"I have disassembled (read: cannibalized for parts) an old Cannon laser printer a few months back. I found a mysterious part in it. Perhaps somebody can enlighten me as to what it was thrown in for...
It looks just like a standard fuse, as found in power supplies. The usually-glass part is made of white plastic. There is a window on one side, made of a clear material(glass, plastic?) And there is a scale next to the window, 0 to 10.
In the window is some substance that looks like mercury from end to end, but there is a "break" somewhere in the middle (around 3 or 4 on the scale)."
(From: Tony Duell (email@example.com).).
The substance is mercury, AFAIK, and in the 'gap' there's a small amount of some electrolyte. Passing current (very low current) through the device electroplates mercury from one column to the other, thus making the gap appear to move along the tube.
It can be used in 2 ways - if a constant current is passed through it, it records the total time that the unit has been in use. That is the more normal way to use it.
However, in the CX, it's fed with a short pulse of current at the start of each page. Thus, a small amount of mercury is transferred for each page printed, and the device does, indeed, operate as a page counter.