After the relaunches of the Flash, Green Lantern and Aquaman in Showcase and Hawkman in Brave and Bold, the next Golden Age character to get a face lift for the Silver Age was the Atom. In the 1940s, the Atom had no super powers, but was simply Al Pratt, a short man who was an excellent fighter. For the Silver Age, DC decided to make him a shrinkable superhero.
Ray Palmer is introduced in Showcase #34 (September-October 1961) as a graduate student in physics at Ivy University. He's studying ways to shrink matter using a piece of a white dwarf star, but his experiments explode, limiting the utility of his process. He's got a girlfriend (Jean Loring), who refuses his offers of marriage because she wants to prove herself as a lawyer before settling down.
While hiking in a cave with some youngsters, Ray and Jean are trapped by a cave-in. Ray discovers a tiny hole to the outside, and using a crystal from the white dwarf, manages to shrink himself down in size (fortunately his clothes also shrink) and enlarge the hole so he can help Jean and the children escape.
In the second story, Ray puts his newly discovered power to work fighting crime. He designs a costume for himself:
We learn that he has the capability of changing his weight in addition to his size, but retains his full strength. He helps Jean win her first case, proving innocent a young woman named Alma Wilson. Along the way we learn one of the Atom's patented tricks: He can dial a number on the phone, then hop on the sound waves and travel to the other end of the line.
Ray has an interesting reason for fighting crime. Since Jean won't marry him until she's succesful, the Atom's help by solving the crimes for which her defendants have been accused may end up getting her to become Mrs. Palmer.
In the third tryout issue, we meet Doctor Loring, Jean's dad, who's another scientist. Alma Wilson, Jean Loring's client, returns as well.
The tryout seems to have succeeded, as the Atom was awarded his own magazine during the summer of 1962.