Theory:

In day-to-day life, so many changes are taking place across many things around us. For example, day turns to night and vice versa.
 
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Time measuring devices
People used different time measuring devices with different designs in various parts of the world.
  
Ancient days
Sand clock, sundial and water clocks were the time measuring devices commonly used in the olden days.
 
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Sand clock
  
In the olden days, people calculated the time of the day with the help of shadows. The shadow cast by a stick was used to measure time.
 
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Sundial
 
Any vessel or bottle with a small hole filled with water can be used as a measuring device. The water that drains from the vessel or bottle is used for measuring time.
 
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Water clock
 
Even pulse can be used to measure time, but not accurately. The number of pulses can say the time elapsed. Even while spelling 'one second', 'two seconds' at a normal rate counts the passage of time. These techniques and methods for measuring the passage of time do not provide an accurate value.
 
In modern times
All these devices were used before the introduction of clocks. However, improvement in technology has led to the development of clocks that can measure the precise time intervals within a day.
 
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Electronic clock and stopwatch are the other devices used to measure smaller durations of time.
 
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Electronic clock
 
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Stopwatch
Units of time
Time is measured in the units of \(second\) (\(s\)), \(minute\) (\(min\)) and \(hour\) (\(h\)). The larger units of time are \(minutes\) (\(min\)) and \(hours\) (\(h\)), whereas the \(second\) (\(s\)) is a smaller unit.
 
\(1\ hour\) \(= 60\ minutes\)
 
\(1\ minute\) \(= 60\ seconds\)
 
One second is the smallest known interval of time that is available in clocks.
Reference:
https://cdn.pixabay.com/photo/2015/06/25/17/21/smart-watch-821557_1280.jpg
https://upload.wikimedia.org/wikipedia/commons/3/37/Ancient_water_clock_used_in_qanat_of_gonabad_2500_years_ago.JPG