How Does The Telephone Work?

Now, let's get to what is inside the telephone. Remember that these components described are commonly found in the older traditional telephones. We are providing this information to lend an historical perspective. Most of today's telephone systems substitute electronic components to emulate these functions.

1. The transmitter. ( Figure 1) The transmitter is the ear of the telephone in that it "hears" the voice of the person speaking into it. The transmitter is a miniature carbon pile rheostat. A rheostat is a device that controls an electric current by varying the resistance in the circuit, similar to the action of a dimmer switch control. The variations in sound pressure from the voice vibrating against the diaphragm change the compression of the carbon granules. This varies the resistance of the transmitter. The transmitter has two contacts that are insulated from each other. Current can only flow through the carbon granules. As sound pressure from the voice presses against the diaphragm, the carbon is more closely compressed within the chamber. Compressing the carbon granules lowers the resistance of the transmitter resulting in more current flow through the transmitter circuit. When the pressure on the diaphragm is released, it momentarily snaps out farther than its original position. The carbon is under less pressure than normal and the resistance of the transmitter is momentarily greater. The current flow decreases.

   Figure 1: The Transmitter

   The diaphragm of a transmitter is made of lightweight phosphor bronze, duraluminum or a similar material. Either an extra inner cone of the same material strengthens the center or it is corrugated to act as a stabilizer. The flexible outer edge is securely clamped in the transmitter housing. This design enables the diaphragm to move in and out at the center like a piston. Since the diaphragm is sensitive to sound waves, the carbon granules are compressed and released as the corresponding pressure from the sound wave's changes.

   The telephone transmitters in use today are, in principle, like the ones invented more than 100 years ago by Thomas Edison. Many modern electronic telephones use real microphones connected to related speech processing equipment to vary the line current. Small microchips allow economy and space saving, enabling inexpensive, high quality "throwaway" telephones. The output now generated by microchip-based telephones must emulate the same variations created by the carbon granule type of transmitter.

   What is known as the basic 500 set, a single-line telephone like the one that was in use in most homes, has dictated the industry's electrical standard for the telephone instrument and all related signal processing equipment.

   All types of 2- and 4-wire circuits are still designed around that 500 set.

2. The receiver { Figure 2). The receiver is the "mouth" of the telephone in that it speaks into the ear of the person using the telephone. It also contains a diaphragm whose movement is caused by the strengthening and weakening of the field created by the magnet within the receiver. The receiver converts the varying electrical current representing the transmitted speech signal to variations in air pressure perceived as sound by the human ear. An electromagnetic receiver consists of coils of many turns of fine wire wound on permanently magnetized soft iron cores that drive an armature. The armature is a diaphragm made of a soft iron material.

   Figure 2: The Receiver

   When someone speaks a word into a transmitter, the current flow in the circuit is alternately increased and decreased as the moving electrode moves in and out of the carbon chamber. A requirement for an electromagnetic receiver is a permanent magnet to provide a constant bias field for the varying electromagnetic field to work against. Otherwise, both positive and negative currents would push the armature in the same direction. The varying electrical current representing speech flows through coils and produces a varying electromagnetic field. It alternately aids and opposes the permanent magnetic field; thus, it alternately increases and decreases the total magnetic field acting on the diaphragm. This causes the diaphragm to vibrate in step with the varying current and moves the air to reproduce the original speech that caused the current changes. Other types of receivers operate similarly, except that the armature is a separate part and is connected to a conical non-magnetic diaphragm. The rocking action of the armature causes the aluminum diaphragm to vibrate to reproduce the original speech. In some telephones this receiver is created with the use of microprocessors.

   The electromagnetic receiver was a central element of Alexander Graham Bell's original telephone patent.

   Part of the design of the telephone handset that enables you to hear your own voice while talking is called side tone or side noise. The reason for this is to give you some feedback that the telephone is working. Too much side tone causes an echo.

3. The ringer. There is a wide variety in types of ringers. Telephones run on DC (direct current) where electrons flow in one direction. The bell or ringer operates on AC (alternating current), which means that electrons are moving in two different directions to activate the bell. This AC sent on the local loop (telephone line) is called ring generator (90 to 105 volts AC at 20-Hz). Minus 48 volts DC is always on the line, which is used to operate the telephone after is answered.

   There is a good analogy for understanding these electrical signals. Envision a garden hose. The hose represents the wire. The water is the current. The water pressure is the voltage (electrical pressure). Stepping on the hose with your foot is equivalent to resistance on an electrical circuit.

4. Microprocessors. The microprocessors in electronic telephones may replace any of the above internal components and may also add additional capabilities and functions to the telephone, such as speed dialing, etc.

Many telephones look the same, but there is wide variation in the capabilities and prices. You can buy a throwaway single-line telephone for less than ten dollars or a multi-line multi-featured telephone to work with a business telephone system for six hundred dollars. As with many manufactured items, there is variation in the quality of the components that is reflected in the price. The price also tends to be higher on the proprietary telephones, which work with a specific manufacturer's system, even though they may look the same as those you, buy in your neighborhood telephone store.