Agricultural Mechanics 8231-A Principles of Electricity, Electronics, Magnetism, & Electromagnetic Induction PDF

# AGRICULTURAL MECHANICS 8231-A ## PRINCIPLES OF ELECTRICITY, ELECTRONICS, MAGNETISM, AND ELECTROMAGNETIC INDUCTION ### INTRODUCTION The use of electricity as a source of energy in the production, processing, and distribution of agricultural commodities has increased dramatically in recent years. To...

# AGRICULTURAL MECHANICS 8231-A ## PRINCIPLES OF ELECTRICITY, ELECTRONICS, MAGNETISM, AND ELECTROMAGNETIC INDUCTION ### INTRODUCTION The use of electricity as a source of energy in the production, processing, and distribution of agricultural commodities has increased dramatically in recent years. To realize the full extent of the use of electrical energy, stop and think how a typical day at your home, on a farm or ranch, or at an agricultural business or industry would progress if all the electrical power was suddenly turned off one morning. ### PRINCIPLES OF ELECTRICITY, MAGNETISM, AND ELECTROMAGNETIC INDUCTION Electricity by definition is the flow of electrons from one atom to the next along a conductor. Electricity or electrical current is used to: - produce light as it flows through the filament (a conductor) of a bulb - produce heat as it flows through a conductor with high resistance - produce mechanical power with the help of magnetic force for a variety of labor saving tasks through the use of electrical motors, etc. - interpret information. To fully understand the safe use of electricity, one must review and understand many scientific terms, principles and concepts. As you may recall from previous science courses, all matter is made up of different elements. The smallest unit part of all matter is the *atom*. All atoms have particles called *electrons* arranged in an orbital pattern around a core of *protons*. Each separate element has its own atomic structure or number of electrons and protons. In a neutral state, each atom has an equal number of electrons and protons and is considered to be electrically neutral. Electrons can be made to leave the outer ring of some elements and attach to the outer ring of an adjacent or neighboring atom. When an "extra" electron attaches to the neighboring atom, it then has "too many" electrons to be neutral and is thus negatively charged. The first atom that gave up the electron, has "too few" electrons to be neutral and is thus positively charged. Atoms with like charges repel and unlike charges attract. A positively charged atom (too few electrons) will continue to attract an electron from the next atom to become neutral again. The flow of electrical current is the movement or flow of electrons. This flow continues as long as positive and negative charges are maintained at each end of a "chain of atoms" or conductor. The flow of electrons from the atom with too many electrons (negatively charged) to an atom with too few electrons (positively charged) is called the *electron theory of current flow*. Before the electron theory was discovered in the late 1890s and early 1900s, scientists had agreed that electrical current was generated and that it flowed from one point to another through a conductor. They also agreed that current flowed from a place of many charges to a place of fewer charges. However, they called the point or source of current flow the positive end since it had "too much or an excess charge" and the end of the conductor with a "lack of charge" the negative end. This theory is called the *conventional theory of current flow*. Both theories agree that current flows from the point of too much charge or more electrons, to a point of less charge or fewer electrons. The disagreement comes in what to call the source, or point from which the current flows. The electron theory calls the source the negative (-) terminal while the conventional theory calls the source the positive (+) terminal. Both theories are still used today, and most authors or editors of books and references will indicate which theory was used in the discussion and diagrams. The important point is not to mix the two in the design and discussion. An alternative is just to indicate and remember the end of the conductor at which the current is generated as the source, and the other end as the return or ground. The flow of electrons or electrical current is casier in certain elements or materials than in others. Any tendency of an element or material to prevent electrical flow is called *resistance* and produces heat. This is because of the number of electrons being held in the outer ring and their bond or attraction to their own core of protons. Elements that have less than four electrons in their outer ring are said to be generally good conductors and have low resistance. All metals are considered conductors with the best being silver, copper, gold, aluminum, and bronze -- in that order. Because of the expense, gold and silver are seldom used in electrical wiring and devices except in special conditions that will be discussed in later topics. Copper remains the most commonly used conductor because it has only one electron in its outer