Donald H Sullivan 
The Magical Earth 

Jason Devlin, scientist, inventor, and karate master, invents a teleporter, but it malfunctions and sends him and his nephew/assistant, Stu, into an alternate world. The world is several hundred years behind Earth in technology, but magic is commonplace here. He meets a beautiful sorceress who believes him to be sent to save her people from an impending invasion by a barbaric horde led by an evil sorcerer. Jason and Stu must use their wits and scientific knowledge to survive in a world of magic.