A while back there was discussion of non-volatile displays. I found
one that nobody thought of:
The Mercury Coulometer

An obscure, antique, highly toxic device that has a glass tube, two
electrodes, and two columns of mercury separated by a liquid
electrolyte.
Electrical current passing through the device transfers mercury from
one side of the electrolyte bubble to the other, creating a linear
indicator - albeit very very slow, fragile, toxic, etc.

See the diagram at the end of this paper for a clearer picture:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700023619_1970023619=
..pdf

--=20
Martin K.
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