Battery voltage and size:

All silver-oxide watch batteries have the same voltage (about 1.65v).  The problem is getting one that fits.  You can expect that batteries will last approximately one year under normal conditions. Here is a reference table for battery replacement:

BATTERY REPLACEMENT TABLE FOR TUNING FORK WATCHES

APPLICATION
Accutron 214
Accutron 218, 230, 224, 219 series,
Universal Geneva Unisonic,
Omega f300, most others
Accutron 221
BATTERY
Mercury: #387
Silver Oxide: #394 or #387s
(save your old plastic
gromit for the #394)
Mercury: #343
Silver Oxide: #344
Mercury: #388
Silver Oxide: None
(none fit right, although some
smaller like the #329 can work)
ORIENTATION
(+) or engraved side towards green plastic
(-) side touches battery hatch cover
(-) side shows through hole in plastic gromit
Accutron & Unisonic:
(+) side down,
(-) side touches battery strap.

Omega f300 & Others:
(-) side down,
(+) side touches battery clip.

(+) side down, (-) side touches
battery strap.
ILLUSTRATIONS