If you search for "mercury battery replacement" you'll find a host of information. You have 3 issues to deal with 1) physical dimensions of the battery, 2) voltage, 3) discharge curve (how the voltage drops over time).
Alkaline cells decline in voltage over time (as used). Mercury, silver oxide, and zinc air have a constant voltage until they abruptly die. Zinc air batteries don't last long (once the tab is removed) but are cheap. Some devises require a constant voltage; including most meters. You can often compensate for the difference in 1.5 vs 1.35 volts, but difficult to compensate for varying discharge of alkaline cells.
In general, options are:
Use a Wein cell, which is a zinc air battery with the proper dimensions (expensive)
Use a 675 zinc air hearing aid battery and a rubber O-ring (#9 works well). The O-ring brings the 675 battery to the proper diameter.
Use an adapter such as the one from Cris Camera. Expensive, but a one time cost. Or build one (there are instructions on the web).
Use a 1.5v silver oxide battery (such as
http://www.batterymart.com/p-s625px-silver-oxide-button-cell-battery.html) and compensate for 1.5v.
Personally, I use the 675 zinc air batteries. One of these days I'll try the silver oxide 625 battery; but I find the quality of the offbrand Chinese made batteries suspect and variable.
