Theory:

Hot and Cold Objects:
In our daily life, we may come across many objects which are hot or cold.

How do we say which object is hotter or cooler than the other?
In many cases, we will use our fingertip to determine whether the object is hotter or cooler than the others.

But actually, it is not correct.

We can do it through the measurement of temperature through thermometers.
 
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Temperature measurement
  
Temperature
The measurement of the hotness or the coldness of a substance is known as its temperature. It determines the direction of the flow of heat when two bodies are placed in contact.
SI unit of temperature is Kelvin. Celsius and Fahrenheit are the other units used. Celsius can be called Centigrade as well.
 
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Temperature measurement through thermometer
  
Temperature Scales:
Temperature is measured through thermometers according to well-defined scales of measurement, which use pre-defined reference points to help compare quantities.
Kelvin, Celsius, and Fahrenheit scales are the three most common temperature scales used.
 
A temperature scale can be formulated by identifying two easily reproducible temperatures. The freezing and boiling temperatures of water at standard atmospheric pressure are commonly used as reference temperatures.
 
Kelvin scale:
The Kelvin scale is a temperature scale commonly used in science. It is an absolute temperature scale defined to have \(0 K\) at the lowest possible temperature, called absolute zero. The official temperature unit on this scale is the kelvin, abbreviated as \(K\).
 
Celsius scale:
The Celsius scale (which replaced the slightly different centigrade scale) has the freezing point of water at \(0 ºC\) and the boiling point at \(100 ºC\). Its unit is the degree Celsius (\(ºC\)).
Fahrenheit scale:
On the Fahrenheit scale (still, the most frequently used in the United States), the freezing point of water is at \(32 ºF\), and the boiling point is at \(212 ºF\). The unit of temperature on this scale is the degree Fahrenheit (\(ºF\)).
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Temperature scales
  
Temperature Conversion:
The relations between the three standard temperature scales are shown in the below figure. The temperature on one scale can be converted to another using the equations shown in the figure.
 
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Relations between temperature scales
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Temperature conversion
Reference:
https://commons.wikimedia.org/wiki/File:Thermometer_0.svg
https://commons.wikimedia.org/wiki/File:Hand_with_surgical_gloves_aiming_infrared_thermometer_at_a_man_with_mask_on_blue_background.jpg