Voltage, more formally known as electric potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a static electric field, it represents the work needed per unit of charge to move a test charge between the two points. In simpler terms, voltage is the "push" that causes charges to move in a wire or other electrical conductor.
The SI derived unit for voltage is the volt (V), named in honor of the Italian physicist Alessandro Volta, who invented the voltaic pile, arguably the first chemical battery.
The Volt (V)
The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. It can also be expressed in terms of energy, where one volt is equal to one joule of energy per coulomb of charge.
In practical applications, voltage is typically measured using a voltmeter, which is connected in parallel with the component or circuit section being evaluated. Accurate voltage measurement is absolutely critical for the safe and efficient operation of everything from microscopic microprocessors to massive high tension power transmission lines.
Quantum Realization: The Josephson Effect
For decades, the standard for the volt was maintained using standard chemical cells, such as the Weston cell. However, these artifacts were highly sensitive to environmental conditions, specifically temperature fluctuations and physical shock, and they drifted over time.
In 1990, metrology achieved a monumental breakthrough by redefining the practical realization of the volt using a macroscopic quantum phenomenon: the Josephson effect. Discovered theoretically by Brian D. Josephson in 1962, the effect describes the phenomenon of supercurrent, a current that flows indefinitely without any voltage applied, across a device known as a Josephson junction.
A Josephson junction consists of two superconductors separated by a very thin non-superconducting barrier. When this junction is irradiated with microwave frequency radiation, the current voltage characteristic develops discrete, highly precise voltage steps. The voltage ($V_n$) of the $n$-th step is directly proportional to the applied microwave frequency ($f$) and fundamental physical constants.
Where $h$ is the Planck constant, $e$ is the elementary charge, and $K_J$ is the Josephson constant. Since $h$ and $e$ are now exact fixed values following the 2019 SI redefinition, the Josephson constant is exactly $483597.8484... \times 10^9 \text{Hz/V}$.
Modern primary voltage standards use arrays of thousands of Josephson junctions connected in series, cooled to cryogenic temperatures, to generate practically usable, macroscopically precise voltages up to 10 V. Because frequency can be measured with extraordinary precision using atomic clocks, the Josephson array provides an intrinsically stable, universally reproducible voltage standard that is entirely immune to the environmental drift that plagued chemical cells.