NORTH POLE SOUTH POLE (A) Unmagnetized
N N S N N FIGURE 8-81. One end of magnetized strip N IN S IN S IN points to the magnetic north pole.
N S N S It can best be described by the effects it produces.
Examination of a simple bar magnet similar to that illustrated in figure 8-81 discloses some basic characteristics of all magnets. If the magnet is suspended to swing freely, it will align itself with the earth's magnetic poles. One end is labelled "N," meaning the north-seeking end or pole of the magnet. If the "N" end of a compass or magnet is referred to as north-seeking rather than north, there will be no conflict in referring to the pole it seeks, which is the north magnetic Lines of Force FIGURE 8-82. Magnetic field around magnets.
(B) Magnetized FIGURE 8-83. Arrangement of molecules in a piece of magnetic material.
pole. The opposite end of the magnet, marked "S," is the south-seeking end and points to the south magnetic pole. Since the earth is a giant magnet, its poles attract the ends of the magnet.
These poles are not located at the geographic poles.
The somewhat mysterious and completely invisible force of a magnet depends on a magnetic field that surrounds the magnet as illustrated in figure 8-82. This field always exists between the poles of a magnet, and will arrange itself to conform to the shape of any magnet.
The theory that explains the action of a magnet holds that each molecule making up the iron bar is itself a tiny magnet, with both north and south poles as illustrated in A of figure 8-83. These molecular magnets each possess a magnetic field, but in an unmagnetized state the molecules are arranged at random throughout the iron bar. If a magnetizing force, such as stroking with a lodestone, is applied to the unmagnetized bar, the molecular magnets rearrange themselves in line with the magnetic field of the lodestone, with all north ends of the magnets pointing in one direction and all south ends in the opposite direction. This is illustrated in B of figure 8-83. In such a configuration, the 299
Tracing out a magnetic field by means of iron filings
• Iron filings Lucite How the iron filings arrange themselves Magnets FIGURE 8-84. Tracing out a magnetic field with iron filings.
magnetic fields of the magnets combined to produce the total field of the magnetized bar.
When handling a magnet, avoid applying direct heat, or hammering or dropping it. Heating or sudden shock will cause misalignement of the molecules, causing the strength of a magnet to decrease. When a magnet is to be stored, devices known as "keeper bars" are installed to provide an easy path for flux lines from one pole to the other.
This promotes the retention of the molecules in their north-south alignement.
The presence of the magnetic force or field around a magnet can best be demonstrated by the experiment illustrated in figure 8-84. A sheet of transparent material such as glass or lucite is placed over a bar magnet and iron filings are sprinkled slowly on this transparent shield. If the glass or lucite is tapped lightly, the iron filings will arrange themselves in a definite pattern around the bar, forming a series of lines from the north to south end of the bar to indicate the pattern of the magnetic field.
As shown, the field of a magnet is made up of many individual forces that appear as lines in the iron-filing demonstration. Although they are not "lines" in the ordinary sense, this word is used to describe the individual nature of the separate forces making up the entire magnetic field. These lines of force are also referred to as magnetic flux. They are separate and individual forces, 300
Air Gap FIGURE 8-85. Like poles repel.
since one line will never cross another; indeed, they actually repel one another. They remain parallel to one another and resemble stretched rubber bands, since they are held in place around the bar by the internal magnetizing force of the magnet.
The demonstration with iron filings further shows that the magnetic field of a magnet is concentrated at the ends of the magnet. These areas of concentrated flux are called the north and south poles of the magnet. There is a limit to the number of lines of force that can be crowded into a magnet of a given size. When a magnetizing force is applied to a piece of magnetic material, a point is reached where no more lines of force can be induced or introduced. The material is then said to be saturated.
The characteristics of the magnetic flux can be demonstrated by tracing the flux patterns of two bar magnets with like poles together, as shown in figure 8-85. The two like poles repel one another because the lines of force will not cross each other.
As the arrows on the individual lines indicate, the lines turn aside as the two like poles are brought near each other and travel in a path parallel to each other. Lines moving in this manner repel each other, causing the magnets as a whole to repel each other.
By reversing the position of one of the magnets, the attraction of unlike poles can be demonstrated, as shown in figure 8-86.
