Leyden Jar
Leyden jars were invented in the mid 1700s, named after the place – the University of Leiden – where much of the early experimentation took place. These devices, though quite humble, represented a tremendous breakthrough in the history of electricity, as they were the very first capacitors, and as such were able to store electric charge. Scientists had been creating static electricity with electrostatic generators for a century; now they finally had someplace to put it!
If you’re unfamiliar with them, please pay a quick visit to our electrostatic generators tutorial, where you’ll get the basic idea about how these devices filled Leyden jars with the charge they created. Then return to the tutorial below, which goes into more detail about how that happens and how, afterwards, the jars are discharged.
Above is our unassuming Leyden jar. This is not a replica of the very first one, which consisted (reportedly) of a beer glass filled with water (which, unless distilled, is a conductor) into which a nail was inserted through a stopper made of an insulating material such as cork. Rather, this Leyden jar reflects improvements to the prototype that followed, incorporating two metal capacitor plates, not just one.
So our Leyden jar is made up of a glass jar, which insulates our two conductors. The conductors themselves are in the form of thin sheets tin foil, one wrapped around the outside of the jar, the other lining the inside. Inside the jar hangs a metal chain. This chain is connected to a brass rod extending up through the insulating wooden lid and terminating in a ball. This whole setup is grounded, meaning it’s attached to the earth (or to something else that’s attached to the earth) to complete the circuit.
We will use a voltaic pile (unseen) to charge our jar. (Bear in mind that the first scientists who experimented with Leyden jars would not have used a voltaic pile, as it wasn’t invented until several decades later). Click the blue charge button to start this tutorial, and observe as the electrons in the current (depicted as little yellow particles) travel through the wire to the brass rod. As you can see, these electrons are conducted down the metal rod, down the chain, and to the inner lining of the jar to which the chain is attached. But there they hit a roadblock, for their way is barred by the glass, acting as an insulator (also called a dielectric), and they accumulate in the internal metal lining. Meanwhile, on the other side of the glass, electrons in the outer metal lining are repulsed by the accumulating electrons on the internal lining. The repulsed electrons leave behind a net positive charge (depicted by red plus signs) on the conductor. So you wind up with two metal plates of equal but opposite charges.
Another interesting thing going on here (but not depicted in the tutorial) is that the molecules in the glass jar respond to the charges, too; the negative charges in the molecules shift toward the outside, positively-charged metal, and the positive charges orient themselves toward the negative charges on the inside. This is called polarization.
Now what do you do with the electricity once you trap it in the jar? Well, people used to use collections of Leyden jars like batteries, to power any number of things. They are also used, then as now, to demonstrate basic electric principles (shocking audiences, sometimes, in more ways than one). We, too, shall demonstrate here (without the shock).
Use the wand position slider to approach the discharge wand to the charged Leyden jar. As you see, one arm of the wand will draw near with the negatively-charged metal ball, the other will draw near the positively-charged lining outside the jar. Furnished this pathway, these oppositely-charged particles will, when the wand gets close enough, jump across the spark gap with a dramatic little flash and rush towards each other.
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