Introduction

In this chapter we shall consider some general principles for making measurements. These are principles which should be borne in mind, first in selecting a particular method and second in getting the most out of it. By the latter we mean making the method as precise or reproducible as possible, and – even more important – avoiding its inherent systematic errors.

We shall illustrate the various points by describing some specific examples of instruments and methods. Though chosen from several branches of physics, they are neither systematic nor exhaustive. The idea is that, having seen how the principles apply in these cases, you will be able to apply them yourselves in other situations. As always there is no substitute for laboratory experience. But experience without thought is a slow and painful way of learning. By concentrating your attention on certain aspects of measurement making we hope to make the experience more profitable.

Metre rule

We start with almost the simplest measuring device there is – a metre rule. Its advantages are that it is cheap to make and convenient to use. It can give results accurate to about ⅕ mm. However, to achieve this accuracy certain errors must be avoided.

Parallax error. If there is a gap between the object being measured and the scale, and the line of sight is not at right angles to the scale, the reading obtained is incorrect (Fig. 6.1a).