Everyone knows that the sound can not propagate far. How would we not loudly shouting our voices be heard farther than 1 km. Besides, the sound travels relatively slowly - for 3 sec. it is about 1 km.

If a resident of Leningrad could scream so loud, it chgo might hear in Vladivostok, he would be heard only after 10 hours. and only about a day before he heard the first words to answer. This is very slow, and this speed can not satisfy us.

How do we get so that on the radio sound was heard thousands of miles away and almost at the same time, it is pronounced? This was possible because it was found convenient and fast "carrier" of sound. The main credit for this belongs to our scientist Alexander Popov, the inventor of radio. On the question of what the radio that is broadcast, the answer may be not quite normal, but pretty accurate? broadcast - it's invisible light, which carries the inaudible sound.

We are all accustomed to the light, but not all quite aware of what light is. From the standpoint of physics, light is electromagnetic waves, electromagnetic waves that propagate in space with great speed - 300 thousand miles per second. The path from Leningrad to Vladivostok, on that sound must spend about 10 hours., Light flies so fast that we can not observe this short period of time - about three-hundredths of a second (0.03 sec.). The sound during this time will be only 10 m.

Electromagnetic waves in terms of their length in different ways manifest themselves and some of them may even be perceived by our senses. For example, the wave length from four to eight thousandths of thousandths of a millimeter are light waves, and we see and feel them as light. Hot oven or heated body radiates heat. This is also electromagnetic waves but longer than a few of light. We do not see them, but feel as heat. Waves even longer than the heat, we do not perceive any feelings, learn about their presence, we can only by special instruments. These electromagnetic waves ranging in length from a few millimeters to a few kilometers we use to broadcast. They are so wonderful, "carrier" sounds - radio waves, which give us the opportunity to transfer to the sounds of thousands of kilometers.

How can we receive the electromagnetic waves used for radio? It turns out that if the wire is alternating current, the wires around there are electromagnetic waves that, breaking away from the wires are carried away in all directions at light speed. What is alternating current, which generates electromagnetic waves that we call radio waves?

Alternating current is the current that passes through the wire in one direction, then in reverse. The time during which the current time to go over the wire in the forward and reverse directions, called the period. The number of periods per second is called frequency and current is measured in "Hertz" (Hz). If, for example, a current is half a second in one direction and half a second to the contrary, its period is 1 sec. and its frequency is 1 Hz. Alternating current, which is used for illumination, usually has 50 periods, ie it 50 times a second place in the forward direction and 50 times - in reverse. Its frequency is 50 Hz. Radio waves are excited by alternate currents of very high frequencies - from the hundreds of thousands to many millions of cycles per second - high-frequency currents.

There is a definite relationship between frequency and wavelength. If the velocity of propagation - 300 000 km per second - to divide the frequency in kilohertz (1 kHz = 1000 Hz), we obtain the wave length in meters. For example, if the frequency of the AC exciting radio wave is equal to 1000 kHz, the wavelength is 300 000 / 1000 = 300 m. Conversely, if 300 000 divided by the wavelength in meters, then we get the frequency in kHz. Radio waves can be characterized as the wavelength and frequency, both values ​​are equal.

Now we can construct the most general idea of ​​the transmitting station. At this station there is equipment that excites the alternating current required frequency and sends it to the antenna - a wire suspended on masts. Formed around the antenna electromagnetic waves, which are carried away in all directions. But how to get radio obediently carry the sounds that we want to throw thousands of miles away? To do this we must first convert sound into electrical current. This makes the device called a microphone. If the microphone to speak, sing or play, it makes sounds, ie, vibrations of air in an electric current, the strength of which varies according to changes in sound. Fluctuations in electrical current of low frequency sound or derived from the microphone, superimposed on the high frequency produced to create the radio waves. In line with this change and the nature of radio waves that are breaking away from the transmitting station antenna, are made with a microphone in front of sounds. But now the sounds are heard no more. It turns out that the invisible airwaves carry the inaudible sounds.

Radio waves have an interesting property. If they are on their way met any metal object, then they call it the appearance of the same high-frequency alternating currents which gave rise to the appearance of these radio waves at the transmitting station. Their frequency will be exactly the same and will remain imposed on them sound current. This property and are to receive the broadcasts. On the roof, on the masts or suspended on trees receiving antenna, in which the incoming radio waves excite currents rapidly. These currents come from the antenna to the radio, they are extremely weak and the weaker the farther the receiver is from the transmitter.

It is not hard to imagine that the receiving antenna currents will be excited many transmitting stations and receiving stations will therefore be necessary practically impossible. Of all of these currents must select only one who is excited by radio waves required us to the station. This can be accomplished by means of the so-called resonant circuits. It turns out that if you make a coil of wire and attach it to the condenser (that is, and how do condenser coil, the reader will learn later), we obtain an oscillating circuit, which responds to the alternating current of a certain frequency. By changing the data coil and a capacitor, you can configure the path to the desired frequency. So to address the issue of drop-out unwanted stations.

But it also is not all. When the set of currents running through the antenna, the desired current selected station from it must also provide a sound current, mixed with high-frequency current. This is done by the so-called detector. Once we get the detector is a sound current, which in turn is easy to sound. This makes the usual handset. When the phone passes through the sound current, the magnetic tube begins to beat with the current changes shake a thin iron plate - the membrane, the membrane is in turn shakes the air, and the oscillation of the air - this is the sound that is audible to human ears.

Everything described above is performed at an incredible rate and so the word spoken in front of a microphone in Moscow, almost at the same moment will be heard by man, sitting at the receiver over a thousand kilometers.

In conclusion, it should be noted that the distinguish the desired station, we can only, if the other stations will not work on the same wave. If two equally loud audible stations operate on the same or close to the waves, it is impossible to separate them, and they will interfere with each other.

The idea of ​​radio, which the reader has received from this story, of course, is only the most common, but it's enough to make a more conscious to begin construction of a simple receiver.

AM Transmitter maded by hands