The PX625 (also referred to by other names such as PX13) is a small button-shaped 1.35volt mercury battery which great number of camera models were designed to use.

Before the manufacture of mercury batteries was banned, many cameras were designed to use them. Because mercury batteries produced a constant and reliable voltage, many cameras were built without any voltage regulation in the metering circuitry. Unless preventative measures are taken, use of other kinds of batteries gives erratic exposures and meter readings.

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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