Air Gap FIGURE 8-86. Unlike poles attract.
FIGURE 8-87. Bypassing flux lines.
As the unlike poles are brought near each other, the lines of force rearrange their paths and most of the flux leaving the north pole of one magnet enters the south pole of the other. The tendency of lines of force to repel each other is indicated by the bulging of the flux in the air gap between the two magnets.
To further demonstrate that lines of force will not cross one another, a bar magnet and a horseshoe magnet can be positioned to display a magnetic field similar to that of figure 8-87. The magnetic fields of the two magnets do not combine, but are re-arranged into a distorted flux pattern.
The two bar magnets may be held in the hands and the north poles brought near each other to demonstrate the force of repulsion between like poles. In a similar manner the two south poles can demonstrate this force. The force of attraction between unlike poles can be felt by bringing a south and a north end together, These experiments are illustrated in figure 8-88.
Figure 8-89 illustrates another characteristic of magnets. If the bar magnet is cut or broken into pieces, each piece immediately becomes a magnet itself, with a north and south pole. This feature supports the theory that each molecule is a magnet, since each successive division of the magnet produces still more magnets.
Since the magnetic lines of force form a continuous loop, they form a magnetic circuit. It is impossible to say where in the magnet they originate or start. Arbitrarily, it is assumed that all lines of force leave the north pole of any magnet and enter at the south pole.
There is no known insulator for magnetic fiux, or lines of force, since they will pass through all materials. However, it has been found that they will pass through some materials more easily than others. Thus, it is posible to shield certain areas, such as instruments, from the effects 301
REPULSION REPULSION ATTRACTION FicuRE 8-88. Repulsion and attraction of magnet poles.
of the flux by surrounding them with a material that offers an easier path for the lines of force.
Figure 8-90 shows an instrument surrounded by a path of soft iron, which offers very little opposition to magnetic flux. The lines of force take the easier path, the path of greater permeability, and are guided away from the instrument.
Materials, such as soft iron and other ferrous metals, are said to have a high permeability, the measure of the ease with which magnetic flux can penetrate a material. The permeability scale is based on a perfect vacuum with a rating of one.
Air and other nonmagnetic materials are so close to this that they are also considered to have a rating of one. The nonferrous metals having a permeability greater than one, such as nickel and cobalt, are called paramagnetic, while the term ferromagnetic is applied to iron and its alloys, which have by far the greatest permeability.
Any substance, such as bismuth, having a permeability of less than one, is considered diamagnetic.
Reluctance, the measure of opposition to the lines of force through a material, can be compared to the resistance of an electrical circuit. The reluctance of soft iron, for instance, is much lower than that of air. Figure 8-91 demonstrates that a piece of soft iron placed near the field of a magnet can distort the lines of force, which follow the path of lowest reluctance through the soft iron.
The magnetic circuit can be compared in many respects to an electrical circuit. The magnetomotive force (m.m.f.), causing lines of force in the magnetic circuit, can be compared to the electromotive force or electrical pressure of an electrical circuit. The m.m.f. is measured in gilberts, symbolized by the capital letter "F." The symbol for the intensity of the lines of force, or flux, is the Greek letter phi (ø), and the unit of field intensity is the gauss. An individual line of force, called a maxwell, in an area of one square centimeter produces a field intensity of one gauss.
Using reluctance rather than permeability, the law for magnetic circuits can be stated: A magnetomotive force of one gilbert will cause one maxwell, or line of force, to be set up in a material when the reluctance of the material is one.
SOFT IRON FIGURE 8-89. Magnetic poles in a broken magnet. FIGURE 8-90. Magnetic shield.
SOFT IRON FIGURE 8-91. Effect of a magnetic substance in a magnetic field.
Types of Magnets Magnets are either natural or artifical. Since naturally occurring magnets or lodestones have no practical use, all magnets considered in this study are artifical or man-made. Artifical magnets can be further classified as permanent magnets, which retain their magnetism long after the magnetizing force has been removed, and temporary magnets, which quickly lose most of their magnetism when the external magnetizing forcé is removed.
Hard steel has long been used to make permanent magnets, but magnets of even better quality are now available from various alloys.
