Traction motor
On an electric train, the train driver originally had to control the speed by changing the resistance manually. By 1914, automatic acceleration was being used. This was achieved by an accelerating relay in the motor circuit. This was often called a notching relay. The relay would watch the fall of current and control the resistance. All the driver had to do was select low, medium or full speed. These speeds are called shunt, series and parallel from the way the motors were wired.