That's the distinctive sound of radiation hitting a Geiger counter. As a physics teacher, you'll know it's a Geiger Mulla tube and that sound is completely artificial and I can turn it off. However, those clicks are a useful safety measure. They let us know that ionizing radiation is being detected and that's a useful consideration for this practical which is investigating the inverse square law for gamma radiation. This is one of the core A-level physics practicals and it provides an opportunity to assess your student's safe handling of apparatus which is a third of the common practical assessment criteria or CPAC 3. School should have a standard operating procedure for the safe handling and storage of radioactive materials by teachers and handling by students and as a teacher you should be familiar with Kleeps document L93. It's important to think about how you set this up in the lab. The source should always be pointing away from students and they should avoid getting in front of it. It might be a useful activity to get students to calculate their exposure for the duration they use the apparatus. Most schools would use cobalt 60 which has a half life of about five years and if that's what you've got it's worth getting it out of the cupboard and making sure it's still active enough for the practical. I'm using CZM137 which has a half life of 30 years and a quoted activity of 74 kilobeckerels which makes it safer use for students and also happens to be a bit cheaper. I've got a Geiger-Miller tube attached to a scalar timer but you can buy one of these which is a self-contained unit which are a lot more portable and easier to operate. You could give students a worksheet telling them how to carry out this practical and do the data analysis but it is a year 13 practical so you might want to think about asking them to research this for themselves. It all depends which other practical competencies you want to assess. If like me you've only got one set of this apparatus it's important to think about what students are doing while only some of them are collecting data. You could give them some written work but I've got another practical which you can do at the same time which reinforces the practical skills and physics related to this. I stumbled across this on the Exploratorium's website. It's a lovely way to demonstrate the inverse square law for light. All you need is a piece of card with a square centimetre cut out of it, a lamp and a bigger piece of card with some graph paper on it. If you put the square centimetre in front of the lamp what you get is a square of light shining on the graph paper and it only takes a couple of minutes of playing around with this to see that if you double the distance of this from the lamp you could ruple the area of the light covers. What's surprising about this is that you can use this very equipment to collect data of a sufficiently high quality to show the inverse square law graphically and I've done this myself. There are obviously some issues around uncertainties with the precise area that this light covers but you can help reduce that by taking a photograph with your phone and looking at a close up on that. Now this isn't a core practical but I'm looking forward to doing it with my students to help deepen their understanding of a key principle in physics. Hope you found that useful. In the description below you'll find links to lots of other useful stuff like teachers notes and worksheets and please don't forget to subscribe so you can watch the other films in the series. Okay, this doesn't feel weird at all.