February 22, 1995
One Minute Papers - Questions and Answers
Do the two wires have to do with North and South Poles?
The two wires are not magnetic, so that they do not have poles. But they do carry electric current and do have electric charges. The current flowing through the wires as you talk to your friend is DC, meaning that it always flows in one direction through the wires. One of the wires delivers current to your phone and the other returns it to the telephone company. The wire delivering current to your telephone has more positive charge on it than the wire returning current to the phone company, so you could say that the two wires have to do with plus and minus charges.
If you press two numbers at the same time on a touch tone phone, how does the telephone company respond?
In the basic, old-fashioned touch tone phones (not the high-tech electronic ones that are becoming more and more common), the buttons control 7 different electronic tone generators. When you press the "1" key, the left vertical column tone turns on and so does the top horizontal row tone. These tones create small fluctuations in the current passing through the two wires to and from your telephone company. The telephone company senses these two tones and uses that information to control its switching equipment. But when you press two buttons at once, such as "1" and "2", only the top horizontal tone generator turns on. The telephone company doesn't know how to responds so it does nothing. It assumes that what it is hearing is just noise entering the microphone of your telephone. It only responds to the simultaneous presence of two specific tones; a feature that makes the telephone switching system pretty immune to background noises from the telephone's microphone.
How do the two telephones share or not share the voltage drop.
The telephones are connected in a parallel circuit so that they share the current coming from the telephone company. In a parallel circuit, the two devices (i.e. the telephones) experience identical voltage drops. The electric charges that flow out of the telephone company have a certain amount of energy per charge, also called "voltage". The charges arriving at your phone have the same energy per charge (voltage) as those arriving at your friends telephone so the voltage drops across your two telephones are identical. [Actually, the charges may lose some energy en route to your houses, so that it is possible for the voltage drops across the telephones to become slightly different. Occasionally, you have a bad connections and the voltage drop across one of the telephones is seriously reduced because energy is being wasted in the wires.]
How does a cordless phone work?
In a cordless telephone, the base unit pretends to be a convention telephone. It sends out sound information to your friend's telephone by changing the amount of current it passes from one telephone wire to the other. It receives sound information from your friend's telephone by sensing the changes in current passing through the telephone wires. But instead of using a speaker and microphone to do this work, it interacts with a small radio transmitting system. Sound is transmitted back and forth between the base unit and a small, portable radio transceiver via radio waves. We will talk about radio transmission in the next chapter.
How does a telephone hook up with a modem to send information to a computer?
The modem sends and receives tones through the telephone wires in a manner not unlike the touch dialing tones. The modem sends and receives information across the telephone wires by creating or sensing small fluctuations in the current passing through those wires. At a basic level, your modem could simply emit or not emit a particular sound to indicate one bit of information to the computer at the other end of the phone line. But to pack information much more tightly, it uses very complicated schemes that create and sense very subtle variations in the current passing through the telephone wires. At present, the best standard modems change exchange up to 28,000 bits of information each second on conventional telephone wires.
Why does a telephone sometimes use DC current and sometimes use AC current.
The telephone uses DC current when you talking to a friend. That way, any fluctuations in the amount of current flowing through your telephones is caused by someone's voice. But when the telephone company is trying to make your telephone ring, it sends a brief pulse of AC current through the two wires and causes the bell on your telephone to ring. Because the speaker of your telephone is disconnected during the ring process, the speaker does not turn these current fluctuations into sound.
How does a multiple channel cordless phone work?
The multiple channel cordless telephone is an improvement on the single channel cordless telephone. The FCC has allocated a certain group of radio frequencies for use by household cordless telephones. As we will see when discussing radio, two nearby telephones that try to share the same frequency will interfere with one another. To keep them working properly, they must use different frequencies for their radio transmissions. The base unit and the telephone itself have a signaling protocol that allows them to switch simultaneously from one frequency to another as you look for a vacant channel.
What happens after the telephone company sends alternating currents through the two wires in order for the phone to ring?
Before you pick up the telephone, the telephone company sends several brief pulses of AC current through the two wires. Each pulse lasts about half a second and these pulses are spaced about 4 seconds apart. Each pulse causes the clapper to swing back and forth as the current flows back and forth. The bell rings. But when you pick up the telephone, its speaker and microphone are suddenly connected across the two telephone wires and the total amount of current that the telephone carries suddenly increases. The telephone company recognizes this change in current flow and stops ringing your telephone. It connects your telephone with your friends telephone and you can begin to talk.
When you dial someone's house, how is it that you are connected with their house and not someone else's house?
The telephone company has huge switching systems that work hard to connect the wires of your telephone to those of the correct house. Generations ago, this switching system was an operator with a wall full of electrical plugs. The first automated telephone exchanges were built with electromechanical relays, devices that used electromagnets and motors to open and close electrical switches. These relays systems are elegant works of art; wonderful engineering but not entirely reliable. Because they involve moving electrical contacts, they eventually wear out or break. Modern telephone exchanges are computer controlled electronic switches that involve no moving parts. Once you signal the computer where you want your telephone call to go, the computer arranges all of the switches so that your telephone wires are connected with those of the right house.
Why when you have a power outage does the phone still work?
