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. It was replaced by the thermometer in the early 18th century by the improvement of an added scale, the basis of modern temperature measurement.
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. 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.
Galileo's friend and colleague of medicine at Padua, 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.
- The Galileo Project, "The Thermometer"
- Benedict, Robert P., 1984. Chapter 1, "Early attempts to measure degrees of heat", in Fundamentals of Temperature, Pressure and Flow Measurement, 3rd ed, Wiley ISBN 0-471-89383-8.
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