The frequent mishaps that resulted because of the unsecured doors – people falling into the shaft or being struck and killed by an arriving elevator – made the coordination of the shaft doors with the elevator cab an urgent necessity, so that they could open only when the cab had reached the same height as the doors and would only allow the cab to resume its travel once they had closed. This dilemma was not solved until the late 1890’s with the advent of electric controls, for electric contacts established a reliable link between the door’s locking device and the position of the cab. With the almost universal adoption of electrically powered elevators by the early twentieth century, all traces of this kind of accident disappeared. The threat to users of an elevator shaft becoming a deadly abyss in the middle of a building in the absence of a door-locking device was eliminated for good by the simple binary nature of an electric circuit: closed or open.