Superman gave James Olsen the watch just in case he ever got into trouble, so the latter would be able to contact him. James is also able to use the watch to communicate with Supergirl when she is in the field.
At some point, Kara gave Alex a signal watch for in case her sister ever needed help. Kara and Alex also arranged a special code between them for whenever the latter uses the signal watch; pressing it once means that Supergirl should come while pressing it twice means that she shouldn't because it's a trap.
At Kelly and Alex' wedding, James gave Kelly an upgraded signal watch which also serves as a portal device, so James and Kelly can watch each other's backs should the need rise. James then asked Kelly not to tell their mom about it, fearing that she would make them portal in everything for dinner.
- Signal Frequency: The watch's main purpose is to signal Superman or Supergirl in emergency needs.
- Warning: A trick the Supers came up with. Pressing the button twice means the user is in a trap for Kryptonians and not to come.
- Communication: James is also able to use the watch to communicate with Supergirl when she is in the field.
- Portal: An upgraded signal watch added the special effect that would allow the wearer to open a portal.
- Post-Crisis, Lois Lane and Sam Lane use box-like devices known as ELTs to call Superman instead of signal watches, though it's possible the watches were simply replaced with these more durable devices.
- If a signal watch is activated in National City, Superman leaves it to Supergirl (except during her rookie days); additionally, J'onn J'onzz can telepathically contact her, while the rest of the Superfriends can contact her on an earpiece.
Behind the scenes
- Jimmy Olsen's signal watch was first featured in Action Comics #238 (March, 1958) and was created by Otto Binder and Wayne Boring.
- Supergirl Season 3 Deleted Scene