A thermometer is an instrument used to measure temperature. Developed initially by the Italian astronomer Galileo at the end of the sixteenth century the prototype was a rather crude affair and was completely different from the instruments that we are familiar with. Galileo’s thermometer worked however and was the first “gas thermometer” known to have been developed.
The principle that was used is the same that our modern day instruments are based upon. Galileo used a vertical glass tube which he stood in a bowl of water with an air filled glass bulb attached to the top end. Capillary attraction made the water rise up the tube and when the bulb at the top was heated the water descended, when the bulb was cooled the water level rose again. Basically what was happening was that the water was acting as an indicator of the air temperature inside the glass bulb. Galileo had discovered the fact that gas or liquid if cooled or heated will contract or expand more quickly than glass. After the initial discovery was made it was really a short step forward to develop the mercury filled graduated thermometers that we know today.
Mercury is ideal for use in thermometers because it has a low freezing-point and a high boiling-point and it expands uniformly when heated. For measuring extremes of temperature, alcohol, electrical or gas instruments are used and are known as pyrometers.
Thermometers in everyday use are still based on the instrument developed by Galileo and the measurement of temperature is shown either in the Fahrenheit or centigrade scale that we have all become familiar with. Development has continued of course and extremely accurate scientific instruments that utilise modern technology to measure temperature are now available.
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