If you use a magnificent old Canonet or Leica CL camera (you look the complete list ), your camera requires the mercury PX625 battery.

PX625 works several years (usually 10 years!) also has stable 1,35 V during all the life!

Russia is the only country which manufacture original PX625 mercury battery now.

From the message of one American :

The reason these batteries are so beloved, is that they start off at 1.35 Volts, and stay there until they're about to die.
That's really a big deal, when we're talking about a precision measurement device like a light meter.
You don't have to keep calibrating it. It starts out correct, and you can depend on it for a decade or more.

Alkaline batteries start at 1.5, and drop right away.
The recommended alternative for your mercury cells, are zinc-air cells, like they use in hearing aids.
They start at 1.35V and stay constant until they die, but death comes in just a week or so.
So you're constantly feeding them new batteries.

In a camera that doesn't use current when you're not taking a picture,
and in light meters where they only use current when you push the button, these batteries will last for a very long time.

The ones in my Gossen Luna Pro light meter, and my Soliger analog spot meter lasted for twenty years. Twenty years.

That's why I'm thrilled to get more batteries for my wonderful meters.
The Luna Pro will give you a precise reading from tree bark at midnight, and the spot meter
has Zone System marking (I added them, but hey), and a 1 degree reading.
For those of us that study lighting and exposure... what wonderful tools.
They are some of my best and longest-known friends. It's silly to become
so attached to things, but they've done so much for me for over 40 years.

I figure the ones you have so wonderfully sent to me will last me for the rest of my life. I am very grateful.

As for the mercury.
We have far more mercury in our home in the fluorescent lamps that we have in every room, than in all of these tiny batteries.
We are encouraged to buy the lamps, and warned not to dispose of them incorrectly, but
they don't make it easy to do that. So tons of mercury is going into our landfills.
But we can't buy a tiny battery that will last us ten years?

It is total silliness. Think of it... To access the mercury in the battery would take tools and determination.
To access the mercury in the lamp would just take dropping it.
I know people who put the lamp in a bag and smash it, so it won't look like a fluorescent lamp in the trash.

Some laws should be given the "common sense" test. And, if they don't make sense, they should go away.

Attention!
Battery not fit in Leicaflex SL/ SL2 and Leica M5
There are a little to big. +- 0,3mm