Definition
A primary (non-rechargeable) electrochemical cell that uses a zinc anode, a mercuric oxide cathode, and an alkaline electrolyte (typically potassium hydroxide) to produce a steady output voltage of about 1.35 volts. The cell is known for its very flat discharge curve, meaning its voltage stays nearly constant until the cell is almost fully depleted.
Plain English
A small, sealed battery cell that holds its voltage steady for almost its entire life, then drops off quickly at the end. It was commonly used where a stable, reliable voltage was important.
Context Anchor
Seen in discussions of aircraft electrical components, older portable aviation equipment, and battery-powered instruments or test devices.
Derivation
Named for its chemistry: 'mercury oxide' (mercuric oxide, HgO) is the active material at the positive terminal. 'Cell' comes from the Latin cella, meaning a small chamber — here, the small sealed compartment where the chemical reaction takes place.
Why Pilots Care
It supplies constant voltage to sensitive equipment even as the battery ages.
Intuition Check
Do not read “cell” here as a living cell or a compartment. In this term, a cell is one battery unit that produces electrical power.
Example Sentence 1
The light meter in the older aircraft camera was originally powered by a mercury oxide cell, which had to be replaced with a modern equivalent after mercury batteries were banned.
Example Sentence 2
Replacement of the mercury oxide cell restored steady voltage to the instrument panel gauges.