Wet bulb thermometer by Thomas Jones, 1816-1850.
© Science Museum / Science & Society Picture Library
This wet-bulb maximum and minimum thermometer has a bulb that is covered with moist muslin and this is used in a psychrometer to measure humidity. The thermometer was made by instrument-maker, Thomas Jones of 62 Charing Cross between 1816 and 1850. The temperature is measured using a Fahrenheit scale. This type of thermometer gives a record of the current temperature and also the wet bulb temperature which is used to calculate humidity. It is also called a dry and wet bulb psychrometer. This takes the form of two separate thermometers filled with measuring fluid, which are mounted parallel to each other. One of these has a small muslin cloth covering the bulb which is dipped in water to keep it moist, this is the wet bulb thermometer.