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Special Exhibition Label: 'Atmospheres: Investigating the Weather from Aristotle to Ozone' (19/10/2012 - 07/04/2013)

Moisture Measurement

Two of the devices here are wet and dry bulb hygrometers (left and centre). Each has a pair of mercury thermometers, one dry and exposed to the air, the other with a bulb kept wet by a wick fed from a water reservoir. The evaporation of water from the wet bulb causes its temperature reading to drop, giving a lower reading than the dry bulb. The temperature difference is entered into a set of tables to read off the relative humidity.

The central instrument is Mason's hygrometer. Made by A. Pastorelli in London between 1829 and 1848, the instrument conveniently screws into its carrying case. On the left is a more recent 20th-century whirling hygrometer by Casella. The principle is the same but, like an old-fashioned football rattle, the two thermometers are swung vigorously to encourage evaporation from the wet bulb.

The example on the right is a Daniell dew-point hygrometer, first published in 1820 and made here by J. Newman, London. Ether is poured on the muslin-covered globe, and the temperature at which condensation first occurs on the black globe is read from the thermometer sealed within the glass tube. Humidity is worked out by comparison with the temperature of the air indicated by the thermometer on the brass stand.

[Inv 61478, 30481 & 91652]

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