ELECTRIC BELLS AND ALARMS
The most familiar of all electrically worked appliances is probably
the electric bell, which in some form or other is in use practically
all over the world. Electric bells are operated by means of a current
of electricity sent through the coils of an electro-magnet, and one of
the very simplest forms is that known as the single-stroke bell. In
this bell an armature or piece of soft iron is placed across, but at a
little distance from, the poles of an electro-magnet, and to this piece
of iron is fixed a lever terminating in a sort of knob which lies close
to a bell or gong. When a current is sent round the electro-magnet the
armature is attracted, so that the lever moves forward and strikes a
sharp blow upon the gong. Before the gong can be sounded a second time
the current must be interrupted in order to make the magnet release the
armature, so that the lever may fall back to its original position.
Thus the bell gives only one ring each time the circuit is closed.
Bells of this kind may be used for signalling in exactly the same way
as the Morse sounder, and sometimes they are made with two gongs of
different tones, which are arranged so as to be sounded alternately.
[Illustration: FIG. 24.--Mechanism of Electric Bell.]
[Illustration: FIG. 25.--Diagram showing principle of Bell-push.]
For most purposes however another form called the trembler bell is much
more convenient. Fig. 24 is a rough diagram of the usual arrangement
of the essential parts of a trembler bell. When the circuit is closed
by pressing the bell-push, a current flows from the battery to the
electro-magnet EE, by way of terminal T. The electro-magnet then
attracts the soft iron armature A, thus causing the hammer H to strike
the gong. But immediately the armature is pulled away from the terminal
T¹ the circuit is broken and the magnet loses its attraction for the
armature, which is moved back again into contact with T¹ by the spring
S. The circuit is thus again closed, the armature is again attracted,
and the hammer strikes the gong a second time. This process goes on
over and over again at a great speed as long as the bell-push is kept
pressed down, resulting in an extremely rapid succession of strokes
upon the gong. It will be noticed that the working of this bell is very
similar to that of the automatic contact-breaker used for induction
coils (Chapter VIII.). For household purposes this form of bell has
completely driven out the once popular wire-pulled bell. Bell-pushes
are made in a number of shapes and forms, and Fig. 25 will make clear
the working principle of the familiar form which greets us from almost
every doorway with the invitation, “Press.” In private offices and
elsewhere the rather aggressive sound of an ordinary trembler bell is
apt to become a nuisance, and in such cases a modified form which gives
a quiet buzzing sound is often employed.
It is frequently necessary to have an electric bell which, when once
started, will continue ringing until it is stopped. Such bells are
used for fire and burglar alarms and for many other similar purposes,
and they are called continuous-ringing bells as distinguished from
the ordinary trembler bells. In one common form of continuous-ringing
bell two separate batteries are used, one to start the bell and the
other to keep it ringing. When a momentary current from the first
battery is sent over the bell lines the armature is attracted by the
electro-magnet, and its movement allows a lever to fall upon a metal
contact piece. This closes the circuit of the second battery, which
keeps the bell ringing until the lever is replaced by pulling a cord
or pressing a knob. Continuous-ringing bells are often fitted to alarm
clocks. The alarm is set in the usual way, and at the appointed hour
the bell begins to ring, and goes on ringing until its owner, able to
stand the noise no longer, gets out of bed to stop it.
There is another form of electric bell which has been devised to do
away with the annoyance of bells suddenly ceasing to work on account
of the failure of the battery. In this form the battery is entirely
dispensed with, and the current for ringing the bell is taken from a
very small dynamo fitted with a permanent steel horse-shoe magnet. The
armature is connected to a little handle, and current is generated by
twisting the handle rapidly to and fro between the thumb and finger. A
special form of bell is required for this arrangement, which is not in
general use.
In the days of wire-pulled bells it was necessary to have quite a
battery of bells of different tones for different rooms, but a single
electric bell can be rung from bell-pushes placed in any part of
a house or hotel. An indicator is used to show which push has been
pressed, and, this like the bell itself, depends upon the attraction
of an armature by an electro-magnet. Before reaching the bell the wire
from each bell-push passes round a separate small electro-magnet, which
is thus magnetized by the current at the same time that the bell is
rung. In the simplest form of indicator the attraction of the magnet
causes a little flag to swing backwards and forwards over its number.
Another form is the drop indicator, in which the movement of the
armature when attracted by the magnet allows a little flag to drop,
thus exposing the number of the room from which the bell was rung. The
dropped flag has to be replaced, either by means of a knob fixed to a
rod which pushes the flag up again, or by pressing a push which sends
the current through another little electro-magnet so arranged as to
re-set the flag.
The electric current is used to operate an almost endless variety
of automatic alarms for special purposes. Houses may be thoroughly
protected from undesired nocturnal visitors by means of a carefully
arranged system of burglar alarms. Doors and windows are fitted with
spring contacts so that the slightest opening of them closes a battery
circuit and causes an alarm to sound, and even if the burglar succeeds
in getting inside without moving a door or window, say by cutting out
a pane of glass, his troubles are not by any means at an end. Other
contacts are concealed under the doormats, and under the carpets in
passages and stairways, so that the burglar is practically certain to
tread on one or other of them and so rouse the house. A window may be
further guarded by a blind contact. The blind is left down, and is
secured at the bottom to a hook, and the slightest pressure upon it,
such as would be given by a burglar trying to get through the window,
sets off the alarm. Safes also may be protected in similar ways, and
a camera and flashlight apparatus may be provided, so that when the
burglar closes the circuit by tampering with the safe he takes his own
photograph.
