Galileo thermometer
The Galileo thermometer (also called Galilei thermometer ) is a thermometer that consists of a glass cylinder and is filled with liquid and floats. It was named after the (first) name of the physicist Galileo Galilei , who discovered the principle that the density of liquids changes with temperature .
history
This type of thermometer was invented by Grand Duke Ferdinando II. De 'Medici , who also invented the first thermometer with alcohol as a measuring liquid in 1654 .
Working principle
A Galileo thermometer shows the room temperature based on the buoyancy of various bodies in a liquid. Such measuring devices are offered by various manufacturers as room decoration .
The thermometer consists of a narrow glass cylinder that is filled with a liquid (e.g. oil ). In the liquid there are several (usually 5 or 10) small glass balloons with slightly different densities, which for decorative reasons are often filled with a colored liquid (water or ethanol ) and each with a small sticker attached as a weight. If the temperature rises, the liquid expands and thus reduces its density and thus the density-dependent static buoyancy of the individual spheres changes.
The glass spheres used for this purpose, the diameter of which is greater than half the inner diameter of the cylinder (so that they remain in the stratification and do not "overtake" one another), are arranged one above the other so that their mean density increases from the top sphere to the bottom sphere.
- At a certain temperature, all spheres rise up, the mean densities of which are smaller than the mean density of the liquid surrounding them.
- All spheres sink, the mean densities of which are greater than the mean density of the surrounding liquid.
- If the liquid then heats up, i.e. its density decreases, a ball sinks from above,
- If the liquid cools down, the density of the liquid increases and a ball rises upwards.
Depending on the calibration , the glass bodies are calibrated in such a way that they float in the middle of the liquid at the temperature noted on the sticker or that the temperature can be read from the ball floating at the top at the bottom .
The measuring range of standard Galilean thermometers is around 18 to 25 degrees Celsius in steps of one or two degrees Celsius. When calibrating at 2 ° intervals, the mean value between the temperature values of the lowest floating and the uppermost sunken glass body can also apply as the current temperature . The type of calibration is usually described in the instructions.
Since glass is a poor conductor of heat and the heat of the liquid is also transferred to the inner glass balloons and their filling liquids (and vice versa), the thermometer follows changes in the air temperature only slowly and reacts slowly. For this, the temperature can be read easily "from a distance" using the " large scale " and the different colors of the liquids.
One difficulty in production is balancing the density of the glass bodies. For this purpose, the glass bodies should have exactly the same volume or the displacement volume is determined individually and the necessary amount of liquid is added for the correct buoyancy. One-sided heating and melting of the (alcohol) -filled glass body also requires craftsmanship.
See also
literature
- Christian Ucke, Hans-Joachim Schlichting: The Galilei thermometer. Termometro Lento. In: Physics in Our Time. 25 (1994), issue 1, pp. 44-45 (pdf)
Web links
Individual evidence
- ^ WEK Middleton: A History of the Thermometer , Johns Hopkins Press, Baltimore, 1966; quoted at: Games, Physics and Fun. P. 87 ( limited preview in Google Book search).
- ↑ a b Games, Physics and Fun. P. 87 ( limited preview in Google Book search).