My first year classes have been doing the conductivity paper experiment this week so I had to get out the box of 9V batteries. The problem is the box contains a mix of new and old so the first thing students have to do is find the batteries that still have some life left in them. Whilst doing this one of my students (David) found something interesting, a battery with reverse polarity, how did that happen? In the ever changing IB list of required practicals (which isn't in the subject guide but is somewhere else by the way) there used to be an experiment to measure the discharge of a battery. I always thought this was a strange experiment. If you do it safely and let the battery discharge over a long period of time then the experiment could take weeks, short circuit the battery and you'd be teaching the students bad practice. The conductivity paper works quite well getting across the concepts of resistivity, potential difference and the potential divider all in the same experiment. I wonder what would happen if the reverse pole battery was discharged, would it go the right way again? Maybe it would oscillate ? The problem is I have no idea which battery it was since it was returned into the battery bucket, I'll have to wait for it to be rediscovered next year.