What was Nikola Tesla's first patented invention?
List of Nikola Tesla patents
 - Method of Insulating Electric Conductors - 1900 October 23 - Expounding on Faraday's hypothesis of freezing substances make them possess a higher dielectric level to insulate transmission conductors; Improvements in the method set out by Faraday; Method of insulating electrical conductors which consist in surrounding or supporting said conductors by a material which acquires insulating properties when frozen or solidified; Method of maintaining a conductor within gaseous cooling agent by the continuous application of said agent; Trough or conduit with circulating cooling agents.