Theory:

Digital Thermometer:
Clinical thermometers have the practical challenge of disposal of mercury while they are broken down. Mercury is a toxic substance, so it is necessary to dispose of it correctly and safely. To overcome this situation, nowadays, Digital thermometers are used to measure the human body temperature in hospitals.
 
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Digital thermometer
 
Working:
Digital thermometers work on the principle that the electrical resistance of the material changes with temperature. Digital thermometers are different to conventional thermometers, such as liquid-filled and bimetallic thermometers, as they need a sensor. These sensors are producing resistance, current or voltage in reaction to a change in temperature. These changes are referred to as analogue output signals, and they are converted to digital to make them easy to visible.
Infrared thermometer:
Infrared thermometers are typically used to measure the temperatures that are necessary to be measured from a distance.  They give accurate temperatures without ever having to touch the object you are measuring (and even if the object is in motion).
 
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Infrared thermometers