Thordarson 1 MV transformer
Chester Thordarson of the Thordarson Electric Manufacturing Company of Chicago obtained almost 100 US patents on transformers, inductors, high voltage coils, etc. In 1904 he demonstrated the first 1 million volt 60 Hz transformer at the St Louis World's Fair.
For a description of his demonstrations of high voltage phenomena see Thordarson and Rock Island, a book by Richard Purinton published in 2014 by Island Bayou Press of Washington Island, Wisconsin. This book also covers Thordarson's life story from this birth in Iceland in 1867 through his death in Chicago in 1945.