Text 1. Types of Electric Current

The electric current was born in the year 1800 when A. Volta constructed the first source of continuous current. Since that time numerous scientists and inventors, Russian and foreign, have contributed to its development and practical application. According to¹ electronic theory it is the electron that sets up a flow of electric current. In other words an electric current is a flow of electrons in a circuit per second of time. The flow of electricity through a circuit is called the electric current and is measured in amperes. The instrument for measuring electric current is called an ammeter.

The electric current flows in circuits formed of metallic conductors. Sometimes the circuit includes, in addition to² metal conductors, electrolytes (as in batteries), gaseous conductors (as in fluorescent lamps), and streams of electrons and ions (as in electric tubes). In all these cases the passage of the current results in the production of heat and light, or both and in the case of electrolytes it causes chemical changes.

There are different types of current having great importance for our industry, but we shall discuss only some of them. An electric current, which always flows in the same direction through a conductor and does not change its polarity, is called a direct current (d.c. or D.C.). A direct current generally remains at constant voltage, that is, the voltage neither rises nor falls. A pulsating direct current is that kind of electric current that flows in the same direction through a circuit but varies in its voltage value.

An alternating current is an electric current, alternating or varying both in amount and direction, its abbreviation is A.C. The alternation may occur (take place) from a few per second up to several million per second. Two alternations make a cycle. The number of cycles in one second is called the frequency of an alternating current.

An oscillating current is similar to A.C. in shape but has a constant frequency and its amplitude is damped. Transient currents usually damp away extremely rapidly. One should mention unidirectional transient and A.C. transient types of electric current.

(1800 t.un.)

¹ according to – согласно, в соответствии с.

² in addition to – кроме.

Text 2. Sources of Electrical Energy

To produce an electric current requires employing a chemical, as in the battery, or a mechanical, as in the electromagnetic generator, source of energy.

Energy is not created by an electric energy source, but is only converted into an electrical form from another one, such as mechanical, chemical, thermal or radiant energy. In waterpower plants, the energy of falling water is converted into electrical or into heat when coal is burnt. Chemical sources of current have limited application as great quantities of electric energy generated today come from various forms of mechanical energy.

For the first time mechanical energy was converted into electrical one by means of electrostatic machines. As industrial application of energy increases greater amount of energy is needed every year. The energy needs will continue to grow while the energy sources of the world are decreasing.

Coal oil and gas have been man’s main sources of heat from ancient times and today they are our basic sources of energy. However, coal and other kinds of fuel are often replaced by atomic energy. Man has learned to split atoms with the help of a nuclear reactor in order to get great quantities of energy. For example, a ton of uranium can give us as much energy as about 3 million tons of coal.

The Earth contains inside it a practically inexhaustible supply of thermal energy, which can be used for economic needs.

Another unlimited source of energy is the Sun. But at present only a small part of solar energy is being used. Solar power station will permit to generate cheap electricity in large quantities in the near future.

An enormous energy of the wind, exceeding millions of kilowatt-hours, has not yet found effective application. Utilization of tidal energy has been studied in many countries. Using tidal forces and wind energy will certainly make considerable contribution to future electricity production.

(1605 t.un.)