Master Electrician: More Electricity Concepts You Should Know

Meters and Scopes

Oscilloscopes
The oscilloscope is a very useful instrument for measuring and analyzing the performance of electrical and electronic circuits.  It presents a visual image of voltage waveforms.  The normal display mode provides a representation of a signal versus time.
Inputs to the oscilloscope are connected via a coaxial cable or a probe, a special device that clips to a wire or circuit element and matches the instrument impedance to prevent distortion of the viewed waveform.
Most scopes display multiple channels so more than one signal can be viewed simultaneously.  The sweep speed can be varied so that either high or low frequency signals can be viewed.
Most modern oscilloscopes are digital and have sampling circuits so that single events like line transients may be viewed, or samples of repetitive signals can be used to greatly increase the time resolution.
Oscilloscopes measure peak or instantaneous voltages and time differences in a waveform or between waveforms.

Galvanometer
The galvanometer is essentially a very sensitive ammeter. 
It detects and measures currents in the microampere range.  Normally, they consist of a coil that converts the small electric current to a magnetic field that deflects a needle.  It may also be a recording instrument if the needle is attached to a pen that touches a moving paper.
Galvanometers are often used to indicate and measure physical quantities by calibrating the output of a transducer or other signal source to read the desired units directly on the meter face.

Hipot Testers and Meggers
Hipot is short for high potential, and these meters are used to measure insulation resistance at very high voltages to determine the breakdown voltage.  The applied test potential may be in the hundreds or thousands of volts.  Hipot tests are desirable to verify switchgear installations.  The test is also referred to as dielectric breakdown or dielectric withstand test.
A megger is an instrument that measures very high resistances.  It is used to verify the isolation of two circuits or of a circuit to ground by quantifying the resistance which may be a hundred megohms or higher.  This is very useful in a finished wiring installation or to verify the quality of a cable assembly or motor winding.
Both instruments are used to vary the integrity of insulation to guarantee safe operation.

Valence Electrons & Conductivity
Valence electrons are those that reside in the outer shell of atoms.  This determines both the chemical and electrical properties of the element composed of the atoms.  If the valence electrons are plentiful and free
(not bonded to those of other atoms) the material will conduct electricity easily and present low resistance.
The periodic table used in chemistry has elements grouped according to the number of valence electrons.  Those that are good conductors reside in the same area and have similar chemical properties.
The elements copper, iron, silver, and gold are known as transition metals.  They are grouped together in the periodic table because they have 1 or 2 valence electrons.  The electrons of these atoms are free to migrate in response to an applied electric field, which gives the materials a low electrical resistance. These materials are known as conductors.

Electromagnetic Waves Change Directions
Electromagnetic waves include visible light, x-rays, and radio waves.  The direction of travel of all electromagnetic energy can be changed by reflection and by refraction.
In the case of radio waves, reflection is caused by striking materials of different electrical characteristics especially conductors such as metal surfaces.  This is useful in radar.
Waves may also be directed by refraction, which occurs when they strike a transition between materials of different densities at an angle. 
In visible light, the most common example of this is in lenses.  The light slows down in the glass, which is denser than the air and the curves surfaces of the lens causes the light to bend.
Prisms are another example of devices that refract light. 
They are shaped so that the varying speeds of the different wavelengths of light are broken into their components and all the colors can be seen.  This phenomenon is known as diffraction.

Ions
Ions are atoms or molecules that have lost or gained electrons and therefore carry an electrical charge.  Positively charged ions have a shortage of electrons and are called anions.  Negatively charged ions have an excess of electrons and are known as cations.
In a battery, ions give up electrons at the negative electrode and accept them at the positive electrode.  Conventions differ as to which terminals of a battery are termed the anode and cathode, but external current flow is always from the positive terminal to the negative one. 
Electron flow is just the opposite, from negative to positive.

Anode and Cathode
In electroplating, the anode is connected externally to the positive side of the voltage source and electrons flow from the anode through the circuit to be delivered at the cathode where they combine with the anions and the resulting neutral metal atoms are deposited.  Currents through any device from an externally applied potential flows into the anode and out of the cathode.

Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals are in electrical contact and there is also an external environment that allows for the transmission of ions.  This environment is basically wherever any moisture is present.  The moisture together with any impurities serve as an electrolyte and creates a galvanic cell (battery) that destroys the more active of the two dissimilar metals.
Some control of this corrosive action can be made by preventing the electrical contact of different materials, always keeping the materials dry, or by using metals that are very close to same level of activity; that is they are close in the galvanic series.
Cathodic protection is sometimes used where these conditions are unavoidable.  In this case, an externally applied potential is used to counteract the cell created by the dissimilar metals and no chemical action occurs.
The principle of Galvanic corrosion can actually be used to prevent corrosive damage by purposely setting up a galvanic call with dissimilar metals.  In this case, a coating of a sacrificial active metal like zinc is added to another metal.  The cell created will cause the zinc to react and combine with other molecules before the less active protected metal does. 
Usually the result of the zinc reaction creates a passive layer of a composite material that prevents further loss of the zinc.
This process is called galvanizing.  Materials may be galvanized by the addition of a layer of zinc or other material by one of two methods.  Hot dip galvanizing deposits a very thick layer by dipping the steel part in a vat of molten zinc.  Electrogalvanizing is the electroplating of the part which imparts an extremely thin layer of zinc.  The hot dip process is often used for larger more critical parts where extreme protection is needed, and electrogalvanizing is used for less critical applications on cheaper parts like nails.

Diode
A diode is an electronic device that restricts current flow to one direction.  It blocks DC current if the polarity does not match the direction of the diode, and in AC circuits it eliminates one half of the voltage.
Diodes were originally made from vacuum tubes but now consist of a single semiconductor junction.  In order for the diode to conduct current and pass the voltage, the P side of the junction must be connected to the more positive voltage and the N side to the more negative voltage.  These are called the anode and cathode respectively.
Diodes are used to block unwanted voltages and currents based on their polarity.  A specific case is in converting AC to DC, known as rectifying.

Charge
Electrical charge is measured in coulombs.  One coulomb is the charge that is transferred by the flow of one ampere of current for one second, and it is equal to a very large number of electrons (about
20,000,000,000,000,000,000).

Capacitance
Capacitance is the tendency of circuit elements to resist changes in voltage as the electric field builds and recedes.  Current through a capacitor initially is at a maximum, limited only by the resistance in the circuit.
Capacitors can be used to prevent arcing in movable contacts.  The measurement unit of capacitance is the farad, with the microfarad or picofarad being the more practical units. A good capacitor will have a very low conductance (high resistance) to minimize leakage currents when the capacitor is charged.

Lamp Ballasts

Types and Ratings
There are two technologies of ballasts: electromagnetic and electronic.  Electronic ballasts generally are smaller, lighter, and quieter than the older electromagnetic variety.  They operate with less noise and heat and are more efficient.
Rapid start ballasts heat the lamps to the optimum temperature before producing an arc.  A modified start ballast operates like a rapid start device and then removes the heating current.  Instant start devices initiate the arc without preheating by using a higher starting voltage.
Sound ratings are provided by the manufacturers.  Class A ballasts produce the least hum and are for use in offices and homes.  Class F sound rating devices are the noisiest and are used only in outdoor areas or in factories.
Fixtures installed indoors are required by the code to be thermally protected, and carry a class P designation.  This means that the ballast turns power off when the bulbs exceed a maximum temperature.

Voltmeter & Ammeter
Ammeters are inserted into in series with the current path after the conductor being measured is disconnected.  The resistance of the ammeter’s internal shunt will add to the circuit resistance and slightly decrease the current flow.  That is why the shunt resistance must be very nearly zero.  Of course, the smaller this resistance the more difficult it is for the meter to sense and read the current.
A voltmeter on the other hand is connected in parallel with a circuit or circuit element.  Its resistance must be very high to prevent significant current flow which would tend to reduce the voltage at the point being measured.

