Harold Schmidt was a scientist working on a shrinking formula his young friend Jimmy Everett drank some of the mixture while the professor was sleeping causing the boy to shrink. Thankfully the the scientist had the fore thought to create an antidote allowing the boy to return to normal size. Jimmy insisted that he be Harold’s assistant going forward.
Trivia
It’s worth noting that Harold Schimidt predates another Marvel character by the name of Hank Pym who is known for their work on size manipulation as well with his use of “Pym Particles“.