The task of sending messages over long distances has been solved with the invention of the telegraph. However, the telegraph could send only written dispatches. Meanwhile, many inventors have dreamed of a more perfect and the outgoing communication method by which it would be possible to transfer at any distance live sound of the human voice or music. The first experiments in this direction undertaken in 1837 by the American physicist Paige. The essence of Page's experiment was very simple. He collected an electrical circuit, which consisted of a tuning fork, the electromagnet and galvanic cells. During his fork oscillations quickly opens and closes the circuit. This intermittent current is transmitted to the solenoid, which is also quickly attracted and let a thin steel rod. As a result of these fluctuations rod produced a singing sound, similar to that published a tuning fork. Thus, Paige revealed that transmit sound with an electric current is possible in principle, it is only necessary to create a better transmitting and receiving device.
The next important stage in the development of the telephone is connected with the name of the English inventor flights. While still a student Flight interested in the problem of sound transmission over a distance by means of an electric current. By 1860 he constructed up to a dozen different devices. Most of them had committed the following form.
The transmitter is a hollow box fitted with a front sound hole which had a A and its top opening closed by a thin membrane taut. On this thin membrane lying p platinum plate, and the top edge of the elastic n was a platinum needle, which has been adapted so that the plate touches p when membrane is at rest. Touching is interrupted when vibration membrane. As a result of the cross touches of make and break the current flowing from the battery B via a clip in platinum plate p and through the needle n on the second clip from the last wire going to the receiver, passing through CC spiral and back to the battery through the terminal d, and connected with it e wire. Inside spiral placed a thin steel needle that its two ends fastened to two resting on the resonator board racks gg ff. Parts and hi ki formed at both stations devices, which aimed to know the remote listener to start negotiations. Audio playback, sung in the socket A, was based on the fact that the steel needle, magnetization and demagnetization passing an electric current in a spiral, beginning oscillate; They feel like the sound to match the sound that was perceived by the receiver and the fluctuations which are driven membrane. Resonance board served to amplify the sound.
With Reis phone it was already possible to transmit not only the individual sounds, but also complex musical phrases, and even some human speech. But the transmission quality was so low that it was often quite impossible to make out anything. Side noises made by closing and opening the circuit, drowned transmission and sounds reproduced steel needle, were very far from the modulations of the human voice. For the distinct sound transmission was necessary to ensure that the records of both the sender and the receiver were derived from its rest position to the extreme position of the current, the strength of which would increase gradually, and to decrease when the current again passes through the original position of rest. All these smooth fluctuations in sound levels, constituting the wealth of human speech, were completely inaccessible phone Reis - attraction here occurred rapidly and remained the same for some time, and then completely stopped.
To solve the problem of sound transmission only closing and opening the circuit was impossible. It took another 15 years before a Scottish inventor Alexander Graham Bell has found a better way to convert sounds into electrical signals. By profession Bell was a teacher of the deaf and dumb children. From childhood he was busy acoustics, the sound doctrine, and wanted to reinvent the phone. In 1870, Bell moved to Canada, and in 1872 - in the US. Settled in Boston, he brought in there a school for deaf children developed their system "visible speech". It was a great success, and Bell soon became a professor at Boston University. Now he had a laboratory and enough money to devote himself to the work on the invention of the telephone. Forgetting about the dream, Bell sat all night on their experiences. His first experiments were repeated Page's work.
In the summer of 1875 Bell and his assistant Thomas Watson made an installation consisting of magnets with movable tongues, which is driven by current fluctuations. The chain magnets included a variety of devices. Bell and Watson were in adjoining rooms. Watson passed, and Bell took. Once, when Watson pushed the button at the end of the wire to activate the call, corrupted contacts and electromagnet pulled him hammer ring. Watson tried to pull it, so that any vibrations around the magnet. The movement of the spring produced by Watson, changed the current intensity and caused oscillatory motion in the spring opposite the station in Bell's room, and a wire passed very faint sound of the first phone. So, quite by accident, Bell discovered that a magnet with a light anchor may be a transmitter and a receiver. After that implement transmission and reproduction of sound with the help of an electric current is not that difficult.