Resistor Color Code
Resistance values are commonly identified with a standard color code.  A typical color coded resistor is shown below along with a table that defines the code:

 


In order to read the resistor value correctly the electrician must identify the starting color band.  This is usually close to one end of the device, but in any case is at the opposite end of the resistor from the tolerance indicator, which is gold or silver to indicate 5% or 10%.
There are three color bands that are used to indicate the resistance.  The first two represent the resistor value and the third is the multiplier, which defines how many zeroes to add. In the example shown above the BROWN-RED-ORANGE bands mean 1-3-3(zeroes), or 13 Ohms.
Sometimes there will be an additional band following the tolerance stripe that identifies either the quality level or the temperature coefficient of the resistor.
A tap is a point in a wiring system where an electrical connection is made in order to draw current for another use.  It may be used to add parallel connections in a branch circuit such as to power additional outlets or devices.
An air terminal is a device that is used to intentionally draw lightning to it and away from structures or equipment.  It is also called a lightning rod system.
The term pothead is used for a connection point of separate electrical conductors that continue as a cable.  This could be in power distribution system switchgear or at the point where an appliance is connected to its power source.
A pilot lamp, or pilot light, is a small lamp used to indicate that a circuit or device has power applied to it.  It is usually installed on a panel near the switch that activates the circuit.
An actuator is any device that uses electrical energy to perform an action. Examples of actuators might be relays, solenoids, motors, etc.
A device that applies electrical power to another device is called a controller. Specifically, a motor controller is a device used to stop and start a motor by applying and removing the motor current.  A motor controller is also called a contactor, though the device may also be used to control other high power circuits such as for outdoor lighting.
Contactors are rated for the motor’s voltage and current or horsepower.  They have a magnetic coil that is energized by a remote switch, and when the coil is energized it pulls and holds the load contacts against the line voltage contacts.

Duty Ratings
If a device is considered continuous duty, it is designed and constructed to operate under a more or less constant load for an indefinite length of time.  Sometimes this time interval is defined as 3 hours or more.
An intermittent duty situation occurs when a motor is operated under some combination of varying conditions of power and load.  These conditions may be full load, rest (powered off), and powered on but without a load.
Periodic intermittent operation is a special case where the variations in load are cyclical.
Short-time duty involves operation under a constant load but for a specific limited length of time.
Varying duty occurs when there are wide variations in both load and on-time.

Relays

Operation
The typical relay contains an arm that is held in place by a spring or sometimes only by gravity.  When the relay coil is energized, the arm is pulled so that the common contacts linked to it connect electrically to the normally open contacts.  At the same time, the normally closed contacts are pulled away from the commons and these connections are broken.
Relay contacts are often made of silver to resist corrosion and maintain a low resistance.  A lower than normal voltage applied to the coil may cause a relay or contactor to make intermittent connections and “chatter.”

Types of Relays
Reed relays are hermetically sealed and either contains a vacuum or is filled with an inert gas.  They are used where current demand is low and faster switching speeds are required.  Mercury wetted relays are reed relays that contain mercury that is used to make the contact.  They are used to switch low voltages where contact resistance or contact bounce must be minimized.  Mercury wetted relays must be mounted vertically in order for the mercury to make proper contact.
Latching relays are devices that have two possible stable states, and retain the last commanded state when de-energized.  The relay is switched either by energizing one of two coils, or by applying a pulse to a single coil which has a mechanical linkage that alternates the relay position.
Solid state relays contain electronics that make the connections instead of moving mechanical parts.  They are faster and have no contact bounce but their application is limited by some parameters such as high contact resistance.
A qualified person is one who has skills and knowledge related to specific equipment and who has been trained on the hazards associated with the use of that equipment.
The Authority Having Jurisdiction (abbreviated AHJ) is any governmental or insurance office that has the legal authority to approve equipment, installations, or procedures.  An AHJ could be a fire chief, fire marshal, health department official, building inspector, or the commanding officer of a military installation.
The term “approved” refers to equipment, procedures, and installations that have been accepted by the authority having jurisdiction.

