Mercury cells

Mercury cells (also referred to as mercury batteries) are batteries with a mercury oxide electrode. Mercury batteries were discovered in the 19th century, and a practical method to make small button batteries was discovered by Ruben and Mallory. These type of battery use the reaction between mercuric oxide and zinc electrodes in an alkaline electrolyte. The biggest advantage of these batteries is that the voltage during discharge remains practically constant at 1.35 volts.

After WWII these batteries were very popular, but in the 1970s, and due to the toxicity of Mercury they were banned in most countries. In the 21st century, manufacture, transport or sale of these batteries is banned or restricted nearly worldwide, due to the metal's toxicity; although some are still made in Russia. Before such bans, many cameras and lightmeters were designed to use them. Because mercury batteries produced a constant and reliable voltage, these devices were built without voltage regulation circuitry. Use of other kinds of batteries, that have different discharge voltages over time, without regulation circuitry may lead to inaccurate meter readings and therefore incorrect exposures.

Cell names and sizes

386, PX625 (x2), PX400 mercury button cells compared to AA batteries image by AWCam (Image rights)

Mercury batteries came in number of different sizes including the 32, 400, 625, 640, 675 and the like. Most of the mercury batteries are a single electrolytic cell - usually small and circular. However, the 32 is a set of 4 stacked 640 cells. The greatest number of cameras, finders and light meters were designed for the PX625.

Common number	Alternative names	Diameter (mm)	Height (mm)	Weight (g)
625	1124MP, MR9, PX625, PX13, EPX625, RPX625, KX625, HD625, PN625, RM625, 625, H1560, H-D, MR9	15.6	6.05	4.5
675	1128MP, MR44, PX675, HD675, KX675, RPX675, H-C	11.6	5.35	2.6
400	1116M, MR42, E400N, EPX400, KX400, PM400R, PX400, RM400, T400N, V400PX	11.6	3.6	1.4
640	1105M, NR52	16	11.2	8
32	1404M, 4NR52, PX32, E164, TR164, 4NR52, 1404M. HM-4N, V164PX, PX32A, PX32B	17	44.5	36

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Devices requiring a PX625 battery

Many cameras and lightmeters where designed for and require a 625 mercury cell (see a list of cameras and list of exposure meters). Some cameras and exposure meters were designed for 625 sized cells but include voltage regulation circuitry and thus do not require a PX625 to function properly (list of devices).

External links

• Suggestions from Rick Oleson (based on ideas from Leon Schoenfeld) to make classic cameras give acceptable meter readings using zinc-air hearing aid batteries.
• The mercury cell problem and its solutions (PDF file) by F. de Gruijter. Also discusses PX675 and other mercury batteries.
• The mercury oxide battery problem, at Konica SLR system, providing information about the PX625 and PX675 battery that is not specific to Konica models
• Quirks of using zinc-air hearing aid batteries to replace mercury cells part 1 and part 2 from Silverbased (Archived)
• Energizer Battery Online Data
• Russian mercury px625 battery Seller of PX625 mercury batteries.

References

Popular Photography, October 1993, p. 130; January 1994, p. 144; September 1994, p. 144; and August 1995, p. 66.