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Some questions about telephones:1. Do some telephones have capacitors and other telephones don't? 2. Do all phones work the same? 3. Why do you have to have a particular kind of portable phone to use in a car? 4. Can you call from an ordinary phone into space?
Answer 1:

> 1. Do some telephones have capacitors and other telephones don't?

Capacitors are very common components in electronic circuits and can probably be found in all telephones. Older phones used capacitors to help cancel out the noise on the line, making it easier to hear the other person. Modern digital phones perform this function with different electronic circuitry, but use capacitors to help make their digital
circuits more resistant to static.

> 2. Do all phones work the same?

No. There are several types of phone. The early phones (especially those with rotary dials) sent pulses (or "clicks") down the telephone line to specify the number they were calling. Modern phones send tones down the line to do the same thing. In the US,
the phone system can understand both types of phone.

Phones in large office buildings are sometimes digital. These phones turn your voice into a coded series of numbers (or "data") and send the data down the phone line. The data are transmitted using different tones for each digit. The phone at the far end decodes the data and turns them back into voice sounds which are heard in the speaker.

There are also cellular phones, which transmit voice or data over radio waves rather than through a telephone wire.

> 3. Why do you have to have a particular kind of portable phone to use in a car?

You could use a regular cellular phone in a car - many people do. The car might cause a bit of interference though, and a phone specially designed for a car would work a bit better. Some car phones also connect to microphones in the sun visor
so that you don't have to hold up the phone to talk. Some also allow you to dial by speaking the numbers so you don't have to use your hands.

> 4. Can you call from an ordinary phone into space?

Not from an ordinary phone. Last year a new phone system, called Iridium, was started. This looks like a slightly larger cellular phone and is powerful enough to transmit directly to one of a network of satellites around the world. This means that it can be used anywhere on the planet. It is still very expensive.



Answer 2:

> 1. Do some telephones have capacitors and other telephones don't?

Yes, some do and some don't -- as you may have seen, telephones vary widely as far as their internal parts. However, essentially they all perform the same function-- Creating a modulated electrical signal matching the sounds produced when you speak, and translating the electrical signals from the wire (the person you are speaking to) into sounds for your ear. Early telephone systems consisded of little more than a battery to supply the current to be modulated, a speaker, a microphone, a bell and a small generator to ring the bell. (The battery was usually placed at the switch-house where the phone operator lived).
When you turned the crank of the phone it generated several pulses of about 75V which ran all of the bells it was connected to. When you lifted the reciever (the local speaker) it connected the speaker to the phone wires. Because of the cost of the wire, most early phone systems had several people on the same loop -- a "party-line" in which you could hear everyone talking on the loop. The operator had access to several loops and could connect them via repeater coils to further stations.

In more modern phones, from 4 to 6 wires are run, although a basic phone needs only 3 (Tip, Ring, and return) and some phones only use 2, just the tip and ring although this is not safe where lightning can occur. (It is never safe to talk on the phone during a lightning storm! -- several people are killed each year that way). A modern phone uses an amplifier running off the phone wire power to make a much clearer signal -- and another to make the speaker louder. Further amplifiers and filters are in place at the sub-station where are the wires from a neighborhood come together.

> 2. Do all phones work the same?

All phones in the U.S. that hook to the phone lines directly work basically the same-- they all meet the same specifications although how they do this may be quite different. Other countries have similar, but not necessarily compatible systems.

> 3. Why do you have to have a particular kind of portable phone to use in a car?

Typically, in a car, you are out of range of all but Cell phonesand the old single-sideband shortwave car phones. Cell phones have line of sight range, but many substations (or cells) which can be switched as you drive. There were too few channels in the old radio scheme for many callers, and they interfered with some marine calls, so they have not been used for years except in very remote locations. (In fact, the ATT single-sideband marine radio phone service will go off the air on 2/28/99). It will be repleaced by a satellite system.

> 4. Can you call from an ordinary phone into space?

4. Both cell and remote wireless phones have signals that could be received from space, but their signal would be very weak and without some special techniques, all the calls in a city would tend to blend together... However, Motorola and other companies are launching several fleets of satellites which can decode such a multitude of calls -- i.e. the Iridium system. In this system, you could be anywhere on the earth with a clear access to the sky and it would work. However, if you mean "can a standard wire phone call into space?", there were several calls uplinked by NASA to the Apollo spacecraft -- the first telephone call to the moon was in 1968...
To find the range of a wireless phone, you might try walking away from the base station and talking to a friend until the signal stops -- most modern wireless phones go out to about 500 feet if they have little obstruction.


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