Expansion occurs when the hair bundle absorbs water vapor. This makes the hair bundle heavier and moves the pen up the chart. When the hair bundle loses water vapor it contracts. This makes the hair bundle lighter and moves the pen lower on the chart. Human hair is a very accurate way to measure humidity and has been used for many years.
Blond hair works best.
An even more accurate way to measure humidity is the Sling Psychrometer . A sling psychrometer measures the relative humidity of surrounding air. It does this by comparing the temperatures of a dry bulb thermometer and a wet bulb thermometer. The sling psychrometer contains two mercury filled thermometers. One has a damp wick around the bulb at the bottom.