Definition
An ammeter whose needle rests at the center of the scale rather than at one end, allowing it to indicate current flow in either direction. In an aircraft electrical system, it is typically installed between the battery and the main bus to show whether the battery is being charged (needle deflects one way) or discharged (needle deflects the other way), with the magnitude of deflection showing the rate of current flow.
Plain English
A current meter with the zero mark in the middle of the dial. The needle moves right or left depending on which way electricity is flowing, so it can show both charging and discharging.
Context Anchor
Seen on some aircraft instrument panels and in maintenance descriptions of the aircraft charging system.
Derivation
The name describes the design directly: the zero point sits at the center of the scale, unlike a standard ammeter where zero is at the left end. This layout is what allows the meter to show current going in two directions.
Why Pilots Care
Lets the pilot see at a glance whether the alternator or generator is keeping the battery charged during flight.
Intuition Check
Do not read zero-center as meaning the aircraft has no electrical current. It means the gauge is built with zero in the middle so it can show current flowing in either direction.
Example Sentence 1
After engine start, the zero-center ammeter showed a brief deflection toward charge as the battery replaced the energy used by the starter.
Example Sentence 2
When the zero-center ammeter needle moved left of center, the pilot knew the battery was discharging and checked the alternator.