ring which can be easily dislodged. It is soft and can be easily shaped, and it is relatively inexpensive. Because of the slightly higher resistance and possible heat build-up, aluminum is used primarily in outdoor applications where heat can be readily dissipated to the air. Elements with more than four electrons in the outer ring are considered insulators and do not readily allow electrical current to flow through them. Common materials made up of different elements used as insulators include: glass, paraffin, rubber, porcelain, silk, cotton, wood, and most plastics. However, when many of these insulators become wet or even damp, they may lose all or part of their insulating ability. Semiconductors were discovered in mid to late 1940s. Semiconductors are elements with only four electrons in their outer ring. These elements are neither good conductors or insulators. Silicon and germanium are elements commonly used as semiconductors. By certain mixing and treatment procedures with other elements, these semiconductors are used to make diodes and transistors. Diodes and transistors are used to control electrical current by allowing it to flow in only one direction and/or under only certain conditions. These conditions include the amount and intensity of current, temperature, light and other factors. An electrical circuit is the completed path for the exchange of electrons. Circuits are usually conductors in the form of copper wires surrounded by some type of insulation. A simple circuit must contain: - a source to start the flow of electrons - a conductor for the electrons to flow through - a consumer or load to use the flow of electrons - a conductor connected from the consumer, back to the sources. If a complete path for current flow is not provided, the circuit is not complete. Additional components such as switches and protection devices can be added to a simple circuit. Before going any further in the discussion of electricity, you should be introduced to the use of schematics and symbols to simplify drawings and illustrations of electrical circuits. Schematics are simple drawings which use symbols to show different component parts and often indicate the colors of the wire in each circuit. ## GENERATING ELECTRICAL CURRENT Causing electrons to flow through the circuit is referred to as generating electrical energy. Electrical energy is not created, but converted or "generated" from other forms or sources of energy. The generated current flow is of one of two types: *direct current (D.C.)* or *alternating current (A.C.)*. Direct current is electrical current that flows in only one direction through the conductors. Alternating current reverses or alternates the direction of travel at regular intervals as it flows through the conductors. ### From Chemical Energy Since atoms in the conductor are normally neutrally charged, something must occur to "start" the movement of electrons. Storage batteries are called chemical generators because they generate electrical current from chemical energy through a chemical reaction. Two wires or electrodes, one positive and one negative, are submerged in a chemical solution (or chemical paste in the case of a "dry cell" battery). A reaction between different compounds or chemicals in the solution or paste and the electrodes will dislodge electrons. If the two ends of a conductor are attached to the electrodes, the electrons will flow as long as the circuit is complete or the chemical reaction continues. ### From Mechanical Energy Electrical energy can also be generated from mechanical energy. It was discovered that a magnetic force could dislodge the electrons on the atoms of good conductors. By rotating a loop of copper wire within a magnetic field, electrons can be dislodged and electrical current induced in the wire. The wire or conductor must cut across the lines of force between the N and S poles of the magnetic field and not parallel to them. The magnitude of the electrical current induced is controlled by: - the strength of the magnetic field - the speed at which the lines of force are cutting across the conductor - the number of conductors that are cutting across the lines of force. This process is known as *electromagnetic induction*. Both D.C. and A.C. current can be generated by a mechanical generator. A basic D.C. generator is made by rotating a single loop of wire with a commutator attached to each end between the N and S poles of a magnetic field. The induced current will flow in a definite relationship to the N S polarity of the magnetic field through brushes that ride on the commutator to the rest of the circuit. As the loop rotates, it moves out of the magnetic field or to "static neutral" for a split second. At the same instant, the brushes come in contact with the opposite commutator so that as the wire loop re-enters the magnetic field, the induced current then continues to flow in the same direction through the circuit. A basic A.C. generator is constructed in a similar manner with a magnetic field and a rotating loop of wire. However, instead of a split commutator attached to ends of the wire loop, continuous