Category Science

How does a video-tape recorder work?

          The unique feature of a video-tape recorder (VTR) is that it plays back both sound and picture. It is mainly used to record television programmes as magnetic patterns and play video cassettes. But how does the video-tape recorder work?

          A video-tape is a band of plastic tape. On one side, it is coated with a film of magnetic iron oxide whose thickness is about one-five thousandth of a centimetre. The width of the tape is about 1.25 to 2.5 cm. For recording a programme, the tape is run by a magnetic video tape recorder.

          A television camera changes an image into electrical signals. At the same time, a microphone changes sound into electrical signals. These signals are then fed into the recorder. The VTR contains recording heads that convert the signals into varying magnetic fields. As the magnetic tape passes these heads, they produce magnetic patterns on the tape. This tape can then be used to reproduce the original sound and picture. When the tape is played back, the changing magnetic fields of the pattern of iron oxide particles create weak currents which exactly correspond to the recorded sound and picture.

          The sound and picture signals are kept separated in the recorder, and are recorded on to different parts of the tape. Usually, the sound signal is recorded on to a narrow track at the top of the tape. The image signal is recorded on to a wider track in the middle of the tape. A control signal is recorded along the bottom of the tape. Television studios generally use 5 cm-wide tape. The tape moves at a speed of 37.5 cm a second.

          The head that records the image signal rotates, as the tape passes by it. As a result, the recording is made in diagonal bands across the tape. This allows more information to be stored on a given length of tape.

          Video tapes are used to record and reproduce various television programmes. They are also used for the reproduction of sport events during a live broadcast. Video tapes are also used in slow motion and stop-action techniques. Nowadays video discs having pictures as well as sound recordings are also available to see a film on the disc, by playing it on a video disc player connected to a television set. 

How does a microphone work?

          Radio and television stations make use of microphones. They are also used in public address systems and in motion pictures and phonograph records. The mouth piece of a telephone is a simple type of microphone. Let us see what exactly a microphone is.

          A microphone is a device which converts sound waves into electrical signals. These signals can then be broadcast through the air or sent over to distant points, where they can again be converted back into sound.

          Microphones can be divided into two groups depending upon how they respond to sound waves. These are: the pressure type and the velocity type.

          The pressure type microphones contain a thin metal plate called a diaphragm. This is stretched like a drumhead inside a rigid frame. The diaphragm is a part of the electrical circuit. When the sound waves strike the diaphragm, it starts vibrations at the same rate as the sound waves. These vibrations produce corresponding electric signals by changing the electric current that flows through the circuit.

          The pressure microphones are of several types, such as condenser microphone, moving coil or dynamic microphone, the crystal microphone and the carbon microphone.

 

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What is a Mass Spectrograph?

               A mass spectrograph is an instrument used to analyze the constituents of substances. It not only detects different kinds of atoms and molecules present in the substance, but also finds out their relative amounts. By the use of electric and magnetic fields, it separates ions of different masses. Do you know how this instrument works?

               The working of the mass spectrograph first involves the change of the substance into a gas, which is passed into a vacuum chamber. A beam of electrons is bombarded to change the gas atoms and molecules into ions. The ions are then accelerated, by passing them through an electric field. Then the ions are passed through a magnetic field, where they get deflected. The positive ions are deflected one way, and the negative ions in the opposite direction. The amount of deflection is inversely proportional to the masses of the ions. The heavier the mass, the lesser the deflection. This separates ions of different masses. Ions of the same mass and charge stay together. The ions are then allowed to fall on a photographic plate. Different ions hit the plate at different places and as a result, this photographic plate records the amounts of various atoms and molecules. Photographic plate is used to identify different ions which have hit it. From the intensity variations on the plate, we can know the relative amounts of atoms or molecules present in the substance. 

               The mass spectrograph was developed by a British scientist, William Francis Aston. He was awarded the Nobel Prize in 1922 for this invention. After this, several other mass spectrographs were developed by many leading scientists like Dempster, Bainbridge, Nier, etc but all were just modifications of Aston’s mass spectrograph.

              The mass spectrograph is widely used in geology, chemistry, biology and nuclear physics. It is a very useful instrument for isotopic studies. Aston himself discovered 212 of the 287 naturally occurring isotopes. Mass spectrographs are also used as vacuum leak detectors.

 

What are Quasars?

In 1960, very strong radio emissions were observed by an American astronomer, Allan R. Sandage to be coming from certain localized direction in the sky. When viewed on the photographic plate, they appeared like stars. But they were not stars, as proved by their other characteristics including a large red shift. The accurate position measurement of these star like objects on optical photographs, led to the discovery of a new class of objects in the universe, the quasars (quasi-stellar sources).

