Nucleus . more than two electrons. In an atom containing (1 Proton) more than two electrons, the excess electrons will Electron be located in the outer shells. The second shell can have a maximum of eight electrons. The third shell can hold up to 18 electrons, the fourth 32, etc. It should be noted, however, that in some large complex atoms electrons may be arranged in outer shells before some inner shells are filled.
FIGURE 8-2. Hydrogen atom.
The neutrons in a nucleus have no electrical charge. They are neither positive nor negative but are equal in size and weight to the proton.
Since a proton weighs approximately 1,845 times as much as an electron, the overall weight of an atom is determined by the number of protons and neutrons in its nucleus. The weight of an electron is not considered in determining the weight of an atom. Indeed, the nature of electricity cannot be defined clearly because it is not certain whether the electron is a negative charge with no mass (weight) or a particle of matter with a negative charge.
Electricity is best understood in terms of its behavior, which is based in part on the charge an atom carries. When the total positive charge of the protons in the nucleus equals the total negative charge of the electrons in orbit around the nucleus, the atom is said to have a neutral charge. If an atom has a shortage of electrons, or negative charges, it is positively charged and is called a positive ion. If it possesses an excess of electrons, it is said to be negatively charged and is called a negative ion.
Electron Movement In a state of neutral charge, an atom has one electron for each proton in the nucleus. Thus, the number of electrons held by the atoms making up the various elements will vary from one, in the case of hydrogen, to 92 for uranium.
The electrons revolving around a nucleus travel in orbits, sometimes called shells or layers. Each shell can contain a certain maximum number of electrons, and if this number is exceeded, the extra electrons will be forced into the next higher, or outer, shell.
The shell nearest the nucleus can contain no
STATIC ELECTRICITY Electricity is often described as being either static or dynamic. Since all electrons are alike, these words do not actually describe two different types of electricity; rather, they distinguish between electrons at rest and those in motion. The word static means "stationary" or "at rest," and refers to the deficiency or to the excess of electrons. Originally it was thought that static electricity was electricity at rest because electrical energy produced by friction did not move. A simple experiment, such as running a dry comb through hair, will produce cracking or popping sounds, indicating static discharges are taking place. The charges thus built up consist of electrons transferred to the comb as the result of friction. The discharge is caused by the rapid movement of electrons in the opposite direction from the comb to the hair as the charges neutralize each other. In the dark it is possible to see these discharges as tiny sparks.
Static electricity has little practical value, and often causes problems. It is difficult to control and discharges quickly. Conversely, dynamic, current electricity, is generated and controlled easily and provides energy for useful work.
A summary of that part of the electron theory 8 Protons 8 Neutrons 7 FIGURE 8-3. Oxygen atom.
dealing with charges will help explain static Charged rod electricity. All electrons are alike and repel each other. Similarly, all protons are alike and repel each other. Electrons and protons are not alike, but attract each other. Hence, the fundamental law of electricity is that like charges repel and unlike charges attract. Pithballs Generation of Static Electricity Static electricity can be produced by contact, friction, or induction. As an example of the friction method, a glass rod rubbed with fur becomes negatively charged, but if rubbed with silk, becomes positively charged. Some materials that build up static electricity easily are flannel, silk, rayon, amber, hard rubber, and glass.
When two materials are rubbed together, some electron orbits of atoms in one material may cross the orbits or shells of the other, and one material may give up electrons to the other. The transferred electrons are those in the outer shells or orbits and are called free electrons.
When a glass rod is rubbed with silk, the glass rod gives up electrons and becomes positively charged. The silk becomes negatively charged since it now has excess electrons. The source of these electric charges is friction. This charged glass rod may be used to charge other substances.
For example, if two pith balls are suspended, as shown in figure 8-4, and each ball is touched with the charged glass rod, some of the charge from the rod is transferred to the balls. The balls now have similar charges and, consequently, repel each other as shown in part B of figure 8-4. If a plastic rod is rubbed with fur, it becomes negatively charged and the fur is positively charged.
By touching each ball with these differently charged sources, the balls obtain opposite charges and attract each other as shown in part C of figure 8-4.
