You will have a rough idea of what you want to do, but will need to 'tweak' your idea by trying things out practically. You therefore sketch out a rough experiemental procedure and test out the best way to do it in a preliminary session.

You may want to:

- choose materials to work with: check that you will get a big enough range of readings with the ones you have chosen to investigate.

- find out if you are controlling the other variables well enough to have a 'fair test'.... maybe you will spot some you hadn't thought of!

- practise using the equipment, and see if you need to make adjustments to avoid or minimise errors.... or make it safer!

- spot dangers in your procedure that you ought to avoid.

Always check with a teacher before you carry out preliminary experiments - they have more experience at spotting potential dangers than you do!!

What you find out from your preliminary readings will influence your final design of your experiment.

Remember to say in your report if you found out a better way to do it from preliminary work.... and how you checked your ideas were sound before proceeding.