The operator would answer, ask for the destination of the call and 'patch' them through. As you can imagine, this was far from an efficient system. But the lack of technological alternatives meant this became the norm for many decades.
During the war, military and emergency services always had priority of this network. This meant civilians might have to wait as much as 2 hours to be connected. After this, it soon became apparent that there was a need for some form of user input for phone numbers. A way had to be found to literally cut out the middleman or more likely woman. Inventors soon picked up on the potential for this and from Patents for systems flooded in. At this time several, 26 in fact, patents were filed for various dial and push-button telephones.
All of these turned out to be either too impractical to use of expensive to manufacture. The first true rotary phone appeared in and was installed in La Porte, Indiana. This rotary dial phone was built around Almon Brown Strowger's patent design. These early rotary phones used lugs on the finger plate rather than the more recognizable holes that came later. By companies like the American Bell Company began to roll out national service for rotary dial telephones.
Their golden age was about to begin. Apart from the basic working of telephone technology in general, the rotary dial component is actually both complex and simple. For the user, the operation of the phone was pretty intuitive. Well, we say intuitive. But you should never underestimate the naivety of youth. If you have never seen one, each rotary phone had a prominent disc, or dial, on the front.
They could either be made from plastic or metal or any combination of the two. While designs did vary, most were around 7. These holes needed to be large enough for a user's fingertip to be inserted. Each of the holes corresponds to a number, and in some cases letters, from In case you're wondering we didn't miss out the letter Q, it was never usually there. Z, if present, was often included on the zero dial hole.
The order of the numbers also varied too. When two people pick up the phones together, they can talk to each other just fine. This sort of arrangement will work at distances of up to several miles apart. The only thing your little intercom cannot do is ring the phone to tell the person at the other end to pick up. The "ring" signal is a volt AC wave at 20 hertz Hz. If you go back to the days of the manual switchboard, it is easy to understand how the larger phone system works.
In the days of the manual switchboard, there was a pair of copper wires running from every house to a central office in the middle of town. The switchboard operator sat in front of a board with one jack for every pair of wires entering the office. Above each jack was a small light. A large battery supplied current through a resistor to each wire pair in the same way you saw in the previous section.
When someone picked up the handset on his or her telephone, the hook switch would complete the circuit and let current flow through wires between the house and the office. This would light the light bulb above that person's jack on the switchboard. The operator would then send a ring signal to the receiving party and wait for the party to pick up the phone.
Once the receiving party picked up, the operator would connect the two people together in exactly the same way the simple intercom is connected! It is that simple! In a modern phone system, the operator has been replaced by an electronic switch.
When you pick up the phone, the switch senses the completion of your loop and it plays a dial tone sound so you know that the switch and your phone are working. For more information on tones, see How Guitars Work. The dial tone sound is simply a combination of hertz tone and a hertz tone, and it sounds like this. Click here to hear a dial tone. You then dial the number using a touch-tone keypad. The different dialing sounds are made of pairs of tones:.
Click here to listen to a touch-tone number. If the number is busy, you hear a busy signal that is made up of a hertz and a hertz tone, with a cycle of one-half second on and one-half second off, like this:. Click here to listen a busy signal. In order to allow more long-distance calls to be transmitted, the frequencies transmitted are limited to a bandwidth of about 3, hertz.
All of the frequencies in your voice below hertz and above 3, hertz are eliminated. That's why someone's voice on a phone has a distinctive sound. Compare these two voices:. Call up someone you know and play the 1,hertz sound file on your computer. The person will be able to hear the tone clearly.
The person will also be able to hear the 2, and 3,hertz tones. However, the person will have trouble hearing the 4,hertz tone, and will not hear the 5, or 6,hertz tones at all! That's because the phone company clips them off completely. For lots more information on telephones, telephone networks and related technologies, check out the links below.
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Mobile Newsletter chat subscribe. Telephone Technology. The DC resistance of any device attached to the phone line is often quoted in telephone company specifications as Ohms; this will vary in practice from between to Ohms. You can measure the DC resistance of your phone with an Ohmmeter. Note this is DC resistance, not impedance. Using these figures you can estimate the distance between your telephone and the telephone exchange.
In the United States, the telephone company guarantees you no lower current than 20 mA - or what is known to your phone company as a "long loop.
Some countries will consider their maximum loop as low as 12 mA. In practice, United States telephones are usually capable of working at currents as low as 14 mA. Some exchanges will consider your phone in use and feed dial tone down the line with currents as low as 8 mA, even though the telephone may not be able to operate. Although the telephone company has supplied plenty of nice clean DC direct to your home, don't assume you have a free battery for your own circuits.
