Definition
A temperature indicating instrument that measures the ratio of currents flowing through two parallel circuits — one containing a fixed resistor and the other containing a temperature-sensitive resistance bulb. Because the reading depends on the ratio between the two currents rather than their absolute values, the indication remains accurate even when supply voltage varies. Commonly used for cylinder head temperature, oil temperature, and similar engine temperature readings.
Plain English
A gauge that shows temperature by comparing two electrical currents against each other rather than measuring just one. Because it works on the comparison, the reading stays correct even if the aircraft's electrical voltage drifts up or down.
Context Anchor
Seen in aircraft engine instrument systems, especially in maintenance discussions of electrically operated temperature indicators.
Derivation
From 'ratio' (Latin ratio, meaning a comparison or proportion between two things) plus 'meter' (a measuring device). The name describes exactly what it does: it measures a ratio rather than a single value, which is the key reason it stays accurate when voltage varies.
Why Pilots Care
Delivers dependable indications of critical quantities when electrical system voltage fluctuates during flight or engine start.
Intuition Check
Do not read ratiometer indicator as just another name for any indicator. The important point is that it works by comparing two electrical currents, not by reacting to one current by itself.
Example Sentence 1
The cylinder head temperature gauge is a ratiometer indicator, so the reading stays accurate even when the bus voltage dips during engine start.
Example Sentence 2
During preflight the mechanic verified that the ratiometer indicator circuit for the oil temperature gauge was within tolerance.