Theory:

Curved line measurement
There are two methods to find the length of a curved line.
  1. Using a thread or a string
  2. Using a divider
Measuring a curved line using a thread
The length of a curved line can be found using a string or a thread.
 
measure5.svg
  1. Draw a curved line \(AB\) on a sheet of paper.
  2. Place the thread along the curved line covering every bit.
  3. Now, mark the points where the curved line starts (\(A\)) and ends (\(B\)) on the thread.
  4. Then, keep the thread on the metre scale and measure the length between the thread marks.
  5. Thus, the length of a curved line is measured.
Measuring a curved line using a divider
dividers-37921_1280.png
  1. Draw a curved line \(AB\) on a sheet of paper.
  2. Using a ruler or a scale, separate the divider legs by \(0.5\ cm\) or \(1\ cm\).
  3. Place the divider on the curved line from one end and mark the other end.
  4. Move the divider along the line again and repeat this process.
  5. In this manner, cut the line into segments of equal lengths and keep a count.
  6. If there is any remaining part of the line, measure it using a scale.
Therefore,
Length of the line = (Number of segments × Length of each segment) + Length of the leftover part
Reference:
https://cdn.pixabay.com/photo/2012/04/18/21/21/dividers-37921_1280.png