What has the author Hal Hellman written?

It seems you might be mistaken. There is no widely known author named "Hal Hellman." It's possible you're thinking of a different name or have a typo.

Could you please double-check the name or provide more context?

For example, are you looking for:

* A specific book title?

* An author who writes in a particular genre?

* An author who has a similar name?

With more information, I can help you find the author you're looking for.