Ignition distributor for internal combustion engine

In an ignition distributor for an internal combustion engine comprising:

• a distributor cap;
• an ignition distributor rotor rotatable about its axis within said distributor cap;
• said distributor cap having a center electrode extending along the rotor axis of rotation and a plurality of cap electrodes circumferentially disposed about the rotor axis of rotation;
• said ignition distributor rotor comprising a rotor main body of an electrically insulating material rotatable about the rotor axis of rotation and a rotor electrode supported by said rotor main body;
• said rotor electrode including a tip plate formed with a discharge face which, when said ignition distributor rotor is rotated with said rotor main body, traces a circular path inwardly from the circumferentially disposed distributor cap electrodes by a predetermined distributor gap, said rotor electrode also including a center plate in contact with said center electrode, said tip plate protruding from said center plate, the improvement wherein said tip plate is made of semiconductive alumina ceramics comprising a resistance element of alumina and a semiconductive element of titania (TiO.sub.2), said semiconductive alumina ceramics containing from 10% to 40% by weight titania, and said tip plate is a rectangular elongate plate which protrudes from said center plate along the rotor axis of rotation by a distance falling within a range from 3 mm to 15 mm.