slip rings are used. As a loop of wire makes a single rotation through the magnetic field, it completes one cycle which is divided into four parts. With the loop at a right angle or static neutral to the magnetic field, a cycle starts. Through the first 1/4th revolution, current is induced flowing in a definite relationship to the N - S magnetic field. The voltage peaks as the loop moves through the strongest part of the magnetic field. Through the second 1/4th revolution, the voltage gradually decreases until the loop reaches static neutral and the loop is temporarily out of the magnetic field. As the loop starts the third 1/4th cycle, voltage again increases, but current flow is in an opposite direction because the loop is now in an opposite relationship with the N S poles of the magnetic field. Because the brushes are riding on continuous slip rings, the connection with the rest of the circuit has also remained continuous, but the current flow has suddenly reversed. The voltage continues to rise again until the end of the third 1/4th cycle. During the final 1/4th cycle the voltage again decreases until it reaches 0. If the rotation continues, the cycle starts over again. Modern A.C. generators produce 60-cycles per second, but the change is so rapid that one cannot detect the zero voltage even in a light bulb. ### From Other Forms of Energy Electrical current can also be generated by other forms of energy. Friction can be used to mechanically dislodge electrons. Static electricity is often generated on the soles of your shoes as you walk across a carpet. The charge is discharged when you touch someone or a door knob. It is difficult to predict and control when static electricity is generated. ## HOW ELECTRICAL ENERGY IS MEASURED The battery, generator, or other device used to produce the electrical energy is similar to a pump used to pump water through a series of pipes. The flow of water through the pipe is measured in gallons per minute. The flow of electrons through the conductor is called a current and is measured in amperes (A or amps for short). One ampere is an electric current of approximately 6.28 billion electrons passing a certain point in a conductor a second. Thus, amperage is the rate of electron flow or electrons per second. Voltage (V or volts for short) is the electromotive force or potential force that causes the flow of current in the conductor. Voltage is compared to pounds per square inch of pressure in the water pipe. A storage battery or generator can have the potential of producing so much voltage or pressure. Voltage can exist without current flow, but current cannot exist and flow without the push or pressure of the voltage. Voltage is produced between two points when a positive charge exists at one point and a negative charge exists at the other. The greater the difference in the charges, the greater the voltage will be. One volt is the amount of electrical pressure required to push one amp of electrical current through one ohm of resistance. Some common voltages include 1.5 volts for flashlight batteries, 6 volts for older tractors and automobiles, 12 volts for newer farm equipment and automobiles, 120 volts for most household and agricultural lights and small motors, and 240 volts for larger household and agricultural heating elements and larger electrical motors. Electrons do not flow freely through even good conductors such as copper, just as water must overcome the friction and scale on the inside walls of the pipe. *Ohm* (shown by the symbol Ω) is a unit of measure of the resistance to electrical current flow. Resistance is caused by the attraction of an atom's electrons to its own core of protons and by the collision of electrons as they move through a conductor. These collisions create resistance or friction and cause heat in the conductor. It is sometimes desirable to have high resistance. The filament in a light bulb is a short piece of high resistance wire that heats when current flows causing the gas inside the bulb to glow. The heating element in an electrical stove, hair dryer, or space heater is also a special high resistance wire. Total resistance in a circuit is thus affected by: - the materials or elements from which all the conductors are made - the length of the conductors - the cross sectional area of the conductors - the temperature of the conductors. One ohm is the amount of resistance overcome by one volt to cause one amp of current to flow. Resistance is proportional to the length and diameter of the wire. If the length of wire is doubled, the resistance is doubled. If the size of the wire is reduced by half, the resistance is also doubled. A loose or corroded terminal is in effect reducing the size of the conductor. The "effective" wire size at that point is increasing resistance and also decreasing the potential for current flow. The hotter a wire or conductor, the more the resistance to current flow. *Watts* (W for short) is a measure of electrical power, or the amount of electrical energy "used" or converted back to light, heat or mechanical energy. The higher the wattage rating, the faster the electrical