The modern professional burglar is a bit of a scientist in his way, and
he is wily enough to find and cut the wires leading to the contacts, so
that he can open a door or window at his leisure without setting off
the alarm. In order to circumvent this little game, burglar alarms are
often arranged on the closed-circuit principle, so that the alarm is
sounded by the breaking of the circuit. A burglar who deftly cut the
wires of an alarm worked on this principle would not be particularly
pleased with the results of his handiwork. The bells of burglar alarms
may be arranged to ring in a bedroom or in the street, and in the
United States, where burglar and in fact all electric alarms are in
more general use than in England, large houses are sometimes connected
to a police station, so that the alarm is given there by bell or
otherwise.
[Illustration: PLATE X.
_By permission of_ _Vickers Limited._
WHERE ELECTRICAL MACHINERY IS MADE.]
When an outbreak of fire is discovered it is of the utmost importance
that the nearest fire-station should be notified instantly, for
fire spreads with such rapidity that a delay of even a few minutes
in getting the fire-engines to the spot may result in the total
destruction of a building which otherwise might have been saved. In
almost all large towns some system of public fire alarms is now in use.
The signal boxes are placed in conspicuous positions in the streets,
and sometimes also in very large buildings. The alarm is generally
given by the starting of a clockwork mechanism which automatically
makes and breaks a circuit a certain number of times. When this occurs
an alarm bell rings at the fire-station, and the number of strokes on
the bell, which depends upon the number of times the alarm mechanism
makes and breaks the circuit, tells the attendant from which box
the alarm has been given. One well-known form of call box has a glass
front, and the breaking of the glass automatically closes the circuit.
In other forms turning a handle or pulling a knob serves the same
purpose.
It is often required to maintain a room at one particular temperature,
and electricity may be employed to give an alarm whenever the
temperature rises above or falls below a certain point. One arrangement
for this purpose consists of an ordinary thermometer having the top
of the mercury tube fitted with an air-tight stopper, through which
a wire is passed down into the tube as far as the mark indicating
the temperature at which the alarm is desired to sound. Another wire
is connected with the mercury in the bulb, and the free ends of both
wires are taken to a suitable battery, a continuous-ringing bell
being inserted in the circuit at some convenient point. If a rise in
temperature takes place the mercury expands and moves up the tube, and
at the critical temperature it touches the wire, thus completing the
circuit and sounding the alarm. This arrangement only announces a rise
in temperature, but by making the thermometer tube in the shape of a
letter U an alarm may be given also when the temperature falls below
a certain degree. A device known as a “thermostat” is also used for
the same purpose. This consists of two thin strips of unlike metals,
such as brass and steel, riveted together and suspended between two
contact pieces. The two metals expand and contract at different rates,
so that an increase in temperature makes the compound strip bend in one
direction, and a decrease in temperature makes it bend in the opposite
direction. When the temperature rises or falls beyond a certain limit
the strip bends so far as to touch one or other of the contact pieces,
and the alarm is then given. Either of the preceding arrangements can
be used also as an automatic fire alarm, or if desired matters may
be arranged so that the closing of the circuit, instead of ringing
a bell, turns on or off a lamp, or adjusts a stove, and in this way
automatically keeps the room at a constant temperature.
Electric alarms operated by ball floats are used to some extent for
announcing the rise or fall beyond a pre-arranged limit of water or
other liquids, and there is a very ingenious electrical device by which
the level of the water in a tank or reservoir can be ascertained at any
time by indicators placed in convenient positions any distance away.
In factories and other large buildings a watchman is frequently
employed to make a certain number of rounds every night. Being human,
a night-watchman would much rather sit and snooze over his fire than
tramp round a dark and silent factory on a cold winter night; and
in order to make sure that he pays regular visits to every point
electricity is called in to keep an eye on him. A good eight-day
clock is fitted with a second dial which is rotated by the clockwork
mechanism, and a sheet of paper, which can be renewed when required, is
placed over this dial. On the paper are marked divisions representing
hours and minutes, and other divisions representing the various
places the watchman is required to visit. A press-button is fixed at
each point to be visited, and connected by wires with the clock and
with a battery. As the watchman reaches each point on his rounds he
presses the button, which is usually locked up so that no one else
can interfere with it, and the current passes round an electro-magnet
inside the clock case. The magnet then attracts an armature which
operates a sort of fine-pointed hammer, and a perforation is made in
the paper, thus recording the exact time at which the watchman visited
that particular place.
The current for ordinary electric bells is generally supplied by
Leclanché cells, which require little attention, and keep in good
working order for a very long time. As we saw in Chapter IV., these
bells soon polarize if used continuously, but as in bell work they
are required to give current for short periods only, with fairly long
intervals of rest, no trouble is caused on this account. These cells
cannot be used for burglar or other alarms worked on the closed-circuit
principle, and in such cases some form of Daniell cell is usually
employed.