Today the thermometer is perhaps the best known physical instrument. And just few centuries ago it used to be an absolutely unknown thing. The temperature was determined by corporal feeling and while manufacturing the metals and ceramics the people followed the colour of the incandescent objects or molten metals.

Not until the beginning of the 17^th century Galileo Galilei – the famous professor of the Padova university in Italy put to use the thermal extensibility of the air for measuring the temperature. This primitive thermometer was made up from a glass tubule about 30 cm long with a bulb on its end. He warmed up the bulb with his hand and put the “thermometer” (they started to call it an air thermoscope) with the open end of the tubule into a pot with coloured water. The air that was getting colder was shrinking and the water was entering the tubule thanks to the pressure of the surrounding air. After it got cold the bulb was accepting the temperature of the surrounding air and the height of the water-column in the tubule was changing according to the changes in the air-capacity in the bulb, which was changing according to the temperature of the air. In contrast to the nowadays thermometers the level was sinking when it got warmer and it was rising when it got colder. The instrument did not have a scale.

After Galilei also Otto von Guericke and Gaspar Schott were experimenting with similar thermometers. They improved the thermoscope by using a closed system with two bulbs and a connecting tubule in an U shape, which was filled with some liquid.

Still in the same century appear such thermometers, in which the thermometric substance is liquid. Apparently the first one was constructed in 1631 by a French physician Jen Rey, who used the water as the thermometric substance. The disadvantage of this thermometer was the small extensibility of water. Therefore some other liquids were searched. As the most suitable ones appeared mercury and alcohol. First alcohol thermometer was constructed in 1641 by the Tuscan grand duke Ferdinand II. At that time the thermometer had already had the scale, but these were not integrated, so that the data measured with individual thermometers could not have been compared. The first thermometer with a “normalised” scale weren’t constructed until 1650.

Except the integrated scale it was also necessary to assign its beginning. The English physicist Robert Boyle assigned the temperature of the melting ice as the basic point of the scale on his thermometer. This happened in 1664. In 1665 a Dutch scientist Christian Huygens assigned another firm point on the scale. It was the boiling-temperature of water (when the air pressure is normal – 1013 hPa, because as we know the lower the air pressure is the lower is the boiling-temperature of water. And so Huygens suggested to consider the boiling-temperature of water or the melting-temperature of ice to be the basis of the thermometer-scale. Actually this is the way, how we use it up to now.

About 50 year later Daniel Gabriel Fahrenheit started to produce alcohol and afterwards even the mercurial thermometers in Holland. Very individually he chose as the beginning of his thermometer-scales the temperature of this mixture: ice, water an salmiac. As the upper basic temperature he assigned the temperature of a healthy person and signed it with a number 96 (?). He divided the distance between the both temperatures into 24 parts and each of them into other four, so that he finally got the grades! The melting-temperature of the ice is signed on this scale 32 and the boiling-temperature of water with a number 212. It is very strange that so complicated and absolutely illogical scales are being used up to now (e.g. in USA). A more reasonable René de Réamur the Parisian zoologist, drafted the scale with a zero for the melting-temperature of ice and with 80 for the boiling-temperature of alcohol (later this value corresponded with the boiling-temperature of water).

The Swedish mathematician and geodetist Anders Celsius establishes the decimal system, where the boiling-temperature of water has got the number 0 and the melting-temperature of ice the number 100. The name of the man, who later turned these values as we know them and use them up to now was not discovered with certainty. The unit of this scale, as we know, is a thermal degree of Celsius (° C).

World's Tallest Thermometer (Baker, California)
(taken over from http://www.cagedigital.com/jandm/excursions/thermometer.html)

All these thermometers are based on the same principals, on the thermal extensibility of liquids. Therefore they are called dilative. Celsius actually ended up their development. During the following years they only were modified. E.g. the temperature over the boiling-temperature of mercurium (356° C) up to 1100° C is measured with a mercurial thermometer, where the capillary is filled e.g. with nitrogen and the thermometer is made of quartz glass. The medical mercurial thermometers (35 - 42° C) have got the capillary narrowed over the pot with mercury, so that the thermometer shows the maximal temperature that was measured (before another use it has to be knocked down).

	Above infra-thermometer with thermometer-layrer (-30...+3000°C) On the left semiconductor thermometer (-20...+1000°C)

Later on the thermometers based on different physical principles appeared. E. g. the adversative thermometers measure the temperature according to the changers of the electric resistance of the conductor and the semiconductor (temistor). The thermoelectric thermometers are based on a thermoelectric phenomenon, when the changes in temperature of an attachment of two different metals change the arose thermoelectric voltage. The temperature can be measured even “without touching”, e.g. by means of so called infra-thermometers, that measure the radiance emitted with objects into their surroundings (the same principle is used by infra-sensors and unfortunately the guided missiles, too). All these thermometers can have except the classical analog scale also a digital scale and above all they can have an electric output, which allows the electric processing of measured valued or even the automatize control of the process

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