This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
99. In apartment houses the mechanical door opener is frequently replaced to advantage by one operated electrically. A view of one of these is given in Fig. 79, which shows the position of the different parts when the door is open. The latch a slides horizontally, being guided at the back by the rod r, and is moved by the lever l, which is pivoted at its center o. A cam c is connected to the pin p, which has, at its lower end, a crank b, held in the position shown by means of a powerful spiral spring s; when the door is closed, the cam is forced around against this spring, thereby turning the crank b, so that the coil spring d is extended. If the latch lever l were free to move, it would at once fly back on account of this tension, carrying the latch with it; but the lower end of the lever is held in a notch in the arm e, the far end of which is supported by the armature f, working on a pivot at the lower end, and placed in front of the poles of a double electromagnet m, m. The wires from the battery and push button are connected to the terminal g and the wire w. On closing the door, the cam is pressed inwards by a projection on the door frame, and, in turning, it moves the crank around and extends the spring, as already noted. On then closing the circuit through the electromagnet m, n, the armature f is attracted; the arm e, being no longer supported, is forced down by the pressure of the end of the lever l in the notch, and the lever turns around the pivot o, thereby drawing back the latch a and permitting the door to be opened. As soon as this is done (the pressure being removed from the cam), the spiral spring revolves the crank, and, compressing the spring d, pushes the latch lever l back to its original position; the arm e also returns to a horizontal position, by spring pressure, as soon as the lever enters the notch. The armature f flies back from the magnet face as soon as the current is cut off, so that the apparatus is ready for operation again after the door has been opened and closed. Since, in closing the door, the latch will not move back, an extra spring latch is provided on the door frame; this is also convenient when opening the door by hand from the inside.