How does the Telegraph Fire Alarm System Work?

     The fire boxes operate using telegraph lines. Rather than having short beeps and long beeps in a series to type out letter (as in Morse Code), a fire box "taps" out the number on the fire box, corresponding to a code wheel within the fire box.  What happens is this:  The entire wired fire alarm system circuit is normally a closed circuit, meaning that electricity flows through the system without any interruptions.  When the box is activated, it causes a series of "opens" or taps in the pattern of the box number.

     A municipal telegraph fire alarm box, such as the style discussed on this site, has a "cottage style" appearance.  That is, it looks somewhat like a cottage house.  Inside the fire alarm box is a spring-wound clockwork mechanism which transmits four "rounds" of a different identifying code for each location.  This code is cut into a removable code wheel, which is spun by the mechanism.  On the mechanism is a striker which rides along the code wheel.  The code wheel has a series of notches which correspond to the alarm box number.  When the fire alarm box is activated, the code wheel starts to spin.  The striker rides each notch and falls in a valley between each notch. On the back of the striker is a chock which pushes a contact against another contact, completing the circuit.  When the striker falls in a gap, the chock can no longer push the one contact.  The contact then separates from the other contact and breaks the flow of electricity causing an open circuit, which in turn creates a single "tap."   See picture below.

      Using the above picture as an example, let's say that fire alarm box 5531 is activated.  The code wheel in the box will have five notches, a slightly longer valley after the fifth notch, five notches with a slightly longer valley, three notches with a slightly longer valley ,then one notch, with a long valley.  The slightly longer valley shows the separation between numbers.  The long valley indicates the end of the round, which completes the box number.  So, when box 5531 is "pulled" there will be 5 taps, a pause, 5 taps, a pause, 3 taps, a pause, then 1 tap, then a long pause.   The process will then repeat itself 3 more times before stopping. This signal is transmitted through the circuit to peripherals in the system, such as the form 4, digitizer and the bells and punch registers in the fire stations.   This punch register has a cash register type paper tape which has holes punched through it for each tap transmitted.  Using box 5531 as an example, the box will tap out the code on the tape.  If box 5531 was pulled, it would look like this on the tape:


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     At each firehouse and the dispatch center is a listing of boxes.  Each box has its own unique location.


Peripherals of the System

Aside from the fire boxes themselves, there are other parts of the system that are vital to the operation of the telegraph system.  Among them are the Form 4 house gongs, digitize receiver and air horns.  Now, Form Four may actually be the name of the model of equipment.  It may be called by other names.  For our discussion, we'll stay with the name Form Four.  The Form Four is the brain center of the system.  It supplies the power to the circuits and also allows for breaks up all the loops into circuits.  In other words, all the fire alarm boxes are not one big loop or one big circuit.  It is divided up into multiple loops or circuits.  The reasons for this is to not overload a circuit with too much equipment and by having multiple loops, if a break should occur, you don't wipe out the entire system.  From here, the fire alarm technician can take circuit readings and check the health of the system by checking for ground faults.

To protect the systems, a protective entrance cabinet with lightning arrestors and a series of fuses are installed at firehouses and the fire alarm center.  Should a surge in power occur or a lightning strike some part of the system, the fuse near the strike would blow out, preventing the surge from damaging or potentially blowing out the Form Four.

Digitize is a company that manufactures alarm monitoring equipment.  It is a popular brand of equipment for telegraph fire alarms.  Older digitize units had to have fire alarm box information burned into them at the factory.  Newer units are programmable in the field, allowing for growth of systems.  These units are found at the fire department dispatch center and may be found at outlying fire stations.  Fire alarm box information is programmed into the unit and interprets the incoming taps from the fire box.  When the first round is completed, whatever information is programmed in the unit for that box number will be displayed on the unit's screen.  This information will tell the fire dispatcher the location of the alarm and will dispatch the fire department accordingly.

Found primarily in towns with volunteer fire departments, are fire horns.  Two common type of horns are Diaphones, made by the Gamewell Company, and diaphragms, made by the Leslie Company and others.  The difference between can be found on a great site .  Rather than rehash everything about it, I would recommend that you visit that site for great information regarding the horns.


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