Physics paper 6 tomorrow!!! help needed

I've been working on a number of paper 6's now, and I came across some questions which I found difficult to answer:

1. The micrometer screw guage is a very accurate measurement. Sugguest why the string and rule method of finding the circumference was innacurate. [1]

2. What is the range of the ammeter shown? [1] (what is a range? - the diagram shows 0-0.5A)

3. Why is it better to find the value of the constant of the relationship between pressure and volume using a graph than from a single measurement? [1]

4. Give two precautions that you would take to obtain an accurate path for the ray of light. You should consider the positioning of the pins P3 and P4 and how they are viewed. You may assume that simple laboratory apparatus is available.

(diagram shows a transparent preplex block used in an experiment to determine the value of a quantity called the refractive index of perpex. The black is placed on a piece of plain paper that is mounted on a cork mat. Two pins P1 and P2 are viewed through the block and two additional pins P3 and P4 and the images of P1 and P2 appear to be exactly in line, one behind the other.)

5. Why is it a good procedure to stir the cold water after adding the brass mass (hot)?



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That's it. Any help on any of these questions is really appeciated!!!! My exam is tomorrow morning and I want to get things straight before I sit it.

thnx.