Alnico, an alloy of iron, alluminum, nickel and cobalt, is considered one of the very best. Others with excellent magnetic qualities are alloys such The old method of producing a magnet by stroking a piece of steel or iron with a natural magnet has been replaced by other means. A piece of metal placed in contact with, or even near, a magnet will become magnetized by induction and the process can be accelerated by heating the metal and then placing it in a magnetic field to cool. Magnets can also be produced by placing the metal to be magnetized in a strong magnetic field and striking it several times with a hammer, This process can be used to produce permanent magnets from metals such as hard steel.
The ability of a magnet to hold its magnetism varies greatly with the type of metal and is known as retentivity. Magnets made of soft iron are very eaily magnetized but quickly lose most of their magnetism when the external magnetizing force is removed. The small amount of magnetism remaining, called residual magnetism, is of great importance in such electrical applications as generator operation.
FIGURE 8-92. Two forms of horseshoe magnets.
Horseshoe magnets are commonly manufactured in two forms, as shown in figure 8-92. The most common type is made from a long bar curved into a horseshoe shape, while a variation of this type consists of two bars connected by a third bar, or yoke.
Magnets can be made in many different shapes, such as balls, cylinders, or disks. One special type of magnet is the ring magnet, or Gramme ring, often used in instruments. This is a closed.
loop magnet, similar to the type used in transformer cores, and is the only type that has no poles.
Sometimes special applications require that the field of force lie through the thickness rather than the length of a piece of metal. Such magnets are called flat magnets and are used as pole pieces in generators and motors.
Electromagnetism In 1819, the Danish physicist, Hans Christian Oersted, discovered that the needle of a compass be detected. When brought near a current-carrying conductor would be deflected. When the current flow stopped, the compass needle returned to its original position.
This important discovery demonstrated a relationship between electricity and magnetism that led to the electromagnet and to many of the inventions on which modern industry is based.
Oersted discovered that the magnetic field had no connection with the conductor in which the electrons were flowing, because the conductor was made of nonmagnetic copper. The magnetic field around the conductor was created by the electrons moving through the wire. Since a magnetic field accompanies a charged particle, the greater the current flow the greater the magnetic field. Figure 8-93 illustrates the magnetic field around a current-carrying wire. A series of concentric circles around the conductor represent the field, which, 303
MAGNETIC FIELDS Conductor Conductor FIGURE 8-93. Magnetic field formed around a conductor in which current is flowing.
if all the lines were shown, would appear more as a continuous cylinder of such circles around the conductor.
As long as current flows in the conductor, the lines of force remain around it, as shown in figure 8-94. If a small current flows through the conductor, there will be a line of force extending out to circle A. If the current flow is increased, the line of force will increase in size to circle B, and a further increase in current will expand it to circle C. As the original line (circle) of force expands from circle A to B, a new line of force will appear at circle A. As the current flow increases, the number of circles of force increases, expanding the outer circles farther from the surface of the current-carrying conductor.
If the current flow is a steady nonvarying direct current, the magnetic field remains stationary. When the current stops, the magnetic field collapses and the magnetism around the conductor disappears.
A compass needle is used to demonstrate the direction of the magnetic field around a currentcarrying conductor. A of figure 8-95 shows a compass needle positioned at right angles to, and approximately one inch from, a current-carrying conductor. If no current were flowing, the northseeking end of the compass needle would point toward the earth's magnetic pole. When current flows, the needle lines itself up at right angles to a radius drawn from the conductor. Since the compass needle is a small magnet, with lines of force extending from south to north inside the metal, it will turn until the direction of these lines agrees with the direction of the lines of force around the conductor. As the compass needle is moved around the conductor, it will maintain itself in a position at right angles to the conductor, indicating that the magnetic field around a
• Magnetic Lines FreURE 8-94. Expansion of magnetic field as current increases.
current-carrying conductor is circular. As shown in B of figure 8-95, when the direction of current how through the conductor is reversed, the compass needle will point in the opposite direction, indicating the magnetic field has reversed its A method used to determine the direction of the lines of force when the direction of the current flow is known, is shown in figure 8-96.
conductor is grasped in the left hand, with the thumb pointing in the direction of current flow, the fingers will be wrapped around the conductor in the same direction as the lines of the magnetic field. This is called the left-hand rule.
