Figure 81 comprises a drawing labelled 'a' and a diagram labelled 'b'. Drawing 'a' depicts a cat being rubbed with a rod. There is a doubled headed direction arrow situated parallel to the rod indicating the rod is being moved back and forth. The fur on the cat is raised. Diagram 'b' is a section through a container containing 'H subscript 2 S0 subscript 4 (dilute sulphuric acid)'. There are two vertical rods suspended in the liquid, one near each end of the container. One rod is labelled 'Zn' and '-' and the other rod is labelled 'Cu' and '+'. A length of wire is attached to the top of each rod.