Although most objects become charged with static electricity by means of friction, a charged substance can also influence objects near it by contact. This is illustrated in figure 8-5. If a positively charged rod touches an uncharged metal bar, it will draw electrons from the uncharged bar to the point of contact. Some electrons will enter the rod, leaving the metal bar with a deficiency of electrons (positively charged) and making the rod less positive than it was or, perhaps, even neutralizing its charge completely.
FIGURE 8-4. Reaction of like and unlike charges.
A method of charging a metal bar by induction is demonstrated in figure 8-6. A positively charged rod is brought near, but does not touch, an uncharged metal bar. Electrons in the metal bar are attracted to the end of the bar nearest the positively charged rod, leaving. a deficiency of electrons at the opposite end of the bar. If this positively charged end is touched by a neutral object, electrons will flow into the metal bar and neutralize the charge. The metal bar is left with an overall excess of electrons.
Electrostatic Field A field of force exists around a charged body.
This field is an electrostatic field (sometimes called a dielectric field) and is represented by lines extending in all directions from the charged 269
Positively charged rod almost touching.
uncharged bar + + Electrons are attracted by positive charge + When rod touches bar, electrons enter rod + The rod is now less Metal bar now has positively charged positive charge FIGURE 8-5. Charging by contact.
body and terminating where there is an equal and opposite charge.
l'o explain the action of an electrostatic field, lines are used to represent the direction and intensity of the electric field of force. As illustrated in figure 8-7, the intensity of the field is indicated by the number of lines per unit area, and the direction is shown by arrowheads on the lines pointing in the direction in which a small test + Electrons are attracted toward charged rod.
• ...
Electrons are attracted off finger and enter bar.
FIGURE 8-7. Direction of electric field around positive and negative charges.
charge would move or tend to move if acted upon by the field of force.
Either a positive or negative test charge can be used, but it has been arbitrarily agreed that a small positive charge will always be used in determining the direction of the field. Thus, the direction of the field around a positive charge is always away from the charge, as shown in figure 8-7, because a positive test charge would be repelled. On the other hand, the direction of the lines about a negative charge is toward the charge, since a positive test charge is attracted toward it.
Figure 8-8 illustrates the field around bodies having like charges. Positive charges are shown, but regardless of the type of charge, the lines of force would repel each other if the charges were alike. The lines terminate on material objects and always extend from a positive charge to a negative charge. These lines are imaginary lines used to show the direction a real force takes.
It is important to know how a charge is distributed on an object. Figure 8-9 shows a small metal disk on which a concentrated negative charge has been placed. By using an electrostatic Finger is removed. Positive and negative charges are mostly neutralized. + Rod is removed and excess electrons remai.
FIGURE 8-8. Field around two positively FIGURE 8-6. Charging a bar by induction. charged bodies.
FIGURE 8-9. Even distribution of charge on metal disk.
detector, it can be shown that the charge is spread evenly over the entire surface of the disk.
Since the metal disk provides uniform resistance everywhere on its surface, the mutual repulsion of electrons will result in an even distribution over the entire surface.
Another example, shown in figure 8-10, is the charge on a hollow sphere. Although the sphere is made of conducting material, the charge is evenly distributed over the outside surface. The inner surface is completely neutral. This phenomenon is used to safeguard operating personnel of the large Van de Graaff static generators used for atom-smashing. The safest area for the operators is inside the large sphere, where millions of volts are being generated.
The distribution of the charge on an irregularly shaped object differs from that on a regularly shaped object. Figure 8-11 shows that the charge on such objects is not evenly distributed. The greatest charge is at the points, or areas of sharpest curvature, of the objects.
The effects of static electricity must be considered in the operation and maintenance of aircraft. Static interference in the aircraft communication systems and the static charge created by the aircraft's movement through the air are examples of problems created by static electricity.
Parts of the aircraft must be "bonded" or joined together to provide a low-resistance (or easy) path for static discharge, and radio parts must Greatest Charge FIGURE 8-11. Charge on irregularly shaped objects.
be shielded. Static charges must be considered in the refueling of the aircraft to prevent possible igniting of the fuel, and provision must be made to ground the aircraft structure, either by staticconducting tires or by a grounding wire.
