The two main types of hygrometers used in households are electronic hygrometers and mechanical hydrometers. Other types include psychrometers, sling psychrometers and hygrothermographs.
A hygrometer will show the relative humidity (RH) by measuring the moisture content of a gas. They do not produce instant results; it can take up to two hours to get an accurate reading.
A hygrometer needs to be calibrated before the first use, and then you should try to calibrate it approximately once a year after that. It should come with instructions, but if it is mechanical, there is usually a small screw or some other adjustment in the back (see Resources).
Place the hygrometer where you see signs of humidity. Don't place it near heat sources such as heat registers, chimneys, radiators and stoves.
According to "The Columbia Electronic Encyclopedia" (sixth edition), using an electric hygrometer is the most accurate way to measure humidity.