How will you measure the length of a curved line by using a thread?

A curved line or surface cannot be measured by a straight scale, a measuring tape or thread must be used instead.To measure a curved line using a thread, follow the below steps:

Tie a knot at one end of the thread.
Place the thread at the beginning of the line and try to measure a small initial portion of the line which is relatively straight.
Place your thumb at the other end of the measured portion and measure next straighter portion.
Repeat the above step till the end of line is reached. Make a knot at the end of the line.
Now, straighten the thread and measure the length of the two knots on a scale.