_Alarms_
=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.
[Illustration: FIG. 38]
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 _S R_, which will
give warning of a decrease in temperature, as the rubber contracts more
than the metal strip.
[Illustration: FIG. 39]
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.
[Illustration: FIG. 40]
The mercurial thermostat shown in Fig. 39 has a glass tube _T_ and bulb
containing mercury. Into each end is sealed a platinum wire _P P_. Upon
the temperature rising to a predetermined degree, the expanded mercury
completes the circuit between _P P_ 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 _A B_, 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 _S S_ 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_.
[Illustration: FIG. 41]
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_.
[Illustration: FIG. 42
FIG. 43]
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.
[Illustration: FIG. 44]
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.
[Illustration: FIG. 45]
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_.
[Illustration: FIG. 46]
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.
[Illustration: FIG. 47]
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.
[Illustration: FIG. 48]
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.
[Illustration: FIG. 49]
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.
[Illustration: FIG. 50]
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.
[Illustration: FIG. 51]
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.