For measuring the curved line with a cotton thread, the following steps are followed :-
Take a thread and tie a knot at its one end. This knot will act as a starting point for measuring the length of the curved line.
Place the knot of the thread at a point A on the left end of the curved line with the help of thumb and forefinger.
Hold the thread a little distance away from the knot and keep it along the curved line with the help of right thumb and forefinger.
In this way, run the thread all along the curved line in little steps at a time, keeping the thread taut, till the other end B of the curved line is reached. Make an ink mark on the thread where it touches the other end B of the curved line. Now, straighten the thread and measure its length between the knot and the ink mark by keeping it along a scale. This will give us the length of the curved line.