ELECTROMOTIVE FORCE The flow of electrons from a negative point to a positive point is called an electric current; this current flows because of a difference in electric pressure between the two points.
If an excess of electrons with a negative charge exists at one end of a conductor and a deficiency of electrons with a positive charge at the other, an electrostatic field exists between the two charges.
Electrons are repelled from the negatively charged point and are attracted by the positively charged point.
The flow of electrons of electric current can be compared to the flow of water between two interconnected water tanks when a difference of pressure exists between two tanks. Figure 8-12 shows the level of water in tank A to be at a higher level than the water level in tank B. If the valve in the interconnecting line between the tanks is opened, water will flow from tank A into tank B Inside Surface Neutral FIGURE 8-10. Charge on a hollow sphere. FIGURE 8-12. Difference of pressure.
until the level of water is the same in both tanks.
It is important to note that it was not the pressure in tank A that caused the water to flow;
rather, it was the difference in pressure between tank A and tank B that caused the flow. When the water in the two tanks are at the same level, the flow of water ceases because there is no longer a difference of pressure.
This comparison illustrates the principle that causes the electrons to move, when a proper path is available, from a point of excess electrons to a point deficient in electrons. The force that causes this movement is the potential difference in electrical energy between the two points. This force is called the electrical pressure or the potential difference or the electromotive force (electron-moving force) which can all be considered the same thing. Electromotive force, abbreviated e.m.f., causes current (electrons) to move in an electric path or circuit. The practical unit of measurement of e.m.f., or potential difference, is the volt. The symbol for e.m.f. is the capital letter "E."
If the water pressure in tank A of figure 8-12 is 10 p.s.i. and the pressure in tank B is 2 p.s.i., there is a difference in pressure of 8 p.s.i. Similarly, it can be said that an electromotive force of 8 volts exists between two electrical points. Since potential difference is measured in volts, the word "voltage" can also be used to describe amounts of potential difference. Thus, it is correct to say that the voltage of a certain aircraft battery is 24 volts, another means of indicating that a potential difference of 24 volts exists between two points connected by a conductor.
CURRENT FLOW Electrons in motion make up an electric current.
This electric current is usually refered to as "current" or "current flow," no matter how many electrons are moving. When the current flow is in one direction only, it is called direct current. Later in the study of electrical fundamentals, current that reverses itself periodically, called alternating current, will be discussed. In the present study all references are to direct current.
Since an electric current may consist of varying numbers of electrons, it is important to know the number of electrons flowing in a circuit in a given time. Electrons can be counted by measuring the basic electrical charge on each electron.
FIGURE 8-13. Electron movement.
Since this charge is very small, a practical unit, the coulomb, is used to measure an amount, or quantity, of electrical charge. The accumulated charge on 6.28 billion billion electrons is called one coulomb. When this quantity of electrons flows past a given point in an electrical circuit, one ampere of current is said to be flowing in the circuit. Current flow is measured in amperes or parts of amperes by an electrical instrument called an ammeter. The symbol used to indicate current in formulas or on schematics is the capital letter "I," which stands for the intensity of current flow.
The drift of free electrons must not be confused with the concept of current flow that approaches the speed of light. When a voltage is applied to a circuit, the free electrons travel but a short distance before colliding with atoms. These collisions usually knock other electrons free from their atoms, and these electrons travel on toward the positive terminal of the wire, colliding with other atoms as they drift at a comparatively slow rate of speed. To understand the almost instantaneous speed of the effect of electric current, it is helpful to visualize a long tube filled with steel balls as shown in figure 8-13.
It can be seen that a ball introduced in one end of the tube, which represents a conductor, will immediately cause a ball to be emitted at the opposite end of the tube. Even if the tube were long enough to reach clear across the country, this effect could still be visualized as being instantaneous. Thus, electric current flow can be viewed as occurring instantaneously, even though it is a result of a comparatively slow drift of electrons.
