Accession No

2299

Brief Description

wet and dry bulb hygrometer, by Negretti and Zambra, English, 1880 (c)

Origin

England; London

Maker

Negretti and Zambra

Class

meteorology

Earliest Date

1880

Latest Date

1880

Inscription Date

Material

metal (brass, oxidised brass, mercury, white metal); wood (mahogany); glass; cloth (velvet, satin, cotton); rope (string)

Dimensions

length 144mm; breadth 50mm; height 109mm box length 16mm; breadth 77mm; thickness 31mm

Special Collection

Provenance

Inscription

‘NEGRETTI & ZAMBRA LONDON’ (stand for thermometers)
‘NEGRETTI & ZAMBRA
INSTRUMENT MAKERS
TO
HER MAJESTY.
LONDON’ (embossed in lid of box)

Description Notes

Wet and dry bulb hygrometer, by Negretti and Zambra, English, c. 1880.

Dismountable wet and dry bulb hygrometer stored in fitted mahogany box. Two round-bulb mercury in glass Fahrenheit thermometers mounted on graduated plate. The wet bulb is wrapped in cotton and positioned lower than the dry bulb, allowing water to be absorbed from a glass reservoir.

Circular brass stand into which screws an oxidised brass standard. This standard carries the two mercury-in-glass thermometers in a groove near the top and below them has a ring for supporting a glass tube with an opening at one end. The thermometers are mounted on a white metal base with the dry bulb above the wet bulb. Both have scales reading in degrees Fahrenheit, divided [0] - [140], numbered by 10, subdivided to 1.

Fitted mahogany box lined with blue velvet and blue satin; white metal hinges and push-button fastener.

Condition fair; complete.

References

Events

Description
A hygrometer is an instrument that measures the amount of moisture in the air; its dampness or humidity. Hygrometers were originally devised as aids to weather forecasting, but are now more often used to monitor air-conditioning, in museums, for instance.

There are several different types of hygrometer. The earliest type uses the fact that some natural materials are sensitive to dampness. Hair, whalebone and catgut, for example, will all stretch or contract in damp or dry air, so can be made the basis for simple indicators of humidity.

Another type is the wet and dry bulb hygrometer. This consists of two identical thermometers, the bulb of one being wrapped in a fabric sleeve; the sleeve dips into a small amount of water, and is permanently wet. The second thermometer will show a slightly lower temperature due to the evaporation of water from the wet sleeve. The rate of evaporation and the amount of cooling depend on humidity. Evaporation is quick on a dry day, but much slower when it is wet. The difference in temperature between the two thermometers gives a measure of the humidity.

A more precise instrument is the dew-point hygrometer. The dew point is the temperature at which moisture in the air starts to condense as liquid. This depends upon the amount of moisture in the air in the first place, and therefore provides a measure of the original humidity. In the dew-point hygrometer, a glass tube with a gilded bulb is cooled until a mist of condensation is first seen on the gilded surface. A small thermometer inside the tube then shows what the dew point is. By comparison with another thermometer outside the tube it is possible to work out the humidity from a table.

01/03/2001
Created by: Chris Lewis on 01/03/2001

FM:43456