To understand how this happens, imagine a permanent magnet, and near it a flexible iron plate, which varies under the influence of sound waves. Nearing the pole of the magnet, it will strengthen its magnetic field, and moving away from it - to weaken. (Without going into details, we note that the reason for this is the same phenomenon of electromagnetic induction, which was mentioned in the previous chapter, it is clear that in the plate, which moves in a magnetic field is an electric current, this current will create around a plate own magnetic field, which will be imposed on the magnetic field of the magnet, the strengthening, then weakening it.) Now we put on our imaginary magnet coil wire. When the magnetic field oscillations will arise in the coil an alternating electrical current, and in one or the other side. Passing the resulting current through the other magnet coil, we will affect its magnetic field, which will then also increase, then decrease, and exactly repeating all of the changes in the magnetic field of the first magnet. If the pole of the second receiving magnet placed iron plate, it will then be attracted to the magnet under the influence of the increasing magnetic field, then move away from it under the influence of its resilience and thus generate sound waves in all similar to that caused to oscillate first plate. Actually, this has happened under circumstances described above. The role played by the iron plate is flexible armature magnet. But it was too rough device, not able to convey the many nuances of sound. Bell began to look for, what can replace it.
A doctor friend suggested him to use for experiments to the human ear, and got hold of his ear from the corpse. Carefully studying its structure, Bell found that the sound waves cause the eardrum to vibrate, by which they are transmitted to the auditory ossicles. This led to the idea to make it thin metal membrane, placing it near the permanent magnet and thus transform sound vibrations into electrical. It took several months of hard work before the phone said. Only 10 March 1876 Watson clearly heard at the receiving station to Bell, "Mr. Watson, please come here, I need to talk to you." Earlier, on February 14 Bell made a patent application for his invention. Just two hours after a similar application for an identical unit filed another inventor - Elisha Gray. However, the patent was issued in March, Bella, because he first announced his discovery. (Later, Bella had to conduct several trials with Gray and other inventors, claiming the championship. In the end, Bell bought Gray right on your phone.) The exhibition in Philadelphia, held in the same year, Bell's telephone has become the main exhibit. Since that time, despite the fact that the first phones were still very imperfect, phones have become quickly spread. In August 1876 it was in use for nearly 800 phones, and demand for them is increasing.
The device first devices were very primitive. A permanent magnet in the form of a rod was surrounded at one pole short induction coil B of thin copper wire, ending two thicker wires of the CC, which by means of DD terminals are connected to the wires LL. One pole of the magnet was placed on the edges sandwiched EE plate of soft iron sheet. It was inserted into a wooden frame, which is part of the GG was on the plate EE funnel-shaped opening, which served as a sound cone. Below the wooden frame narrowed, as here, it only contained a magnetic rod mounted in a screw position, and two wires CC. This apparatus can serve as both transmitter and receiver. At the originating station and the receiving station at such a phone there. Their induction spiral connected to each other through the terminals LL and DD wires. When the cone GG used as a tube and said to him, EE plate in front of the magnet pole comes into vibrations; therefore arose in the helix B induction currents, which change in line with the current record on the sound vibrations. These currents flowed through the wire in the LL receiving phone spiral and cause oscillation of the membrane. Pressing the cone to your ear, you could hear the voice of the speaker at the other end of the phone wire. Induction currents generated by the movement of the diaphragm, were very weak, so the stable communication can be establish only a few hundred meters away. Then the voice spoke became so quiet that drowned in the din of noise. It took the work of many, many inventors before the phone has become a reliable means of communication.
In general, telephone Bella was more adapted to convert current waves into sound waves than the back. It is therefore very important in the history of telephony was the discovery in 1877 by the English inventor Hughes microphone effect. In its original form was a microphone next device.