Although it has been stated that the lines of force have direction, this should not be construed to mean that the lines have motion in a circular direction around the conductor. Although the lines of force tend to act in a clockwise or counterclockwise direction they are not revolving around the conductor.
Since current flows from negative to positive, many illustrations indicate current direction with a dot symbol on the end of the conductor when the electrons are flowing toward and a plus sign when the current is flowing away from the observer. This is illustrated in figure 8-97.
When a wire is bent into a loop and an electric current flows through it, the left-hand rule remains valid, as shown in figure 8-98.
If the wire is coiled into two loops, many of the lines of force become large enough to include both loops. Lines of force go through the loops in the same direction, circle around the outside of the two coils, and come in at the opposite end.
(See figure 8-99.)
When a wire contains many such loops, it is called a coil. The lines of force form a pattern 304
Magnetic field
CHECKING DIRECTION
OF MAGNETIC FIELD,
USING N S COMPASS
AND LEFT-HAND RULE
- COMPASS
•Current-carrying conductor B FIGURE 8-95. Magnetic field around a current-carrying conductor.
through all the loops, causing a high concentration of flux lines through the center of the coil.
(See figure 8-100.)
In a coil made from loops of a conductor, many of the lines of force are dissipated between the loops of the coil. By placing a soft iron bar inside the coil, the lines of force will be concentrated in the center of the coil, since soft iron has a greater permeability than air. (See figure
8-101.) This combination of an iron core in a coil of wire loops, or turns, is called an electromagnet, since the poles (ends) of the coil possess the characteristics of a bar magnet.
The addition of the soft iron core does two things for the current-carrying coil. First, the magnetic flux is increased, and second, the flux lines are more highly concentrated.
When direct current flows through the coil, the core will become magnetized with the same polarity (location of north and south poles) as the coil would have without the core. If the
• current is reversed, the polarity will also be reversed.
The polarity of the electromagnet is determined by the left-hand rule in the same manner as the polarity of the coil without the core was determined. If the coil is grasped in the left hand in such a manner that the fingers curve around the coil in the direction of electron flow (minus to plus, the thumb will point in the direction of the north pole. (See figure 8-102.)
The strength of the magnetic field of the electromagnet can be increased by either increasing the flow of current or the number of loops in the wire. Doubling the current flow approximately doubles the strength of the field, and in a similar manner, doubling the number of loops approximately doubles magnetic field strength. Finally, the type metal in the core is a factor in the field strength of the electromagnet.
Direction of Current Direction of Field FIGURE 8-96. Left-hand rule.
Dot indicates current is flowing out of the conductor. Cross indicates current is flowing into the conductor.
FICURE 8-97. Direction of current flow in a conductor.
A soft-iron bar is attracted to either pole of a permanent magnet and, likewise, is attracted by a current-carrying coil. As shown in figure 8-103, the lines of force extend through the soft iron, magnetizing it by induction and pulling the iron bar toward the coil. If the bar is free to move, it will be drawn into the coil to a position near the center where the field is strongest.
Electromagnets are used in electrical instruments, motors, generators, relays, and other devices. Some electromagnetic devices operate on the principle that an iron core held away from the center of a coil will be rapidly pulled into a center position when the coil is energized. This principle is used in the solenoid, also called solenoid switch or relay, in which the iron core is springloaded off center and moves to complete a circuit when the coil is energized.
The application of the solenoid is shown in figure 8-101, where it is a solenoid relay. When the cockpit switch is closed, the energized coil pulls the core switch down, which completes the circuit to the motor. Since this solenoid relay operates on low current, it eliminates high-amperage wiring in the cockpit of the aircraft.
The solenoid-and-plunger type of magnet in various forms is used extensively to open circuit breakers automatically, when the load current becomes excessive, and to operate valves, magnetic brakes, and many other devices. The armature-type of electromagnet also has extensive applications. For this type of magnet, the coil is wound on and insulated from the iron core;
the core is not movable. When current flows through the coil, the iron core becomes magnetized and causes a pivoted soft-iron armature located near the electromagnet to be attracted to it. These magnets are used in doorbells, relays, circuit breakers, telephone receivers, and many other devices.