energy is being converted. Some books and references may refer to amperes as I, volts as E, and ohms as R. Amperes = Amps = A = I. Voltages = Volts V = E. Ohms = R. ## RELATIONSHIP BETWEEN VOLTAGE, AMPERAGE, WATTS, AND OHMS OF RESISTANCE Ohms' law states that the electromotive force or voltage is always equal to the amperage times the ohms of resistance in a circuit. In other words, the flow of electricity through the conductor is always directly proportional to the force or voltage that produces it. If the resistance in a circuit increases while the voltage remains constant, the amount of current flow or amperage is decreased proportionally. When any two values are known, the other can be calculated. Just as the most water will flow through the pipe with the least scale or restrictions, electrical current will follow the path of least resistance. This is an important safety factor to remember when working around electricity. Water and the human body normally have a lower resistance to current flow than copper and aluminum which are used for most conductors. Electrical safety is discussed in greater detail in other topics. ## TYPES OF CIRCUITS Series circuits have several lamps or other consumers of electricity (resistors) connected so that current can flow along only one path. If one of the lamps is disconnected or burns out, the current will stop flowing through the rest of the circuit. The current flow through each lamp (resistor) in a series circuit is the same. The voltage drops across each lamp (or other resistor) will be different if the resistance in the lamp is different. The sum of the voltage drops equals the source voltage. Series circuits are not common in residential or agricultural applications except in appliances, controls, motors, and in lower voltage automotive, tractor and equipment electrical systems and electronics applications. Parallel circuits have more than one path for current to flow. The resistors are side by side and provide separate routes for current. The voltage across each resistor is the same. The current through each resistor will be different if the resistance in the resistors (lamps) are different. The sum of the separate currents equals the total current in the circuit. If one lamp or other consumer is turned off or burns out in a parallel circuit, the rest will stay on. Series-Parallel circuits have some resistors connected in series and some in parallel. In a series-parallel circuit, the laws for the series are obeyed in that portion of the circuit. Likewise the parallel laws are obeyed in the parallel circuit portion. It is often casier to redraw the schematic so each type of circuit is easily recognized. Then combine the total resistance in each parallel circuit to one value. Keep redrawing and inserting the total resistance of each parallel until the circuit is reduced to a simple series circuit. It is beyond the scope of this topic to go into greater detail at this time. Series-parallel circuits are used primarily in control and sensing units. When an electrical current flows through any circuit, part of the voltage or pressure is lost or "used" trying to overcome the resistance in the conductor. This reduction in voltage is called voltage drop. This is in addition to the voltage used by the actual consumers or loads in the circuit. This is similar to the pressure supplied by a water pump that is lost due to the friction and scale on the inside of a pipe. The longer the conductors, the smaller the conductors, or the more resistance through faulty or corroded connections in the circuit, the more the voltage drop. Voltage drop must be planned for and kept to an acceptable minimum of four percent or less for an electrical system to operate properly. ## RELATIONSHIP BETWEEN ELECTRICITY AND MAGNETISM It was previously mentioned that by passing a loop of wire through a magnetic field, electrical current could be induced in the wire or conductor. This process was called generated voltage from *electromagnetic induction*. Another relationship also exists between electricity and magnetism. A magnetic field is produced around a wire or conductor as electrical current passes through it. The magnetic lines of force are in a definite relationship with the direction of current flow through a conductor. By pointing the thumb of the right hand in the direction of current flow from positive to negative as in the conventional theory, the fingers point in the direction in which the lines of magnetic force surround the conductor. The stronger the current through the conductor, the stronger the magnetic field around it. Now consider the principles of electromagnets. A single loop of wire carrying current is a basic electromagnet. When a number of loops are combined to form a coil, the magnetic field is strengthened proportionally since each wire adds its magnetic force to the total field. When the coils are wound around an iron bar, the magnetic field is strengthened still more and the assembly becomes an electromagnet. Iron is approximately 2,500 times better at conducting the lines of force in a magnetic field than air. ## OPERATION OF A