It measures relative humidity and air temperature. This is a spring-wound or battery powered eight day clock instrument that records on a chart the relative humidity and air temperature. The temperature sensor is an alcohol filled metal tube. It is fairly accurate but slower to respond to fast temperature changes but it gives a good idea of the temperature history on a chart for a week. The humidity sensor measures relative humidity by stretching a human hair bundle between mechanical linkages and through expansion and contraction moves a pen on a calibrated chart recording relative humidity in percent.
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.