# What is the purpose of this gadget that came with a laser printer? **URL:** \ **Category:** Factual Questions\ **Created:** [March 30, 2012, 4:22am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235 "2012-03-30T04:22:23Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![SenorBeef](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senorbeef/32/2220_2.png) [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)\ **Post date:** [March 30, 2012, 4:22am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/1 "2012-03-30T04:22:23Z")
I was setting up a laser printer the other day, an HP LaserJet something or other, and there was a small cylindrical device that the instructional diagram showed to clip onto the ethernet cable on the printer. What possible purpose could it serve? Here’ the [instruction sheet](http://www.senorbeef.net/photos/001.JPG) it came with. I thought maybe it was some sort of product identifier to authenticate it in the event that they ever needed service, but it didn’t seem to be labelled in any obvious way. It serves no functional purpose. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [March 30, 2012, 4:35am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/2 "2012-03-30T04:35:42Z")
Is it a [Ferrite core](http://www.google.com/products/catalog?q=Ferrite+Core%3F&hl=en&prmd=imvns&resnum=4&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.,cf.osb&biw=1680&bih=935&ix=seb&ion=1&um=1&ie=UTF-8&tbm=shop&cid=761843538070174409&sa=X&ei=yDd1T_TVMoSTiALdrtWnDg&ved=0CG0Q8wIwAA)? They’re for noise suppression. > **[Ferrite bead](https://en.wikipedia.org/wiki/Ferrite_bead)** > > A ferrite bead (also known as a ferrite block, ferrite core, ferrite ring, EMI filter, or ferrite choke) is a type of choke that suppresses high-frequency electronic noise in electronic circuits. > Ferrite beads employ high-frequency current dissipation in a ferrite ceramic to build high-frequency noise suppression devices. > Ferrite beads prevent electromagnetic interference (EMI) in two directions: from a device or to a device. A conductive cable acts as an antenna – if the device produces radio-... ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [March 30, 2012, 4:39am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/3 "2012-03-30T04:39:42Z")
Looks like a [ferrite bead](http://en.wikipedia.org/wiki/Ferrite_bead). It actually does serve a functional purpose — it reduces noise in the signal from the cable, allowing for better data transmission. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [March 30, 2012, 5:22am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/4 "2012-03-30T05:22:05Z")
As the others said, it’s a ferrite core/bead. They generally mean the manufacturer screwed up the design and that it didn’t meet FCC electromagnetic interference requirements–especially in the case where the user installs it, like what you had to do. They’re pretty common on power and USB cables; this is the first time I’ve heard of one on an ethernet cable. I’m pretty surprised to see this on an HP device; I can’t imagine that happening 20 years ago, but their product quality has definitely gone downhill over the years. ---
**Author:** ![SenorBeef](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senorbeef/32/2220_2.png) [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)\ **Post date:** [March 30, 2012, 5:31am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/5 "2012-03-30T05:31:55Z")
How odd. How much noise can an eithernet cable have? They’re shielded, and the medium has lots of error checking. Seems strange that they’d have ethernet cables that didn’t meet FCC requirements. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [March 30, 2012, 5:37am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/6 "2012-03-30T05:37:05Z")
They’re shielded? I’ve never seen a Shielded Twisted Pair cable around the home or office. ---
**Author:** ![SenorBeef](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senorbeef/32/2220_2.png) [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)\ **Post date:** [March 30, 2012, 5:39am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/7 "2012-03-30T05:39:30Z")
Shielded was the wrong word - but the twisting itself provides pretty good EM interference. I’ve never seen an ethernet cable that needed additional shielding. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [March 30, 2012, 11:01am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/8 "2012-03-30T11:01:09Z")
How fascinating. In 15 years of working with computers, I’ve never seen a ferrite around an ethernet cable. I wonder if it impacts transmission rates… ---
**Author:** ![BorgHunter](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@BorgHunter](https://boards.straightdope.com/u/BorgHunter)\ **Post date:** [March 30, 2012, 11:19am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/9 "2012-03-30T11:19:23Z")
> [@AaronX](#): > > They’re shielded? I’ve never seen a Shielded Twisted Pair cable around the home or office. The class F (also known as “category 7”, but TIA hasn’t so named it yet) cables I have are screened and shielded. Plain ol’ cat 5e cable might not be, but unshielded twisted pair usually has too much crosstalk to sustain gigabit speeds. ---
**Author:** ![Khendrask](https://avatars.discourse-cdn.com/v4/letter/k/3e96dc/32.png) [@Khendrask](https://boards.straightdope.com/u/Khendrask)\ **Post date:** [March 30, 2012, 11:45am UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/10 "2012-03-30T11:45:11Z")
Are you absolutely sure they said to put it on the Ethernet cable? The page is a bit vague, but that looks like the power cable to me. ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [March 30, 2012, 12:21pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/11 "2012-03-30T12:21:10Z")
[QUOTE=Khendrask] The page is a bit vague, but that looks like the power cable to me. [/QUOTE] That’s how it looks to me as well. The ferrite core should be fairly snug - on the power cord, it will probably be snug enough to stay in place and not slide down the cord towards the floor. ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [March 30, 2012, 2:35pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/12 "2012-03-30T14:35:11Z")
> [@Khendrask](#): > > The page is a bit vague, but that looks like the power cable to me. It’s definitely the power cable. ---
**Author:** ![SenorBeef](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senorbeef/32/2220_2.png) [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)\ **Post date:** [March 30, 2012, 2:41pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/13 "2012-03-30T14:41:07Z")
They either used a generic drawing or one for the wrong model them, because IIRC on the one I was installing it to, the power cable connection wasn’t where they drew it to be, but the ethernet connection was. The power cable connector also looked different. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [March 30, 2012, 3:08pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/14 "2012-03-30T15:08:45Z")
It’s not clear from the [Wikipedia page](http://en.wikipedia.org/wiki/Ferrite_bead), but I think that affects the common-mode current much more than the signal current. In Ethernet cable, the wires come in pairs, and the _signal_ current in one wire of a pair is opposite the current in the other wire of the pair. The fields from the signal current are largely confined between the two wires, and the ferrite bead won’t have much effect. If you put the _same_ current on all the wires, this is the common-mode current. This current can radiate, with the wire acting just like an FM wire antenna (like a car antenna). The magnetic field due to this current will go around the wires, and the ferrite bead will damp this out very well. Without the bead, the signals inside the printer can be radiated by this “antenna”, causing interference. I don’t work in this field, so I’m willing to be corrected by someone who does, but I’m pretty sure that’s how they work. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [March 30, 2012, 8:23pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/15 "2012-03-30T20:23:28Z")
Similar HP printer config. PDF link see page 6 . As others said it’s the power cable not the ethernet cable. [http://h10032.www1.hp.com/ctg/Manual/c01883340.pdf](http://h10032.www1.hp.com/ctg/Manual/c01883340.pdf) ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [March 30, 2012, 8:38pm UTC](https://boards.straightdope.com/t/what-is-the-purpose-of-this-gadget-that-came-with-a-laser-printer/617235/16 "2012-03-30T20:38:25Z")
I should add\* that the same explanation above applies to other cables. In power cables, the hot and neutral 60 Hz current (or for a laptop, the DC power from the brick) will be equal and opposite, but you can still have common mode RF currents that the ferrite bead will prevent. Similarly for USB or monitor cables, to name a couple other cables with ferrite beads on the computer I’m currently typing on. - ETA: especially since I’ve never seen a ferrite bead on an Ethernet cable.