Soft iron is used for making the armature of an electric bell because soft iron is magnetised easily on passing the current and completely demagnetised easily on withdrawing the current through the solenoid.

An electric bell contains an electromagnet, consisting of coils of insulated wire wound round iron rods. When an electric current flows through the coils, the rods become magnetic and attract a piece of iron attached to a clapper. The clapper hits the bell and makes it ring.

Striker is moved under influence of electromagnet when circuit is complete.

When the switch of electric bell is pushed,circuit completes and current flows leading an electromagnet to attract armature and ring the bell.

Electric bell works on the mechanism of electromagnets i.e, temporary magnets, and no permanent magnet is used.

In a buzzer, the simplest sort of doorbell, an electromagnet is used to operate a self-interrupting circuit. You can see how this system works in the diagram below. Click and hold the doorbell button to see how the buzzer works. ... This breaks the doorbell circuit, which shuts off the electromagnet.characteristic of a buzzer It has higher frequency of collision of gong and hammer

Electric bell works on the mechanism of electromagnets and for this any type of power either AC or DC can be used.

The bell works on the principle of electromagnetism which says that when an electric current flows through the coil, the rods become magnetic and attract a piece of iron attached to a clapper. And the clapper hits the cap and comes back as the current stops, and hence the bell rings. So the current flows and stops in succession. And in this way the bell rings.

Make and break arrangement causes striker to repeatedly hit the metal gong as long as switch is pressed. When Circuit is complete, electromagnet attracts the striker which hits metal gong. As soon as the striker moves, circuit breaks and electromagnet is inactive. Because of flexibility of striker, it moves back to its original position. As soon as it reaches original position, circuit is complete. This process repeats continuously. 

Because of lack of flexibility, striker will resist change of motion more and may break if current is large.

Without the flexibility strip, the striker will not come back to original position when the electrical circuit breaks.

When switch is pressed, circuit completes and ringing begins. Hence, electrical energy is converted to sound energy. As switch is released, no current flows and no energy is consumed,

Mechanical Damage to metal gong or striker may hinder working of the bell.

Striker repeatedly hits the metal gong when doorbell is ringing. 

The working of an electric magnet is based on the magnetic effect of current. When the key is pressed the circuit gets completed and current passes through it causing the electromagnet to get magnetized which in turn attracts the armature. This movement of the armature causes the hammer to strike the gong. However when the armature moves towards the gong the connection between the spring strip and adjusting screw is lost which breaks the circuit and the current flow is stopped thus demagnetizing the electromagnet. This causes the armature to return to its original position and it is thus pulled back due to the spring effect of the spring strip. This process is called a make and break circuit.

The working of an electric magnet is based on the magnetic effect of current. When the key is pressed the circuit gets completed and current passes through it causing the electromagnet to get magnetized which in turn attracts the armature. This movement of the armature causes the hammer to strike the gong. However when the armature moves towards the gong the connection between the spring strip and adjusting screw is lost which breaks the circuit and the current flow is stopped thus demagnetizing the electromagnet. This causes the armature to return to its original position and it is thus pulled back due to the spring effect of the spring strip. This process is called a make and break circuit.

The working of an electric magnet is based on the magnetic effect of current. When the key is pressed the circuit gets completed and current passes through it causing the electromagnet to get magnetized which in turn attracts the armature. This movement of the armature causes the hammer to strike the gong.

In his experiment Oersted observed that the deflection of the needle compass changed according to the direction of the current and also as the current in the wire increased, the deflection of the needle increased. Thus he inferred that the strength and direction of the magnetic field depends on the magnitude and direction of current.

Heating effect and lighting effect does not depend on the direction of current.

Oersted conducted an experiment to study the magnetic effect of electric current. In this experiment he observed the deflections of a compass needle placed in close proximity of an electric wire carrying current. He observed that in the absence of current the needle came to rest in Earths North-south direction under the influence of Earths magnetic field. However, when the needle was kept in close proximity of an electric wire carrying current it showed deflections in the east and west direction depending on the direction of current. Thus, indicating that the strength and direction of the magnetic field depends on the magnitude and direction of the current.

