Chemistry
Mercury batteries use either pure mercuric oxide or a mixture of mercuric oxide with manganese dioxide as the cathode. Mercuric oxide is a non-conductor so some graphite is mixed with it; the graphite also helps prevent collection of mercury into large droplets. The anode is made of zinc and separated from the cathode with a layer of paper or other porous material soaked with electrolyte. During discharge, zinc oxidizes to zinc oxide and mercuric oxide gets reduced to elemental mercury. A little extra mercuric oxide is put into the cell to prevent evolution of hydrogen gas at the end of life. Mercury batteries are very similar to silver-oxide batteries.
The overall reaction is:
Sodium hydroxide or potassium hydroxide are used as an electrolyte. Sodium hydroxide cells have nearly constant voltage at low discharge currents, making them ideal for hearing aids, calculators, and electronic watches. Potassium hydroxide cells, in turn, provided constant voltage at higher currents, making them suitable for applications requiring current surges, e.g. photographic cameras with flash, and watches with a backlight. Potassium hydroxide cells also have better performance at lower temperatures. Mercury cells have very long shelf life, up to 10 years.
A different form of mercury battery uses mercuric oxide and cadmium. This has a much lower terminal voltage around 0.9 volts and so has lower energy density, but it has an extended temperature range, in special designs up to 180 C. A 12 volt battery of this type was formerly used for residential smoke detectors, where the two-step voltage characteristic gave a useful warning for replacement.
Read more about this topic: Mercury Battery
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