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.
93. When a number of push buttons is used in a call-bell system, as in the different rooms of a house or hotel, it becomes necessary to provide some means of determining 6C which of these buttons has been pressed when the bell rings, in order that the call may be answered. The instrument employed is called an annunciator; the ordinary house style is shown in Fig. 72. On the face are rows of small windows, before one of which an indicator appears when the bell rings, showing from which room the signal has been sent. A handle h at the side is intended to be used to restore the indicators to their normal position when the call is answered. A view of the indicator itself is given in Fig. 73. A hinged arm a carries a card, bearing the name or number of the room to which the drop is connected, and is held up, in the position shown, by a counterbalanced trip t in front of an electromagnet m. As soon as the current passes through the electromagnet, the trip is attracted and the indicator falls, being then visible from the outside through one of the openings in the front.
04. The needle annunciator, Fig. 74, is a style much used in hotels and for elevators. The current, on passing through the electromagnet of an indicator, attracts a pivoted, iron armature carrying a pointer P on the outside dial, causing it to set in an oblique position, in which it is held by a catch until released by pressing the knob k below the case.