Jimmy Olsen wears a specially designed wrist watch that is capable of emitting a high frequency ultra-sonic signal that only Superman can hear. Jimmy relies upon the watch to contact Superman should his life be threatened or if he is present during an emergency that requires Superman's assistance. Jimmy has been known to abuse his use of the wrist watch and has occasionally contacted Superman for rather mundane matters. Superman has criticized him in the past for such casual use of the device as it often draws him away from a more pressing emergency.
When Jimmy Olsen first came to Metropolis to live in the same city as his idol Superman he would first get a job working for Professor Crane. One of Crane's experiments was a time machine that would allow someone to travel back in time. One of the devices flaws was that it would due to the constant expansion and rotation of the universe, the user would likely end up on another planet when arriving at their intended destination. Traveling back in time some 20 years, Jimmy would end up on Krypton prior to the planets destruction. Jimmy would help keep an eye on baby Kal-El for Jor-El and Lara while Jor-El put the finishing touches on the rocket that would see Kal-El sent safely to Earth before Krypton would explode.
Returning to his own time, Jimmy would lose all memory of the incident and get laid off due to the devices failure. When looking for other employment, Jimmy would be greeted by Superman who would remember how Jimmy Olsen helped him all those years ago. In reward, Superman would become Jimmy's pal and would gift him with the signal watch and land him his job with the Daily Planet.
Since then, Jimmy has used it to summon Superman whenever there is trouble, ranging from the mundane tasks to universal threats. Over the years the watch has been subject to a series of malfunctions and damages that would cause it to act irregularly, or cause strange transformations, changes in behavior, and other bizarre effects.
Following Crisis on Infinite Earths this watch has since been erased from history.
Jimmy first developed the watch exactly one year after Clark Kent began working at the Daily Planet. A friend of Jimmy's named Chrissie Walker had attempted to take her own life by overdosing on pills. Regretting the decision, she came to Jimmy's house and passed out in his bedroom. Jimmy and his mother Sarah were unable to contact emergency personnel and neither of them had a vehicle to take Chrissie to the hospital. As the girl lay dying on Jimmy's bed, the quick-thinking copy boy cannibalized several old radios and created a device that emitted a hypersonic signal. It was his hope that Superman's super-hearing would perceive the signal and that he would investigate its source. The plan worked and Superman arrived at the Olsen household and rushed Chrissie to the hospital. Superman complimented Jimmy on his ingenuity and suggested that he should miniaturize his device so he would always be able to contact should another emergency arise. Jimmy took his advice and used the radio parts to create a special wristwatch that he wore at all times. 
- The signal watch was first shown in Action Comics #238 (1958). After Crisi on Infinite Earths, the watch was reintroduced in World of Metropolis #4 (1988).
- Appearances of Jimmy Olsen's Signal Watch
- Item Gallery: Jimmy Olsen's Signal Watch
- Images featuring Jimmy Olsen's Signal Watch
Links and References