RESISTANCE The property of a conductor of electricity that limits or restricts the flow of electric current is called its resistance. Electrical pressure is required to overcome this resistance, which is the attractive force holding the electrons in their orbits. The materials from which electrical conductors are manufactured, usually in the form of extruded wire, are materials that offer very little resistance to current flow. While wire of any size 272
or resistance value may be used, the word "conductor" usually refers to materials which offer low resistance to current flow, and the word "insulator" describes materials that offer high resistance to current. There is no distinct dividing line between conductors and insulators; under the proper conditions, all types of material conduct some current. Materials offering a resistance to current flow midway between the best conductors and the poorest conductors (insulators) are sometimes referred to as semiconductors," and find their greatest application in the field of transistors.
The best conductors are materials, chiefly metals, which possess a large number of free electrons; conversely, insulators are materials having few free electrons. The best conductors are silver, copper, gold, and aluminum, but some nonmetals, such as carbon and water, can be used as conductors. Materials such as rubber, glass, ceramics, and plastics are such poor conductors that they are usually used as insulators. The current flow in some of these materials is so low that it is usually considered zero.
The unit used to measure resistance is called the ohm. The symbol for the ohm is the Greek letter omega(Q). In mathematical formulas, the capital letter "R" refers to resistance. The resistance of a conductor and the voltage applied to it determine the number of amperes of current flowing through the conductor. Thus, 1 ohm of resistance will limit the current flow to 1 ampere in a conductor to which a voltage of I volt is applied.
Factors Affecting Resistance Among the four major factors affecting the resistance of a conductor, one of the most important is the type of conductor material. It has been pointed out that certain metals are commonly used as conductors because of the large number of free electrons in their outer orbits.
Copper is usually considered the best available conductor material, since a copper wire of a particular diameter offers a lower resistance to current flow than an aluminum wire of the same diameter However, aluminum is much lighter than copper, and for this reason as well as cost considerations, aluminum is often used when the weight factor is important.
A second resistance factor is the length of the conductor. The longer the length of a given 2 Feet.
2 Ohms KAmp 1 Foot 1 0hm . Amp FIGURE 8-14. Resistance varies with length of conductor.
size of wire, the greater the resistance. Figure 8-14 pictures two wire conductors of different lengths. If 1 volt of electrical pressure is applied across the two ends of the conductor that is I foot in length and the resistance to the movement of free electrons is assumed to be 1 ohm, the current flow is limited to 1 ampere.
If the same size conductor is doubled in length, the same electrons set in motion by the 1 volt applied now find twice the resistance; consequently, the current flow will be reduced by onehalf.
A third factor affecting the resistance of a conductor is cross-sectional area, or the end surface of a conductor. This area may be triangular or even square, but is usually circular. If the crosssectional area of a conductor is doubled, the resistance to current flow will be reduced in half This is true because of the increased area in which an electron can move without collision or capture by an atom. Thus, the resistance varies inversely with the cross-sectional area of a conductor.
To compare the resistance of one conductor with that of another having greater cross-sectional area, a standard, or unit, size of conductor must be established. The most convenient unit of measurement of wire diameter is the mil (0.001 of an inch). The most convenient unit of wire length is the foot. Using these standards, the unit of size will be the mil-foot. Thus, a wire will have unit size if it has a diameter of 1 mil and the length of 1 foot. The resistance specified in ohms of a unit conductor of a certain material is called the 273
. 1 Mil FIGURE 8-15. Circular mil.
specific resistance, or specific resistivity, of the substance.
The square mil is a convenient unit of crosssectional area for square or rectangular conductors. A square mil is the area of a square, each side of which measures 1 mil.
To compute the cross-sectional area of a conductor in square mils, the length in mils of one side is squared. In the case of a rectangular conductor, the length of one side is multiplied by the length of the other. For example, a common rectangular bus bar (large, special conductor) is 8 inch thick and 4 inches wide. The 8 inch thickness may be expressed as 0.375 inch. Since 1,000 mils equals 1 inch, the width in inches can be converted to 4,000 mils. The cross-sectional area of the rectangular conductor is .375 X 4,000 or 1,500 square mils.
