The word “thermo” means heat, and the word “meter” means to measure. Thermometers measure temperature, by using materials that change in some way when they are heated or cooled. Heat and temperature are not the same though, temperature actually is the degree of ‘hotness’ or ‘coldness’ of the body and in that sense it is related to the heat contained by the body. Heat is a form of energy and no energy can be directly measured, energy is measured by the changes it brings about in matter. Heat is no exception, change in temperature of the body is one of the changes caused by heat in matter and it is measured by the physical change it causes in the body like change in size of the object on receiving or loosing heat.
The invention of the simple thermometer dates back to the times of Galileo, who first invented a water thermometer. But the thermometer of today is the invention of Daniel Gabriel Fahrenheit, a German physicist.
A simple bulb thermometer, which is used to check whether you have fever, or to check the temperature inside or outside the house, has a bulb at the base of the thermometer with a long glass tube containing a liquid, usually mercury. The volume of a liquid increases with temperature and this change in volume is recorded as change in the level of liquid in the long narrow tube. Mercury is the most commonly used liquid in thermometers because it expands and contracts evenly with temperature changes and also its rate of expansion is quite high. This change in mercury levels is calibrated to tell us the temperature.
Another type of common thermometer is a "spring" thermometer. A coiled piece of heat sensitive metal is used, with one end of the metal attached to the pointer. As the air heats, the metal expands and the pointer moves higher. When the air cools, the metal contracts and the pointer moves lower
An interesting thermometer is the ear thermometer. Just the way a tongue is used to detect our body’s temperature, the eardrum too can be an extremely accurate point to measure body temperature. However we cannot insert a thermometer into the ear, as it is a very fragile organ. The heat of the eardrum is measured by detecting subtle changes in infrared emissions from the ear, which is converted to temperature readings.
All thermometers are calibrated into two common scales, Fahrenheit scale or Celsius scale.