Bringing Old Thermometers Out of Retirement

In early 2009, MIT’s Safety Office made a clean sweep of its mercury thermometers. Suddenly, Deborah Douglas, science and technology curator at the MIT Museum, found herself in possession of a chest full of 18th-century old thermometers stored for more than a century in the Rohsenow Heat and Mass Transfer Lab in mechanical engineering. The lab had acquired its collection from the predecessors of physicists who worked on heat measurements in the late 1800s—along with a wooden cabinet they’d been using since then. As physics research moved away from thermal properties of materials like mercury, the thermometers gathered dust in the cabinet until last year when a request from the MIT Museum brought them out of retirement.

Before there were thermometers, there were the thermoscopes that Galileo Galilei invented in the 17th century—instruments for gauging temperature but with no scales. These didn’t show the changes in temperatures, only the difference in heat between two points; they could be very different and still not indicate a precise temperature in degrees. It was also impossible to compare the results of different thermometers, as air expands and contracts differently with change in temperature, and Sanctorius’ unsealed thermometers were particularly sensitive to atmospheric pressure.

Galileo’s thermoscopes were a good start, but they weren’t very accurate. In 1642, the Grand Duke of Tuscany had an enclosed liquid-in-glass thermometer made for him that used alcohol instead of mercury and did use a numerical scale. This new device benefited from the fact that alcohol has much more sensitive expansion to change in temperature than water, but it was still not perfect. Alcohol was not easy to find in absolute purity, and it’s low boiling point meant that the device had to be carefully cooled to avoid a leak.

In 1701, Danish astronomer Olaus Romer came up with the idea of calibrating the degree mark system to something easily accessible—the freezing and boiling points of water—and his innovation became the standard thermometer scale we still use today. His freezing-point calibration of 7.5 degrees Celsius, while not perfect, was convenient and practical; for boiling, he chose 212 degrees Fahrenheit.

In modern times, the thermometer continues to evolve and is often electronic, rather than mercury. It’s important to remember that mercury is toxic, especially when it’s vaporized, and that even a small amount of exposure can be dangerous. The toxicity of mercury is why the old mercury-in-glass thermometers are no longer being sold—the safer metal alloys now used in oral and rectal thermometers are less susceptible to breaks, which allow the poisonous vapors to escape. The vapors can be inhaled, causing symptoms such as a sore throat, nausea, coughing, or even vomiting. It’s also why we never vacuum up mercury spills and why you should not place any type of mercury in the refrigerator. You can recycle a mercury thermometer at a local hazardous-waste facility, but if you have one in the home, it should be stored safely and not left near children.