Definition
An electrical device that converts alternating current (AC) into direct current (DC) by using ionized mercury vapor inside a sealed glass or metal envelope. Current passes from a heated cathode through the mercury vapor to an anode, but only in one direction, producing rectified DC output. Once common in older aircraft and ground power equipment, mercury vapor rectifiers have largely been replaced by solid-state semiconductor rectifiers.
Plain English
A device that turns AC electricity into DC electricity by passing the current through mercury vapor sealed inside a tube. The vapor lets current flow one way only, which is what makes the output DC.
Context Anchor
Seen in older aircraft electrical system descriptions, maintenance manuals, battery chargers, and power-supply equipment, not usually in normal cockpit operation.
Derivation
Mercury vapor' refers to mercury heated until it becomes a gas inside the tube. 'Rectifier' comes from the Latin 'rectus,' meaning 'straight' -- it 'straightens out' the back-and-forth flow of AC into the one-way flow of DC.
Why Pilots Care
Most pilots will not encounter one in service today, but the term appears in older maintenance manuals and electrical theory references. Knowing what it is prevents confusion when reading legacy material.
Analogy
Think of it like a one-way gate for electricity. The mercury vapor inside the tube helps the current go through in the useful direction and blocks most of the reverse direction.
Intuition Check
Do not read this as a mercury battery or a mercury sensor. In this term, the mercury vapor is part of the device that changes AC electrical power into DC electrical power.
Example Sentence 1
The vintage ground power unit in the hangar still uses a mercury vapor rectifier to supply DC to the aircraft.
Example Sentence 2
Early test benches relied on a mercury vapor rectifier to supply steady direct current for checking aircraft electrical components.