Leslie's thermoscope is a differential air thermometer allowing the measurement of small differences in temperature. It was invented at the beginning of the 19th century by John Leslie (1766-1832), a professor of natural philosophy and mathematics at Edinburgh, who used it in his important studies on radio energy. The instrument in our museum is composed of a glass capillary tube in the shape of a U (width 11 cm, height 22.6 cm) fixed on a wooden board, it too in the shape of a U and supported by as column (height 21.5 cm) on a circular base (diameter 13.5 cm).
The capillary is filled up to approximately half of the vertical parts of the tube with a liquid having a high vapour pressure that does not evaporate at ordinary temperatures, such as sulphuric acid, and terminates at the top with two large bulbs (diameter 2.44 cm) filled with air. The air is the thermometric fluid, while the sulphuric acid simply acts as an index and is generally dyed red. In this instrument it is colourless. The amount of liquid is regulated so as to reach the same height in the two branches when the bulbs are at the same temperature and zero is indicated on the two scales. A difference in temperature between the two bulbs raises the liquid above zero in one branch and lowers it by the same amount in the other branch. The scales on the two branches are graduated from 0 to 20 going downwards and from 0 to 50 going upwards.
The fact that the two bulbs are blackened indicates that this instrument was used in research on radio energy.

Boato - Bruzzaniti (1993), p. 117
Ganot (1864), p.206
Jamin (1886), T. II, p. 133
Turner (1983), pp. 113, 119
Verdet (1868), T. I , p. 75

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