The present edition retains the basic outlook of the book, namely to demonstrate the purposive and critical approach which should be made to all experimental work in physics. But I have made a number of changes and additions in response to new experimental methods and the widespread use of computers, which I hope will add to its usefulness.

Substantial changes have been made in chapter 7, in which a selection of techniques are analysed to show the art and craft of the experimenter. I have added a section on the measurement of time and frequency, which includes an account of the caesium atomic clock and the present universal time scale. This is followed by a description of the Global Positioning System, which, based on atomic clocks, enables position on the surface of the Earth to be determined to very high precision. Timing techniques have a number of elegant and ingenious features, which, combined with the importance of their practical applications, make them instructive and interesting, both at the elementary and advanced level.

I have added an appendix on the χ2 distribution. The goodness of fit test based on this distribution finds widespread application in the physical, biological, medical, and social sciences. Many students have had an introduction to the topic at school and have learnt to apply the formulae of the test, but I feel that valuable insight is gained from seeing a derivation of the distribution, which I have given in a not too formal manner.