What Is That Little Cylinder on My Computer Wire?

Brow Beat
Slate's Culture Blog
Nov. 1 2012 2:50 PM

What’s That Thing? Lump on a Wire Edition

USB cable end.
USB cable end.

Stwalkerster/Wikimedia Commons.

It’s the definitely-not-wireless edition of our quest to dispel the mysteries of the modern visual landscape. For previous columns, click here; to submit your own suggestions, see the e-mail address below.

The conundrum of the suddenly-useless-because-unmatched sock is a small but stubborn cultural meme. (Yes, there’s a book.) The digital era has its own less mysterious but more annoying version of such low-grade frustration: the purposeless connector or power cable from an obsolete electronic device.

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Many Americans will have a drawer brimming with such coaxial vexations. And those who upgrade their iPhones have no choice but to add fistfuls of suddenly démodé “dock connectors” to the pile. Until recently I’d never actually thrown out such cables, even when I replaced the device to which they were meant to attach. Even when I’d long forgotten what device they went with. The result: a decade-old pile of useless black and grey electrical spaghetti.

But recently, in a fit of decluttering that would make Oprah proud, I sorted through and disposed of nearly all these severed but lingering connections. In the process I learned that decluttering can cause anxiety, as Oprah sagely cautions. She’s right, too, that “letting go leaves space for more to come.” That’s just the kind of mental space I needed to contemplate the mystery e-mailed in by Russell Yee, a seminary professor and author in Oakland, California.

Many cables—especially older ones—have a small cylinder on them. What does it do? First, four guesses, this time with a little help from Oprah:

a) “Own your power.” The cylinders are called accumulators—they’re effectively small batteries. They store a few milliseconds’ worth of power and keep data flowing through the cable even during minute fluctuations in electrical supply in the devices at either end.

b) “Everything and everybody is vibrating at different frequencies.” If only. The cylinders are designed to reduce the possibility of electrical interference with other electrical sources and devices.

c) “Real integrity is doing the right thing.” The cylinders ensure accuracy of data transmission by briefly delaying a portion of it. About 5 percent of the wires in the cord enter a coil in the cylinder while the rest go straight through; that path takes just slightly longer, a delay which makes it much easier for the receiving device to check the integrity of the data.

d) “When I look into the future, it’s so bright it burns my eyes.” The cylinders are a kind of fuse whose sole purpose is to protect the devices they connect. If one device experiences a power surge, the fuse-like technology in the cylinder prevents the surge from damaging whatever device is connected to it.

And the answer is… b). The cylinder is called a ferrite bead, ferrite core, or, more generally, a choke. Cables can act like unintentional antennas, broadcasting electrical interference (“noise”) or picking it up. The appointed task of a ferrite core is to prevent such interference.

The most common use of ferrite cores, according to Dwayne Campbell, Senior Director of Regulator and Product Development for Radio Shack, “is to suppress noise emanating from the product through cords or cables.” So the ferrite core is typically working to reduce interference from a cable—often to comply with regulatory requirements that specify how much interference a device is permitted to emit.

Ferrite cores can also be deployed to reduce electrical interference to a cable, especially if it’s carrying data. The household blender, and the otherwise innocuous electric can opener, are particularly virulent sources of such interference, according to John Drengenberg, Consumer Safety Director for UL (Underwriters Laboratories), the company that develops safety standards, tests products, and provides manufacturers with the UL safety mark to place on products that pass. A ferrite core on data cables can “enhance the quality of your data stream,” says Drengenberg.

How does a ferrite core work? Drengenberg: “Think of your cell phone not working well in a particular building. The building is acting as a filter and reducing the signal from the cell tower while likely allowing AM radio signals, for example, to pass easily.” The ferrite core acts as such a filter. (You can find more detailed technical explanations on the web.)

Not every such cable has a ferrite core. In fact, only about one-quarter of the power and data cables I recently purged had a ferrite core on them. Why’s that? According to Radio Shack’s Dwayne Campbell:

Usually when you see this on a consumer product such as a computer or printer it was added as a means of fixing an EMI (electromagnetic interference) problem. In most cases since it adds cost, designers will try and fix the problem such that these are not necessary. In some cases, these fixes may be incorporated into the internal design of the product.

UL’s John Drengenberg also notes that some important cables—those that connect medical equipment, for example—are more likely to feature ferrite cores.

So there you go. If any of this has inspired you to consider going through your cable drawer, I warn you that it can be tough going. Start the job off right with some Oprah-caliber gratitude for, say, the miracle of the ferrite core. And then get sorting, keeping in mind that, as Oprah put it, “the thing you fear most has no power.” Especially if it’s not plugged into anything.

If you see something and wonder what it’s for—or even what it should be plugged into—drop a note and a pic to whatisthat@markvr.com.

Mark Vanhoenacker is a writer in New York. Follow him on Twitter.

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