Complete the following sentences using the information from the text
1In a conductor the outer electrons of the atoms … .
2An external influence will cause … .
3Atoms of insulating materials … .
4Insulators are characterized by … .
5Copper is the most popular material used for wire because … .
6Most non-metals are … .
Find in the text synonyms to the following words. Use them in your own sentences.
- to spread
- to repulse
- loosely
- opposition
- outer
- to isolate
- to grasp
6 
Put questions to the underlined words in the text and let your partner answer them.
7 
Speaking task. Tell about the properties of conductors and insulators using the illustration from the text. (see Fig. 1). While speaking use the following expressions.
| It is known that… | Moreover… |
| As a matter of fact… | To sum it up… |
| However… | As a result… |
| Nevertheless… | To summarize… |
| Besides… |
Text III
Useful terms. Read and memorize the following words and word combinations.
chord хорда окружности
circular mil круговой мил
circumference длина окружности
cross-sectional area площадь поперечного сечения
decimal десятичный
dimension размер, измерение
fraction дробь
mil мил
rectangular conductor провод прямоугольного сечения
square conductor провод квадратного сечения
square mil
strand жила, скрутка
stranded wire многожильный/скрученный провод
unit of measurement единица измерения
Before reading the text find the sentences where the terms mentioned above are used.
3 
You are going to read the text about conductor sizes. For questions (1-5) choose the answer (A, B, C or D) which you think fits best according to the text.
Conductor sizes
Conductors are the means used to tie components of electrical circuit together. Many factors determine the type of electrical conductor used to connect components. Some of these factors are the physical size of the conductor, its composition, and its electrical characteristics. Other factors that can determine the choice of a conductor are the weight, the cost, and the environment where the conductor will be used.
To compare the resistance and size of one conductor with that of another, we need to establish a standard or unit size. A convenient unit of measurement of the diameter of a conductor is the mil (0.001, or one-thousandth of an inch). A convenient unit of conductor length is the foot. The standard unit of size in most cases is the MIL-FOOT. A wire will have a unit size if it has a diameter of 1 mil and a length of 1 foot.
Square Mill
The square mil is a unit of measurement used to determine the cross-sectional area of a square or rectangular conductor (views A and B of Figure 2). A square mil is defined as the area of a square, the sides of which are each 1 mil. To obtain the cross-sectional area of a square conductor, multiply the dimension of any side of the square by itself. For example, assume that you have a square conductor with a side dimension of 3 mils. Multiply 3 mils by itself (3 mils X 3 mils). This gives you a cross-sectional area of 9 square mils.
To determine the cross-sectional area of a rectangular conductor, multiply the length times the width of the end face of the conductor (side is expressed in mils). For example, assume that one side of the rectangular cross-sectional area is 6 mils and the other side is 3 mils. Multiply 6 mils X 3 mils, which equals 18 square mils. Here is another example. Assume that a conductor is 3/8 inch thick and 4 inches wide. The 3/8 inch can be expressed in decimal form as 0.375 inch. Since 1 mil equals 0.001 inch, the thickness of the conductor will be 0.001 X 0.375, or 375 mils. Since the width is 4 inches and there are 1,000 mils per inch, the width will be 4 X 1,000, or 4,000 mils. To determine the cross-sectional area, multiply the length by the width; or 375 mils X 4,000 mils. The area will be 1,500,000 square mils.
Circular Mill
The circular mil is the standard unit of measurement of a round wire cross-sectional area (view C of Figure 2).
This unit of measurement is found in American and English wire tables. The diameter of a round conductor (wire) used to conduct electricity may be only a fraction of an inch. Therefore, it is convenient to express this diameter in mils to avoid using decimals. For example, the diameter of a wire is expressed as 25 mils instead of 0.025 inch. A circular mil is the area of a circle having a diameter of 1 mil, as shown in view B of Figure 3.
The area in circular mils of a round conductor is obtained by squaring the diameter, measured in mils. Thus, a wire having a diameter of 25 mils has an area of 252, or 625 circular mils. To determine the number of square mils in the same conductor, apply the conventional formula for determining the area of a circle (A = pr2). In this formula, A (area) is the unknown and is equal to the cross-sectional area in square mils, p; is the constant 3.14, and r is the radius of the circle, or half the diameter (D). Through substitution, A = 3.14, and (12.5)2; therefore, 3.14 X 156.25 = 490.625 square mils. The cross-sectional area of the wire has 625 circular mils but only 490.625 square mils. Therefore, a circular mil represents a smaller unit of area than the square mil.
If a wire has a cross-sectional diameter of 1 mil, by definition, the circular mil area (CMA) is A = D2, or A = 12, or A = 1 circular mil. To determine the square mil area of the same wire, apply the formula A = pr2; therefore, A = 3.14 X (.5)2 (.5 representing half the diameter).
When A = 3.14 X .25, A = .7854 square mil. From this, it can be concluded that 1 circular mil is equal to. 7854 square mil. This becomes important when square (view A of Figure 3) and round (view B) conductors are compared as in view C of Figure 3.
When the square mil area is given, divide the area by 0.7854 to determine the circular, multiply the area by 0.7854 to determine the square mil area.
A wire in its usual form is a single slender rod or filament of drawn metal. In large sizes, wire becomes difficult to handle. To increase its flexibility, it is stranded. Strands are usually single wires twisted together in sufficient numbers to make up the necessary cross-sectional area of the cable. The total area of stranded wire in circular mils is determined by multiplying the area in circular mils of one strand by the number of strands in the cable.
Circular-mill-foot
A circular-mil-foot (Figure 4) is a unit of volume. It is a unit conductor 1 foot in length and has a cross-sectional area of 1 circular mil. Because it is a unit conductor, the circular-mil-foot is useful in making comparisons between wires consisting of different metals.
For example, a basis of comparison of the resistivity of various substances may be made by determining the resistance of a circular-mil-foot of each of the substances.
In working with square or rectangular conductors, such as ammeter shunts and bus bars, you may sometimes find it more convenient to use a different unit volume. A bus bar is a heavy copper strap or bar used to connect several circuits together. Bus bars are used when a large current capacity is required.
Unit volume may be measured as the centimeter cube. Specific resistance, therefore, becomes the resistance offered by a cube-shaped conductor 1 centimeter in length and 1 square centimeter in cross-sectional area. The unit of volume to be used is given in tables of specific resistances.
1Why has a “unit size” for conductors been established?
ATo compare the size and resistance of one conductor with that of another.
BTo establish a uniform style for conductors.
CTo determine the requirements for conductors.
DTo ensure all conductors are interchangeable.
2What is the decimal equivalent of one mil?
A1.000 in
B0.100 in
C0.010 in
D0.001 in.
3What is the definition of a mil foot?
AA conductor 0.001 foot in length with a diameter of 0.001 millimeter.
BA conductor 1 foot in length with a diameter of 0.001 foot.
CA conductor 1 foot in length with a diameter of 1 mil.
DA conductor 0.001 foot in length with a diameter of 0.001 inch.
4A square mil is defined as the area of a square, the sides of which are each equal in length to what dimension?
A1 mil-foot
B1 mil
C1.0 inch
D0.001 mil
5A circular mil is defined as the area of a circle having what dimension?
AA radius of 1 mil
BA diameter of 1 mil
CA circumference of 1 mil
DA chord of 1 mil