List of Commonly Used Electrical Appliances | Electrical Engineering

Some of the commonly used electrical appliances are as follows: 1. Electric Bell 2. Single-Stroke Bell 3. Electric Bell with Indicators 4. Electric Buzzer 5. Electric Heater – Hot Plate 6. Electric Iron.

Electrical Appliance # 1. Electric Bell:

In all such places as a house, a hotel, a hospital etc., the electrical appliance which is mostly in use—apart from lamps and fans—is the Electrical Calling Bell.

In an electric bell two magnetic coils are wound generally on a wooden bobbin. At the centre of each coil there is a soft iron magnetic core. This increases the intensity of the magnetic field produced by the coils. The iron used for the core should be such as would leave practically no residual magnetism in the core whenever the current flowing through the coil is stopped.

One end of each core is fixed to an iron frame and the other end is left open (Fig. 197). As soon as the current flows through the magnetic coils, the two cores become electro-magnets. But the two coils are so connected in Series that they carry current in opposite directions simultaneously.

As a result, whenever the open end of one electro-magnet develops North Pole, that of the other electro-magnet develops South Pole. Now, if a piece of iron is kept near the open ends, the two electro-magnets together will attract the piece when current flows through the coils, and this attracting power of electro-magnets will no longer exist as soon as the flow of current through the coils is stopped.

Fig. 197 shows the connections of an ordinary electric bell. In this diagram a spring S, an armature A and a contact C have been shown. When no current flows through the magnetic coils, the spring keeps the armature pressed against the contact. The tip of the contact which remains in touch with the armature is made of tungsten or platinum. At the time of separation of armature from the contact surface, spark appears between them. If the contact tip is made of tungsten or platinum, it will not be burnt out or oxidized due to sparking.

In the circuit of an electric bell, usually a push button is provided. The electric circuit is closed as soon as the push-button is pressed. Electric current then enters the bell through the live wire and comes to the spring S after flowing through two magnetising coils—through one coil in clockwise direction and through the other in anti-clockwise direction.

From the spring the current goes to the armature A and then to the contact C, and finally flows back to the neutral wire. As soon as the current flows through the magnetising coils, the two iron cores placed inside the wooden bobbin become electro-magnets and attract the armature towards their open ends.

At this time a hammer attached to the armature strikes a bell as a result of which the bell rings. But on the other hand, the armature being detached from the contact surface, the electric circuit opens. Hence, the current cannot flow through the magnetic coils. The attracting power of electro-magnets then disappears and the armature springs back to the contact surface again. Immediately the electric circuit is restored and the electro-magnets attract the armature again.

In this way, as long as the push-button is kept pressed, the armature continues to ply between open ends of the electro-magnets and the contact tip at a fast speed. The hammer attached to the armature continues to strike the bell again and again at a very quick rate and the bell also continues to ring incessantly.

Precautionary Arrangements for Electric Bell:

In our country electricity is supplied usually at 230-volt a.c. or 225-volt d.c. In most cases the electric bells are made suitable for this pressure, although bells rated 110 volts or even less are available in plenty from the market. Even if the bell is suitable for 230-volt pressure, it should not be connected directly across the supply line.

An electric lamp rated for supply pressure should be connected in series with the bell, and the series combination is then connected across the supply. This arrangement helps to maintain the bell in good condition even after long period of use.

While ringing an electric bell, many people repeats pressing the push-button and releasing it instead of keeping it pressed for some time. This gives rise to a ‘surge voltage’ in the circuit. The surge voltage being much higher than the line pressure, the insulation of the magnetic coils of the bell deteriorates rather quickly.

But in case there is a lamp in series with the bell, some pressure will be dropped across the lamp due to its resistance. As a result, the entire surge voltage cannot work across the magnetic coils. Under this circumstance the pressure across the magnetic coils is somewhat less than even 230 volts. Hence, the coil insulation cannot be deteriorated so quickly.

Electrical Appliance # 2. Single-Stroke Bell:

The type of bell, continues ringing as and when the push-button is kept pressed. Another type of electric bell is also in use. In this type the push-button may be kept pressed, but the hammer will strike the bell only once and only one sound will be heard. The connection diagram of such a bell is shown in Fig. 198. It is called Single-Stroke Bell.

All its construction and internal details are much the same as the ordinary electric bell, the difference lies only in the connection of the magnetic coils with the supply line. Here the series combination of the two magnetic coils is directly connected across the supply line. The electric circuit is not completed through the contact C as is the case with an ordinary bell. Hence, as the push-button is pressed, electric circuit is closed and the current- starts flowing through the magnetic coils.

