How was temperature measured in the past

Three of the world’s most complete temperature tracking records – from NASA’s Goddard Institute for Space Studies, the National Oceanic and Atmospheric Administration’s National Climactic Data Center and the UK Meteorological Office’s Hadley Centre – begin in 1880. Prior to 1880, temperature measurements were made with instruments like thermometers. The oldest continuous temperature record is the Central England Temperature Data Series, which began in 1659, and the Hadley Centre has some measurements beginning in 1850, but there are too few data before 1880 for scientists to estimate average temperatures for the entire planet. Data from earlier years are reconstructed from proxy records like tree rings, pollen counts, and ice cores. Because these are different kinds of data, scientists generally don’t put proxy-based estimates on the same charts as the “instrumental record.”

The above-mentioned agencies and others collect temperature data from thousands of weather stations worldwide, including over the ocean, in Antarctica, and from satellites. However, instruments are not perfectly distributed around the globe, and some measurement sites have been deforested or urbanized since 1880, affecting temperatures nearby. Each agency uses algorithms to filter the effects of these changes out of the temperature record and interpolate where data are sparse, like over the vast Southern Ocean, when calculating global averages. Generally, all four datasets agree quite closely (see graph above) and are in agreement on the global warming trend since the Industrial Revolution.

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The Measurement Challenge

That was answered by Gay-Lussac and other scientists working on the gas laws. During the 19th century, while investigating the effect of temperature on gas at a constant pressure, they observed that volume rises by the fraction of 1/267 per degree Celsius, (later revised to 1/273.15). This led to the concept of absolute zero at minus 273.15°C.

Observing Expansion: Liquids and Bimetals

Galileo is reported to have built a device that showed changes in temperature sometime around 1592. This appears to have used the contraction of air in a vessel to draw up a column of water, the height of the column indicating the extent of cooling. However, this was strongly influenced by air pressure and was little more than a novelty. The thermometer as we know it was invented in 1612 in what is now Italy by Santorio Santorii. He sealed liquid inside a glass tube, observing how it moved up the tube as it expanded. A scale on the tube made it easier to see changes, but the system lacked precise units. Working with Roemer was Daniel Gabriel Fahrenheit. He began manufacturing thermometers, using both alcohol and mercury as the liquid. Mercury is ideal, as it has a very linear response to temperature change over a large range, but concerns over toxicity have led to reduced use. Other liquids have now been developed to replace it. Liquid thermometers are still widely used, although it is important to control the depth at which the bulb is immersed. Using a thermowell helps ensure good heat transfer.

The bimetallic temperature sensor was invented late in the 19th century. This takes advantage of the differential expansion of two metal strips bonded together. Temperature changes create bending that can be used to activate a thermostat or a gauge similar to those used in gas grills. Accuracy is low — perhaps plus or minus 2 degrees — but these sensors are inexpensive, so they have many applications.

Thermoelectric Effects

Early in the 19th century, electricity was an exciting area of scientific investigation, and scientists soon discovered that metals varied in their resistance and conductivity. In 1821, Thomas Johann Seebeck discovered that a voltage is created when the ends of dissimilar metals are joined and placed at different temperatures. Peltier discovered that this thermocouple effect is reversible and can be used for cooling. In the same year, Humphrey Davey demonstrated how the electrical resistivity of a metal is related to temperature. Five years later, Becquerel proposed using a platinum-platinum thermocouple for temperature measurement, but it took until 1829 for Leopoldo Nobili to actually create the device. Platinum is also used in the resistance temperature detector invented in 1932 by C.H. Meyers. This measures the electrical resistance of a length of platinum wire and is generally considered the most accurate type of temperature sensor. RTDs using wire are by nature fragile and unsuitable for industrial applications. Recent years have seen the development of film RTDs, which are less accurate but more robust.

The 20th century also saw the invention of semiconductor temperature measurement devices. These respond to temperature changes with good accuracy but until recently lacked linearity.

Thermal Radiation

Today, inexpensive infrared pyrometers are used widely, and thermal cameras are finding more applications as their prices drop.

Temperature Scales

Today, temperature measurement scales are defined in a document titled International Temperature System 90, or ITS-90 for short. Readers that would like to check or better understand their measurement units should obtain a copy.