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.

wet-and-dry hygrometer
Image 1 Wh.2498. This wet-and-dry hygrometer was made in Paris by Fastre Aine in 1851. Image © the Whipple Museum.
wet-and-dry hygrometer
Image 2 A De Saussure type hair hygrometer, made circa 1820. Image © the Whipple Museum.
Lambrecht's Hygienischer Rathgeber
Image 3 Lambrecht's Hygienischer Rathgeber, made in Göttingen, Germany. Image © the Whipple Museum.

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

Wet and dry bulb hygrometers were more reliable than condensation hygrometers for measuring air humidity (Image 1). 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.

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 2). 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 3).

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'.

» Read more about a curious evaporimeter.

Allison Ksiazkiewicz

Allison Ksiazkiewicz, 'Measuring Air Humidity', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, [http://www.hps.cam.ac.uk/whipple/explore/meteorology/measuringairhumidity/, accessed 25 November 2017]

 
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