How Solenoids Work

By: Mark Mancini  | 
The guy delivering pizza to your house activates the magnetic field lines of the solenoid valve coil every time he rings your doorbell. Pekic/Getty Images

"Ding-dong!" Sounds like the pizza's here. The delivery guy is out on your porch and he just rang the doorbell. By doing this, he's activated the magnetic field of one of the most useful devices in our electronics-driven world: A solenoid.

Life would be a lot harder without these things. Modern cars depend on solenoid valves; they're an essential part of certain door locks; and hey, did you know you've got solenoids in your washing machine?


What Is a Solenoid?

A solenoid is a long piece of wire that is wound in the shape of a helical coil. When an electrical current signal passes through the linear solenoid coil it creates relatively uniform magnetic field lines inside the solenoid coil.

Definitionally, every solenoid includes a coiled piece of metal wire. That's how a solenoid capitalizes on electromagnetism, one of the fundamental forces of the universe.


"Opposites attract" might be dubious dating advice, but it's a steadfast rule in magnetism. All magnets include north and south poles. They also have a magnetic field that flows from the former to the latter.

Two solenoids made of copper wire are shown wound in the shape of a helical coil.

Two north poles, or negative poles, in a solenoid will naturally repel each other. Ditto for two south poles. But if you take a pair of magnets and put them close together, the north pole on the first magnetic core will become attracted to the second magnet's south pole. It's physics, baby.

A solenoid is helpful because it gives mechanics and engineers some control over this electrical signal process. When an electrical current flows through a metal wire, it produces a uniform magnetic field, a ferromagnetic core and mechanical energy.

The charged particles within that field of magnetic materials move in circles around the outside of said wire.

Coiling the wire makes its magnetic field stronger. With different configurations and each new solenoid coil that's added, the field gets more powerful. And here's another thing to remember: The magnetic field is going to be more concentrated in the space within those coils — and the magnetic field less so in the area surrounding your wire.

A solenoid is a cylindrical coiled wire that acts as a magnetic field when it carries electric current and creates an electrical signal.
Fouad A. Saad/Shutterstock


The Solenoid and Electric Slides

Now here's where it gets interesting. For the most part, solenoid wires come wrapped around a metal rod. (The word "solenoid" is a derivative of the Greek word sōlēnoeidēs, which means "pipe-shaped.")

When the solenoid wire receives an electric current, this piece of metal will become attracted to — and get pulled toward — one end of the solenoid. But the effect of this immediate action on the solenoid is temporary. Cut off the electrical current the solenoid created, and you kill the magnetic field. Then, thanks to spring-loading, your unit should revert back to its original position.


Basically, we can have our cake and eat it, too. A solenoid allows us to magnetize wires, creating a magnetic field, and then demagnetize them whenever we like (pretty much). All with the push of a button. Or the twist of a key.

Like we said before, cars use solenoids. Turning the ignition key relays electrical energy from your battery into a starter solenoid. Once the starter solenoid is activated, several things happen.

Electric current in the solenoid wire attracts a movable iron rod. The circuit between the starter motor and the car's battery is completed. And a "pinion" gear wheel engages with a disc called the "flywheel."

Within a couple of seconds, your once-dormant car engine comes alive. At least that's how it's supposed to work.

Has this ever happened to you? You're sitting in your car and just turned the ignition key, but the engine won't start. Instead, you hear an unpleasant clicking noise. The culprit might be a dead battery or a compromised alternator. Or perhaps your starter motor solenoid's the real culprit here.

Any mechanic should be able to give your solenoid a test if (s)he's got a circuit tester to test the circuit breakers or multimeter. Sometimes, these parts are fixable. Sometimes, they're not — and need to be replaced. So goes the life of a car owner.

Turning the ignition key in your car relays electrical energy from your battery into a starter solenoid.


Cars and Appliances Use Solenoids

By the way, many cars also use a solenoid in their electric door locks. Separate solenoids are called into action to either lock or unlock the door handles, using the same principles we've already discussed.

That's not to say all solenoids are created equal. Plenty of solenoid variations exist, each with its own strengths.


Manufacturing and water treatment plants make good use of hydraulic solenoids. Just as the name implies, a hydraulic solenoid regulates the flow of water and other liquids. Also, let us not forget the pneumatic solenoids that affect contained gasses in the same manner.

Coming full circle, we now return to the pizza dude on your front step. Not all doorbells use a solenoid; newer designs tend to leave the solenoid out entirely. But even in the golden age of "smart" devices, many doorbells still incorporate a solenoid.

Let's assume yours is one of them. When our pizza-carrier pressed the button, he sent electricity flowing through a built-in solenoid.

The magnetic field this simple action produced dragged an iron core into the coiled solenoid wire. Next the metal hit a tiny chime, releasing a "Ding!" noise.

Obviously, the delivery guy couldn't keep his finger on the button forever. Right when he let go of it, the magnetic field vanished and a spring shot that iron core in the opposite direction. Then the metal hit a second chime that went "Dong!"

Should give you something to think about while you enjoy those complimentary breadsticks.