Anders Celsius conducted a series of precise experiments that demonstrated that the melting point of snow or ice and the boiling point of water, when adjusted for atmospheric pressure, were universal constants. He used these results to establish a uniform temperature scale, which allowed the calibration of thermometers worldwide.
Although it is relatively easy for humans to determine that the temperature in a room is “hot” or “cold,” or to compare the temperatures of two objects by touching them, the scientific measurement of the temperature of an object developed relatively recently. Galileo,
The German physicist and inventor Daniel Gabriel Fahrenheit significantly improved on the design of the thermometer. He filled capillary tubes with alcohol, beginning in 1709, or with mercury, beginning in 1714, so the expansion of the fluid produced a significant change in its height. Fahrenheit’s thermometers were capable of precise temperature measurements.
Even with Galileo’s invention of the thermometer, there was no simple way to ensure that thermometers built by different people in different places around the world would read the same numerical value for the temperature of an object. Thermometers must be calibrated in order for their readings to be compared. Such calibration requires that readings be taken at two standard temperatures, which are called fixed points. The positions of these fixed points are noted and the number of divisions between the fixed points is specified, providing a numerical scale for the thermometer. Thermometers were used in meteorology, in agriculture, and sometimes indoors, so it was natural that scientists chose fixed-point temperatures within the temperature range of interest in those fields.
A major difficulty in the initial development of thermometer scales was deciding how to define the fixed points such that thermometers could be calibrated easily and reproduced all over the world. In the early years of temperature measurement, there were many different fixed points in use, but no one knew with certainty if any of these points were truly fixed. Fahrenheit constructed mercury thermometers with scales that used fixed-points at the freezing point of water,
It was easy to demonstrate, however, that normal body temperature varies a bit during the day, even in a single individual, and it can vary significantly in an individual who is ill. In addition, two individuals are likely to have slightly different body temperatures. Thus, human body temperature is not truly a fixed point, and two thermometers calibrated using human body temperature as a fixed point will not necessarily record exactly the same temperature for an object. A variety of other fixed points, including the melting point of butter, had been suggested by other scientists. Scientists recognized that they needed to find physical phenomena that occurred at precisely the same temperature all around the world. These phenomena could then be used as the fixed points to calibrate thermometers.
Anders Celsius, a Swedish astronomer, became interested in problems of weights and measures, including temperature measurements, early in his career. As a student, Celsius assisted Erik Burman, a professor of astronomy at Uppsala University, in meteorological observations. At that time, there was no accepted standard for thermometers. Thus, it was impossible to compare temperature readings taken in different places, unless the same thermometer was taken from place to place or the two researchers used thermometers that had been calibrated against each other, a tedious system similar to synchronizing watches.
In 1702, Ole Rømer,
Since human body temperature had been shown to vary from individual to individual and from time to time in the same individual, it was important to demonstrate that any fixed points that were selected were truly universal standards. It had been established by this time, for example, that the freezing point of water varied if contaminants were dissolved in the water. For example, salt water from the ocean freezes at a significantly lower temperature than does pure water. Celsius attacked the problem of establishing a universal temperature scale by conducting a series of careful experiments. He determined that the temperature at which pure water, in his case newly fallen snow, melts is independent of latitude and also independent of atmospheric pressure. Thus, the melting point of snow, or of pure water ice, could serve as one of the fixed points of a reliable temperature scale.
The boiling point of water posed more of a problem. Celsius showed that it did not depend on latitude, but it did vary with atmospheric pressure. Celsius measured the dependence of the boiling point of pure water on atmospheric pressure. Atmospheric pressure could be measured accurately using a barometer, which had been invented by the Italian physicist Evangelista Torricelli,
In 1742, Celsius published his results in the annals of the Royal Swedish Academy of Sciences
There was one major difference between the temperature scale developed by Celsius and the temperature scale used in the twenty-first century. Celsius created an inverted scale, in which higher numbers indicated increased cold rather than increased heat. Carolus Linnaeus may have been the first scientist to reverse Celsius’s scale, putting the freezing point of water at 0 degrees and the boiling point at 100 degrees. However, Linnaeus’s thermometers, as well as many of those used by Celsius, were built by Daniel Ekström, and it is unknown if Linneaus or Ekström actually initiated the scale reversal. A Frenchman named Jean Pierre Christin has also been suggested as the source of the inversion.
The modified temperature scale became popular in Sweden and France in particular. It was referred to as the centigrade scale,
From the scientific point of view, the most important contributions to the modern temperature scale were those of Celsius, because his careful experiments established two universal fixed points, which allowed thermometers built in different laboratories to be calibrated using the same temperature scale. After Celsius developed this universal temperature scale, it was possible to measure temperature very accurately, allowing comparison of results from one laboratory to another. Scientists were then able to determine how various physical properties of materials vary with temperature.
Thus, the development of a universal temperature scale was essential, for example, to the understanding of the expansion of gases as a function of their temperature and of similar changes in the volumes of liquids and solids that depend on temperature. Later, it was established that thermal and electrical conductivity also vary with temperature. The development of a universal temperature scale allowed weather records to be compared from one location to another, resulting in the eventual understanding of weather patterns and the development of weather forecasting.
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Asimov, Isaac. The History of Physics. New York: Walker, 1966. Designed for nonspecialists, this book includes an excellent chapter on temperature, including a section on temperature scales that describes Celsius’s contribution and its historical context. -
Chang, Hasok. Inventing Temperature: Measurement and Scientific Progress. New York: Oxford University Press, 2004. A 304-page account that traces the history of the development of temperature scales, including an excellent table of the various fixed points employed before Celsius established a universal standard. -
Gardner, Robert, and Eric Kemer. Science Projects About Temperature and Heat. Berkeley Heights, N.J.: Enslow, 1994. An excellent, 128-page collection of experiments allowing students in middle school and higher to perform many of the classic experiments in the development of ideas about temperature and heat.
Fahrenheit Develops the Mercury Thermometer
Gray Discovers Principles of Electric Conductivity
Linnaeus Creates the Binomial System of Classification
Bernoulli Proposes the Kinetic Theory of Gases
Invention of the Leyden Jar
Daniel Gabriel Fahrenheit; Carolus Linnaeus.