CHAPTER VI

Three-wire return call system--Installing elevator annunciators--Burglar alarm annunciators--Clock alarm circuit--Bells for high voltages--Bell-ringing transformers--Combination bell, door opener and telephone circuits--Fire alarm circuit--Interior fire alarm system--Fire alarm system for considerable areas 64

LIST OF ILLUSTRATIONS

FIG. PAGE 1 Electric bell, push, and battery x

2 Leclanche cell 1

3 Dry cell 4

4 Gravity cell 5

5 Vibrating bell 10

6 Single stroke bell 10

7 Shunt or short circuit bell 10

8 Continuous ring bell 13

9 Waterproof bell 14

10 Dome gong 15

11 Tea gong 15

12 Cow gong 15

13 Sleigh bell gong 15

14 Spiral gong 15

15 Relay and circuit 16

16 Door push 19

17 Pear push 19

18 Door push 19

19 Wall push 19

20 Floor push 20

21 Door pull attachment 22

22 Wire joint first operation 25

23 Wire joint second operation 25

24 Wire joint insulating 25

25 Section of house showing wiring 29

26 Bell with ground return 30

27 Pushes in multiple 31

28 Bells in series 31

29 Bells in multiple 31

30 Two bells and two pushes 32

31 Two bells and two pushes 32

32 Two bells, two pushes and one battery 33

33 Double contact push 33

34 Grounded bell 34

35 Tongue test of wiring 38

36 Knife test of wiring 38

37 Knife test of wiring 39

38 Metallic thermostat 40

39 Mercury thermostat 41

40 Mercury thermostat circuit 42

41 Water level alarm 44

42 Lever water level alarm 45

43 High or low water level alarm 45

44 Window spring for burglar alarm 47

45 Burglar alarm--closed circuit 47

46 Special bell connection for burglar alarm 48

47 Special bell connection for burglar alarm 49

48 Burglar alarm and relay 50

49 Window-shade contact spring 51

50 House wired for burglar alarm 52

51 Door trip alarm 53

52 Annunciator drop 55

53 Needle drop 56

54 Needle drop indicating 56

55 Pendulum drop 57

56 Annunciator drop circuit 58

57 Simple annunciator circuit 59

58 Annunciator and fire call circuit 60

59 Single-wire room and fire call 61

60 Three-wire return call circuit 65

61 Elevator bells and annunciator circuit 67

62 Burglar alarm annunciator circuit 69

63 Clock alarm circuit 71

64 Bell-ringing transformer 73

65 Bell-ringing transformer with three secondary voltages 73

66 Western Electric interphone system 75

67 Western Electric interphone system for more extensive service 77

68 Fire alarm circuit 79

69 Interior fire alarm circuit 81

70 Fire alarm circuit for considerable areas 82

INTRODUCTION

An electric bell depends for its action on the fact that a piece of iron wound with insulated wire becomes a magnet and will attract another piece of iron just so long as an electric current is allowed to travel through the wire.

The instant the current ceases, the magnetism also ceases, and the attracted piece of iron (termed the armature) is no longer held in contact.

The general construction of an electric bell is shown in Fig. 1. _M M_ are coils of insulated wire wound on soft iron cores. _A_ is a soft iron armature mounted on a flat spring so that it is normally kept a slight distance away from the soft iron cores. _S_ is a brass screw with a platinum tip touching a platinum disc on a spring attached to the armature.

When the push button _P_ is pressed down, its two brass springs touch each other, the current from the battery cell _B_ then flows through the wire _W_, through the push _P_, through the coils _M M_, along _A_ to the platinum disc, out at _S_, which touches this disc, and back to the battery.

The instant this is done the current causes the iron cores to become magnets, they attract _A_, which then breaks contact at _S_. The spring mounting of _A_ causes it to jump back to its first position, _S_ then touches the platinum disc again, the current flows as before, and the armature is again attracted only to break contact with _S_ and fly back.

This continual making and breaking of the circuit keeps up as long as the push is pressed, a ball mounted on _A_ by means of a rod strikes against the gong _G_ causing a continuous ringing of the bell. The wires leading between the bell, battery cell and push must all be insulated, that is, covered with cotton, rubber, etc., which prevents the leakage of current should two wires cross each other. Copper wire is mostly used for circuits indoors, the details of the kind and size of wire will be given later on.

The main parts of an electric bell circuit are then--the battery to supply the electric current; the circuit, or wires, to carry this current; a push, or circuit breaker, to control the current flow; and a bell to utilize the current.