Barometers Realm

This late 19th-century hair hygrometer, manufactured by the German company Lufft, is labeled as a Präzisions-Hygrometer. In fact, it is a multifunctional instrument known as a Polymeter. The polymeter was developed and patented by the Lambrecht company in 1880. It combines the functions of a thermometer and a hygrometer into a single measuring device. In addition to determining air temperature and relative humidity, it also allows the user to calculate the saturation pressure and the dew point.

At the top of the instrument is a mercury thermometer. To the left of the capillary tube, the scale indicates temperature in degrees Celsius, while to the right, the corresponding saturation pressure is given in millimeters of mercury. This pressure value is numerically almost equal to the maximum water vapor content in grams per cubic meter.

Below the thermometer is a circular hair hygrometer, with the hair element stretched the full length of the rear panel. The hygrometer’s dial, protected by a convex mineral glass, features two scales: the lower one displays the relative humidity as a percentage, while the upper scale is used to determine the dew point. The pointer indicates the temperature difference between the current air temperature and the dew point. To obtain the dew point temperature, one subtracts the value shown on the upper hygrometer scale from the air temperature indicated by the thermometer.

Unlike Lufft’s more streamlined designs, Lambrecht polymeters typically feature a trident-shaped pointer with three prongs at the tip. The right prong is used for readings around 0 °C, the middle for about 10 °C, and the left for approximately 20 °C. This design allows for further calculation of water vapor partial pressure, absolute humidity, and saturation deficit.

Altogether, the polymeter stands as a fine example of scientific ingenuity of the era, offering a compact and elegant solution for complex atmospheric measurements.