A. Take the thread and wrap it around the metre scale. It should be wrapped without giving any space between the coils. Assume that the diameter of the thread is d.

Then number of turns, take n (coils) multiplied with d will give the length upto which you’ve used the thread (the value which you have, take it l). So, d = l/n

B. It is possible theoretically to increase the accuracy of the screw gauge arbitrarily by increasing the number of divisions on the circular scale as the least count is given by pitch/no. of divisions. So, on increasing the no. of divisions, least count will decrease and thus, increasing the accuracy. But practically, it depends on the resolution of eye (to see to different things distinctly and clearly). You can increase no. of divisions upto a certain limit only.

C. The mean diameter of a thin brass rod measured by vernier callipers from a set of 100 measurements is more reliable a more reliable estimate than a set of 5 measurements because the chances of making a negative error and positive error are equally likely and thus they cancel each other giving results with less error.