Definition
An electrical instrument movement that displays a measured value by comparing the ratio of currents flowing through two coils, rather than by responding to the absolute amount of current in a single coil. The two coils are wound so they produce opposing magnetic forces on a pivoting armature, and the armature settles at the position where those forces balance. Because the reading depends on the ratio between the two currents, the indication remains accurate even if the aircraft's supply voltage varies.
Plain English
A gauge movement that reads by comparing two electrical currents against each other instead of measuring just one. Because it works on the balance between the two, changes in battery or bus voltage do not throw the reading off.
Context Anchor
Seen in aircraft instrument and maintenance discussions, especially with remote-reading gauges such as fuel quantity or temperature indicators.
Derivation
From 'ratio' (Latin ratio, meaning a relation or proportion between two things) and 'meter' (a measuring device). The name describes exactly what it does: it measures a ratio rather than an absolute quantity.
Why Pilots Care
Provides stable instrument readings despite normal variations in aircraft electrical system voltage, reducing the chance of misleading indications during flight.
Analogy
Think of a tug-of-war between two ropes pulling on a pointer. It does not matter how strong both teams pull in absolute terms -- what matters is which side pulls harder and by how much. The pointer settles wherever the two pulls balance.
Grounding Statement
The pointer moves to the angle where the two coil effects balance each other.
Intuition Check
Do not read ratiometer as rate-meter. It is about a ratio, or comparison, between two electrical currents.
Example Sentence 1
The cylinder head temperature gauge uses a ratiometer indicator mechanism, so the reading stays accurate even when the bus voltage drops during engine start.
Example Sentence 2
During preflight the technician verified that the ratiometer indicator mechanism in the oil temperature gauge produced steady pointer movement across the full range.