They appear star like on the photograph because their angular diameters are less than about 1 second of an arc, which is the resolution limit of ground-based optical telescopes. Since stars also have angular diameters much less than this, they too appear unresolved or point-like on a photograph.

In 1962 a much brighter star like object 3C273 was identified by Maarten Schmidt with the help of a radio telescope in Australia. Its red shift was found to be 0.158. This red shift turned out to be far larger than any other that had been detected for ordinary galaxies. These observations established the existence of quasars beyond doubt.

Quasars are generally much bluer than most of the stars, except white dwarf stars. The blueness of quasars, as an identifying characteristic, led to the discovery that many blue star like objects have a large red shift, and are therefore quasars. Till today scientists have studied more than 1000 quasars but their nature and distance from earth remain a puzzle.

Quasars consist of a massive nucleus with a total size of less than a light year, which is surrounded by an extended halo of gas excited by the energy radiated by the central object. The central object emits radiation over a wide spectral range. Some quasars emit significant amount of energy at radio frequencies ranging from about 30 MHz to 100 GHz. It is believed that the energy emitted by quasars is gravitational and not thermonuclear in origin. More than ergs of energy are released in quasars over their life-time.

Till to day scientists have not been able to measure the exact distance of quasars from the earth. Various similarities of quasars with radio galaxies strongly suggest that quasars are also active nuclei of galaxies might be associated with the birth of some galaxies. Studies have shown that quasars must have been much more common in the universe about many years ago.

 

How can we extinguish fires?

          We are all aware of the damage and disaster a fire can cause in certain situations. Now let us see how to control a fire and prevent it from spreading.

          A fire is basically a chemical reaction during which heat and light are produced. Three factors are necessary for a fire to start – fuel, oxygen or air, and heat to raise the temperature of the fuel to its ignition temperature.

          A fire can be extinguished when one or more of these agents is removed, i.e. fuel, supply of air and lowering the temperature of the combustible substance. All fire extinguishing methods make use of these principles.

          The original fire extinguisher, a bucket of water, is still useful in controlling many types of fires. The principal effect of water on a fire is to cool the burning material, thus removing the heat – one of the factors without which combustion cannot continue. It can be applied in a variety of ways such as by flooding the fire with water. Jets of water are used to knock down the flames of fire, and sprays are used to absorb heat and drive back smoke and gases.

           Another common extinguisher is the soda-acid type. It sprays a mixture of water and carbon dioxide on the fire. This is based upon the principle of cooling the burning material and cutting the supply of air by non-combustible carbon-dioxide.

           In this extinguisher a solution of sodium bicarbonate is placed in a cylindrical vessel of steel. Sulphuric acid is kept in a bottle in a small compartment made within the cylinder, near the top. When required, the knob is hit against the floor. This brings the sodium bicarbonate and sulphuric acid in contact with each other. Immediately carbon dioxide is formed and it comes out of the fire nozzle which is directed towards the fire. These extinguishers are useful only for small and localized fires. They are not effective against gasoline, oil and electrical fires.

           Foam extinguishers are based upon the principle of cutting off the supply of air by forming a fire-proof coating of foam around the burning material. In this, a mixture of sodium bicarbonate and aluminium sulphate containing licorice extract is sprayed. It produces foam and extinguishes the fire.

           The other types of extinguishers that are used on oil and electrical fires are: Carbon dioxide extinguishers, dry-chemical extinguishers and vaporizing liquid extinguishers.

           Water should never be used for extinguishing electrical or oil fires. In case of electrical fires, it can cause electrocution. If water is used on burning oil, the oil simply floats on top of water and continues to burn. As the water flows away, it can carry the oil with it and so spread the fire.

           Fire extinguishers are provided by law in all public buildings, factories and schools. Most of the big cities have fire brigades for fire prevention and control.

 

How does a polaroid camera take instant photographs?

          The polaroid camera is also known as the ‘instant camera’ because it takes pictures and develops them in a matter of minutes. It was invented by Edwin H. Land of the United States and the first polaroid camera was sold in 1948. At that stage, it took only black and white photographs. Later, another camera was built that could take pictures and develop colour photographs.

          Polaroid cameras are loaded with a double picture roll. One part is a negative roll of the film, and the other a positive roll of a special printing paper. Small pods (containers) of chemicals are joined to the positive roll. After exposure to light through the camera’s lens, the negative and positive rolls are made to pass through a pair of rollers that break the chemical pods. The chemicals flow over the exposed portion of the negative roll and develop a negative image on the roll – the parts of the picture that should be black are white, and the parts that should be white are black. More chemical reactions take place between the pod chemicals and the chemicals coated on the positive roll, and a positive photograph is made – the white areas in the photograph are printed white and the black areas black. This process takes about 10 seconds for a black and white photograph and upto a minute for a colour one. 

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