More common than the square or rectangular shape is the circular conductor. Because the diameters of round conductors may be only a fraction of an inch, it is convenient to express these diameters in mils to avoid the use of decimals. The circular mil is the standard unit of wire cross-sectional area used in American and English wire tables. Thus, the diameter of a wire that is 0.025 inch may be more conveniently expressed as 25 mils.
Figure 8-15 illustrates a circle having a diameter of 1 mil. The area in circular mils is obtained by squaring the diameter measured in mils. Thus, a wire with a diameter of 25 mils has an area of 25 squared, or 25 X 25, or 625 circular mils.
In comparing square and round conductors, it should be noted that the circular mil is a smaller unit of area than the square mil. To determine the circular-mil area when the square-mil known, the area in square mil is divided by
0.7854. Conversely, to find the square-mil area when the circular-mil area is known, the area in circular mils is multiplied by 0.7854.
Wires are manufactured in sizes numbered according to a table known as the American wire gage (AWG). Wire diameters become smaller as the gage numbers become larger. This table is available to aviation technicians for reference, not only on wire size but also resistance and cross-sectional area.
The last major factor influencing the resistance of a conductor is temperature. Although some substances, such as carbon, show a decrease in resistance as the ambient (surrounding) temperature increases, most materials used as conductors increase in resistance as temperature increases. The resistance of a few alloys, such as constantan and maganin, change very little as the temperature changes. The amount of increase in the resistance of a 1-ohm sample of a conductor per degree rise in temperature above 0° Centigrade (C.), the assumed standard, is called the temperature coefficient of resistance. For each metal this is a different value; for example, for copper the value is approximately 0.00427 ohm.
Thus, a copper wire having a resistance of 50 ohms at a temperature of 0° C. will have an increase in resistance of 50 X 0.00427, or 0.214 ohm, for each degree rise in temperature above 0° C. The temperature coefficient of resistance must be considered where there is an appreciable change in temperature of a conductor during operation. Charts listing the temperature coefficient of resistance for different materials are available.
BASIC CIRCUIT COMPONENTS AND SYMBOLS An electrical circuit consists of: (1) A source of electrical pressure or e.m.f.; (2) resistance in the form of an energy-consuming electrical device;
and (3) conductors, usually in the form of copper or aluminum wires, to provide a path for electron flow from the negative side of the power source through the resistance and back to the positive side of the power source. Figure 8-16 is a pictorial representation of a practical circuit.
This circuit contains a source of e.m.f. (storage battery), a conductor to provide a path for the flow of electrons from the negative to the positive terminal of the battery, and a power-dissipating device (lamp) to limit the current flow. Without some resistance in the circuit the potential difference between the two terminals would be neutralized very quickly or the flow of electrons would become so heavy that the conductor would become overheated and burn.
At the same time that the lamp acts as a 274
Resistive Device
- (Lamp) Conductor - + EMF FIGURE 8-16. A practical circuit.
current-limiting resistance in the circuit, it is also accomplishing the desired funtion of creating Figure 8-17 is a schematic representation of figure 8-16, in which symbols rather than pictures are used to represent the circuit components.
All components used in electrical circuits are represented in drawings, blueprints, and illustrations in schematic form by symbols. The components commonly used in basic circuits, together with their schematic symbols, are discussed to provide the necessary background for interpretation of circuit diagrams.
Source of Power The source of power, or applied voltage, for a circuit may be any one of the common sources of e.m.f, such as a mechanical source (generator), a chemical source (battery), a photoelectric source (light), or a thermal source (heat). Figure 8-18 illustrates two schematic symbols for a generator.
Most electrical components have only one symbol; however, in the case of the generator and a few others, more than one symbol has been developed to represent a single electrical component. These symbols are normally very similar
- Lamp Conductor OR FIGURE 8-18. Electrical symbols for a d.c.
generator.
in design. Figure 8-18 illustrates that the two symbols for a generator are so nearly alike there is little chance for confusion.
