Definition
A trade name for a variable autotransformer used to adjust alternating current (AC) voltage smoothly from zero up to (or slightly above) the input line voltage. It uses a single coil with a sliding contact that taps off a selectable portion of the winding, allowing the output voltage to be dialed in by hand.
Plain English
A device with a knob that lets you dial AC voltage up or down to whatever level you need, instead of being stuck with a fixed wall-outlet voltage.
Context Anchor
Seen in aircraft maintenance shops during electrical bench testing, troubleshooting, and equipment checks.
Derivation
Variac is a contraction of 'variable AC.' It began as a brand name from General Radio Company in the 1930s and, like 'Kleenex' or 'Thermos,' became the common name for the whole class of device.
Why Pilots Care
A technician working on aircraft electrical or avionics equipment may use a Variac to slowly bring power up on a unit being tested, or to simulate low-voltage conditions. Knowing the term helps when reading shop manuals, test procedures, or maintenance write-ups.
Intuition Check
Do not assume a Variac converts AC power into DC power. It normally keeps the power as AC and only changes the voltage level.
Example Sentence 1
The technician used a Variac to gradually raise the voltage on the inverter while watching for any sign of overheating.
Example Sentence 2
They used the Variac to simulate low-voltage conditions during the alternator test.