Hygrometers
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. To measure the amount of humidity, 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. Hygrometer and thermometerThe French hygrometer and thermometer shown at the top of the page 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. Dew point hygrometerEther 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. In this section
|
