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Wooden framed thermometer by W. E. Pain.

Although a type of thermometer called the thermoscope had been invented in the late sixteenth century, it was not until the seventeenth and eighteenth centuries that natural philosophers attempted to develop a standard temperature-scale according to fixed points, such as the degree of heat in boiling or freezing water. While many scales were developed, only a few types persisted in the design and manufacturing of thermometers into the nineteenth and twentieth centuries. Still familiar to us today, temperature scales developed by Anders Celsius (1701-44) and Daniel Gabriel Fahrenheit (1686-1736) are two such examples.

Galilean thermometers

In a letter to Balthasar de Monconys (1611-1665) dated 1646, Evangelista Torricelli (1608-1647) described the earliest known sealed liquid-in-glass thermometer. Developed in Florence, a vibrant commercial center with a rich glass-blowing culture, the phial of these early thermometers contained glass balls of different weight to volume ratios.

Small air bubbles trapped in the glass balls would shrink or expand according to the surrounding liquid, causing the balls to rise or fall in the tube and indicate temperature. The principles of the Galilean thermometer are based on the air thermoscope as invented by Galileo Galilei (1564-1642); however, Galileo did not invent the Galilean thermometer.

Florentine thermometers

17th century Florentine thermometer
Image 1 Charles Babbage (1791-1871) was presented with this seventeenth-century Florentine thermometer in 1834 by Vincenzo Antinori (1792-1865), Director of the Imperial and Royal Museum of Physics and Natural Philosophy at Florence (Wh.1116).

The later Florentine thermometer was also a sealed liquid-in-glass design (Image 1). In this instance, the expansion and contraction of the thermometer's liquid was measured against a scale of glass beads that marked the neck of the phial. A variety of liquids were used to measure temperature, but most thermometers were filled with water or spirit of wine. This was a colourless spirit, as red dye tended to soil the tube.

Andrea Mariani, master glass-blower to Ferdinand II, Grand Duke of Tuscany, produced many of the thermometers used at the Accademia del Cimento (the Academy of Experiments), which was founded by Ferdinand II in 1657 to conduct extensive experiments on the conditions of the atmosphere. He claimed his workshop could produce 50 degree thermometers with uniformity; however, manufacturing consistent products measuring 100 and 300 degrees proved too challenging.

Although Florentine scales did not use fixed points, such as the heat of boiling water, to establish the amount of spirit sealed inside the tube, thermometers produced in Mariani's workshop are remarkable in that their temperature readings consistently agree between each other.

Read more: Ferdinand II and the development of hygrometers

Thermometer scales

Réamur thermometer
Image 2 Mercury in glass Reaumur thermometer, French, c. 1780 (Wh. 1889).

Due to Mariani's excellent glass-blowing skills, his Florentine thermometers produced the same temperature readings with regularity. However, thermometers made by different glass-blowers used different scales that did not agree when compared.

In the 1660s, Robert Hooke (1635-1707), at the Royal Society in London, attempted to construct thermometers that would universally agree at one fixed point: that at which distilled water started to freeze. He experimented with several thermometric liquids, such as spirit and mercury, to use inside his thermometers.

The polymath Isaac Newton (1643-1724) used linseed oil in his sealed-in-glass thermometers. His scale used three fixed-points: the temperature of air when water begins to freeze, the heat of blood in the human body and rapidly boiling water. On Newton's scale, freezing air was 0º, blood heat was 12º and boiling water was 34º.

Thermometers produced in the eighteenth century sometimes marked the temperatures of various 'standards' for medical, economic and meteorological purposes (Image 2). For example, the long-bulb mercury thermometer made by René-Antonie Ferchault de Réaumur (1683-1757) listed the temperatures - in his own scale - of freezing and boiling water, the human body (30ºR), a warm bath (25ºR), silkworms (19ºR), hothouses (15ºR), a temperate atmosphere (9.5ºR), an orangery (6ºR), black ice (-4ºR), as well as the coldest temperature registered in Paris in 1740 (-10.5ºR) and 1776 (-15ºR).

Réaumur divided his thermometer scale according to two standard temperatures: when water freezes (0º) and boils (80º). The distance between these two points on the neck of the thermometer was divided into equal increments. Throughout his career, Réaumur continued to develop and modify his temperature scale, so that his thermometers from the 1730s differed numerically from those produced in 1770.

Read more: Early thermometers: further designs

Allison Ksiazkiewicz

Allison Ksiazkiewicz, 'Early thermometers and temperature scales', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge

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