Insulators and Insulation
Insulators of course must not conduct electricity.  They should also be non-toxic, durable, and resistant to environmental conditions, especially heat.
The conductivity of an insulator under normal conditions is classified by its resistance or resistivity.  Unlike conductors, the resistance of an insulator generally decreases when the temperature increases.
When the voltage applied to an insulator reaches the breakdown value, a rapid transition occurs and the material becomes conductive.  Insulators must not be exposed to voltages higher than this dielectric breakdown strength.  The highest breakdown voltage is achieved with impregnated paper, which is higher than thermoplastic, woven cloth, or rubber.
Breakdown can occur at the peak value of an applied voltage during each AC cycle.
The durability of insulation is affected by heat.  The type least able to withstand heat is rubber, which is inferior to mica, fiberglass, or enamel.  Mica is used for commutator bar separators in motors and generators where considerable heat is generated.

Wire Material
Copper clad aluminum conductors are made with an outer surface of copper that is metallurgically bonded to an aluminum core.  A minimum of 10% of the cross sectional area is copper.  It has the weight and cost advantages of aluminum, but the copper cladding reduces corrosion problems.  Aluminum and copper clad aluminum of the same size and insulation have same ampacity.
Nichrome wire, made from an alloy of nickel and chromium, is used for heating coils in appliances such as hair dryers and toasters.  It is corrosion resistant even at high temperatures, and has a high melting point and a high resistivity.

Neon Test Lamp
Neon lamps are made with low pressure mixture of noble gasses such as neon and argon.  They glow when a small threshold voltage is reached, which ionizes the gases and makes them conductive.
Neon lamps are used as simple test devices by connecting them with a resistor to limit current.  Lamps connected in this way can operate across a wide range of AC and DC voltages from 70 volts up to several hundred volts.  The lamps are sometimes housed in the handle of a probe that resembles a screwdriver.
A very low current causes the lamp to flicker, and too high a current causes the metal electrodes to emit metal particles which coat the inside surface of the lamp making it darker.

Power Factor of a Load
The power factor of a load can be measured using an ammeter, a wattmeter, and a voltmeter.  The power factor is defined as the cosine of the angle between the apparent power and real power, or more simply the ratio of the real power to the apparent power. The wattmeter reads real power and the product of the RMS values from the ammeter and voltmeter readings is the apparent power, so the power factor can be determined from these readings as follows:
Power Factor =
Power(Watts)/(Current (Amps RMS) x Voltage(Volts RMS))

Three Phase Power
The most common lower voltage three phase power distributed for commercial applications is of the 3 phase 4-wire wye connected configuration.  The 4th wire is the neutral for all phases.  Each voltage of the three phases is 120 degrees from the others with 208 volts (RMS) between the phases and 120 volts from each phase to neutral.  Two different voltage levels and the grounded neutral are available for various purposes, including standard 120 volts single phase power.
Other systems have higher voltages, up to 277 volts per phase with 480 volts phase to phase.  With this power form, lights may operate on the individual phases while three phase motors are driven with the higher voltages.

DC Motors
DC motors are used where small size, high torque, or bi-directional operation is needed; or where electrical noise generation needs to be minimized or only DC power is available.  Common applications include precision or portable devices, and in electronic instruments and automobiles.
The speed of the DC motor is controlled by varying the field current while keeping the armature current constant.  This can be done with a rheostat in the field circuit.  The direction of rotation is established by the polarity of the applied voltage.  Armature current in DC motors is proportional to the voltage applied.

Electronic Switches
The disadvantages of a mechanical switch is that it takes time to make or break contact, and it tends to “bounce”, that is have multiple make and break cycles each time it is operated.  In some circuits this is very undesirable.
Electronic switches overcome some of these shortcomings. 
They are solid state devices with no moving parts that are functionally equivalent to relays.
Electronic switches are easily integrated into modern automated control equipment.  They are fast and reliable, but do have limitations such as high contact resistance and low current handling capability.

