Solenoid
Definition
A coil of wire wound into a helix — when current flows, it acts like a bar magnet with a nearly uniform field inside.
Symbol: · Unit: T
Technical definition
A helical coil producing an axial magnetic field B = μ₀nI inside (n = turns per unit length), nearly uniform away from the ends. Used in electromagnets, relays, valves, and MRI machines.
Example
A solenoid with 1000 turns/m carrying 2 A produces about 2.5 mT inside — roughly 50 times Earth's magnetic field.
Common confusion
The field is strong and uniform inside the solenoid but weak and spreading outside — the opposite of what many expect. And it is the current that makes it magnetic; switch off the current and (for an air core) the field vanishes.