We use a divider to measure the length of a straight line. Measuring the length of a curved line using a divider is a time consuming method. In this process, the length of a curved line is the sum of the lengths of small line segments contained in it. The technique of measuring curved line will be study in higher classes.
You can also take the divider , and mark the line into equal amount of arc on it, as we already know the each length of the segment, we can add up to calculate the final result.