Optical Fibers and Cables
Optical fiber is used for control, communication, and signaling.  The fibers are made of glass or plastic which transmit visible or other types of light with information encoded on it.  Because nearly all of the light is reflected within the fiber and very little is blocked, almost no loss of signal occurs even over very long distances.  Due to the high frequency of the light as compared to the information signals, the signal carrying capacity
(bandwidth) is much greater than that of copper wire.  Adding these benefits to the lower weight and cost than copper wire makes optical transmission very desirable.
Fiber optic cables are usually bundled and covered with a jacket to protect the fibers and add rigidity.  There may be up to a thousand fibers in a single cable.
To transmit and receive the light signals, LEDs and light sensitive devices are used. Modulators and demodulators are used to encode and decode the information on the light signals.  For extremely long runs, amplifiers can be used to boost the signal levels.

Circuit Breakers
A circuit breaker is a device that opens a circuit automatically to prevent damage to itself and the circuit when an overcurrent condition occurs.  It also allows opening and closing of the circuit by nonautomatic means.
Adjustable indicates that the circuit breaker can be set to trip at various values of current, time, or both.  The setting is the value of current, time, or both, where an adjustable circuit breaker is set to trip.
Instantaneous trip indicates that no delay is introduced in the tripping of the circuit breaker.  Inverse time indicates that a delay is introduced in the tripping of the breaker, and the delay decreases as the magnitude of the current increases.

Autotransformer
An autotransformer is a transformer with one winding that is shared between the primary and secondary.  Separate taps are used on the winding but there may be a common connection that is used for both primary and secondary.
Autotransformers are generally lighter and less expensive that multiple winding transformers for small ratios, 3:1 or less.  They are also more efficient.
Autotransformers are often used for high power applications and to start large induction motors where the line voltage must be reduced for starting.  On long power distribution lines, autotransformers are connected to automatic monitoring and switching equipment to regulate the line voltage.

Current Draw
The motor load can be converted to watts and with the line voltage a theoretical current can be calculated.  The inefficiency of the motor increases that current, as does the deviation from a unity power factor.

If a 10 horsepower motor is running on a single phase 220 volt circuit, is 80% efficient, and has a power factor of 90% the current is calculated as follows:
10 horsepower = 7460 watts.
7460 watts divided by 220 volts is about 34 amperes.
The 0.8 efficiency increases the current to 34 Amps / 0.8 = 42.5 amps
The power factor further increases the current to 42.5 amps / 0.9 = 47.2 amps.

Delta and Wye Configurations
With a delta connection, the loads are wired so that they are between each line. The load phase voltages are equal to the line voltages and the phase current is the vector sum of the current in each load.  No ground connection is needed.  If one phase opens, the other phase voltages and currents are unaffected.
With the wye connection, the loads are wired so that each connects between one line and the neutral which is required. The load phase voltages are less than the line voltages and the line and load currents are the same.  If one phase opens the neutral wire is necessary to prevent the other phases from changing values.
Various configurations of transformers are available to convert between delta and wye connections as well as adjust the voltage and current levels.
The delta connected system delivers power without the added neutral wire, and it is tolerant of the loss of a phase.  Because the line currents are higher, the wye system delivers more power for the same voltage or the same power with less line current.

Electric Lamp Bulbs
Incandescent lighting is provided by traditional light bulbs that have a conductive metal (usually tungsten) filament inside a glass enclosure that is either partially evacuated or filled with an inert gas to prevent the filament from burning up.  Light and heat are produced when they are lit.
The halogen lamp, also called the quartz lamp, is an innovation in which the bulb is filled with a halogen gas such as iodine or bromine.  The gas inhibits consumption of the filament and so the bulbs can be run at much higher temperature.  This produces brighter light of a whiter (less yellow) color.  The name for these lamps comes from the special glass or quartz used to enclose the filament.
Gas discharge lamps include the familiar neon lights used in store fronts and fluorescent lamps.  Fluorescent lamps contain a coating of mercury or other material that produce a visible glow when the energized gas emits invisible ultraviolet radiation.
Light-emitting diodes (LEDs) emit narrow band (single color) visible or invisible light.  These are semiconductor devices that glow when electrically stimulated.

