Burnt thumb
A couple of weeks ago there was a question on the OCC forum about whether an experiment to measure the internal resistance and EMF of a battery would be a good DCP practical. Provided the students use a graphical method to analyse the data then it is a good method. It's a classic experiment but I've never done it partly because most methods require an Ammeter and a Voltmeter and I don't have enough multimeters to give each pair of students two of them, so I thought of an alternative way. I bought 1500 resistors of resistance ranging from about 1 to 10MΩ (actually the advert says the lowest is 0Ω but I'm ignoring that piece of information). The idea is to measure the current through a range of resistors and by plotting a suitable graph find the EMF and internal resistance. I tried it myself with a limited range (the 1500 haven't arrived from China yet) and got a good linear correlation and reasonable values. The resistors are all quoted as being 1/4 W and 1%. This makes the error bars easy to plot but students need to be careful which they use so as not to overload them. I didn't take this precaution, since I only had 5 resistors and had to use them all, and ended up burning my thumb. I'll write a worksheet etc. when my thumb gets better.