Definition
A type of primary (non-rechargeable) electrochemical cell using a zinc anode, a mercuric oxide cathode, and an alkaline electrolyte, producing a nominal voltage of about 1.35 volts. Mercury cells are noted for their stable output voltage over a long service life and were historically used in applications requiring a steady reference voltage, such as photographic light meters and certain aviation instruments. Their use has been largely discontinued due to the environmental hazards of mercury.
Plain English
A small, sealed battery that uses mercury compounds to produce electricity at a very steady voltage. It holds its voltage almost constant until it suddenly dies, which made it useful in instruments that needed a reliable reference.
Context Anchor
Seen in discussions of older battery-powered aircraft equipment, small electrical components, and replacement battery specifications.
Derivation
‘Mercury’ refers to the metallic element used in the cathode (as mercuric oxide), and ‘cell’ comes from the Latin ‘cella’ meaning a small chamber or compartment — here, a single self-contained electrochemical unit. Knowing this helps distinguish a single ‘cell’ from a ‘battery,’ which is two or more cells combined.
Why Pilots Care
In legacy aircraft these cells supplied drift-free reference voltages critical for accurate instrument operation before solid-state alternatives became standard.
Intuition Check
Do not assume a mercury cell is a rechargeable aircraft battery. Here, it means one small non-rechargeable battery cell with a very steady output voltage.
Example Sentence 1
The older light meter in the flight bag still uses a mercury cell, which gives a more stable reading than a modern alkaline replacement.
Example Sentence 2
Mechanics verified the mercury cell output remained within tolerance before returning the aircraft to service.