STORAGE BATTERIES There are two sources of electrical energy in an aircraft: (1) The generator, which converts mechanical energy into electrical energy, and (2) the battery, which converts chemical energy into electrical energy. During normal engine operation, electrical energy is taken from the engine-driven generator. The storage battery is used as an auxiliary source of power when the generator is not operating.
When the generators are operating at a speed too low to supply electrical energy for the airplane, electrical power is taken from the battery and the FIGURE 8-98. Magnetic field around a looped battery discharges, losing some of the chemical conductor. energy stored in it. During flight, the airplane 306
DIRECTION
OF CURRENT FIGURE 8-99. Magnetic field around a conductor with two loops.
generator charges the battery over a long period of time and restores the chemical energy. Leadacid and nickel-cadmium batteries are the types of storage batteries in general use.
LEAD-ACID BATTERIES Lead-acid batteries used in aircraft are similar to automobile batteries. The cells of a battery are connected in series. Each cell contains positive plates of lead peroxide, negative plates of spongy lead, and electrolyte (sulphuric acid and water).
In discharging, the chemical energy stored in the battery is changed to electrical energy; in charging, the electrical energy supplied to the battery is changed to chemical energy and stored.
It is possible to charge a storage battery many times before it deteriorates permanently.
Lead-acid Cell Construction The components of a typical lead-acid cell are shown in figure 8-105. Each plate consists of a framework called a grid, made of lead and antimony, to which the active material (spongy lead or lead peroxide) is attached. The positive and negative plates, (1) of figure 8-105, are so assembled that each positive plate is between two negative plates. Thus, the end plate in each cell is a negative plate. Between the plates are FIGURE 8-100. Magnetic field of a coil.
FIGURE 8-101. Electromagnet.
porous separators (7) which keep the positive and negative plates from touching each other and and shorting out the cell. The separators have vertical ribs on the side facing the positive plate.
This construction permits the electrolyte to circulate freely around the plates. In addition, it provides a path for sediment to settle to the bottom of the cell.
Each cell is sealed in a hard rubber casing through the top of which are terminal posts and a hole into which is screwed a nonspill vent cap
(4). The hole provides access for testing the strength of the electrolyte and adding water. The vent plug permits gases to escape from the cell with a minimum of leakage of electrolyte, regardless of the position the airplane might assume.
In figure 8-106, the construction of the vent plug is shown. In level flight, the lead weight permits venting of gases through a small hole. In inverted flight, this hole is covered by the lead weight.
The individual cells of the battery are connected in series by means of cell straps, as illustrated in figure 8-107. The complete assembly is enclosed in an acid-resisting metal container (battery box), which serves as electrical shielding and mechanical protection. The battery box has a removable top.
It also has a vent-tube nipple at each end. When the battery is installed in an airplane, a vent tube is attached to each nipple. One tube is the intake tube and is exposed to the slipstream. The other is the exhaust vent tube and is attached to the battery drain sump, which is a glass jar containing a felt pad moistened with a concentrated solution of sodium bicarbonate (baking soda). With this arrangement, the airstream is directed through the battery case where battery gases are picked up, neutralized in the sump, and then expelled overboard without damage to the airplane.
To facilitate installation and removal of the battery in some aircraft, a quick-disconnect assembly is used to connect the power leads to the FIcURE 8-102. Left-hand rule applied to a coil. FIGURE 8-103. Solenoid with iron core.
Relay Bus Bar Battery M Cockpit switch Motor FIGURE 8-104. Use of a solenoid in a circuit.
battery. This assembly, which is shown in figure 8-108, attaches the battery leads in the aircraft to a receptacle mounted on the side of the battery.
The receptacle covers the battery terminal posts and prevents accidental shorting during the installation and removal of the battery. The plug consists of a socket and a handwheel with a course-pitch thread. It can be readily connected to the receptacle by the handwheel. Another advantage of this assembly is that the plug can be installed in only one position, eliminating the possibility of reversing the battery leads.
Upright position Inverted position Gases escape Electrolyte Vent Vent Lead weight Lead weight seated not sealed seated FIGURE 8-106. Nonspill battery vent plug.
