A voltmeter. The volt. The ampere. The ohm. A galvanometer.
Example:
An ammeter. – An ammeter is an instrument used to measure the current.
Conductors
According to their conductivity all materials are divided into conductors, insulators and semiconductors. Conductors allow electricity to pass through them; insulators do not. Conductors are materials having a very high conductivity. A conductor owes its conducting powers to the presence of free electrons which have broken away from their atoms. The most common of them are metals (copper, aluminium, steel and others), carbon and electrolytes. Their coefficient of resistance is different. Copper, for example has a positive coefficient while carbon has a negative coefficient of resistance.
According to their resistivity, conductors are divided into two groups; the first includes materials with low resistivity; copper, aluminium and others. The second group includes materials with high resistivity: one of these is nichrome. Due to its good heat resistance, nichrome is used to produce heaters. Carbon is commonly used to produce electrodes and brushes for electric machines.
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6.1 Test. Choose the correct variant
1. All materials are divided into
a) conductors and semiconductors
b) conductors, insulators and semiconductors
2. Copper has
a) high mechanical strength
b) low mechanical strength
3. Carbon has
a) a positive coefficient of resistance
b) a negative coefficient of resistance
4. Copper is commonly used to produce
a) heaters
b) brushes
Answer the questions
1. Into what groups are materials divided?
2. What do conductors do?
3. What materials are common conductors?
4. What metals have high mechanical strength?
Semi-conductors
The term “Semi-conductors” means “half-conductor”. They include a great variety of elements (selicon, germanium, selenium, phosphorus and others), many chemical compounds (oxides, sulphides) as well as numerous ores and minerals.
The conductivity of semi-conductors increases sharply with heating and falls with cooling. This dependence has opened great prospects for employing semi-conductors in measuring techniquies. Light as well as heat increases the conductivity of semi-conducting materials, this principle being used in creating photo resistances. It is also widely applied for switches on engines, for counting parts on a conveyer belt, as well as for various systems of emergency signals and for reproducing sound in cinematography. Besides reacting to light, semi-conductors react to all kinds of radiations and therefore employed in designing electronic counters.
Sunlight, like heat, can feed our electric circuits. Photocells made of semi-conducting materials are capable of transforming ten per cent of sun-ray energy into electric power. The electricity generated by semi-conductor thermocouples can produce not only heat but also cold, this principle being used in manufacturing refrigerators. Semi-conducting materials are also excellent means of maintaining a constant temperature.
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7.1 Test. Choose the correct item
1. What is a “semi-conductor”? – It is…
a) conductor
b) non-conductor
c) insulator
d) half-conductor
1. The conductivity of semi-conductor …
a) falls with heating
b) increases with heating
2. Light…
a) decreases the conductivity of semi-conductor material
b) increases the conductivity of semi-conductor material
3. Semi-conductors react
a) to all kinds of radiation
b) not to all kinds of radiation
4. Sunlight, like heat
a) can feed our electric circuits
b) cannot feed our electric circuits
5. Electricity generated by semi-conductors
a) can produce heat
b) can produce cold
c) cannot produce heat and cold
d) can produce heat and cold
Answer the questions
2. What does the term “semi-conductor” mean?
3. Do semi-conductors include many elements?
4. Can light increase the conductivity of semi-conductors?
5. Do semi-conductors react only to light and heat?
6. What principle is used in manufacturing refrigerators?
Dynamo
Dynamo is a common device for converting mechanical energy into electric energy. This process depends on the fact that if an electrical conductor moves across a magnetic field, an electric current flows in the conductor.
Usually a dynamo includes an electromagnet, called the field magnet, between the poles of which a suitable conductor, usually in the form of a coil, called the armature, is rotated. The mechanical energy of the rotation, in the form of a current in the armature, is thus converted into electric energy.
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