A thermometer is an instrument used to measure the temperature. Traditionally, a thermometer is a sealed glass tube with a bulb at one end containing colored alcohol that expands and contracts when heated and cooled.
A thermometer is a tool that can be used to measure the temperature of a substance. The name comes from Greek thermos, meaning warm, and metron, meaning measure.
Traditionally, thermometers are made from a sealed glass tube with a bulb containing liquid at one end. As the liquid expanded and contracted as it was heated and cooled, the liquid would move up and down the tube. On the tube are lines following a temperature scale, such as the fahrenheit, celsius, or kelvin scales.
The thermometer wasn’t a single invention because there were a number of discoveries that led to invention. Hero of Alexandria discovered that substances can expand and contract as they are heated and cooled. Notably Galileo Galilei created a thermoscope that could be used to detect changes in temperature. A scale was put on this by a friend of Galileo's, Giovanni Francesco Sagredo. This was the first time quantitative measurements, as opposed to relative measurements, could be made using a thermometer. The first reliable thermometer was created by the Dutch scientist Daniel Gabriel Fahrenheit who used mercury instead of alcohol and water.
The invention of the thermometer has helped humanity in many ways. In catering it allows us to know if food is being stored correctly or if the food has been cooked the correct amount to be safe. In medicine they can be used to check if the body is working outside of its normal temperature range, a sign that something is wrong. They also provided a tool that allowed scientists to quantitatively compare the climate in different parts of the world.
British scientist James Joule famously used a thermometer in 1843 when carrying out experiments looking at the mechanical equivalents of heat. In the experiment, he connected a falling weight connected to a turning paddle using pulleys. The paddle was situated in a sealed container of water. As the weight fell, the paddle started to turn inside the water. Joule wanted to see if the energy from the falling weight would be converted into heat energy in the water. Joule used a thermometer to measure the temperature increase of the water.