Capacitor

Lore & Background

The earliest forms of capacitors were created in the 1740s, when European experimenters discovered that electric charge could be stored in water-filled glass jars that came to be known as Leyden jars. Von Kleist's hand and the water acted as conductors and the jar as a dielectric. The following year, the Dutch physicist Pieter van Musschenbroek invented a similar capacitor, which was named the Leyden jar, after the University of Leiden where he worked.

Reader's Guide

Capacitors are widely used as parts of electrical circuits in many common electrical devices. Unlike a resistor, an ideal capacitor does not dissipate energy, although real-life capacitors do dissipate a small amount. Today, capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass. In analog filter networks, they smooth the output of power supplies. In resonant circuits they tune radios to particular frequencies. In electric power transmission systems, they stabilize voltage and power flow. The property of energy storage in capacitors was exploited as dynamic memory in early digital computers, and still is in modern DRAM. The most common example of natural capacitance are the static charges accumulated between clouds in the sky and the surface of the Earth, where the air between them serves as the dielectric, resulting in bolts of lightning when the breakdown voltage of the air is exceeded.

Did You Know?

• The earliest unit of capacitance was the jar, equivalent to about 1.11 nanofarads.
• Benjamin Franklin concluded that the charge in a Leyden jar was stored on the glass, not in the water.
• The MOS capacitor was later widely adopted as a storage capacitor in memory chips and as the basic building block of the charge-coupled device (CCD).

The Accidental Birth of Energy Storage

By connecting a high-voltage electrostatic generator through a wire to water inside a small glass jar, he found that touching the wire afterward produced a shock far more intense than anything the machine alone could deliver. His hand and the water served as conductors while the glass acted as an insulating barrier, though the precise mechanism was misidentified at the time. The following year, Dutch physicist Pieter van Musschenbroek independently created a similar device at the University of Leiden, and the apparatus became known as the Leyden jar. Musschenbroek was so startled by the force of the discharge that he famously declared he would not accept a second shock for the kingdom of France. Benjamin Franklin later demonstrated that the stored charge resided on the glass itself rather than in the water, and he introduced the term "battery" to describe multiple jars linked together. Daniel Gralath had already pioneered connecting several jars in parallel to boost total storage. The earliest unit of capacitance was simply "the jar," roughly 1.11 nanofarads.

The Physics of Stored Charge

A capacitor is, at its core, a passive two-terminal component engineered to add capacitance to a circuit. While any two nearby conductors exhibit some degree of capacitance naturally, a capacitor is purpose-built to make that effect practical. Its construction typically involves two electrical conductors—metallic plates, foils, thin films, sintered metal beads, or even an electrolyte—separated by a nonconducting dielectric material such as glass, ceramic, plastic film, paper, mica, air, or an oxide layer. When a voltage is applied across the terminals, an electric field forms within the dielectric, drawing net positive charge to one plate and net negative charge to the other. In an ideal scenario, no current passes through the dielectric itself, though charge flows through the external source circuit. If the voltage remains constant, that current eventually stops. However, under a time-varying voltage, the capacitor continuously charges and discharges, presenting the source with an ongoing current. An ideal capacitor stores energy without dissipating it, though real-world components lose a small amount of power in practice.

From Glass Jars to Modern Circuits

For roughly 150 years, the Leyden jar and its flat-plate glass-and-foil descendants were the only capacitors available. Engineers began sandwiching flexible dielectric sheets—such as oiled paper—between layers of metal foil, then rolling or folding the assembly into a small package. Porcelain capacitors served in Marconi's early wireless transmitting equipment for high-voltage applications.

Ubiquity in Everyday Electronics

Capacitors are so deeply embedded in electrical engineering that they appear in virtually every common device. In signal processing, they block direct current while permitting alternating current to pass, a property that makes them indispensable in analog filter networks where they smooth the output of power supplies. In resonant circuits, they help tune radios to specific frequencies. On a much larger scale, electric power transmission systems rely on capacitors to stabilize voltage and manage power flow across the grid. In computing, the energy-storage property underpins dynamic random-access memory—both in early digital machines and in the DRAM chips inside today's devices. Even nature exploits the same principle: static charge builds up between clouds and the Earth's surface, with the intervening air acting as a dielectric, until the breakdown voltage is exceeded and lightning strikes. Whether in a smartphone, a power plant, or a thunderstorm, the fundamental mechanism of separated charges across an insulating medium remains the same.