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What is commonly called the Galileo thermometer today is really the development of the thermoscope by Ferdinando II de' Medici.

The Invention of the Thermometer

In the early 17th century, during the Scientific Revolution when the frontiers of discovery were marked by new ways to quantify natural phenomena, Galileo Galilei was forging new, innovative and empirically-based methods in astronomy, physics and engineering. He also got humanity started toward a lesser-known advance, the ability to measure heat.

During this era, a flurry of measuring devices and units of measurement were invented, eventually forging the standard units we have in place today. While working in Padua in 1596, Galileo invented a device called the ‘thermoscope.’ It was an instrument for gauging heat, but it is not the same as a thermometer. It lacked a critical function that we take for granted today, it had no scale. A brilliant concept at the time, the device indicated differences in temperature, but unlike a thermometer it did not measure the difference.

A statue of Galileo Galilei is one of 28 historic figures that adorn the columns of the Uffizi Gallery in Florence.

In 1612, Venetian scholar Santorio Santorio made crucial advances to Galileo’s thermoscope by adding a scale for measurement. The early thermoscope was a vertically-oriented glass tube with a bulb at the top and a base suspended in a pool of water which ran up a length of the column. As the temperature of the air in the bulb increased, its expansion caused the height of the liquid in the column to change. Santorio set the maximum level by heating the thermoscope’s bulb with a candle flame and the minimum by contacting it with melting snow. Santorio was the first to apply the thermometer to the field of medicine. To take a body temperature measurement, the patient would either hold the bulb with their hand or breathe on it.

The next crucial participant in the development of the thermometer was Ferdinando II de’ Medici, Grand Duke of Tuscany. Like his father before him, Ferdinando was a patron, ally and friend of Galileo who dedicated his “Dialogue Concerning the Two Chief World Systems” to the Duke. Unfortunately, this work led to Galileo’s second set of hearings before the Inquisition. Ferdinando attempted to keep the concerns of the Holy See from leading to a full-fledged hearing and kept Galileo in Florence until December 1632, when the Roman inquisitors finally threatened to bring Galileo to Rome in chains if he would not come voluntarily. In June 1633, the Roman Inquisition convicted the astronomer-scientist for “vehement suspicion of heresy” and sentenced him to imprisonment for life. After this was commuted to house arrest, Ferdinando came to visit the elderly scientist at his villa, where Galileo lived out the remainder of his life, passing away in 1642.

A replica of the thermoscope invented by Galileo.

Ferdinando had long been obsessed with new technology and had several hygrometers, barometers, thermoscopes and telescopes installed in the Palazzo Pitti. In 1654, influenced by Galileo, he invented the sealed-glass thermometer by enclosing the glass tip of a tube filled to a certain height with colored alcohol. Small glass bubbles filled with air at varying pressures hovered, trapped within the liquid. The bubbles changed positions as the temperature rose or fell. Marked off with 360 divisions, like the degrees of a circle, this type of device was called a spirit thermometer, because it was filled with distilled alcohol or spirit of wine, as it was then known. In 1657, Leopoldo de’ Medici, the Grand Duke’s youngest brother, established the Accademia del Cimento, the first scientific society in Italy. It was set up to attract scientists from all over Tuscany to Florence for mutual study. The scientists explored various forms and shapes for the thermoscope. This led to improvements in both the form and function of the instruments and demand grew steadily for the Florentine thermoscopes.

In the years to come, other improvements were made, including the use of mercury rather than alcohol and ultimately the introduction of standardized scales that we still use today, all of which were based on the scientific endeavors over the course of 60 years by Galileo, Santorio and the de Medici’s.