More sterling technology journalism

Regarding the widely reported "new discovery" of "WiTricity", wireless electricity transmission, as mentioned in Austenite's comment the other day:


The people at MIT who came up with this "new invention" would be the first to tell you that the basic technology involved in it isn't new at all. It's just an electrical transformer with a large air gap between the primary and secondary coils. This, inescapably, means its efficiency gets worse the further away the receiving device moves, and it wastes energy heating up conductive things in the area.

I'm sure that the MIT research is doing clever things with the control and detection systems; it may even result in real useful products some time quite soon. It's quite possible that at some point in the future we will indeed have charging pads we can put various devices on to be inductively recharged. The same goes for the bigger versions envisaged by the WiTricity idea, where a laptop will work without a power connection anywhere in a room; to make that work, you'll probably have to build your primary coils into the very walls of the room.

It's not even out of the question that such an arrangement could be used to power transportation systems, though we'd need to start making our cars out of plastic before it'd be workable.

The reason for this is tied to the main problem with these sorts of systems, which are proposed about twice a year by some crank who thinks he's the one who can do what Tesla didn't. The problem is that if you put a conductive loop in the field, like a wristwatch with a metal band, it'll eat most of the power output and get hotter and hotter. It's that problem which I presume the MIT researchers are working on, though it's hard to figure that out from the press coverage.

That big problem, plus the crippling loss of efficiency as your secondary coil(s) get further away from the primary, is why inductive/magnetic "wireless electricity" for powering normal appliances, streetlights and most other straightforward everyday things still seems to be a complete non-starter.

(As usually happens when a subject that's covered in first-year Electrical Engineering comes to the attention of the masses, there's a decent Slashdot thread about all this. I've also written about inductive chargers before, here and here.)

I'm looking forward to some really dumb mutations of this idea over the next few weeks. Given the well-established inability of the "electrosensitive" crowd to tell the difference between milliwatt radio waves and ionising radiation, the notion of "electricity beams" popping up all over the place should make them go absolutely spare.

If we can't make these idiots see sense, we should at least attempt to gain as much amusement from them as we can.

5 Responses to “More sterling technology journalism”

  1. trs80 Says: has a much better explanation - it's not a standard transformer, it's a pair of tuned coils that create an evanescent field to achieve 15% power transfer over a few meters. It's still not pratical, since the coils must be precisely tuned to each other.

  2. bmorey Says:

    Meantime, my humble wireless optical mouse (A4tech "Battery Free") works this way. Coil in the mouse pad, coil in the mouse - works like a charm and it's a good business mouse too (don't know about gaming). Better quality than my Microsoft wireless mouse, which went in the bin because the wheel had worn out.

    My thought on reading the original article was, "Wasn't Tesla working on this, unsuccessfully, a century ago?"

  3. JsD Says:

    Not that unsuccessfully, as it happens . . .

  4. richardgaywood Says:

    Does anyone know what sorts of current these wireless pad chargers can deliver? As they are allegedly going to go on sale next month. Yeah, sure, my toothbrush charges this way but it takes hours and hours to fill a teeny tiny NiCd cell.

    I'm sure these sorts of inductive chargers can work OK (note how carefully that picture doesn't show the dongle that must be plugged into each device to actually deliver the charge, though) but I find it hard to believe they can get enough coil windings in there to deliver enough current to simultaneously charge an iPod and a mobile phone.

    Still, though, I live in hope!

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