TRANSFORMER A basic transformer consists of two windings or coils called 1) a primary coil and 2) a secondary coil. The coils are wound around iron cores. Electrical current from a generated source flows through the primary coil of wire and creates a strong magnetic field. The magnetic field created in the primary coil then induces current flow in the secondary winding coil. The wires of the secondary coil may be wound around the same iron core as the primary coil or they may be wound on a separate core located next to the primary windings. There is no connection between the two windings and current does not flow between them. The metal core(s) is (are) only to hold the coils in shape and strengthen the magnetic fields. If the size of the wire and the number of wraps of wire or "turns" in both windings are equal, the induced voltage and current in the secondary windings will be approximately the same as that in the primary windings. What then is the benefit of a transformer? If the number of turns in the secondary windings is one-half the number in the primary windings, the voltage induced in the secondary winding will be one-half of that flowing into the primary windings. The size of the wire in the secondary windings in comparison to that in the primary side also determines the amperage of the induced current. As wire size in the secondary winding increases, the amperage induced increases. Resistance to high amperage current flow creates heat in the conductors and wastes energy. Both "step-up" and "step down" transformers are used by increasing or decreasing the number and size of wires in either winding where appropriate. For example, power supply companies use step up transformers near the generating plants to increase the voltage or pressure of the current flow to thousands of volts while holding amperage to a minimum. The increased voltage can then be distributed hundreds of miles through smaller wires on cross country high lines more economically. At the point near where the electrical current is to be used by customers, step down transformers are used to reduce the voltage and increase the amperage to safe and usable levels. Transformers are also used in other applications where it is beneficial to increase or decrease current flow such as ignition systems and electronics. ## OPERATION OF AN ELECTRICAL MOTOR Electrical motors also use the principles of electromagnets and electromagnetic induction to operate. The main parts of a basic electrical motor include the "stator" or stationary part and a "rotor" (sometimes called the armature) that rotates inside the stator. Several permanent magnets and/or electromagnets (coils of wires) are placed around the inside of a metal frame in a circle to form the stator. The poles face toward the center. Depending on the motor type and construction, permanent magnets and/or electromagnets are also positioned around the outside of the rotor with poles facing toward the stator poles. By sending or inducing electrical current through the windings of the electromagnets in the stator and rotor, magnetic fields are created. Because unlike poles attract and like poles repel, the magnetic fields created and those from the permanent magnets pull and push against each other to start the rotor turning. If the polarity of the poles is not suddenly changed, the rotor would turn only far enough so that all the unlike poles are aligned and the rotor would then stop. Various means are used to periodically reverse the polarity of the poles. In some types of A.C. motors, the poles in the stator are placed so that as the current alternates, the polarity of the poles are reversed at just the correct time. In D.C. motors, the direction of current flow in the armature windings is reversed by means of a commutator and brush assembly similar to those used in a D.C. generator. Some modern D.C. and A.C. motors use solid state transistors and diodes to reverse the polarity of the magnetic fields. ## ELECTRONIC CIRCUITS The use of electronics has changed the way we think about electricity, its uses and the size of electrical equipment. No longer can we only talk about power generating plants that cover several acres, large copper and aluminum conductors, light bulbs, large multi-horsepower motors, alternators and generators on tractors and automobiles, and large mechanical switches and controls. Attention must also be focused on small electronic devices the size of the tip of a pencil. Electronics use transistors, semiconductors and integrated circuits for information gathering and processing, and to control other electrical circuits and mechanical functions. For most purposes and discussion, electronic circuits use direct current (D.C.) of below 8 volts which is measured in milliamps. Although older tractors and automobiles use 6 volt systems, these systems are not considered electronic circuits. They use mechanical circuit control devices, larger conductors, higher amperage, and not semiconductors and diodes. Many modern electronic devices start out with higher voltage current which is reduced through the use of transformers and have "hybrid circuits." If