magnet phones - how do they work?

November 18, 2010 5:59 AM

My colleague's new mobile phone produces a weak magnetic effect. What's the science at work here?

My colleague recently got an HTC Mozart and we've noticed an interesting phenomenon: the top portion of the reverse side of the phone (image here) is lightly magnetized. The bottom of the reverse side is not, and the magnetized area is where the external speaker, camera, flash and power button are located.

The force is far from refrigerator magnet-strong but is powerful enough to pick up staples and paper clips. The effect persists when the phone is charging and when it is not. We briefly removed the battery and it persisted then too.

We have some theories but we're not scientists by any stretch of the imagination so I'm looking forward to hearing the hypotheses of more learned folks.

posted by CRM114 to science & nature (16 answers total)

Speakers usually have magnets in them. Also, current passing through a coiled wire can create a magnetic effect. See electromagnets.
posted by kalessin at 6:03 AM on November 18, 2010 [1 favorite]

Some Android phones (including my Droid) have a small magnet sensor for "docking modes." I'm not sure if the HTC Mozart does, but that would be a reasonable explanation. (That having been said, my Droid will not pick up the (particularly heavy) paperclips I had in my desk.)
posted by JMOZ at 6:03 AM on November 18, 2010

Note that "Android" is a red herring; I could imagine Windows Phone 7 COULD also have a docking sensor.
posted by JMOZ at 6:04 AM on November 18, 2010

'Magnet sensors' don't have magnets in them, if they did all they would do is activate themselves. See: reed switch
posted by Confess, Fletch at 6:06 AM on November 18, 2010

My Blackberry also has a magnetic area there. It's the back of the speaker which, as kalessin said, has a magnet in it. There are some diagrams here and an explanation of how loudspeakers work.
posted by le morte de bea arthur at 6:13 AM on November 18, 2010

My Blackberry is magnetic there too, it's the speaker.

My Blackberry also has a 'magnet sensor' approximately in the middle, which is used to detect when it's been put in the case, which is held closed with a magnet.
posted by Confess, Fletch at 6:18 AM on November 18, 2010

Another vote for speaker. Find a pair of in-the-ear headphones they will strongly repel each other because of the magnets in them.
posted by Coobeastie at 6:20 AM on November 18, 2010 [1 favorite]

Awesome guys! Thank you.
posted by CRM114 at 6:37 AM on November 18, 2010

Speakers in cell phones are generally piezoelectric speakers which are solid-state and do not have magnets in them. A magnet big enough for a speaker is too big for a cellphone. There are no permanent magnets or voice coils in a cellphone speaker. See this wikipedia article instead.
posted by GuyZero at 7:44 AM on November 18, 2010

my htc hd has a magnet in the stylus, which i guess helps keep it in place; i also guess there is an opposite magnet inside the case..
posted by 3mendo at 7:53 AM on November 18, 2010

Speakers in cell phones are generally piezoelectric speakers which are solid-state and do not have magnets in them. A magnet big enough for a speaker is too big for a cellphone. There are no permanent magnets or voice coils in a cellphone speaker.

maybe for the speaker in a phone that is just a phone, but the sound quality out of that type isn't very good. The phones that are out now which are geared to being media devices as well as phones typically are marketed partly on their sound quality.

my nokia 5800 does this too, and it is localized around the areas the 3 speakers (one for the phone and two for playing music) are in. I would guess that they are using rare-earth magnets that are both strong and small.
posted by ArgentCorvid at 7:59 AM on November 18, 2010

You mentioned that it had a flash. I think that it's common for a flash to be fed by an inductor -- it's how they store up the energy needed for the flash.
posted by Chocolate Pickle at 9:17 AM on November 18, 2010

A common camera flash would be fed by a capacitor - not an inductor. You charge up a capacitor and then quickly discharge through the flash circuit. - maybe a transformer though.
posted by TravellingDen at 10:04 AM on November 18, 2010

I spoke without thinking - apologies for my stupidness. Camera flashes are indeed inductor fed.
posted by TravellingDen at 10:08 AM on November 18, 2010

Well, this says they do usually use a capacitor, so I guess I was wrong.

The power supply inside the phone will be a switcher, which will use both a capacitor and an inductor, but it probably wouldn't be where you're talking about. That inductor would be in the main body of the phone, near the battery.
posted by Chocolate Pickle at 10:43 AM on November 18, 2010

Regardless, the flash circuit wouldn't be producing a magnetic field except when it was charging. (And for efficiency's sake, you usually don't have magnetic fields straying that far from the inductor.)

It's a permanent magnet, the question is why there is a permanent magnet in the phone. Cell-phone-sized voice-coil-type speakers certainly do exist.
posted by hattifattener at 1:26 PM on November 18, 2010