Another common source for the voltage applied to a circuit is the battery, a chemical source of power. Figure 8-19 shows symbols for a singlecell battery and a three-cell battery.
The following statements are true of battery symbols used in schematic diagrams (refer to figure 8-19):
Q) The shorter vertical line represents the negative terminal.
(2) The longer vertical line is the positive terminal.
(3) The horizontal lines represent the conductors connected to the terminals.
(4) Each cell of a battery has one negative and one positive terminal.
Dry cell batteries, such as those used to operate flashlights, are called primary cells. The larger storage batteries containing several primary cells are called secondary cells. The schematic symbol for the primary cell is shown in figure 8-20. The center rod is the positive terminal of the cell, and the case of the cell is the negative terminal.
When more than 1.5 volts are required, cells are connected in series. To connect the cells in series, the negative terminal of each cell is connected to the positive terminal of the succeeding cell as shown in A of figure 8-21. The voltage is then equal to the sum of the voltages of the individual cells. Since the same current must flow through each cell in succession, the current that the battery can supply is equal to the current rating of a single cell. Thus, a battery composed of cells in series provides a higher voltage, but not a greater current capacity.
Battery FIGURE 8-17. Circuit components represented FIGURE 8-19. One-cell and three-cell battery by symbols. symbols.
FIGURE 8-20. Schematic symbol for a dry cell battery.
To obtain a greater current flow than one cell is able to supply, the cells are connected in parallel.
The total current available is equal to the sum of the individual currents from each cell, but the voltage is equal to the voltage of a single cell. To connect cells in parallel, all positive terminals are connected together and all negative terminals are connected together. In A of figure 8-22, a schematic diagram of cells connected in parallel is shown.
B of figure 8-22 illustrates the symbol used to represent this group of cells connected in parallel.
Each cell must have the same voltage; otherwise, a cell with higher voltage will force current through the lower voltage cells.
Another method of arranging cells is to connect them in series-parallel. In this method, shown in figure 8-23, two groups of cells are connected in series, and then these two groups are connected in parallel. This arrangement provides both a greater voltage and a greater current output.
Conductor Another basic requirement of a circuit is the conductor or wire connecting the various electrical components. This is always represented in schematic diagrams as a line. Figure 8-24 illustrates | 1:5V
1.5V 6V
1.5V
1.5V
1.5V
1.5V
1.5V
1.5V FIGURE 8-22. Cells connected in parallel.
two different symbols used to indicate wires (conductors) that cross but are not connected.
While either of these symbols may be used, the symbol shown in B of figure 8-24 is now found more often, since it is less likely to be misinterpreted.
Figure 8-25 illustrates the two different symbols used to represent connected wires. Either of these two symbols may be used, but it is important that no conflict exists with the symbol selected to represent unconnected wires. For example, if the symbol for unconnected wires shown in A of figure 8-24 is selected, the symbol for connected wires must be that shown in A of figure
8-25.
A circuit component found in all practical circuits is the fuse. This is a safety or protective device used to prevent damage to the conductors and circuit components by excessive current flow.
1.5V 1.5V Symbol
1.5V 1.5V
1.51 1.5V 6V
1.5V FIGURE 8-21. Schematic diagram and symbol of FIGURE 8-23. Cells in series-parallel cells connected in series. arrangement.
FIGURE 8-24. Unconnected crossed-over wires.
The schematic symbol for a fuse is shown in figure 8-26.
Another symbol found in basic circuit schematics is the symbol for the switch, shown in figure 8-27. The open switch symbol is shown in A of figure 8-27, and in B of figure 8-27 the closed switch symbol is shown connected in a circuit. There are many different types of switches, but these symbols can represent all but the most complex.
Figure 8-28 illustrates the symbol for "ground" or thè common reference point in a circuit. This is the reference point from which most circuit B FIGURE 8-25. Connected wires.
voltages are measured. This point is normally considered to be at zero potential.
