Determine the ring’s outer diameter.
On Fig. 2.3, place one ring so that it is centred on the $5.0\,\text{cm}$ mark.
Explain why the student adds the rings one at a time with care.
On Fig. 2.4, draw a graph of $N$ on the $y$-axis against $l / \text{cm}$ on the $x$-axis. Begin both axes at $(0, 0)$. Draw the best-fit curve.
Use your graph to estimate the number $N_{18}$ of rings needed to balance the rule when $l$ is $18.0\,\text{cm}$. Show your working clearly on your graph.
Theory predicts that the mass $M$ of the metre rule is given by $M = \frac{3.3 \times N_{18} \times l}{45 - l}$. Using your answer in (ii), calculate a value for $M$. Give your answer to 2 significant figures.
Name a piece of apparatus that can be used to check whether the theory is correct.
Suggest one way in which this method of measuring the mass $M$ of the metre rule can be improved.