CHAPTER II

_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.

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.

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.

=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.

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.

=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.

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.

=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.

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.

=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.