The iron-cores become electro-magnets and attract the armature towards their open ends. At this time the hammer attached to the armature strikes the bell and only a single sound is emitted. But in this case, in spite of the armature moving towards the electro-magnets, the flow of current through the magnetic coils is not stopped and the electro-magnets do not lose their attracting power.

As a result, the armature remains attached to the open ends of the electro-magnets so long the push-button is kept pressed. The electric circuit is opened and the flow of current is stopped when finger is removed from the push-button; only then the armature can spring back to the contact tip. Thus, the electric bell rings once every time-the push-button is freshly pressed.

Electrical Appliance # 3. Electric Bell with Indicators:

In places like office, hotel, hospital etc. often arrangements are provided so that a single electric bell may be rung from two or more locations or rooms. In such cases it is necessary to use &n indicator with the bell so that one can easily understand which location or room is ringing the bell.

Each indicator is made of an electro-magnet. The exciting coil of this magnet is connected in series with the electric bell. When a particular push-button is pressed, current starts flowing through the corresponding magnetising coil as well as through the electric bell which starts ringing.

At the same time the relevant electro-magnet of the indicator gets-excitation and attracts a coloured cross or some other form of indicating element to a suitable position. This helps the indicating element-to attract the eye of a person standing near the bell. As a result, the person can easily understand wherefrom the bell is rung.

As many indicators are used with an electric bell as the number of places or rooms from where it is necessary to ring the bell when required. By the side of the bell all the indicators are set on a board. Fig. 199 shows the connection of an electric bell with a board containing four indicators.

Any single board of indicators is said to have as many ways as the number of indicators fixed on it, e.g. if there are two indicators, the board is said to be a two-way board, with three indicators, it is a three-way board, and so on. It is necessary to have a separate push-switch for each indicator. This push-switch should be handy in the room or at a place from where the bell is to be rung along with the corresponding indication.

On pressing a particular push-switch, the corresponding indicator circuit with the bell is completed. Current then flows through the circuit and thus the bell rings. At the same time the electro-magnet of the indicator gets excited and attracts the indicating element.

Even when the push is released, the indicating element remains in the same position where it has been attracted by the electro-magnet. That is why there is a separate switch attached to every indicator board so as to take back the indicating element to its previous position. This switch is called ‘Re-set’. On pushing the re-set the indicating element is restored to its previous position.

The indicator circuits are connected in parallel to one another so that each circuit is independent to others and complete in itself. It will be clear from a minute observation of Fig. 199. In order to reduce the cost of wiring, often an ordinary electric lamp is used in each push-switch circuit in place of an indicator board for the same purpose of using an electric bell.

On pressing a push-button this lamp glows along with the ringing bell, but it is put out as soon as the push is released. Each lamp carries its own distinct mark on its body. When the lamp glows, this mark helps a person to understand from where the push is being operated.

Electrical Appliance # 4. Electric Buzzer:

The method of construction of an electric buzzer is very much the same as that of an ordinary electric bell. But there is no bell or a hammer attached to the armature of a buzzer like an electric bell.

When the push-button is pressed, the electric circuit is completed and the current flows through the magnetic coils. The electro-magnets, being excited, then attract the armature towards their open ends. This opens out the electric circuit, flow of current is stopped and the magnets lose their attracting property.

As a result, the armature springs back to the contact surface. In this way the armature vibrates very fast so long the push is kept pressed, and during this vibration the armature strikes a piece of non-resonant metal again and again. Owing to this strike the metal emits a buzzing sound.

Electrical Appliance # 5. Electric Heater: Hot Plate:

In Electrical Engineering copper and aluminium are mostly used as conducting metals. But the specific resistance or resistivity of these two metals is very low. Hence, in situations where it is necessary to generate heat, these two metals are of no use. Better result is obtained with conductor having higher specific resistance of its metal. The reason behind it is that with wires of much less length, considerably more amount of heat can be generated.

Besides, the other necessary properties of this metal are:

(i) The melting point of the metal, i.e. the temperature at which it melts must be rather high, otherwise it may melt at the time of producing high heat;

(ii) It must not be oxidized at higher temperature;

(iii) The resistance-temperature co-efficient of the metal must be very low, so that the resistance of the conductor remains more or less the same at different temperatures.

All the above properties are not available from a single metal. That is why it becomes necessary to use a suitable alloy. The alloy which is an amalgam of 80% nickel and 20% chromium exhibits the above properties to the largest extent. But the manufacture of this alloy is very expensive. The economical mix of an alloy suitable for making wires of heater coils contains 65% nickel, 15% chromium and 20% iron. With the reduction in cost owing to use of iron, the melting point of the alloy also shows a downward trend.