alternating current is used, it is converted to direct current through the use of a rectifier. A rectifier is a device using one or more diodes made of semiconductors which allow current to flow in only one direction. Only one side of the alternating positive negative wave pattern of the electrical current is allowed to flow through. Diodes are made by joining positively and negatively charged semiconductors. Zenor diodes are special diodes that will conduct current flow in a reverse direction above a certain voltage. Several other devices are used to smooth and control both voltage and current flow. *Capacitors* are components designed to control a sudden change in the voltage of a circuit and temporarily store or filter a surge. *Resistance* is the opposition to current flow. Resistors are devices with higher resistance than the normal conductor purposely placed in a circuit. By controlling resistance, the current and voltage in a circuit can be controlled. *Inductors* are basically a small coil of wire. They have the ability to oppose a change in amperage and can smooth or filter current flow. *Transistors* combine several semiconductors into one component to act as a switch or switches under certain conditions. The switches can either be on or off, or can be constructed to allow one current switch to vary the current flowing through another circuit. The on or off function becomes very important in understanding the operation of computers and computerized controls as will be discussed later. Some resistors and transistors are temperature sensitive and control current based upon the surrounding ambient temperature. Other devices which have reduced the size of electronic instruments are light emitting diodes or L.E.D.s, and liquid crystal displays or L.C.D.s. L.E.D.s are diodes that emit light as current passes through. They have replaced incandescent lamps as signals in low voltage electronic circuits because of their long-life and fast on and off switching capabilities. L.C.D.s reflect light when no voltage is applied, but they absorb light when voltage is applied and current is flowing. Many of the original electronic devices such as radios and T.V. were large because many of the control devices had to be constructed to operate inside bulky glass vacuum tubes. In the mid 1960s when the use of solid-state semiconductors first began, as many as 1,000 different circuits could be placed on a small three-inch by five-inch circuit board. Today, thousands of mini circuit elements can be made on a chip less than 1/4-inch in size. These chips are made with sophisticated microscopic equipment. Microscopic series and parallel circuits are connected to slightly larger wires and finally to larger terminals. These terminals are then connected to circuit boards and other component leads. Circuit patterns are etched on the surfaces of several layers of silicon semiconductors. These grooves or patterns are chemically treated so the semiconductors act as diodes, transistors, resistors, capacitors, or conductors to form integrated circuits or "chips". Computers and microprocessors are basically thousands and thousands of mini circuits which sense the presence or absence of electronic impulses. It is the ability of electronic circuitry to sense impulses that allow information to be stored and processed by the computer or electronic sensing and controlling devices. Electronic impulses or "input" is fed into a computer or other electronic control device. The input flows in from keyboard switches, sensors, or monitors. Imagine literally thousands of possible circuits in the computer or microprocessor for the input to flow through. Each of these circuits have switches that are either open or closed. Also, imagine literally thousands of things that could happen in life "if, then, or, and" certain things would or could happen. This is basically what happens inside a computer or any other electronic device. The input flows depending upon certain "if, then, or, and conditions" which are the way certain transistor switches are opened or closed. The switches are opened or closed depending upon how they were set by the "program" or the function the electronic device is designed to perform. In electronics terms, these semiconductor transistor switches are sometimes called "flip-flops". As the input flows, it can cause other switches to be opened or closed by the electronic transistors. After the input has flowed through the proper circuits based on the "if, then, or, and conditions," the information then can be stored for future use. Information is stored by the pattern certain flip-flop switches were left open or closed or by the directions or patterns of the N-S polarity of mini magnetic fields left on a magnetic surface. After the input has been processed and/or stored, it can flow back out as "output" in the form of electronic impulses to activate printers, L.E.D.s, L.C.D.s, buzzers, gauges, or other electronic or electrical control devices. ## CIRCUIT FAILURES There are three basic types of failures that occur in electrical and electronic circuits. An open is a break or interruption