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What's in your wallet?

By Chuck Newcombe

Unless you've been living under a rock without access to TV, you probably recognize this last line of some recent funny ads for a major credit card. I've been watching them for several months now without attaching any particular significance to the message - until two weeks ago, that is.

I was preparing to go on yet another trip, and I decided to take stock of the contents of the wallet I'd be carrying with the idea to thin it down a little. No need taking a bunch of cards I wouldn't be using. Well, I took inventory and was amazed at what I discovered.

Not only did I have my driver's license, a few credit cards and medical insurance cards - I'd certainly need those - but I had preferred shopper cards for three stores and to my surprise, forgotten magnetic gift cards for my favorite 'big box' home improvement store and a major bookseller chain. Then, there were my bonus mile cards for several airlines. And that was in just one side of the wallet.

The credit, debit and gift cards carry magnetic information that allows them to serve as money. It seems we're doing everything possible to become a cashless society. And that has its good and bad points. One purpose for this column is to underline how vulnerable we have become when something like hurricane Katrina strikes. Our cards won't work, and neither will the cash machines at our local banks.

But there are other, less dramatic ways we can individually become suddenly crippled from a monetary standpoint. Have you ever stood at the checkout counter at a store where anti-theft tags are deactivated over a certain area in the countertop? We are warned not to place credit cards near these devices because they might be rendered useless. Well, that got me to wondering – how far does the magnetic field extend, and how might I measure it? My Fluke 87-V DMM and i200 current transformer probe to the rescue.

I must admit I got a few strange looks as I wandered into a couple of stores to see what I could discover. I connected the leads of the transformer secondary to the Volts (yes, the voltage) input of the meter instead of the mA input normally used with this type of probe. My goal wasn't to measure current, but rather to detect the magnetic field present on the countertop. Using the current clamp with its jaws held open to act as an antenna, I registered several volts on the meter, and I was able to measure the frequency of the field as well – several kHz in one case. It extended a few inches above the countertop, but only over the pad. I still didn't know how strong the field was, but at least I knew its shape. That's when a manager, worried that I was nosing around their security system, asked me to leave, and so I did.

It turns out the field detection technique I used is also useful for several real industrial applications – like trying to discover the source of magnetic interference that can cause a computer monitor image to waver around, making the operator seasick. Using this gauss probe trick I've actually discovered stray current flowing in the grounded conduit of a power circuit buried in the wall near the offending monitor. The frequency measured is generally 60 Hz, but it could be at a power harmonic frequency as well in some cases, often 180 Hz.

You can also detect the vertical scan field in front of a monitor or television CRT, and measure the vertical scan frequency – 30 Hz in the case of the television, and other values (I've measured from 37 to 82 Hz) from a computer monitor.

Back to the story of the checkout counter – I took a different approach to see if the deactivation field would affect the magnetic stripe on a credit card. I asked a friend at a nearby company to program a few of his magnetic stripe cards used for building entry with some numbers that would not activate their system, and went back to the stores. This time I just walked through a checkout and passed the cards over the deactivator pad and then took them back and asked my friend to read them. The good news was that they all still worked. Because of the warnings placed on these pads, I still won't tempt fate with my credit cards, but now, because I know the shape of the field lurking there, I worry less about reaching across the counter to hand my card to the clerk.

My friend did point out another way that magnetic cards may be disabled, and this one is far more threatening. He has had several instances where entry cards have been corrupted by placing them near magnetic purse catches or in one case, a few refrigerator magnets that had collected in the bottom of an employee's purse. It turns out the real threats are those that seem to be the most innocent – and there's no warning sign to alert us to the danger.