Definition
An electromagnetic switch in the aircraft electrical system that connects or disconnects the battery from the main electrical bus. When energized by a small control current from the master switch, it closes a heavy-duty internal contact that allows the battery's high current to flow to the rest of the aircraft.
Plain English
A remote-controlled switch that lets the master switch in the cockpit turn the battery on and off without running heavy battery cables all the way up to the panel.
Context Anchor
Seen in aircraft electrical and starting system descriptions, especially when explaining what happens after the battery side of the master switch is turned on.
Derivation
Solenoid comes from the Greek 'solen' meaning channel or pipe, plus '-oid' meaning shaped like. It originally described a coil of wire shaped like a tube. When current flows through the coil, it creates a magnetic field that pulls a metal plunger inside, and that plunger closes the heavy electrical contacts.
Why Pilots Care
It lets the cockpit switch safely control the large current required by the starter without routing heavy cables to the instrument panel.
Analogy
Think of it like a doorbell button at the front door that triggers a much heavier latch inside the house. The button (master switch) is small and easy to operate, but it controls something big and powerful (the battery's main current path).
Intuition Check
Do not think of the battery solenoid as the battery itself or the starter motor. It is a switch controlled by electricity, used to connect the battery to the aircraft electrical system.
Example Sentence 1
When the pilot turned the master switch on, the battery solenoid clicked and the panel lights came alive.
Example Sentence 2
The preflight checklist includes verifying that the battery solenoid engages with a distinct click before engine cranking begins.