Winter online exhibition
Dew point hygrometer made about 1850.
Accession number LIV.2000.138.198
A hygrometer measures the amount of humidity in the air.
The atmosphere always carries a certain amount of water vapour, the proportion of which varies with temperature. The warmer the air the more moisture it will contain.
As air becomes colder the water vapour in it condenses into moisture to produce dew or frost. In a mist or fog the air is completely saturated and this is called 100% humidity.
In early scientific instruments such as these, hydroscopic material was used to move a pointer to indicate the amount of water vapour in the air. Common materials used were hair, whalebone, cat gut and impregnated paper.
The hygrometers shown on this page were both originally part of the physics teaching collection at the University of Liverpool. They are now in the physical science collections at World Museum Liverpool.
In the dew point hygrometer shown above, ether is poured over the muslin covered bulb on the right, which cools the fluid inside the tube to cause water vapour to condense on the outside of the darker bulb. The temperature inside the bulb is read and compared with the air temperature recorded by the outside thermometer. It is then possible to determine the relative humidity from tables using these readings.
The French hygrometer and thermometer shown below would have had a hair secured between the pointer on the left and the brass fitting on the right. The pointer would move as the hair expanded or contracted, depending on the amount of humidity in the air. This hygrometer has a thermometer built into it, to allow a temperature reading to easily be taken at the same time.
Hygrometer and thermometer, made about 1830.
Accession number LIV.2000.138.114