Electrification was called "the most important engineering achievement of the 20th century" by the National Academy of Engineering. Electric lighting in factories greatly improved working conditions, eliminating the heat and pollution caused by gas lighting, and reducing the fire hazard to the extent that the cost of electricity for lighting was often offset by the reduction in fire insurance premiums. Frank J. Sprague developed the first successful DC motor in 1886. By 1889 110 electric street railways were either using his equipment or in planning. The electric street railway became a major infrastructure before 1920. The AC motor (Induction motor) was developed in the 1890s and soon began to be used in the electrification of industry. Household electrification did not become common until the 1920s, and then only in cities. Fluorescent lighting was commercially introduced at the 1939 World's Fair.