_Bells and Pushes_
=Electric Bells.= The two main types of house bells are the iron box
and the skeleton.
The iron box has a cast-iron frame, or base, and a cast- or
stamped-iron cover over the mechanism.
The skeleton bell has an iron frame but no cover, and is generally
better finished and more expensive than the iron box bells.
For fire alarm purposes, mechanical bells or gongs are made, in which
a clockwork mechanism causes the hammer to strike the gong upon being
released by electromagnetism.
Marine or waterproof bells have an iron cover fitting tight over a
rubber gasket; they are for marine, or mining, work.
Polarized, or magneto, bells are used in telephone work, and are rarely
operated by a battery, but have a miniature dynamo generator operated
by hand, or power, to supply the actuating current.
Most bells are classed for size by the diameter of the gong, a
four-inch bell being one with a gong four inches in diameter; a
six-inch bell one with a six-inch gong, and so on.
According to the use for which they are intended, bells may
be vibrating, as before described, single-stroke, shunt or
short-circuiting, differential, continuous-ringing, or adapted for
circuits of high voltage.
=The Single-stroke Bell.= The bell before described, and again shown
in Fig. 5, is a vibrating, or trembling, bell. It is often desired to
have the hammer give only one stroke for each pressure of the push, as
in signaling with a code of taps; in this case a single-stroke bell is
used. The circuit from the binding posts is then directly through the
magnet coils without any break at the contact screw, as in Fig. 6.
[Illustration: FIG. 5
FIG. 6
FIG. 7]
In adjusting such a bell to give a clear sound, press the armature up
against the iron magnet cores and then bend back the hammer until it
just clears the gong. The spring of the hammer wire will carry the
hammer sufficiently forward to hit the gong. The tone will be clearer
than if the hammer dampered the gong by pressing against it when the
armature was nearest the core.
By bringing out a third connection, a vibrating bell may be made both
single stroke and vibrating.
=The Shunt Bell.= There is a form of bell, Fig. 7, known as the shunt,
or short circuit bell, which is often used when two or more are to be
connected in series, as will be seen in the description of circuits. In
this bell the circuit through the magnets is not broken at the contact
screw, but the forward movement of the armature short circuits the
coils.
As the short, or shunt, circuit is very much lower in resistance than
the wire on the magnet coils, the main current flows around the latter
and they do not become energized. The sparking at the shunting contact
screw is much less than it would be at the ordinary breaking contact
screw, and the platinum points last longer.
=The Differential Bell.= Sparking at the breaking contacts of an
electric bell is detrimental to the platinum points, and many remedies
have been devised to overcome it.
Sparking is due to the self-induction of one turn of the wire coil
acting on its neighbor, and this property is utilized in the gas
engine, or gas-lighting spark coil, where a fat spark is needed to
ignite gas.
The differential bell has two windings in opposite directions. The
action of one would be to produce an N-pole at one end and an S-pole
at the other. But the second coil produces poles just the opposite, as
the polarity of a magnet depends on the direction in which the current
flows around it.
Where the current flows around the first winding the armature is
attracted and its spring contact meets the contact screw and allows
the current to divide, part flowing through the first coil, the other
flowing in the reverse direction in the opposite way. One coil would
tend to produce an N-pole where the other coil produced an S-pole, and
these opposite poles would so neutralize each other that there would be
no magnetism.
The armature would therefore be pulled back by its spring when both
coils were thrown into circuit. In so doing it would cut out one coil
and the same series of operations would recommence.
As a spark is normally produced where magnetism is _lost_ by a break of
circuit,[B] no spark appears, as magnetism is _produced_ by a break of
circuit in this case.
[B] For a full explanation of self-induction see No. 1 of this
series.
=Continuous-ring Bell.= In some classes of bell work, such as
burglar alarms, it is desired that the bell when once started shall
continue to ring until stopped by the person called. In this case a
continuous-ringing bell is needed, such as in Fig. 8.
[Illustration: FIG. 8]
When the push _P_ is pressed, the current flows in the usual way
through contact screw _L_, armature spring _A_, magnet coils _M M_,
battery _B_, back to _P_, and the bell rings. But on the first forward
movement of the armature it releases the spring contact _S_, which
flies forward and makes contact at _U_. The circuit is now from _B_,
through _M M_, to _A_, thence through _L_ and _S_, to _U_ and back to
_B_.
The bell will continue to ring until the spring contact _S_ is moved
back and caught by the projection on the armature _A_.
A continuous-ring attachment is also made and sold in most electrical
supply stores, which is complete in itself and can be applied to any
bell.
[Illustration: FIG. 9]
=Waterproof Bells.= In Fig. 9 is an example of a waterproof bell where
the mechanism is almost all entirely encased in a waterproof brass case.
The circuit is made and broken inside the case, but the magnet cores
project through it and act on a second armature placed outside. This
second armature carries the hammer which strikes the gong and is
governed in speed by the contact-breaking armature inside.
=Forms of Bell Gongs.= In order to provide a variety of sounds, bells
are provided with gongs of various shapes.
Fig. 10 shows the ordinary form of gong. Fig. 11, a tea gong; Fig. 12,
a cow gong; and Fig. 13, a sleigh bell.
[Illustration: FIG. 10
FIG. 11
FIG. 12
FIG. 13]
A coil of steel wire is also used, as in Fig. 14, which on being struck
by the hammer gives a pleasant but not loud tone.
[Illustration: FIG. 14]
=The Buzzer.= The buzzer is the mechanism of a vibrating bell less the
hammer and gong. As the armature vibrates it makes a buzzing noise
which does not carry as far as the sound from a struck gong. It is used
chiefly for a desk call and in telephone exchange work, or any place
where general attention is not desired to the signal.