In his experiment Oersted observed that as the current in the wire increased, the deflection of the needle also increased, indicating that they are directly proportional.

Oersted conducted an experiment to study the magnetic effect of electric current. In this experiment he observed the deflections of a compass needle placed in close proximity of an electric wire carrying current. He observed that in the absence of current the needle came to rest in Earths North-south direction under the influence of Earths magnetic field. However, when the needle was kept below the wire and current was passed from A to B the north pole of the needle deflected towards the west and on reversing the direction of the current from B to A, it deflected towards the east. Similarly, when the needle was kept above the wire and current was passed from A to B the north pole of the needle deflected towards the east and on reversing the direction of the current from B to A, it deflected towards the west. This indicates that the direction of the magnetic field depends on the direction of current.

The earth acts like a huge magnet. Thus in the absence of electric current a compass needle always comes to rest in Earths North-south direction. However, when current passes through the circuit the needle comes to rests in the direction opposite to the magnetic field of Earth i.e. South-North direction.

If electric current is passed through metal body then it behaves as magnet. passing an electric current through iron does not produce a magnet. But if you pass a direct current through a coil wound around the piece of iron, you will magnetize it. Passing a direct electric current through the same coil with no iron inside the coil will still produce a magnetic field that behaves just like a magnet. Similarly, passing a direct current through a coil around a piece of Bismuth will create the same magnetic field which behaves just like a magnet.

Oersted discovered the relationship between electricity and magnetism.

Thermistor has a resistance range of 100 to 100 k. Thermistor consists of a mixture of metallic oxides of manganese, nickel, cobalt, copper, iron and uranium.

A thermistor material is usually pressed under high pressure to form a flat cylindrical shape. Disks and washers are placed in series or in parallel to increase the power dissipation.

Thermistor generally has a negative temperature coefficient of resistance. With increase in temperature, resistance of a thermistor decreases.

For small changes in the values of current, voltage across a thermistor increases. It attains a peak value. Then the voltage across the thermistor decreases. As a result Ohms law is followed at small variations of current.

I)According to Oersted experiment electric current can produce magnetism.

Electromagnets are formed by passing an electric current through a wire coiled around the substance with iron core. The electric current generates a magnetic field around the  substance and thus magnetize it which convert it into a magnet. Thus electromagnet can be formed due to the magnetic effect of current.

When the direction of the current flowing through the wire is reversed, direction of the magnetic field, so produced, also changes its direction which results in changing the position of pole of needle kept near the wire.

Frequency $\pi=\dfrac{1}{2\pi}\sqrt{\dfrac{BM}I}$

The deflection of the compass needle, whenever there is current in the wire show that a current carrying wire produces a magnetic field around it, which is essentially saying the current carrying wire must be acting as a magnet.

Answer is D.

Electromagnetism is the study of the electromagnetic force which is a type of physical interaction that occurs between electrically charged particles. 
Hans Christian Oersted discovered the relationship between electricity and magnetism in 1820.

Answer is D.

Electromagnetism is the study of the electromagnetic force which is a type of physical interaction that occurs between electrically charged particles. 
Hans Christian Oersted discovered the relationship between electricity and magnetism in 1820.

More is the current in the wire near to the magnetic field, more is the strength of magnetic field generated around the wire and thus more is the vibration in the needle.

Hans Christian Oersted. Hans Christian Oersted began a new scientific epoch when he discovered that electricity and magnetism are linked. He showed by experiment that an electric current flowing through a wire could move a nearby magnet.Oersted's experiment that when an electric current is passed through a conducting wire, a magnetic field is produced around it. The presence of magnetic field at a point around a current carrying wire can be detected with the help of a compass needle

The deflection in galvanometer is the defined by 

Oersted observed while performing the experiment when he passed a current through a wire which caused a nearby magnetic compass needle to move.When current switched off then needle comes to their original position

In the experiment of Oersted's he passed electric current through a wire,which caused a nearby magnetic field so that compass needle move.That shows that magnetic effect of eclectic current.