Sometimes meters for measuring current flow or voltage are temporarily connected to the circuit, and in some circuits these meters are permanent components. In figure 8-29, the symbols for an ammeter and a voltmeter are used in a simple circuit. It is important that these components be connected properly. The ammeter, which measures current flow, is always connected in series with the power source and circuit resistances. The voltmeter, which measures the voltage across a circuit component, is always FIGURE 8-27. Open and closed switch symbols.
connected across (in parallel with) a circuit component, never in a series arrangement.
Resistors The last of the basic component requirements of a complete circuit can be grouped under the FIGURE 8-28. Ground or common reference point symbol.
single heading of resistance. Resistance in a practical circuit may take the form of any electrical device, such as a motor or a lamp, which uses electrical power and produces some useful function. On the other hand, the resistance of a circuit may be in the form of resistors inserted in the circuit to limit current flow.
FIGURE 8-26. Schematic symbol for a fuse. FIGURE 8-29. Ammeter and voltmeter symbols.
Resístor Terminal Mounting Bracket FIGURE 8-30. Fixed wire-wound resistors.
A wide variety of resistors are available. Some have a fixed ohmic value and others are variable.
They are manufactured from special resistance wire, graphite (carbon), or metal film. Wirewound resistors control large currents, while carbon resistors control relatively small currents.
Wire-wound resistors are constructed by winding resistance wire on a porcelain base, attaching the wire ends to metal terminals, and coating the wire for protection and heat conduction. (See figure 8-30.
Wire-wound resistors are available with fixed taps which can be used to change the resistance value in increments or steps. They may also be provided with sliders which can be adjusted to change the resistance to any fraction of the total resistance. (See figure 8-31.) Still another type is the precision wire-wound resistors (figure 8-32) made of manganin wire. They are used where the resistance value must be very accurate.
Carbon resistors are manufactured from a rod of compressed graphite and binding material, with wire leads, called "pigtail" leads, attached to each end of the resistor. (See figure 8-33.)
Adjustable tap 7 Fixed taps.
FIGURE 8-32. Precision wire-wound resistors.
Variable resistors are used to vary the resistance while the equipment is in operation. Wire-wound variable resistors control large currents, and carbon variable resistors control small currents.
Wire-wound variable resistors are constructed by winding resistance wire on a porcelain or bakelite circular form. A contact arm which can be adjusted to any position on the circular form by means of a rotating shaft is used to select resistance settings. (See figure 8-34.)
Carbon variable resistors (see figure 8-35), used to control small currents, are constructed of a carbon compound deposited on a fiber disk. A contact on a movable arm varies the resistance as the arm shaft is turned.
The two symbols used on a schematic or circuit diagram to represent variable resistors are shown in figure 8-36.
The schematic symbol for a fixed resistor is shown in A of figure 8-37. A variation of this Pigtail lead.
FIGURE 8-31. Wire-wound resistors with fixed and adjustable taps. FIGURE 8-33. Carbon resistors.
—Cover Variable contact Resistance element terminal End terminals
- Rotating shaft Variable slider contact FIGURE 8-34. Wire-wound Variable resistor.
symbol represents the tapped resistor, which has a fixed value but is provided with taps from which selected amounts of resistance can be obtained. (See B of figure 8-37.)
Resistor Color Code The resistance value of any resistor can be measured by using an ohmmeter. But this is seldom necessary. Most wire-wound resistors have their resistance value in ohms printed on the body of the resistor. Many carbon resistors are similarly marked, but are often mounted in such a manner that it is difficult or impossible to read the resistance value. Additionally, heat often discolors the resistor body, making the printed marking illegible, and many carbon resistors are so small that a printed marking cannot be used.
Thus, a color code marking is used to identify the resistance value of carbon resistors.
There is only one color code for carbon resistors, but there are two systems or methods used to paint this color code on resistors. One is the bodyend-dot system, and the other is the end-to-center band system.
In each color code system, three colors are used to indicate the resistance value in ohms, and a tourth color is sometimes used to indicate the tolerance of the resistor. By reading the colors in the correct order and by substituting numbers from the color code, the resistance value of a resistor can be determined.
Cover End terminals Variable slider contact
- Resistance Variable contact element terminal
- Rotating shaft FIGURE 8-35. Carbon variable resistor.