Knob and Tube Wiring
Knob and tube wiring was used from about 1880 until the 1930s.  It used single insulated copper wire run in ceilings or walls and passing through wooden joists.  The wires were supported by insulating porcelain knobs that were nailed to wall and floor joists.
The wire was wrapped around the ceramic knobs which isolated it from combustible structural and insulating materials, and then the wires were twisted to securely fasten them to the knobs.
The conducting wires had insulated tubes installed loosely around them.  The wires operated more or less in the open air so they dissipated heat well.
Knob and tube wiring can still be found in some older homes.

Orangeburg Pipe
Orangeburg pipe is a sewer pipe and wiring conduit made of a nonmetallic fiber that is manufactured by combining tar or pitch with paper or wood pulp and forming the material into a circular or oval cross section.
It was used extensively from the 1800s until the 1970s when it was replaced with PVC, ABS, and other synthetic materials.  One disadvantage of Orangeburg is that it softens and deforms with age, so it is susceptible to damage and root intrusion.
Orangeburg pipe is also known as Bituminous Fiber Sewer Pipe or fiber conduit.
Any piece of equipment that is described as having a dead front is built without any live parts exposed to contact by persons on the external operating surfaces.
Bonding is the permanent joining of metallic parts for the purpose of creating electrical conductivity.
A system that automatically supplies light or power when a power failure occurs is called an emergency system.  The term automatic is defined as self-acting; that is, an operation that is initiated without human interaction.
The minimum temperature at which a liquid gives off an ignitable vapor is called its flash point.

Classifications of Electrical Equipment
All electrical equipment is classified as utilization equipment (including appliances), devices, fittings, and fixtures.
Utilization equipment consists of units that use electrical energy.  Appliances are non-commercial utilization equipment.  An appliance that is not easily moved, such as an oven or range, is referred to as stationary.
A unit of an electrical system that carries or controls electricity but does not use it is a device.  Switches and circuit breakers are devices.
Fittings are devices that perform a mechanical function. 
Plastic bushings and locknuts are fittings.
Fixtures are utilization equipment such as luminaries (light fixtures).

Peak Load and Demand Factor
The maximum power actually consumed by a circuit or system during a period of time is the peak load for that time period.
The ratio of this maximum load to the sum total of all the individual loads in a circuit or system is the demand factor, also known as the load factor.  It is a measure of the demand as a fraction of the total possible load if all loads were activated at the same time, which is usually very unlikely to occur.
Peak loads and actual loads are measured in real power and expressed as watts or kilowatts.

Circular Mil
The circular mil (cmil) is a convenient measure of the area of a conductor without having to use pi (π) to actually calculate the true area of a wire with a circular cross section.  One circular mil represents the area of a wire with a circular shape and a diameter of one mil (one thousandth of an inch). The unit Kcmil is 1000 circular mils.
The ampacity of a wire is dependent on the conductor cross sectional area and this area can be expressed in circular mils.  With stranded conductors, the total area of the wire is equal to the number of strands multiplied by the area of each strand.  When comparing different sized conductors, if the value for cmils or Kcmil is doubled, the resistance is reduced to one half and the ampacity approximately doubles.

Incandescent Light Bulbs
The most common light bulb base in the U.S. is the medium base.  It accepts a screw-in bulb with a center contact and the threads provide the other contact.
The candelabra base is similar to but smaller than the medium base.  These are used for night lights, Christmas tree bulbs, and stylized bulbs for candelabras.
The mogul base has a larger diameter than the medium base and is used for higher power industrial applications.
The ab medium base, sometimes used on bulbs in public places, is incompatible with common bulb bases and it used to discourage theft of the bulbs.

Wiring of Light Fixtures
A light fixture mounted in a wet location must be installed so that water will not enter the wiring compartment, and the fixture must be marked for use in damp locations.
Thermal protection is needed for fluorescent ballasts mounted indoors or for recessed incandescent fixtures installed indoors. Where there are lighting fixtures or lighting outlets in a stairway, switches must be provided on each level.
For pendant light fixtures, those that are hung by chains, the conducting wires must not be used to support the light fixture.  Stranded conductors are used for the wiring on fixture chains.

