why does mercury cell give a constant voltage throughout its life?
Mercury cells use KOH(Potassium Hydroxide) or NaOH(Sodium Hydroxide) as their electrolyte, and cells using NaOH 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.