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Mercury batteries were used in the shape of button cells for watches, hearing aids, and calculators, and in larger forms for other applications.
The mercury oxide-zinc battery system was known more than 100 years ago but did not become widely used until 1942, when Samuel Ruben developed a balanced mercury cell which was useful for military applications such as metal detectors, munitions, and walkie-talkies. The battery system had the advantages of long shelf life (to 10 years) and steady voltage output. After the Second World War the battery system was widely applied for small electronic devices such as cardiac pacemakers and hearing aids. Mercury oxide batteries were made in a range of sizes from miniature button cells used for hearing aids and electric wrist watches, cylindrical types used for portable electronic apparatus, rectangular batteries used for transistor radios, and large multicell packs used for industrial applications such as radio remote control for overhead crane systems. In the United States, mercury oxide batteries were manufactured by companies including P. R. Mallory and Co Inc, (now Duracell), Union Carbide Corporation (whose former battery division is now called Energizer Holdings), RCA Corporation, and Burgess Battery Company.
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.
Electrical characteristics 
Mercury batteries using mercury(II) oxide cathode have a very flat discharge curve, holding constant 1.35 V (open circuit) voltage until about the last 5% of their lifetime, when their voltage drops rapidly. The voltage remains within 1% for several years at light load, and over a wide temperature range, making mercury batteries useful as a reference voltage in electronic instruments and in photographic light meters. Mercury batteries with cathodes made of a mix of mercuric oxide and manganese dioxide have output voltage of 1.4 V and a more sloped discharge curve.
Product ban 
The 1991 European commission directive 91/157, when adopted by member states, prohibited the marketing of certain types of batteries containing more than 25 milligrams of mercury, or, in the case of alkaline batteries, more than 0.025% by weight of mercury. In 1998 the ban was extended to cells containing more than 0.005% by weight of mercury.
In the United States, in 1992 the state of New Jersey prohibited sales of mercury batteries. In 1996 the United States Congress passed the Mercury-Containing and Rechargeable Battery Management Act that prohibited further sale of mercury-containing batteries unless manufacturers provided a reclamation facility, effectively banning the sale.
The ban on sale of mercury oxide batteries caused numerous problems for photographers, whose equipment frequently relied on their advantageous discharge curves and long lifetime. Alternatives used are zinc-air batteries, with similar discharge curve, high capacity, but much shorter lifetime (a few months) and poor performance in dry climates, alkaline batteries with voltage widely varying through their lifetime, and silver-oxide batteries with higher voltage (1.55 V) and very flat discharge curve, making them possibly the best, though expensive, replacement after recalibrating the meter to the new voltage. Special adapters with voltage dropping Schottky or Germanium diodes allow silver oxide batteries to be used in equipment designed for mercury batteries. Since the voltage drop is a non-linear function of the current flow, diodes don't represent a very accurate solution for applications where the current flow alters significantly. Currents drawn by old CdS light meters are typically in the 10 µA to 200 µA range (Minolta SR-T range). Various kinds of active voltage regulation circuits using SMD transistor  or integrated circuits have been devised, however, they are often difficult to integrate into the battery compartment space. Replacements must operate on the low voltage of a single cell. Lack of a power switch on many old light meters makes an ultra-low-power design mandatory.
See also 
- C. L. Clarke, US Patent 298175, 1884.
- David Linden, Thomas B. Reddy (ed). Handbook Of Batteries 3rd Edition. McGraw-Hill, New York, 2002 ISBN 0-07-135978-8, chapter 11.
- http://data.energizer.com/PDFs/E146X.pdf Energizer obsolete type E146, in NEDA 1604 size.
- Thomas Roy Crompton Battery reference book page 5-23.
- Rod Hunter, Koen J. Muylle (ed.) European Community deskbook Environmental Law Institute , 1999 ISBN 0-911937-82-X, p. 75.
- Frank Kreith, George Tchobanoglous Handbook of solid waste management, McGraw-Hill Professional, 2002 ISBN 0-07-135623-1, pp. 6-34.
- "Matthias Paul: Using a 7x7 mm SMD transistor-based low-side voltage regulator circuit as Mercury battery replacement". 2009-03-14. Retrieved 2011-02-26.
- "Matthias Paul: Using a Bandgap voltage reference as Mercury battery replacement". 2005-12-12. Retrieved 2011-02-26.
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