# What is temperature?

How did the historical progress of thermometer follow and how was the unit of temperature determined?

Galileo Galilei's thermometer in 1603 utilized the expansion and contraction of air. However, as the air was sealed with the water which was exposed to the outside air, the height of the water surface, which indicates the temperature, was affected by the atmospheric pressure. Later, a thermometer was developed that contained liquid in a tube, and it enabled to be observed temperature changes without being affected by atmospheric pressure.

When the accuracy of the thermometer was improved in this way, there was also a person who tried to measure quantitatively with regular graduations on the tube.

In 1701, Isaac Newton proposed that the liquid level of the thermometer set to zero, when the thermometer is inserted into melted ice, and that the temperature of the human body set to twelve, and divided that temperature difference by twelve. At that time, the thermometer used the expansion and contraction of water and alcohol, and the measurement range and accuracy were quite limited.

## Thermal energy

All substances are composed of atoms and molecules. These atoms and molecules are constantly moving actively when the temperature of the material is high and inactive when it is low. The average value of the kinetic energies of these atoms and molecules is called thermal energy.

Temperature is a numerical representation of the thermal energy of a substance.

## Heat transfer

There are three types of heat transfer: conduction, convection, and radiation.

### Conduction

" Conduction " is the transfer of thermal energy from hot to cold objects in contact with each other.　Conduction causes the temperature difference between the hot and cold objects to gradually decrease, eventually equalizing in temperature and stopping the transfer of thermal energy.

In contact-type thermometers, the temperature is measured when it reaches thermal equilibrium by taking advantage of this conductive nature of the object and the sensor.

### Convection

" Conduction " is the transfer of thermal energy from hot to cold objects in contact with each other.　Conduction causes the temperature difference between the hot and cold objects to gradually decrease, eventually equalizing in temperature and stopping the transfer of thermal energy.

In contact-type thermometers, the temperature is measured when it reaches thermal equilibrium by taking advantage of this conductive nature of the object and the sensor.

### Radiation

"Radiation" is a phenomenon in which the thermal energy of a substance is emitted to the ambients in the form of electromagnetic waves (visible light, infrared rays, etc.), but this may not be intuitive. For example, you can feel warm without touching your hands by simply bringing your hands close to the stove, as your hands have sensed the radiant energy from the stove. In this case, the hand acts as an infrared sensor. In the same principle, an infrared sensor detects the amount of infrared energy emitted from an object and measures the temperature of the object from the amount of infrared energy.