A door closer is defined as any mechanical device that closes a door in a controlled manner, preventing it from slamming, in general after someone opens it, or after it was automatically opened. The force used to open the door is stored in some type of spring and when released this energy is then used to return the door to a closed position. Door closers can be linked to a building's fire alarm system. Where doors need to be held open for the majority of the time they are held back with an electromagnetic device. When the fire alarm is triggered it cuts power to the electromagnetic hold-open device allowing the doors to close. These hold-open devices can be separate from the door closer or part of its design. Early history Gravity alone may be considered to be the first "door closer". Before any mechanical devices were attached to doors to close them, the way in which the door was hung automatically closed the door once opened. This technique to hang doors may have originated with gates because of the need for them to remain closed once traffic has passed through. The method involves an intentional misalignment of the hooks or pintles that traditional strap hinges rest on, the bottom pintle protruding from the door post further than the top one. This configuration ensures the weight of the door and gravity will always close any gate or door opened. It is best used on doors only opening one way, many old church doors for instance can still be seen to be hung in this manner. One of the first references concerning a device to close a door can be found in the writings of Hero of Alexandria who describes his "automata" which controlled the doors of temples, both opening and closing them automatically. Weights and levers have also been used to close doors, Another device for smaller domestic doors used a loop of rope or skein fixed to the door frame, that was twisted, with a piece of wood placed in between the twists to push the door. The opening of the door twists the skein further, when the door is released the rope's torsional force pushes the arm back against the door, thereby closing it. In more modern times the clock manufacturers Thwaites and Reed in 1850 claimed to be the original inventors of the spiral door spring. The