X-ray machine – The X-ray machine was an invention that had its public debut at the 1904 World's Fair. X-rays were first discovered in November 1895 by a German scientist, Wilhelm Conrad Röntgen, who at the time was studying the phenomena accompanying the passage of an electric current through a gas of extremely low pressure. After Rontgen's discovery, he took an x-ray of his wife's hand, showing the bones and her fingers along with her wedding ring, and sent it to several of his scientist colleagues. One of the scientists that learned of the discovery was Thomas Edison and he soon began to experiment with his own x-ray machine with his assistant Clarence Dally. In 1901, Dally brought a test version of the X-ray machine to the 1901 World's Fair but failed to test the machine when President McKinley was assassinated. For the 1904 World's Fair, the X-ray machine was perfected and successfully shown to the public. X-rays are now commonplace in hospitals and airports.