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Capacitors are in every phone and laptop, and they are a JEE favourite. The first one was a glass jar of water held by a professor in Leiden, and the shock it gave made it famous across Europe. This post covers who Pieter van Musschenbroek was, how the jar works, and how to calculate what it stores.
Who was Pieter van Musschenbroek?
The 19th-century Dutch reference work by A. J. van der Aa records that Petrus (Pieter) van Musschenbroek was born in Leiden on 14 March 1692 and died on 19 September 1761. He attended lectures by Herman Boerhaave and others, and in 1717 visited London and heard Isaac Newton lecture. He taught at Duisburg and Utrecht, then returned to Leiden as professor of mathematics and philosophy in 1740.
He was known above all as an experimenter. The same entry credits him with careful work on magnetism, the strength of materials and the weather, and with being among the first to show series of observations as curves on a graph.
How was the Leyden jar discovered?
The Leiden University Instituut-Lorentz page on the Leiden jar explains that the jar was invented in 1745 by Van Musschenbroek and, independently, by the German cleric Ewald Georg von Kleist. It was a glass jar partly filled with water, with a brass wire through its cork. Charge made by friction was led down the wire and stored in the jar, instead of leaking away into the air.
A student, Andreas Cunaeus, received a heavy shock in one demonstration. After feeling it himself, Van Musschenbroek wrote to the French scientist Réaumur that, in the page's paraphrase, the whole kingdom of France could not make him repeat it. That letter spread the news. In France, Jean-Antoine Nollet staged a demonstration with 700 monks joined in a circle. In London in 1746, William Watson improved the jar by coating its inside and outside with metal foil.
Why does a jar store charge?
A Leyden jar is two conductors separated by an insulator. The water or inner foil is one conductor, the outer foil or the hand holding the jar is the other, and the glass is the insulator between them. Charge added to the inside attracts opposite charge to the outside. The glass keeps them apart, so the charge stays.
That is the definition of a capacitor in the NCERT Class 12 chapter Electrostatic Potential and Capacitance. For flat plates of area A a distance d apart, with a dielectric of constant K between them, the chapter gives C = ε0KA/d, and the stored energy as U = ½CV².
Worked example: how much does a jar store?
Treat the foil on a jar as flat plates. These are sample numbers for a classroom estimate, not measurements of a real jar: foil area A = 0.05 m², glass thickness d = 2 mm = 0.002 m, dielectric constant K = 6, charged to V = 10 kV. Take ε0 = 8.85 × 10⁻¹² C² N⁻¹ m⁻², as NCERT does.
Capacitance. C = ε0KA/d = (8.85 × 10⁻¹² × 6 × 0.05) / 0.002 = 1.33 × 10⁻⁹ F, about 1.3 nF.
Charge. Q = CV = 1.33 × 10⁻⁹ × 10⁴ = 1.33 × 10⁻⁵ C, about 13 μC.
Energy. U = ½CV² = 0.5 × 1.33 × 10⁻⁹ × (10⁴)² = 0.066 J.
Which mistakes does this help you avoid?
Forgetting to convert mm to m. Using d = 2 gives an answer a thousand times too small.
Squaring the wrong thing. In U = ½CV², only V is squared.
Leaving out K. Without the glass, the same jar would hold six times less charge at the same voltage.
The JEE (Main) 2026 syllabus lists capacitance of a parallel plate capacitor with and without a dielectric, and energy stored in a capacitor. Check the current syllabus for your year. For practice, read our post on capacitor and dielectric problems, and for what came next historically, see Benjamin Franklin's experiments with electricity.
What can a student take from Van Musschenbroek's way of working?
Report what surprises you. His letter about an unexpected shock spread the discovery faster than any book could.
Record observations in a form others can use. Showing data as curves made patterns easier to see, and you can do the same in your lab file.
Respect the experiment. He said he would not take the shock again. Safety is part of good science.
If the link between charge, field and stored energy is hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Try it now: redo the example with the voltage halved to 5 kV and explain why the energy drops to a quarter.
