The Purple Wizard — John Russell Fearn

To satisfy the belief of Professor Addison that Entropy can be controlled, to produce a backward and forward movement through the condition known as Time, Kitty Addison and her fiance, Arthur Davis, consent to being projected to the year corresponding to Arthur Addison's telephone number - which happens to be 828. Accordingly, the two young people find themselves in the world of the past, back in the darkest medieval times.