Definition
A trade name for a conductive coating made of finely powdered graphite suspended in water, applied to the inside surface of a cathode-ray tube (CRT) to collect secondary electrons emitted from the screen and to provide an electrical path back to the anode.
Plain English
A black, water-based graphite paint used inside the picture tubes of older cockpit displays and instruments to carry stray electrons away from the screen.
Context Anchor
Seen mainly in aircraft electrical, radio, and instrument maintenance references, especially where a surface needs a controlled conductive coating.
Derivation
The name comes from 'aqueous' (water-based) and 'dag,' short for 'deflocculated Acheson graphite' -- a process developed by Edward Acheson for breaking graphite into a fine, evenly suspended powder. Knowing this tells you immediately what it is: graphite dispersed in water.
Why Pilots Care
Pilots themselves do not work with Aquadag, but understanding the term helps when reading maintenance documentation or troubleshooting older avionics. A degraded Aquadag coating can cause display problems in CRT-based instruments.
Analogy
It is a little like making a surface behave electrically more like pencil lead: the coating is thin and dark, but it gives electricity a path to follow.
Intuition Check
Do not read “aqua” as meaning the important part is water. The water is only the carrier; the graphite left behind is what makes the coating conductive.
Example Sentence 1
The technician inspected the Aquadag coating inside the radar CRT after the display began showing intermittent flicker.
Example Sentence 2
After sanding the radome, the mechanic reapplied Aquadag to restore the conductive path needed for static dissipation.