Telephone as Communication Device

Invention of the telephone is credited to Alexander Graham Bell. The telephones are of several types: hand sets, mobile phone, satellite phone and through internet. The basic function of a phone is to allow communication of voice both ways.

Of late, phones with facility of transmitting images have also become available. The telephone may be with or without wire.

wired phone has a microphone and a speaker. The microphone receives our voice and converts it into electrical signal. Similar process occurs inside the mouthpiece of the telephone. A basic telephone has 3 main parts:

  1. Cradle with a hook switch
  2. A mouth piece which houses a microphone
  3. A hearing piece which houses a speaker (usually an 8 ohm speaker)

The phone is rested on the hooks. As soon as the phone is lifted, the hooks pop up and a connection gets made inside the body of the cradle which completes a dial tone ringer circuit. This produces a tone which is actually a mix of 2 tones (2 frequencies). On hearing the dial tone, we know that the phone is connected and we can dial a number.

If the number is busy when we dial, we hear another mix of tones. Over the times, the telephone has undergone many changes including introduction of the cordless (for short range) and mobile phones. But as far as basic structure of a telephone set is concerned, it has remained the same.

Now the dialing is by pressing the keys. We speak into the mouth piece and hear the other person through the speaker. In a basic phone, the speaker and the microphones form the ends of the phone set. In this way, the set can be held close to face such that the speaker is close to our ear and the mic to our mouth.

The speech is controlled by a mouth piece which contains a mic. It includes a diaphragm. In the old phones, the diaphragm was made of a 2 metallic sheets between which carbon granules were filled. As one speaks, the diaphragm gets pressed following the same pattern as the sound of a speaker. In turn, the carbon granules also get compressed and decompressed, coming closer and moving away, thus increasing and decreasing the conductivity.

A current is sent through the diaphragm. The source of this DC current (a few mA) is a battery at telephone exchange and the current comes to our phone. This leads to varying electrical current. This current will depend upon the sound pressure hence this can pattern the signal being sent through the amplifier and the cable.

Now-a-days, there are electronic microphones. This signal (as electrical current) is sent to a junction box outside house using a pair or copper or aluminum wires. There are signals from other houses also reaching this junction box. All of these electrical wires carry voice-signals (sound converted into electrical signal) that are sent through a common coaxial cable, housing many pairs of copper wires, to the telephone company's exchange.

From there, they can be routed either through metallic or fiber optical cables. These days, the signals are also routed from the exchange through microwaves using satellites especially for international calls. To avoid our own voice reaching our ears, a duplex coil is placed in the circuit of the microphone. In addition, there is a ringer. When someone calls, it rings a bell and we know that we have to attend a phone call.

The hearing is controlled by a speaker. It consists of a diaphragm with a permanent magnet attached to it on one end and an electromagnet close to the other end. The electromagnet is a piece of soft iron with a coil wound around it. The signal comes and flows through the coil. This causes the iron core to be magnetized. This naturally causes the diaphragm to vibrate in the same pattern as incoming current (voice). This generates sound that we hear.

Mobile phones have brought great convenience in daily life. The basic working principle remains the same in mobile phones also. But for them, the sound doesn’t travel through cables or wires. It travels as electromagnetic wave through space via antennas, base towers, switching stations (or even satellite) and then again the antenna. When a number is dialed, the (electromagnetic) field is spread all around through antenna of the mobile.

The signal is received by the nearby microwave tower and then by the switching station. This station re-transmits it in all directions (it doesn't know where the intended mobile may be) and a part is available to the other antennas in other places. When an antenna near the intended receiver gets the signal, it also retransmits it and this is received by the antenna of the intended mobile, which rings.