to Cold and Catarrh. To calibrate the hair hygrometer a wet pigeon's feather was used to moisten the hair and the instrument adjusted to read 95% on the dial.

Evaporimeters

Evaporimeters determined the rate of air evaporation from a wet surface to the atmosphere, which is another way of measuring air humidity. Not only were evaporimeters used to study conditions of the atmosphere, but the instrument became useful for early forms of 'climate control'. The "Dragoyle" Air Tester in the Whipple's collection measures evaporation from a wet cloth that is kept damp from water contained within the object. The decreasing level of water over a period of hours or days was used to calculate the rate of air evaporation.

Dragon-shaped evaporimeter — Image 3 The "Dragoyle" Air Tester, a specialty evaporimeter (Wh.5790).

Monitoring air comfort in the work environment

The Dragoyle, which is designed to look like a curious dragon-like creature, measured the "air-comfort" of workplaces such as factories, workshops, and offices. The principles behind this evaporimeter were first developed by the Scottish mathematician and physicist Sir John Leslie (1766-1832). By 1925 the instrument was packaged and sold as the Dragoyle in America. 'Air comfort' was determined by measuring the combined effects of temperature, humidity, and air-circulation.

The Dragoyle is made from a sealed glass piece consisting of a large bulb, which features as the head of the Dragoyle, with a long tube that resembles the creature's tongue. A millimeter of coloured liquid rests in the 'tongue' tube.

As seen in Image 3, the large bulb of glass is covered with a piece of fabric. The ends of the fabric are suspended like a wick in an internal reservoir of water and the wet cloth creates temperature and pressure differences as the water evaporates. The bead of liquid inside the Dragoyle's 'tongue' moves to equalize the pressure between the two sections of the glass vessel, causing the creature's head to rock back and forth. These movements or 'strokes' are counted by the minute and indicate how humidity and air circulation might be adjusted for a more comfortable work environment.

Dragoyle air tester with accompanying booklet — Image 4 The booklet that accompanied the "Dragoyle" Air Tester (Wh.5790).

Depending of the rate of 'strokes' per minute, temperature or draughts could be better regulated. By the early twentieth century, it was recognized that metabolic processes within the worker's body significantly affected closed or confined spaces such as the factory and that better indoor conditions improved the efficiency of the worker. The ideal conditions for the sedentary worker differed from those standing or involved in more physical activity, as described in the accompanying instruction booklet.

Allison Ksiazkiewicz

Allison Ksiazkiewicz, 'Measuring Air Humidity', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge.

Early studies of dew point

Wet and dry hygrometers

Hair tension hygrometers

Evaporimeters

Monitoring air comfort in the work environment

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