By Dave Donovan
Updated March 24, 2022

For hobbyists and professionals alike, understanding the solenoid is essential. This device underpins countless everyday functions—from electric locks to automotive starters—by converting electrical energy into precise mechanical motion.

What a Solenoid Is

A solenoid is a coil of insulated wire wound around a ferromagnetic core, known as the armature. When electric current passes through the coil, it generates a magnetic field that pulls the armature toward the coil, closing a gap and creating a controlled mechanical action.

How It Works

The strength of the magnetic field—and therefore the force exerted on the armature—depends on the number of turns in the coil and the amount of current flowing through it. The armature is typically spring‑loaded so that, when the current is removed, it retracts to its original position, allowing the solenoid to act as an on‑off switch.

Common Uses

Solenoids serve as the heart of many automatic systems:

• Locks & latches: Provide rapid, reliable locking mechanisms in electronic door locks.
• Household appliances: Operate valves and actuators in washing machines, copiers, and coffee makers.
• Automotive: Found in starter relays, fuel injectors, and hydraulic controls.
• Industrial equipment: Control pneumatic and hydraulic valves in manufacturing lines.

Types of Solenoids

• Electromechanical: The standard type discussed above, used in most household and automotive applications.
• Pneumatic: Acts as a valve that directs air flow in pneumatic systems.
• Hydraulic: Similar to pneumatic but controls fluid (often oil) flow in hydraulic circuits.
• Starter (Linear) Solenoid: Used in vehicles to engage the starter motor; the clicking sound when a battery is dead comes from this solenoid.

Expert Insight

In engineering circles, the term “solenoid” often refers to the transducer component in a solenoid valve. In physics, it strictly denotes the wire coil with a movable core. Regardless of context, a solenoid’s core principle remains the same: a magnetic field that moves a metal armature to perform work.