Definition
An electrical circuit containing both resistance (R) and inductance (L). The inductor opposes any change in current flow, causing the current to rise and fall more slowly than the applied voltage. This produces a predictable time delay between voltage being applied and current reaching its steady value, and causes current to lag voltage in an alternating current circuit.
Plain English
A circuit built from a resistor and a coil. The coil resists sudden changes in current, so the current takes a moment to build up when voltage is applied and a moment to die away when voltage is removed.
Context Anchor
Seen in aircraft electrical-system theory, especially when discussing coils, relays, solenoids, and ignition-related circuits.
Derivation
R and L are the standard electrical symbols: R for resistance (from 'resist') and L for inductance (chosen by 19th-century physicists in honour of Heinrich Lenz, whose law describes how inductors oppose changes in current).
Why Pilots Care
Most aircraft electrical components with coils -- relays, solenoids, starter motors, generator field windings -- behave as RL circuits. The built-in time delay and the energy stored in the coil are why these devices switch with a slight lag and why opening the circuit can produce a voltage spike that needs to be managed.
Analogy
Think of turning on water through a hose with a heavy paddle wheel inside it. The narrow hose limits the flow, and the paddle wheel makes the flow take a moment to speed up or slow down. An RL circuit behaves somewhat like that with electrical current.
Intuition Check
Do not read RL as the name of a single part. It describes a circuit behavior caused by resistance and inductance working together.
Example Sentence 1
Because the starter solenoid forms an RL circuit, the current does not reach full strength the instant the switch is closed.
Example Sentence 2
During preflight, the avionics check confirmed the RL circuit was maintaining stable voltage to the instruments.