We all love jars, don't we? And this jar is an early device for storing electricity - generally, the electricity made by an electrostatic generator. Benjamin Franklin used them in his experiments - they're a form of rudimentary battery. They were first invented in the town of Leyden in 1745, in what's now the Netherlands, by a scientist named Pieter van Musschenbroek. It was probably called the Leyden Jar because who in God's name was going to ask for a van Musschenbroek Jar? Sounds like something you'd ask for in a club of ill repute.
The jar's design is fairly simple - mainly because it's a jar. This one is a glass bottle with metal foil layered on the outside and inside - but it represents a huge step forward in the understanding of electricity, because it gave scientists something to work with. Before they could create electricity, but had nowhere to store it - not exactly practical.
I'd be careful around the jar if I were you. You could get a nasty shock.
Come to think of it, have a play with it. Go on. Forget I said anything.