The advent of the police car, two-way radio, and telephone in the early 20th century transformed policing into a reactive strategy that focused on responding to calls for service. In the 1920s, led by Berkeley, California police chief, August Vollmer, police began to professionalize, adopt new technologies, and place emphasis on training. With this transformation, police command and control became more centralized. O.W. Wilson, a student of Vollmer, helped reduce corruption and introduce professionalism in Wichita, Kansas, and later in the Chicago Police Department. Strategies employed by O.W. Wilson included rotating officers from community to community to reduce their vulnerability to corruption, establishing of a non-partisan police board to help govern the police force, a strict merit system for promotions within the department, and an aggressive, recruiting drive with higher police salaries to attract professionally qualified officers.