A thermoscope is a device that shows changes in temperature. A typical design is a tube in which a liquid rises and falls as the temperature changes. The modern thermometer gradually evolved from it with the addition of a scale in the early 17th century and standardisation through the 17th and 18th centuries.
It is thought, but not certain that Galileo Galilei discovered the specific principle on which the device is based, and built the first thermoscope in 1593. The inventor could be his physician friend Santorio Santorio or another person of the learned circle in Venice of which they were members. Devices employing both heat and pressure were common during Galileo's time, used for fountains, nursing, or bleeding in medicine. The device was built from a small vase filled with water, attached to a thin vertically rising pipe, with a large empty glass ball at the top. Changes in temperature of the upper ball would exert positive or vacuum pressure on the water below, causing it to rise or lower in the thin column.
Santorio Santorio wrote a Commentary on the Medical Art of Galen in 1612 that described the device in print. Shortly afterward, in 1617 Giuseppe Biancani published the first clear diagram. The device at this time could not be used for quantitative or standardized measurement, and used the temperature of air to expand or contract gas, thereby moving a column of water.
The device was improved by early German scientist Otto von Guericke in the 17th century. Ferdinando II de' Medici, Grand Duke of Tuscany personally made a further improvement by introducing the use of a colored alcohol, so that the material responding to heat was now liquid instead of gas.
It is possible that Francesco Sagredo or Santorio may have added some kind of scales to thermoscopes, and Robert Fludd may have accomplished something similar in 1638. In 1701 Ole Christensen Rømer effectively invented the thermometer by adding a temperature scale (see Rømer scale) to the thermoscope.
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