in the circuit when a wire or conductor has come lose or a connection has slipped and is not making good contact. An open may occur instantly and completely, or a connection may gradually loosen. As it slowly loosens, corrosion may slowly increase the resistance until the circuit is completely open. A ground occurs when any part of the wiring circuit inadvertently touches a metal frame or another conductor (other than the intended path) that will allow current to flow back to the source. A ground involves accidental or unintentional connection or contact between the intended circuit path and an alternative return path. Depending upon where the ground occurs in the circuit and what the wire comes in contact with, resistance in the circuit is usually increased. Low voltage circuits on tractors, automobiles, and equipment do utilize the metal frame as part of the return circuit. In these cases, the system has been designed to operate in this manner. A short occurs when the insulation on wires in one or more circuits fail and two wires come in contact with each other. A short occurs when part of the circuit or circuits is/are bypassed because wires are touching. When by-passing of a load or consumer occurs, all or part of the resistance in the circuit is eliminated. A short circuit is said to exist when the resistance of a circuit becomes so low that the circuit current increases to the point where it can damage the circuit components. When resistance is reduced, current flow increases. As current increases, heat build-up may increase to the point of further electrical damage, melting insulation, or igniting other flammable materials. Various types of circuit protection devices are used. When current flow increases to unsafe levels, the extra heat will cause a fuse to purposely melt or a circuit breaker to trip stopping current flow. Specific circuit protectors will be discussed in other topics. ## ELECTRICAL TESTING EQUIPMENT The basis of accurate electrical system diagnosis is accurate electrical measurements. Various meters and instruments can be used to measure voltage, amperage, ohms of resistance and watts. However, since all are interrelated and in a definite relationship with each other, it may not be necessary to measure each. The *voltmeter*, like a pressure gauge in the water system comparison, is used to measure the voltage in the circuit. It measures the voltage differential between two points in the circuit. A voltmeter is connected in parallel to the circuits across two points. An *ammeter* measures the rate of current flow similar to the way a flow meter measures fluid flow. Two different types of ammeters are commonly used. In a connected type ammeter, the ammeter leads are connected in series in the circuit to be tested. The circuit must first be turned off and wires disconnected from terminals before the ammeter leads can be installed. An induction type ammeter uses moveable metal jaws which are placed around one of the wires in the circuit. The magnetic field "induced" into metal jaws indicates the amount of current flow through the wire or conductor. An *ohmmeter* measures the ohms of resistance in a wire or conductor. An ohmmeter has its own source of power in the form of small batteries. Before resistance can be measured, a circuit must be turned off and the conductor or component to be tested must be disconnected. The ohmmeter test leads are then connected to each end of the wire or component being tested. Most older type electrical meters used a large permanent horseshoe magnetic, a movable coil with a needle attached, various shunt coils, and multiple scales. As electrical current flowed through the various coils depending upon the settings of various switches, the magnetic fields in the coils and the permanent magnet attracted and/or opposed each other moving the needle accordingly. Many newer electrical test meters use solid state electronic circuits and digital read outs. Digital multimeters often referred to as DMMS, VOAMS, or VOMs, combine three meters into one. They are usually smaller and lighter. Other test equipment is available for more specific applications. The knowledge of these principles, concepts, and relationships is obviously helpful in safe operating, planning, designing, testing, and/or trouble shooting of electrical or electronic circuits and equipment. Their actual application to agricultural mechanics are expanded and discussed in greater detail in other topics. ## Acknowledgements Dr. Joe Muller, Curriculum Specialist, Instructional Materials Service, developed and organized the information in this topic. Mr. Rodney Schmalriede, Graduate Assistant, Department of Agricultural Education, Texas A&M University, developed the student activities. ## References: - Electrical Energy: Utilization, Generation, Transmission, Distribution, Conservation; AAVIM, Athens. GA. - Fundamental of Service: Electrical Systems, Deere and Company, Moline IL. - Crozier, P., *Introduction to Electronics*, Breton Publishers, North Scituate, MA. - Shelly, G. B., and T. J. Cashman, *Computer Fundamentals*, Boyd and Fraser Publishing Co., Boston, MA.