What Do Those Mysterious Lumps on Your Cables Do?

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You have some sense of what the wires going to and from your computer do. Some bring power; others transmit information from one device to another. But some of these cables look a bit...off.

Maybe you've noticed the cylindrical growth that pops up for maybe an inch on one side of a cable? Maybe on your monitor cable? My camera-to-computer connector has one, and it creeps me out-like a cyst that some engineer put there intentionally. Ew. So we decided to get to the bottom of it. What is that weird lump in the cable anyways?

Turns out that lump's called a ferrite bead or, more generically, a choke. It's a fancy name for what's basically an electromagnetic wave-bouncer. Electromagnetic interference is what makes our radios chirp when our cell phones are too close, and something similar turns our televisions fuzzy or pixilated. It's the reason we're not supposed to use cell phones on planes, and the reason some of our cables come with weird beads on them.


If you open these lumps, you wont find any elaborate circuit board-like contraption. Instead you'll find a solid ball or cylinder made of ferrite, which is magnetic and kind of ceramic-like. Ferrite is made out of iron oxide (that's a fancy synonym for "rust") combined with at least one other metal; it's dark, hard, and brittle. But its magnetic qualities are really what help our gadgets get along.

In you have a computer tethered to a camera, there will be electromagnetic interference (EMI) coming from both devices. This could cause your monitor to flicker a little bit. It might make your speakers buzz, or, in very extreme cases, it could play havoc inside of your computer. Our electronics produce alternating electric current—and EMI. Unhindered, the cable running from your camera to computer would behave like antennae, picking up and transmitting the waves produced by the gadgets—and possibly interfering with your kit. Nobody wants to buy a digital camera that screws up their computer screen. This is where the choke comes in.


The choke is responsible for chilling out those EMI waves—basically making sure our cables don't send any signals except in the intended directions. The ferrite's atoms allow the choke to be the mediator by aligning themselves in a couple of different directions. The arrangement acts as an EMI blocker. And the block is most effective when close to the source. "It will always be between device that generates interference and a device you don't want interfered with," explains Greg Winchester, a program manager at the National Electrical Manufacturers Association. The ferrite lumps are positioned at the ends of cables because their job is to take noise from a certain device and turn it into heat. And closer positioning to the source stops the cable from turning into a receiver early on, giving the lumps a purpose.

The good news is, it's not a cyst. The bad news is, it still looks like one.

Rachel Swaby is a freelance writer living in San Francisco. You can keep up with her on Twitter.


Giz Explains is where we break down whatever science or tech questions are scratching at the backs of our noggins. Got questions of your own? Email them to us at explains@gizmodo.com and we'll see about answering them.

Photograph by Joe "Ansel Adams" Brown