In the late 19th and early 20th centuries Phelan worked on invention and development with Goodyear, Monsanto, American Car and Foundry, and Allis-Chalmers, on the Panama Canal project, as well others in the fields of carton making, oil burners, and steam traps.
[1] In the 1910s Phelan invented and developed various forms of the mercury switch, on which he obtained 52 patents.
In 1920 he formed the Absolute Con-Tac-Tor Corporation to manufacture and market contactor mercury switches.
[5] Phelan also invented the "Zest-O-Mat" frozen custard freezer and founded Zesto Products Dairy, which grew into a chain of more than 50 drive-in restaurants.
In the early 1950s, approximately the same time as Colonel Harlan Sanders began franchising his KFC restaurant chain, Phelan developed the broasting method of cooking chicken, for which he invented a modified commercial-grade pressure cooker.