Between two pieces of coal C and C ', fixed on the plate B, installed carbon rod with pointed ends. The current from the element E passes through the carbon rod and winding through the phone T. In horizontal shaking of the plate A, played the role of the resonator, carbon rod shifted. At this point, the resistance decreased in the current locations of contacts, and this, in turn, produced a marked increase in the current strength in the phone. The membrane begins to vibrate with greater amplitude, making the original sound is amplified several times. Faint ticking of the clock laid on the stand, was perceived as the phone is very loud. Even the flies crawling over the plate reproduced in the form of noise is quite noticeable.
A few years after the invention of Hughes, a host of different designs microphones. Widespread microphones, in which instead of the carbon powder used rods. The vibrations of the membrane caused in this case, the powder seal, its loosening, resulting in constantly changing its resistance. Connected with microphone phone just got a lot safer, but it was still imperfect. Weak induction currents were unable to overcome the resistance of the transmission cables. It was necessary to somehow increase their power, without changing the nature of their fluctuations. An ingenious way out of the situation found the famous American inventor Edison, who proposed the use of an induction coil to increase the voltage. Since the telephone was supplemented by a transformer.
About transformers in more detail will be discussed in a later chapter. Now just explain how it works. When applied to the two coils on the same iron core and to pass one of them through an alternating current in the second coil also induced alternating current. Let us examine this phenomenon. Created the first coil changing magnetic field induces in each turn of the second coil current of certain voltage. The turns of the coil, as has already been shown in the previous chapter, can be seen as series-connected current sources. Then the total voltage across the second winding of the coil voltage is equal to the sum of all its windings. If we want to increase the voltage taken from the second reel, we must increase the number of turns. Thus, by varying the number of turns in the second coil, we can get its voltage to less than, equal to or greater than the first. However, it is increasing the number of times the voltage as many times the amperage is reduced so that the product of the first and the second coil is equal to (in fact, due to unavoidable losses in the secondary coil is a few work even less). Transformer effect was discovered simultaneously with the phenomenon of electromagnetic induction, but because the technique has long been used only a direct current, he at first did not find the application. Phone was one of the first devices where the transformer (in the form of an induction coil) has gained some recognition.
The device created by Edison phone and microphone are included in two separate chains. The power source, the microphone and the primary winding of the transformer is connected to one circuit and the other coil telephone receiver - in another. The principle of operation of this phone is clear: as a result of fluctuations of the membrane resistance in the microphone is constantly changing, making the battery direct current converted into a pulsating. This current is supplied to the primary winding of the transformer. Induced in the secondary winding are the same in shape currents but higher voltages. They easily overcame the resistance of the wires and can be transmitted over long distances. Enhanced phone so soon became widespread.
At first the machines were connected to each other in pairs. They did not have switches and calls. To make a call to the machine just knocking a pencil on the membrane. Subsequently, the Edison Electric calls were introduced. In 1877 there was the first central telephone exchange in New Haven (USA). The order of connection here was gone. The subscriber, who wanted to speak to any person or institution in the subscription book looking for a number, and call the central station. When the last answer, he informed him the desired number, and if the number is not busy, the operator connected it to the desired person via special connectors and informed him that the connection is ready. Thereafter, the subscriber is already addressed to the interconnected party. At the end of their conversation will be disconnected.
Contemporaries very quickly appreciated the convenience that gave the phone. Soon telephone exchanges were built in all major cities. At the same time growing demand for handsets. In 1879, Bell set up his production company on the phone, turning quickly into a powerful concern. Within ten years, only in the United States was established over 100 thousand handsets, and in 25 years they have had more than a million. Then, the figure rose by another order of magnitude. Bell had a long life and was able to observe the spread of telephony worldwide. He died in 1922, and his memory is honored kind of a minute of silence when the coffin with the inventor's body was lowered into the grave, all the phone calls stopped. They write that the US was silent at that moment more than 13 million phones.