III. Reading.

Exercise 1. Read and translate the text.

Text A.

Electric current

Ever since Volta first produced a source of continuous current, men of science have been forming theories on this subject. For some time they could see no real difference between the newly-discovered phenomenon and the former understanding of static charges. Then the famous French Scientist Ampere (after whom the unit of current was named) determined the difference between the current and the static charges. In addition to it, Ampere gave the current direction: he supposed the current to flow from the positive pole of the source round the circuit and back again to the negative pole.

We consider Ampere to be right in his first statement but he was certainly wrong in the second, as to the direc­tion of the current. The student is certain to remember that the flow of current is in a direction opposite to what he thought.

Let us turn our attention now to the electric current itself. The current which flows along wires consists of mov­ing electrons. What can we say about the electron? We know the electron to be a minute particle having an elec­tric charge. We also know that that charge is negative. As these minute charges travel along a wire, that wire is said to carry an electric current.

In addition to traveling through solids, however, the electric current can flow through liquids as well and even through gases. In both cases it produces some most impor­tant effects to meet industrial requirements.

Some liquids, such as melted metals for example, conduct current without any change to themselves. Others, called electrolytes, are found to change greatly when the current passes through them.

When the electrons flow in one direction only, the cur­rent is known to be d. c, that is, direct current. The sim­plest source of power for the direct current is a battery, for a battery pushes the electrons in the same direction all the time (i.e., from the negatively charged terminal to the positively charged terminal).

The letters a.c. stand for alternating current. The cur­rent under consideration flows first in one direction and then in the opposite one. The a.c. used for power and light­ing purposes is assumed to go through 50 cycles in one sec­ond. One of the great advantages of a.c. is the ease with which power at low voltage can be changed into an almost similar amount of power at high voltage and vice versa. Hence, on the one hand alternating voltage is increased : when it is necessary for long-distance transmission and, on the other hand, one can decrease it to meet industrial re­quirements as well as to operate various devices at home.

Although there are numerous cases when d.c. is re­quired, at least 90 percent of electrical energy to be gen­erated at present is a.c. In fact, it finds wide application for lighting, heating, industrial, and some other purposes.

One cannot help mentioning here that Yablochkov, Rus­sian scientist and inventor, was the first to apply a.c. in practice.