The telephone company supplies all of the electrical power needed to operate the telephone so that your household electric power is not an issue. Your house may be dark, but as long as the telephone company has power (or backup generators), there will still be current flowing through your telephone wires and you will be able to make calls. However, some modern electronic telephones have an auxiliary power supply to run their fancy electronics (e.g. cordless telephones, message machines, fax machines). When the power fails, these fancy telephones are usually out of action.
How does carbon conduct a current? I did not think that carbon was a conductor. Even so, why does proximity enhance current?
Carbon as graphite is an electrical conductor. It is a semimetal, meaning that has empty conduction levels that are very near in energy to its full valence levels (as is the case for a normal metal) but that the conduction levels and valence levels have some subtle differences. When you apply an electric field to a piece of graphite, its electrons shift from the valence levels to the empty conduction levels and it begins to carry current. Thus graphite can carry electricity. But it is not a great conductor so electricity flow best through it if that electricity doesn't have far to go. When the granules in a carbon microphone are loosely packed, the current must follow a long, circuitous route in order to get from one side to the other. When those granules are tightly packed, the route becomes more direct so current flows more easily. Carbon as diamond is an electrical insulator. Its empty conduction levels are very far in energy above its full valence levels. When you put diamond in an electric field, its electrons cannot responds and it does not carry current.
How does the telephone company compensate for the shared current of many telephones?
The more extensions you use at once, the more current the telephone company sends through the wires. But it makes this adjustment slowly, by using its inductors. An inductor will allow the current passing through it to change slowly but will oppose any sudden changes in current. Thus a few seconds after you all pick up extensions, each telephone will be receiving its normal requirement of current. But when you talk and create sudden fluctuations in current, the telephone company's inductors will not allow the total current in the circuit to change so the other telephones must make up for the changes in current through your telephone.
How is sound made (by the speaker)?
As the current passing through the speaker's coil changes, the speaker cone moves back and forth toward or away from the speaker's permanent magnet. This moving cone pushes or pulls on the air, creating compressions and rarefactions that propagate through the air as sound.
How are people's individual voices so clear when it is just electricity going through a circuit?
As you talk, the fluctuations in current through your telephone reflect the pressure of your voice on the microphone's diaphragm. With a good microphone, these current fluctuations can contain all of the subtle changes that are present in your voice so that the current is an excellent representation of the sound itself. As long as the telephone wires are in good shape and your friend's speaker is in good shape, your voice will sound very clear when the current is turned back into sound. Long-distance telephone calls are no longer handled by electrically connecting the wires of your telephone to those of your friend's telephone. That arrangement uses too many resources: the pair of wires running from your house to theirs. Instead, you voice is converted to digital information at your local telephone company and is sent on high speed wires, fiber optics, or radio waves to your friend's local telephone company. In the early days of new long-distance carriers, you voice might have gone by satellite, but the delays caused by this complicated route made it hard to carry on a conversation. Most long-distance calls now travel on surface cables.
If sound quality or volume is reduced as the number of telephones used increases because the telephones share the current, why is the volume on extensions in the same house so loud? Does it have anything to do with sidetone?
The wires that connect your telephone to the phone company waste some of the power that they carry. When there are two active extensions in one house, the power wasted in the wires becomes even greater. The two extensions share a current that is limited in part by the wires connecting them to the telephone company. When you talk into one of these extensions, the nearby extension experiences the full impact of the resulting current fluctuations. But the telephone at your friends house experiences a small change. The bottom line is that the nearby extension is not equivalent to the distant telephone in your friend's house. The wires themselves lessen the impact for your friend's telephone. Sidetone is only controlled within a single telephone. If it were not reduced, you would practically deafen yourself as your own voice is reproduced at top volume in your ear. Even in the nearby extension, the volume is high enough to be unpleasant.
In a parallel circuit, how does the current decide which device to go to and how much current each device should receive?
Each device draws current according to its electrical resistance. The lower its resistance, the more current will pass through it when it is subjected to a particular voltage drop. When two telephones are connect in parallel to a power source that delivers a steady current, they will start off sharing the current equally. But if one of them experiences a drop in electrical resistance (as something compresses the carbon granules in the microphone), the current passing through it increases. To maintain a steady current, the power source's voltage must go down so that it doesn't push the current through the two telephones as hard as before. The second telephone, which has not changed its electrical resistance, now experiences a reduced voltage drop and the current passing through it diminishes. Thus an increase in the current passing through one telephone has caused a decrease in the current passing through the second telephone.
How will the advent of fiber optics affect the telephone system throughout the world?
Fiber optics are particularly good at carrying digital information long distances because they have enormous bandwidths (can carry lots of information each second) and do not require extensive amplification and signal processing for long distance runs. As a result, fiber optics are quickly replacing electronic cables. To use these fiber optics, the signals from the speaker's telephone must be converted from electrical currents to digital numbers for transmission as digital information. These numbers are sent over the fiber optics cables. When these numbers arrive at the destination, they are converted back into electrical currents and sent to the listener's telephone. Eventually, the conversion to and from currents will occur at the two telephones so that the entire linkage between telephones will be digital. The result will be very clear, static-free transmissions. The telephone "wires" will then be suitable for carrying large amounts of non-voice information, such as video images or computer data. Because one fiber optic cable can carry billions (or even trillions) of bits of information each second, these cables will eventually provide incredible information services to our houses and offices.