The Henry (H) is the unit of measurement in electrical science that quantifies inductance. Inductance is an intrinsic characteristic of any electrical conductor, representing the tendency of a circuit to oppose changes in the electric current flowing through it. The Henry provides a universal standard for engineers and scientists to communicate the precise behavior of these components.
Defining Inductance
Inductance describes the electrical property of a conductor to resist any change in the current that moves through it. This phenomenon is often likened to electrical “inertia,” where the conductor attempts to maintain a constant current flow. The physical basis for this opposition lies in the magnetic field generated when electric current passes through a conductor, particularly when coiled.
When the electric current changes, the magnetic field it creates also changes in strength. This changing magnetic field then induces a voltage, or electromotive force, back across the conductor itself. This induced voltage always acts in a direction that opposes the original change in current, a principle known as Lenz’s law. A higher inductance value means the component generates a greater opposing voltage for a given rate of current change.
The Henry: The Unit of Measurement
The Henry (H) is the official International System of Units (SI) measure for inductance, defining the relationship between the rate of current change and the resulting induced voltage. One Henry is defined as the amount of inductance that generates an induced voltage of one volt when the electric current changes at a rate of one ampere per second. This definition links the Henry directly to the fundamental units of voltage, current, and time, illustrating its derivation as $\text{H} = \text{V} \cdot \text{s} / \text{A}$.
A full Henry represents a relatively large amount of inductance, often exceeding the requirements for small electronic devices. Consequently, inductance values in practical electronics are frequently expressed in smaller metric sub-units. Designers commonly work with the millihenry (mH) and the microhenry ($\mu$H). For high-frequency radio applications, values can even drop into the nanohenry (nH) range.
Joseph Henry: The Scientist
The unit of inductance is named in honor of Joseph Henry (1797–1878), an American scientist who made contributions to the field of electromagnetism in the early 19th century. Henry’s work advanced the understanding of how electricity and magnetism interact, particularly in the context of coiled conductors. His research led to the discovery of self-inductance, the phenomenon where a changing current in a circuit induces a voltage in that same circuit.
Henry also independently discovered mutual inductance, which describes how a changing current in one coil can induce a voltage in a separate, nearby coil. Although Michael Faraday published his findings on electromagnetic induction first, Henry’s foundational research was recognized internationally, leading to the adoption of the Henry unit in 1893. Beyond induction, Henry improved the electromagnet, creating the strongest magnets of his time by insulating the wire and tightly coiling it around an iron core.
Inductance in Everyday Technology
The principle of inductance is put to practical use in a component known as an inductor, typically a coil of wire designed to possess a specific Henry value. These components are fundamental to nearly all electronic and electrical systems, utilizing their current-opposing property for specific functions. One widespread application is in power supplies, where inductors, often called chokes, are used as filters to smooth out fluctuations in the electric current.
Inductors are important in tuning circuits, used in devices like radios and televisions to select a specific frequency from a range of signals. When paired with a capacitor, the inductor’s value determines the resonant frequency, allowing the circuit to efficiently isolate the desired signal.
Inductors play a significant role in energy storage, particularly in switch-mode power supplies, such as those found in computers. In these converters, the inductor stores energy in its magnetic field during one part of the switching cycle and releases it during another, enabling efficient voltage regulation.