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Wikipedia is the elephant in the room in most discussions of digital history. We all know it is there and important but as professional historians we try to ignore it. You know that opinionated 19-year-old in your survey class, the one who is speaking up all the time, the guy with the loud voice who is always correcting you with some wrong historical information he learned on the internet last night? Well he is editing Wikipedia right now. For most teaching historians our approach to Wikipedia is to hope it goes away. (This is also our approach to Rate My Professor.)
Yet our students turn to Wikipedia as the information source of first resort, and we need to understand how it works. In my Public History course we recently read "The Charms of Wikipedia" by Nicholson Baker, a good and amusing introduction to the online encyclopedia. Personally I have no objection to students using Wikipedia exactly as they would any other encyclopedia--as a handy place to check basic facts and to begin research, but absolutely not an acceptable source of research for a college-level paper.
I actually assign some Wikipedia articles from time to time, such as Land Ordinance of 1785 and the related articles Northwest Ordinance, Public Land Survey System, and Northwest Territory. These articles are Wikipedia at its best, giving simple explanations of how the public lands of the United States were divided. It is important basic information in a class on Western History, but too often neglected in textbooks.
More problematic are the articles on controversial topics--Robert E. Lee, Zionism, or Leon Trotsky. The talk page on Trotsky warns users that "This is a controversial topic that may be under dispute." You think? "Please discuss substantial changes here before making them,
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At the end of the day Wikipedia becomes what most of it's users want it to be. So the article on Lee is generally admiring and minimizes and excuses his life-long support of slavery in a way that makes this historian uncomfortable.