=Operating Bells at a Distance.= When it is desired to ring a bell
situated at a considerable distance from the push, the resistance of
the line becomes objectionable.
[Illustration: FIG. 15]
On lines of 500 feet, No. 18 copper wire and upwards, the battery
necessary would be very large, two small batteries and a relay would
prove more satisfactory.
In Fig. 15 the circuit of a simple form of relay is given. An
adjustable contact screw _C_ is placed where an extension _S_ of the
armature _A_ can strike it. This extension is provided with a platinum
contact. The connections are as in the figure.
When the push _P_ is depressed, the current from the main battery _M_
energizes the electromagnet _E_, and the armature _A_ being attracted,
contacts _S_ and _C_ meet. These contacts close the second circuit
containing the bell _B_ and the local battery _L_.
The relay resembles a second push near the bell, but controlled by
current from a distance instead of being depressed by hand. Its
advantage consists in it needing but a very weak current to move the
armature _A_, which is held back by a light spring, or by gravity.
The relay may then be set near the bell and the wires from the push may
be of a very great length. Battery _L_, which actually rings the bell,
will thus only have to work through a few feet of wire.
=Reducing Resistance of a Bell.= Sometimes it is desired to reduce the
resistance the bell coils offer to the current, the bell then working
over a very short line with few cells of battery. Or the bell coils may
have been wound with fine wire for large battery voltage and a long
line.
The bell coils may be put in multiple, the current then dividing and
one-half going through each spool.
Untwist the joint between the spools near the yoke or iron bar to
which the spools are attached. Join one of these ends to the wire at
the armature end of the _other_ spool and the second untwisted end to
the armature end wire of its neighboring spool. Use short pieces of
insulated wire for these extra connections.
The current now instead of having to go through one spool and then the
other, can branch through both at once.
The resistance to the current of one spool is half the resistance of
two, the current through one spool will therefore be twice that through
the two spools as at first connected. And as there are two paths
for it, each one-half the first resistance, the total will be only
one-fourth the resistance of the ordinary series arrangement.
The same size battery will therefore send four times the current
through the spools in multiple than when they are in series.
It is to be noted that the wire on one spool is wound in the reverse
direction to that on the other. The reason will be apparent if the
two spools and yoke are considered as merely one spool bent in a U or
horseshoe form.
If both spools were wound in the same direction they would be in
opposite directions when the U were straightened out, and would cause
like poles at the same ends. These poles would neutralize one another,
so that there would be no magnetic attraction.
This can be readily proved by joining together the two yoke ends and
the two armature ends of the spool wires. Then pass the current through
these two joined connections.
[Illustration: FIG. 16
FIG. 17
FIG. 18
FIG. 19]
=The Push Button.= Push buttons, or pushes, are made in a variety of
forms, with metal, wood, hard rubber, or porcelain bases.
Fig. 16 has a metal base, and is suitable for a front door.
Fig. 17 is a wooden pear push, and is attached at the end of a cord
which has the two conductors braided in it, each, however, having its
own insulation.
Fig. 18 is a plate push for an outside door.
Fig. 19 is either of metal, wood, or porcelain, and is the shape most
commonly used.
A three-point push has three contact springs. One is movable by means
of the button, one is below the movable spring, and the third is above
it.
[Illustration: FIG. 20]
When the push button is not being depressed, the movable spring makes
contact with the upper spring. But when the button is depressed, these
two springs part, and the movable spring makes contact with the lower
one.
This style of push is used for special bell and annunciator work, as
will be described later.
The form of combination floor and table push in Fig. 20 is the most
solidly constructed device of its kind. The lower part is set in a
hole bored in the flooring, the metal flange keeping it in place and
preventing its slipping through.
The floor push attachment works as follows: The central metal rod is
divided into two parts _B D_, by an insulating piece of hard rubber.
When depressed against the action of the spiral spring by the foot, the
upper part _B_ connects together the contact springs _A C_, closing the
circuit of bell and battery. These contact springs are insulated from
each other by a hard rubber block _R_.
From the table push a cord containing two insulated wires leads to the
two parts of the rod at _B_ and _D_. When the push centre is pressed
down, the push springs come together and practically short circuit _B_
and _D_, which completes the circuit of bell and battery. At any time
the centre rod may be removed, leaving a surface almost flush with the
carpet, or floor, over which furniture may be moved without injury to
the mechanism of the push.
For a floor push alone a shorter form of the centre rod is also
sometimes furnished which is not divided by insulation. The spiral
spring keeps it clear of the lower contact _A_ but enables it to always
make connection with the upper contact _B_. Pressing this rod down will
also short circuit the bell and battery so that the signal is given.
A door pull attachment, like Fig. 21, is made so that the ordinary form
of lever pull bell may be changed into an electric bell. Being screwed
up near the door pull, a wire is run from the latter and fastened to
lever _L_. When the pull is drawn out the lever _L_ turns on a pivot
and a projection presses the insulated spring _S_ against the metal
base _B_. The circuit of the bell and battery being thus closed, the
bell rings.
[Illustration: FIG. 21]
=Indicating Push Button.= A push button is made which contains in the
base a small electromagnet in series with the line. An armature on a
spring is fixed near the magnet poles. When the push is depressed,
the current travels through this electromagnet, and as the circuit is
made and broken at the distant bell, it is also interrupted in the
electromagnet. The armature vibrates in unison with the bell and thus
gives an audible indication that the bell is ringing.