Sharp PC-1211

Indeed. this is a well-known problem for owners of many older cameras, and other measuring instruments which depend on the stable voltage from a mercury cell. Even some older electronic multimeters and more particularly analogue plotters [1]/recorders used a mercury cell as a reference [1] I suspect some of the old analogue X-Y flatbeds were used on digital computers. They were certainly used on analogue computers. There, this sort-of scrapes being on-topic ;-) Incidentally, I read somewhere that the Eastern Bloc couldn't make reliable mercury cells at one time, so they designed their cameras not to need them. Things like Prakticas and Exaktas use a balanced [2] weatstone bridge circuit (meaning it's in balance at the correct exposure point) which is independant of the battery voltage, of course. [2] Unlike the Pentax Spotmatic where the correct exposure point is slightly off-balance (the service manual gives a certain current to be flowing through the galvaometer coil to get it to the 0 marker). %deity knows why it was done this way.... Yes. The problem with zinc air cells is that they have a fairly short life once you've let the air in. Fune if you're using the instruemtn every day, not so good if you want to use it for a few days and then put it away for a few nonths. There are other solutions too. One is to use a silver oxide cell (again stable voltage, but not the same voltage as a mercury cell) and drop the excess 0.2V with a shottky diode. Another is to use a micropower regulator circit. They're not PX625-size, that's for sure... I forget what they are. Anyway, with 4 cells, a Si diode drops about the right voltage to use alkaline cells. The unit doesn't depend on the exact battery voltage (not like a measuring instrument), it just doesn't like 6V. And there's plenty of room inside to fit a 1N4148 or whatever. -tony