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The PX625 (also referred to by other names such as PX13) is a small button-shaped 1.35 volt mercury battery, whose case has a wider lip surrounding its negative pole. As other mercury-based batteries, its production and sale is banned in most countries, though some are still made in Russia.

Before the manufacture of mercury batteries was banned, a large number of cameras and light meters were designed to use PX625 cells. Mercury batteries produce a constant and reliable voltage, so light meters were often designed without any voltage regulation in their circuitry. Use of other battery chemistries (such as alkaline or lithium) does not provide the same constant voltage, and will give erratic meter readings and exposures unless preventative measures are taken.

Devices requiring PX625

Many cameras and light meters were designed for and require a 625 mercury cell: 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), and so can use the higher-voltage (and less voltage-stable) alkaline (PX625A, 1.5v) or Silver-Oxide (S625PX, 1.55v) cells. Devices without regulation circuitry (or some method of compensating for other voltages) can use Zinc-Air type cells made especially - with the correct 1.35v potential - to replace the mercury cells.

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