Some reed switches use Rhodium as contact material.
How does this influence key bounce? I always thought in the old days, switch contacts were mercury coated, which effectively eliminates bounce. They could as well be on the reed switches, but this is just speculation; given they need to be encapsulated in glass anyway, I don't see any reason not to coat them--remember, they're old, so mercury toxicity and environmental concerns don't count.
The hall effect sensor has to be powered to work?
Yes, that's how hall-effect sensors work. Basically, you send a current through a block of a special material (I think silver was used before semiconductors became available; today, semiconductors are used because they're better AND cheaper). If a magnetic field penetrates your block perpendicular to the direction of current flow, this leads to a force (Lorentz force) on charge carriers (electrons; if you're using semiconductors, electrons and holes, but this gets somewhat nasty), which are in turn displaced. If you attach some leads perpendicular to the magnetic field and your "feed current," you can then measure a voltage which is dependent on magnetic field strength (the stronger the magnetic field, the stonger the Lorentz force--charge carriers are displaced more severely, leading to a bigger potential difference at the sides of the block), called the Hall Voltage.
So yes, hall effect sensors need to be powered, but they don't really decay. Hall effect doesn't work without auxiliary power, but the sensors themselves are what you'd call "passive." You do need electronics to amplify and interpret the results, though. Hall sensors are also somewhat cheap these days, so they're used in a lot of equipment (as far as I know, standard PC fans use them to synchronize the motor for example)--making a hall-effect based key switch seems to be a somewhat logical decision, while coupling it with a membrane sheet does not.
That means they will fail not only from usage but also from non-usage. It will take years but it adds another element of failure that a mechanical switch does not have. I think they are overrated.
As I wrote, they need to be powered to operate. Hall sensors are basically a chunk of metal (or semiconductor; the effect is present in metals, but the resulting hall voltages are really tiny, so you'll need expensive amplifiers and whatnot to get an acceptable result), so they don't decay over reasonable periods of time. They'll basically last as long as the electronics last, but lifetime most likely seems to be limited by the mechanism to provide tactility anyway, as with mechanical switches. One big advantage of hall sensors is they don't bounce; you'll need electronics for processing the signal anyway, and adding a large enough hysteresis removes bounce effectively.
-huha
