To clean the copper sulfate from the copper contacts in the head torch, I put that part of the case in dilute hydrochloric acid overnight. Somehow it seems to have acted as an electrolytic cell. The top contact seems to have been cleaned and has a layer of new copper. The other contacts have been cleaned to some extent but now have a layer of what is presumably copper chloride. There was no battery connection so there should have been no potential difference between the contacts. I'm surprised at what has happened because there was no difference in metal like there was on the human electrolyte cell I found at Green's Mill two weeks ago. Anyway, the acid has done the trick and the head torch is working again.
