“Professor Henry’s Big Magnet” Click on the image above to view the file of the original article that appeared in Scientific American on Dec. 11, 1880.
In 1833 Joseph Henry constructed an electromagnet at Princeton for his own studies that eclipsed the previous most powerful electromagnet in existence that he had built in 1831 for Yale. The iron core is covered in a nonconducting varnish and the copper wire is wrapped an insulating silk to allow close winding while preventing short circuits along the electromagnet. This new electromagnet had a maximum lifting power of approximately 3,500lbs (1587 kg).
(1) Riedman, Sarah Regal, 1902-. Trailblazer of American Science; the Life of Joseph Henry. Chicago, Rand McNally  Print.