Operation of Lead-acid Cells A lead-acid cell contains positive plates coated with lead peroxide (P602); negative plates made of lead (Pb); and a liquid electrolyte, consisting of sulphuric acid (H2S04) and water (H20). During discharge, lead sulfate (PISO,) is formed on both the positive and negative plates, the acid content of the electrolyte is decreased, and its water content is increased. As discharge continues, the amount of lead sulfate on the plates increases until the sulfate coatings become so thick that the weakened electrolyte cannot effectively reach the active materials (lead and lead peroxide). When this happens, chemical reaction is retarded and the output of the cell is reduced. In practice, the cell is not permitted to be discharged to this extent because thick coatings of lead sulfate are difficult to remove in charging. Additionally, a cell ap-
• Vent plug Cells
1. Plates 3. Supporting ribs 6. Cell cover
2. Cell container 4. Vent cap 7. Separators
5. Terminal post - Cell strap FIGURE 8-105. Lead-acid cell construction. FIGURE 8-107. Connection of storage battery.
To give a high discharge current and a high terminal voltage under load, a battery must have low internal resistance. This characteristic can be achieved through extensive plate area. Therefore, each cell contains several sets of plates. All the positive plates of a cell are connected by one connecting bar, and all the negative plates by another. Thus, the plates are connected in parallel, further decreasing the internal resistance of the cell. The open circuit cell voltage is not affected;
it remains the same as that of a single pair of plates.
FIGURE 8-108. A battery quick-disconnect assembly.
proaching a state of total discharge is of little use because the high internal resistance (IR) caused by the sulfate coatings on its plates reduces the current to a value too low for practical use.
When a cell is being charged, lead sulfate is removed from both the positive and negative plates, and sulphuric acid is again formed. In the process, the water content of the electrolyte is of the deal re is decreased and the density of the electrolyte is increased.
The open-circuit voltage of a lead-acid cell, that is, its voltage when there is no load drawing current, is approximately 2.2 volts. This voltage is the same for every lead-acid cell regardless of its plate size and remains at this value until the cell is practically dead, regardless of its state of discharge. When the cell approaches total discharge, its voltage begins to drop rapidly.
The closed-circuit voltage of a cell, that is, its voltage under load, decreases gradually as the cell is discharged. This gradual decrease in terminal voltage is due to a gradual increase in the internal resistance of the cell caused by sulphation of the plates. At the end of normal discharge, the internal resistance of a lead-acid cell is more than twice as high as it is when fully charged. The difference between the open-circuit and closedcircuit terminal voltages is due to the voltage drop inside the cell. This is equal to the current the load draws multiplied by the internal resistance in the cell. Therefore, the discharging voltage that a lead-acid cell can supply under closedcircuit conditions is equal to the open-circuit voltage of the cell minus the IR drop in the cell.
Lead-acid Battery Ratings The voltage of a battery is determined by the number of cells connected in series to form the battery. Although the voltage of one lead-acid cell just removed from a charger is approximately
2.2 volts, a lead-acid cell is normally rated at only 2 volts, because it soon drops to that value.
A battery rated at 12 volts consists of 6 lead-acid cells connected in series, and a battery rated at 24 volts is composed of 12 cells.
The capacity of a storage battery is rated in ampere-hours (amperes furnished by the battery times the amount of time current can be drawn).
This rating indicates how long the battery may be used at a given rate before it becomes completely discharged.
Theoretically, a 100 ampere-hour battery will furnish 100 amperes for 1 hour, 50 amperes for 2 hours, or 20 amperes for 5 hours. Actually, the ampere-hour output of a particular battery depends on the rate at which it is discharged.
Heavy discharge current heats the battery and decreases its efficiency and total ampere-hour output. For airplane batteries, a period of 5 hours has been established as the discharge time in rating battery capacity, However, this time of 5 hours is only a basis for rating and does not necessarily mean the length of time during which the battery is expected to furnish current. Under actual service conditions, the battery can be completely discharged within a few minutes, or it may never be discharged if the generator provides sufficient charge.
The ampere-hour capacity of a battery depends upon its total effective plate area. Connecting batteries in parallel increases ampere-hour capacity. Connecting batteries in series increases the total voltage but not the ampere-hour capacity. In multiengine airplanes, where more 310