The Rotary Watch Co, founded in Switzerland in 1895, grew in reputation through the decades as a manufacturer of high-quality Swiss-made timepieces. The button battery in a Rotary watch will last about a year, on average. By following these steps you can do the job yourself with a steady hand and a jeweler's screwdriver.
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Place the Rotary watch face down on a padded surface such as a folded blanket to prevent scratching the crystal while you work.
Locate the small notch on the back of the watch opposite the stem.
Slip the flat edge of the jeweler's screwdriver into the notch and gently pry up the back cover until it pops off.
Remove the screw securing the battery cover. The battery underneath is a round button.
Check the battery polarity in the compartment, The positive (+) terminal should be facing downward.
Take the battery to a jewelry store or electronics store such as Radio Shack to match up with the correct replacement.
Insert the new battery in the Rotary watch with the positive side facing down.
Reattach the battery cover with the screw.
Align the cover with the Rotary watch case, making sure the notch is opposite the stem, then snap the cover into place until you hear a solid click.