Definition
An electrical instrument coil arrangement consisting of two windings whose magnetic fields oppose each other, so that the instrument responds only to the difference between two electrical signals rather than to either signal on its own. Commonly used in aircraft pressure ratio gauges, fuel flow indicators, and certain warning systems where a comparison between two values must be displayed.
Plain English
A coil setup with two windings that pull against each other, so the instrument only shows the difference between two readings instead of the readings themselves.
Context Anchor
Seen in aircraft electrical system discussions, especially older DC generator control units, cutout relays, and voltage regulator descriptions.
Derivation
Differential comes from the Latin differentia, meaning a difference. The coil is named for what it measures: not a single value, but the difference between two.
Why Pilots Care
Maintains balanced electrical loads when multiple generators operate in parallel, preventing one unit from overcharging or failing.
Analogy
Think of two people pulling on opposite ends of a rope. The movement depends on which pull is stronger. A differential coil works in a similar way electrically: its effect is judged against another coil’s effect.
Intuition Check
Do not read “differential” here as a car part or as simply “different.” In this electrical use, it means the coil works by opposing or comparing against another magnetic effect.
Example Sentence 1
The pressure ratio indicator uses a differential coil to compare inlet and exhaust pressures, so the needle only moves when the two values diverge.
Example Sentence 2
An open differential coil allowed one generator to supply all the current while the second unit stayed offline.