Fire Alarms.

Thermostats, heat alarms and fire alarms are all practically the same, the term thermostat being applied principally to the apparatus which closes the electrical circuit.

Thermostats act on the principle that heat causes expansion whether of substances, liquids, or gases.

The degree in which different substances expand varies for the same increase in temperature. This fact is used in a common form of thermostat shown in Fig.38. A strip of wood or hard rubber R has a strip of thin sheet metal S riveted to it. This compound strip is held at one end by a lug L screwed fast to a baseboard. Upon an increase of temperature the hard rubber expands more than the metal strip and the compound strip bends towards the adjustable contact screw A. Upon touching the latter, the circuit through the bell B, battery C and the metal strip S is completed, and the bell rings. A contact screw can be arranged at the other side of SR, which will give warning of a decrease in temperature, as the rubber contracts more than the metal strip.

In some thermostats of this character two metals having different coefficients of expansion, such as steel and brass, are used instead of metal and hard rubber.

Thermostats of this nature are much used in incubators, and they can readily be combined with electric apparatus to open or close hot-air valves, dampers, etc., and thus regulate the supply of hot air, hot water, or gas.

A thermostat much used in fire alarm work has a thin metal chamber which is air tight. An increase of temperature causes the air to expand, which swells out the walls of the chamber and closes an electric circuit.

The mercurial thermostat shown in Fig.39 has a glass tube T and bulb containing mercury. Into each end is sealed a platinum wire PP. Upon the temperature rising to a predetermined degree, the expanded mercury completes the circuit between PP and the battery C and bell B are put in operation.

Fig.40 is the open circuit system most used by the fire alarm companies, only one circuit of six thermostats being illustrated.

It will be seen that if any thermostat closes the circuit between the outer and inner wires of the ring AB, current will flow through the corresponding drop of the annunciator and will attract the armature A of the relay. This will cause the bell to ring. As the relay is connected to the annunciator as before shown for the annunciator bell, it offers a common path for any drop to the battery. Thus the bell will ring for any circuit, but the individual drop only will fall. In a simpler circuit the relay may be dispensed with and a vibrating bell only used.

Thermostats may be operated on open or closed circuits, that is, they may give the alarm by closing a circuit and ringing a bell, or by opening one and releasing a contact spring as in the burglar alarm system to be described later.

Water Level Alarms.

Where it is desired to signal the rising or falling of water in a tank above or below a given point, a water level indicator as in Fig.41 may be used.

A hollow ball H is mounted on the end of a rod which slides vertically in guides, not shown. Adjustable stops SS press against a spring arm R, pressing it up or down, according as the water level is rising or falling. If rising, R makes contact with the adjustable screw A, if falling, with D, in both cases completing the electrical circuit of the battery C and bell B.

Another and simpler form is shown in Fig.42, where the ball H is mounted on the end of a lever L pivoted at P, its rise or fall completing the circuit of B and C as before. Where it is desired to give a different signal for the rise and the fall of level, two bells B and E (Fig.43) may be used connected as shown. The rising of the ball will ring bell B, and its fall, bell E.

In both forms of indicator, a means must be provided that an undue rise may not bend the lever. This may be accomplished by using contact springs instead of contact screws; it is, however, then harder to adjust the indicator to fine differences of level.

In all cases the contacts must be faced with platinum to prevent corrosion.

Burglar Alarms.

A burglar alarm is a device for indicating the opening of a door or window, by the ringing of a bell or operation of an annunciator. The contact apparatus at the points to be protected may either open an electrical circuit or close one, in the latter case being mere modifications of push buttons. The simplest form is the latter or open-circuit method.

The spring contact to be inserted in the door jamb or window frame is so constructed that while under pressure the contacts are kept apart and the circuit is open. But when the door or window is opened, the pressure is released and a spring forces the contacts together.

Fig.44 is an open-circuit window spring fitted in the window frame so that when the window is closed the spring lug S is pressed inwards, breaking contact with the base B.

If the window is raised, the lug flies to the position shown by the dotted lines, and making contact with B, completes the circuit through bell and battery. These springs are fitted in the side of the window frame in a vertical position and are entirely concealed when the window is shut.

In the closed-circuit system the reverse happens. The pressure of the closed door or window keeps the contacts together and its opening enables them to spring apart.

In Fig.45 is a diagram of a closed-circuit burglar alarm, C a cell of gravity battery, R a relay, F the fixed contact and M the movable contact of the spring, S a stud projecting through the base of the spring and pushed in by the closed door.

When the door is closed, S being pushed in, the circuit of C, R, F and M is closed. The magnets of the relay hold the armature arm A forward against a hard rubber contact. But when S is released, the relay circuit is opened, R loses its power and A flies back, making contact, and throwing in circuit bell B and battery L.

A form of bell and relay combined is shown in Fig.46. Here the armature A is held against the magnets while the circuit through the spring F and battery G is closed. But on opening this circuit the armature flies back and makes contact with an adjustable contact screw S putting in circuit a local battery C. The bell is now practically a vibrating bell; on a closed circuit it rings until the circuit is again closed or the battery runs down.

A different connection of the same scheme is Fig.47, where only one battery is used. This must be a gravity battery or some other closed-circuit battery. The circuit can be easily traced in the figure and needs no special description. Both of the latter schemes are inferior to one using a separate relay. If the circuit at the spring were quickly closed again the bell would either stop ringing, or be so hampered as to ring very weakly.

A relay made as in Fig.48 has no spring support to the armature A, which falls down by gravity. The adjustable contact C is screwed far back, so that the armature must fall a considerable distance away from the electromagnets before it makes contact. This ensures that the armature will not be attracted and the bell stopped from ringing by a re-closing of the circuit at the door or window spring.

A shade spring (Fig.49), is made for either open or closed circuits. In operation, the shade is pulled down and its string or ring hooked on to H. This draws H up a trifle against a spiral spring and its lower end makes contact with an insulated spring S closing the circuit. If the shade is disturbed, the spiral spring on the lower part of H is released and it causes a break of contact with S in the direction of the arrow.

When made for open circuit, S is bent so that while under tension no contact is made, but release of tension causes the contact.

Fig.50 gives the wiring of two windows and a door on the closed-circuit system. It will be seen that the contact springs are all in series, opening a window or the door will thus break the circuit.

When setting the alarm at night by connecting up the batteries, relay and bell, should any one of these springs be open the relay armature will not hold, and the bell rings.

In this figure the relay is replaced by an electromagnet holding up a drop shutter by magnetic attraction. Upon the circuit opening, this shutter falls, exposing a number painted on it. At the same time it hits a spring contact placed below it and closes the bell and local battery circuit.

Door Trip Alarm.

A swinging contact door trip can be attached over a door to ring a bell when the door is opened.

In Fig.51 the door trip is screwed over the door so that the lowest arm A is struck by the door. When the door is opened, in the direction of the arrow, the arm A is thrust forwards, and in its turn moves the contact arm C, completing the bell and battery circuit. But when the door is being closed, A swinging in the reverse direction does not move C and no alarm is given.

Miscellaneous Alarms.

The Applegate electrical matting is composed of wooden slats with springs so arranged that the weight of any person stepping on it will close a circuit and ring a bell.

It is intended to be put under the ordinary door mat or under stair and room carpeting.

The Yale lock switch is a Yale lock and switch combined. Upon any key but the right one being inserted, a circuit is closed and an alarm bell is rung.

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