A Galileo thermometer is a very simple and surprisingly accurate thermometer named after Galileo Galilei, a noted 16th century scientist. While these thermometers are primarily kept around as novelty items, they can also be utilized as functional measures of the temperature. They also provide teaching opportunities, for those with young children who are just beginning to learn about the sciences.
The design of a Galileo thermometer includes a sealed glass tube, filled with liquid and a series of weights. As the temperature changes, the liquid changes density, causing the weights to rise or fall in the tube. Classically, a cluster of weights which are less dense than the liquid float near the top of the tube, while the denser weights sink to the bottom. The temperature is read by looking for the band between the two sets of weights, and averaging the temperature of the weights on either side of the band. Sometimes, a single weight will be suspended in the middle of the band, and this weight reflects the current temperature.
In the most basic form, the Galileo thermometer simply has weights with attached metal discs indicating the temperature. Many manufacturers, however, attach the weights to glass bubbles which may be brightly colored. The bubbles are calibrated so that they all have the same density, ensuring that they act purely ornamentally, and do not influence the operation of the thermometer. Objects other than glass bubbles can also be used as ornaments in the thermometer.
The very simplistic design takes advantage of the property of buoyancy, and if the weights are calibrated properly, the temperature readings can be extremely accurate. Those who are curious about the accuracy of a Galileo thermometer can compare its performance with another type of thermometer to determine how well the weights were made. The differences between the weights need to be extremely subtle, which can make errors glaringly obvious.
This type of thermometer can be used indoors or outdoors, and may be suspended in a frame or mounted on a block which allows it to sit on a table. Because it reads the ambient temperature which immediately surrounds it, it is important to think about placement. If it is placed in the sun, for example, the fluid will heat up, causing the thermometer to register a very high temperature. Likewise, placing the Galileo thermometer next to a stove or in a very cold area of the house will result in a skewed reading.