Zachary Crockett has written an excellent article entitled "How Mickey Mouse Evades the Public Domain" which contains good data and visualizations to help tell the story of Mickey Mouse and the changes to U.S. Copyright Law. I'll give you this tidbit as encouragement to read the entire article. It is a quote from Paul J. Heald, a professor in the University of Illinois School of Law:
Copyright correlates significantly with the disappearance of works rather than with their availability.
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