Google's new service "Ngram" is cool. It lets you search for any word or phrase through all the books Google has indexed, displaying the count versus year.
Here is the graph for "amateur radio":
and here is the result for "ham radio":
The significance? Ham radio (in books) didn't start until about 1950! The Ngram view page also gives links for the books in particular year ranges. I looked up some of them prior to 1965 and found lots that were in the "popular" category: Popular Mechanics, Popular Science, Boy's Life, etc. Amateur radio, however, has a book record back to 1900 if not before.