I ended up returning my cycling computer/odometer this weekend and getting a new one. I have the Sigma BC 400, a really basic one.
It turns out the first one may have been OK, but Sigma evidently cuts corners (and costs) on the parts. The salesguy at REI mentioned that sometimes the magnets are duff, so when this one worked when I tested it, but didn’t on the bike, I simultaneously figured out how it worked and why it didn’t, if that makes sense.
I pulled a super-powerful ceramic magnet from a box I had on hand from another project (doesn’t everyone have some of these?) and lo, it worked fine. The new magnet has about the same dimensions as a hearing aid/button battery, so some packing tape to hold it in place on top of the other one (it has a fitting to clip onto a spoke) and all is well.
I’d include a URL but the folks at Sigma are so clever, their site is all Flash-driven: must have used all the cash they saved on useless magnets . . . .
The way it works is simple: the computer is about the size of digital watch and connects to a little sensor mounted on your front fork. Once you set your wheel size, the magnet passing the sensor sends a signal to the computer which then calculates speed, records distance, etc.