The telephone company wants the DC resistance of your line to be about 10 megOhms when there's no apparatus in use "on hook," in telephone company jargon ; you can draw no more than 5 microamperes while the phone is in that state.
When the phone is in use, or "off hook," you can draw current, but you will need that current to power your phone, any current you might draw for other purposes would tend to lower the signal level.
The phone line has an impedance composed of distributed resistance, capacitance, and inductance. The impedance will vary according to the length of the loop, the type of insulation of the wire, and whether the wire is aerial cable, buried cable, or bare parallel wires strung on telephone poles.
For calculation and specification purposes, the impedance is normally assumed to be to Ohms. If the instrument attached to the phone line should be of the wrong impedance, you would get a mismatch, or what telephone company personnel refer to as "return loss.
A mismatch on telephone lines results in echo and whistling, which the phone company calls "singing" and owners of very cheap telephones may have come to expect. A mismatched device can, by the way, be matched to the phone line by placing resistors in parallel or series with the line to bring the impedance of the device to within the desired limits. This will cause some signal loss, of course, but will make the device usable. A phone line is balanced feed, with each side equally balanced to ground.
Any imbalance will introduce hum and noise to the phone line and increase susceptibility to RFI. The balance of the phone line is known to your telephone company as "longitudinal balance. If you live in the United States, the two phone wires connected to your telephone should be red and green. In other parts of the world they may be different colors. The red wire is negative and the green wire is positive. Your telephone company calls the green wire "Tip" and the red wire "Ring".
In other parts of the world, these wires may be called "A" and "B". Most installations have another pair of wires, yellow and black.
These wires can be used for many different purposes, if they are used at all. Some party lines use the yellow wire as a ground; sometimes there's 6.
If you have two separate phone lines not extensions in your home, you will find the yellow and black pair carrying a second telephone line. In this case, black is "Tip" and yellow is "Ring.
The above description applies to a standard line with a DC connection between your end of the line and the telephone exchange. Most phone lines in the world are of this type, known as a "metallic line. Other types of lines are party lines, which may be metallic lines but require special telephones to allow the telephone company to differentiate between subscribers.
Very long lines may have amplifiers, sometimes called "loop extenders" on them. Some telephone companies use a system called "subscriber carrier," which is basically an RF system in which your telephone signal is heterodyned up to around Khz and then sent along another subscriber's "twisted pair. If you have questions about your telephone line, you can call your telephone company; depending on the company and who you can reach, you may be able to obtain a wealth of information.
The standard network used all over the world is an LC device with a carbon microphone; some newer phones use discrete transistors or ICs. One of the advantages of an LC network is that it has no semiconductors, is not voltage sensitive, and will work continuously as the voltage across the line is reduced.
Many transistorized phones stop working as the voltage approaches 3 to 4 Volts. When a telephone is taken off the hook, the line voltage drops from 48 Volts to between 9 and 3 Volts, depending on the length of the loop. If another telephone in parallel is taken off the hook, the current consumption of the line will remain the same and the voltage across the terminals of both telephones will drop. Bell Telephone specifications state that three telephones should work in parallel on a 20 mA loop; transistorized phones tend not to pass this test, although some manufacturers use ICs that will pass.
Although some European telephone companies claim that phones working in parallel is "technically impossible," and discourage attempts to make them work that way, some of their telephones will work in parallel. While low levels of audio may be difficult to hear, overly loud audio can be painful. Consequently, a well designed telephone will automatically adjust its transmit and receive levels to allow for the attenuation - or lack of it - caused by the length of the loop.
This adjustment is called "loop compensation. Although some telephones using ICs have built-in loop compensation, many do not; the latter have been designed to provide adequate volume on the average loop, which means that they provide low volume on long loops, and are too loud on short loops. Various countries have different specifications for transmit and receive levels; some European countries require a higher transmit level than is standard in the United States so a domestically-manufactured telephone may suffer from low transmit level if used on European lines without modification.
Because a telephone is a duplex device, both transmitting and receiving on the same pair of wires, the speech network must ensure that not too much of the caller's voice is fed back into his or her receiver. This function, called "sidetone," is achieved by phasing the signal so that some cancellation occurs in the speech network before the signal is fed to the receiver. Callers faced with no sidetone at all will consider the phone "dead.
Can you hear ME? A telephone on a short loop with no loop compensation will appear to have too much sidetone, and callers will lower their voices. In this case, the percentage of sidetone is the same, but as the overall level is higher the sidetone level will also be higher.
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