This device is a vintage aspirating psychrometer, which is a type of hygrometer used to measure relative humidity in the air.
How It Works The Two Thermometers: The instrument relies on a dry-bulb and a wet-bulb thermometer. The dry-bulb thermometer simply reads the current ambient room temperature.
The Wet Sock: The thermometer wrapped in a cloth "sock" (wick) is dipped in water. As air blows over it, the water evaporates from the sock. Because evaporation consumes heat energy, it cools this thermometer down, creating a lower temperature reading.
The Hand Crank and Fan: Turning the hand crank drives the paddle/fan blades, drawing a consistent, steady stream of air directly over both thermometer bulbs. This manual ventilation accelerates evaporation to get a fast, stable reading.
Determining Humidity By comparing the final temperature difference between the dry and wet thermometers (known as the wet-bulb depression), a scientist or weather observer would use a standard psychrometric conversion table to find the precise relative humidity of the room.
Handmade or custom laboratory demonstration model typically dating from the late 19th century to the early-to-mid 20th century (circa 1890–1940s).
Condition is good to fair. The screws could be tightened. Original patina left in tact.
Super cool item to display or as a gift for your weather geek.