A variety of alloys are available in the market for making heater coils. Each of these are specified by a brief name, e.g. nichrome, calido, silichrome etc. The coil made by winding a piece of wire of appropriate length and cross-section for heaters on the form of a suitable insulator is called the ‘heating element’.

In most electric heaters one or two heating elements made of nichrome wire are used. The coil of each element is set in the circular grove of a thick porcelain slab. In some heaters this slab and the upper side of the element remains open. Pots and other utensils for heating are directly placed on it. In others the upper side of the slab is covered with a thick sheet of steel. This latter type is called ‘hot plate’.

Usually there are two heating elements in each hot plate. These two elements can be connected in series or in parallel with the help of a rotary switch so that three different rates of heat can be generated. On the body of the heater four different positions of the switch are marked.

In one position electric circuit remains open and no current flows through the heater coils, and thus no heat is generated. This position of the switch is called ‘Off Position’. In the second position of the switch the elements are connected in parallel. Under this condition maximum current flows through the heater circuit and the heater generates highest amount of heat. This position is known as High Heat.

In the third position of the switch the two ends of one element become short-circuited. Current can then flow through one element only, and the rate of generation of heat becomes less than before. This position is known as Medium Heat. If the switch is placed in the fourth position or Low Heat position, the heater generates the minimum amount of heat. As the two elements are then connected in series, minimum amount of current, flows through the heater circuit.

Electrical Appliance # 6. Electric Iron:

Electric iron is considered to be very useful as a domestic appliance.

Two varieties of iron are available in the market:

(a) Ordinary or non-automatic,

(b) Automatic.

Fig. 201(a) shows an ordinary iron and Fig. 201(b) gives sketches of its different components.

The different components which are assembled together to form an ordinary iron are:

(i) Sole Plate:

This part of the iron has ordinarily chromium plating. Sometimes another plate is used below the sole plate. This plate is known as ‘heel plate’.

(ii) Heating Element:

This is made of long thin strip of nichrome wound on a mica sheet. Two separate mica plates are also set above and below this.

(iii) Pressure Plate:

This is generally made of cast iron and is placed over the heating element. Pressure plate is used for the purpose of keeping the heating element at its proper position.

(iv) Upper Case:

This is also made of iron and has usually chromium plating.

(v) Handle:

In most cases it is made of wood or ebonite.

Before ironing a piece of cloth, shirt, trousers etc. an electric iron should be heated; otherwise the cloth will not take proper folds. But the degree of heating of the iron should not be the same in all cases. In cases of ironing silken or terelene cloth it is necessary that the iron should be heated much less than in the case of cotton or woollen cloth; otherwise there is the probability of burning the cloth.

Now, with an ordinary iron the degree of heating is guessed either by touching the body of the iron lightly or by watching the time duration for which the iron remains connected to the supply line. Often this sort of guesswork may be erroneous. As a result, the cloth does not keep proper folds when iron is insufficiently heated, or it is burnt out with an overheated iron.

In order to avoid this inconvenience, automatic electric irons are nowadays available in the market. The method of construction of an automatic iron is generally the same as an ordinary iron, but includes a thermostat by means of which temperature may be increased or decreased as required.

A thermostat may be said to be an automatic switch. It consists of a bimetallic strip made of two different metals joined together. When the temperature exceeds a certain limit, this bimetallic strip bends and becomes so-curved that the electric circuit opens and the flow of current through the iron stops.

At one end of an insulator a fixed contact and an adjustable screw remain fixed (Fig. 202). With the help of this screw the pressure on the fixed contact can be increased or decreased as required. Another movable contact remains attached to one-end of the bimetallic strip. Under normal condition the movable contact remains in touch with the fixed contact and closes the electric circuit [Fig. 202(a)]. Current then flows through the strip as well as through the heating element and the iron becomes hot.

With the rise of temperature bimetallic strip is heated more and more along with the iron, and each metal increases in length at higher temperature. But in a thermostat two such different metals are joined together that the rise in temperature makes one of them expands more than the other.

As, however, they still remains joined together, the bimetallic strip bends and takes a curved shape. When the temperature exceeds a certain predetermined limit, the strip becomes so curved that the electrical connection between movable contact at one end of the strip and fixed contact is lost. As a result the electric circuit opens, current no longer flows through the heating element and the iron cannot be further heated. This is shown in Fig. 202(b).

As the adjustable screw is lowered more and more on the fixed contact, greater curvature-is required for the bimetallic strip to open the electric circuit (i.e. the iron is heated more and more). The top of the screw is fixed to a knob on the upper case of the iron. Turning the knob one way or the other, the screw can be raised or lowered. Thus, it is possible to have the iron more heated or less heated according to requirement simply by funning the knob.

The sketches of different parts of an automatic electric iron are given in Fig. 203.