Weather Instruments to Make
Relative humidity can be measured by an instrument called a hygrometer. The simplest hygrometer - a sling psychrometer - consists of two thermometers mounted together with a handle attached on a chain. One thermometer is ordinary. The other has a cloth wick over its bulb and is called a wet-bulb thermometer.
When a reading is to be taken, the wick is first dipped in water and then the instrument is whirled around. During the whirling, the water evaporates from the wick, cooling the wet-bulb thermometer. Then the temperatures of both thermometers are read.
If the surrounding air is dry, more moisture evaporates from the wick, cooling the wet-bulb thermometer more so there is a greater difference between the temperatures of the two thermometers. If the surrounding air is holding as much moisture as possible - if the relative humidity is 100% - there is no difference between the two temperatures. Meteorologists have worked out charts of these differences for each degree of temperature so that the observer can find relative humidity easily. A sample is shown below:
Partial Relative Humidity Chart for 30° C
Dry Bulb and Wet Bulb
You can make a sling psychrometer by using two commercial thermometers. Wrap the bulb of one tightly with a piece of cloth. Attach the thermometers to a narrow, thin board with wire or strong tape. Drill a hole in the top of the board and attach a wooden handle to the board with a short piece of chain.
Source: Tannenbaum, Beulah and Harold E. Making and Using Your Own Weather Station.1989:New York. Venture Books.
See also: a website with illustrations