[personal] Shaft encoder in HP2631B printer

[1] I once worked on an IR spectrometer which used a Nernst glower as the light source. THis is a rod or coil of rare earth oxides, and you'd think it eas a good electircal insualtor. At room temperature it is, but get it hot, and it'll conduct (the reisstance drops with increasing temperature, in fact it can thermally 'run away'). Anyway, the nernst glower was connected in series wiht a ballast lamp across the mains. To get it started you used a bit of cototn wool soaked in ethanol and ignited to heat the glower, once it got hot, it would glow quite brightly. Of course the advantage is that the nernst glower runs in air (it can't exaclty oxidise, so you don't have a glass bulb to absorb any IR emission. The nernst lamp was actually proposed for electric lighting at one point (with an electical heater coil to start it), the advantage being that you don't have to have an evauated bulb, and you can just replace the glower when it fails, not the whole lamp. But they're less efficient than vacuum or gas-filled filament lamps for producing visible light (the golwer runs cooler than a tnngsten filament). -tony