Power Distribution Wiring Methods
Many years ago an exposed wiring method was used that involved insulating knobs and tubes that contained the wires.  These were concealed in hollow spaces of walls and ceilings.  Care must be taken with any knob and tube wiring that still exists as the tubes may be brittle and susceptible to damage.
Today wires are either formed into multiple conductor cable that is strung along building supports, or some form of conduit or tubing is used.  Tubing typically already contains the wires while conduit must have the wires added after the conduit is assembled.
Insulated multiple conductor cables come in a variety of sizes and are specified for any number of applications.  Romex is a common trade name for these cables.
The horizontal distance between support points of conductor(s) is called the span. Completed wiring must be free of shorts and unintended grounds.

Reversing Direction
The direction of rotation of direct current (DC) motors is determined by the polarity of the applied voltage so they are reversed simply by interchanging the wire connections.
If a single phase motor has starting windings, the direction of rotation can be reversed by reversing these connections.  By convention the rotation of induction motors is counterclockwise when facing the front of the motor.
To reverse the rotation of a 3-phase motor, reverse the order of any two of the leads.  This reverses the order of the phases applied and causes the motor to rotate in the opposite direction.
Most motors are designed to run in a specific direction and may either not achieve their ratings or be degraded by forcing them to run in the opposite direction.

Magnetic Circuit
A magnetic circuit is a closed path of magnetic energy similar to an electric circuit. All sources of magnetism have two poles (N and
S) and so they form a circuit. 

Analogous to Ohms law (E = IR) is the relation F = ΦR where F is the magnetomotive force, Φ is the magnetic flux, and R
is the reluctance of the path between the magnetic poles.  The circuit must have the two poles formed by a permanent magnet or an electromagnet, and an air gap or separation containing the flux.  Connecting the poles of a permanent magnet there may be a keeper, a ferromagnetic bar whose presence helps maintain the magnetic field strength by completing the circuit.

Magnetic Hysteresis
In magnetic materials such as iron, hysteresis is the physical effect of a lag between the applied magnetizing force and the resulting magnetic flux.  The resulting curve is in the form of a loop with a different response depending on whether the field is increasing or decreasing, as shown in the figure on page 85. (the arrows indicate the direction).


The advantage of this effect is that when an external magnetic field is applied to a ferromagnetic material it absorbs some of the field, so when the external field is removed the material remains magnetized. 
This property is called retentivity.
The disadvantage of hysteresis is that energy is lost if a field is applied repeatedly such as in a coil or transformer with an iron core.

Thermocouples
Thermocouples are precise temperature sensors that operate on the physical principle that junctions of dissimilar metals produce small voltages; these voltages vary with different materials, and they also change with temperature.  With the proper instrumentation, these small potentials (in the microvolt range) can be converted to precise temperature measurements.
There are a wide variety of thermocouples that cover different temperature ranges.  They are classified by type such as type K, E,
J, N, etc.  Each type uses a unique set of alloys.
The difficulty in wiring thermocouples into the instruments is that the same materials must be used from the junctions all the way to the instrument.  This requires extension cables and connectors made of special materials such as chromel (nickel chromium), alumel (nickel aluminum), constantan (copper nickel), etc.
In less precise applications, compensating cables are sometimes used for these connections.  They are special cables made from a single alloy that introduce an acceptable error, but do not require the restrictive connections described above.

Generators
Effects on Output
The output voltage of an AC or DC generator is proportional to its magnetic field strength and the speed at which the rotor is driven.  The voltage is produced by induction as the armature cuts through the magnetic lines of force.  The number of lines per second that are crossed determines the output.  This rate can be increased by strengthening the field by increasing the field current.
The frequency of an AC generator’s output depends on the number of pole pairs built into it and the rotational speed of the armature. 
If there are 6 pole pairs (six N and six S magnetic poles) per revolution, then each rotation of the rotor produces three electrical cycles.

Measurement Performed
The outputs of generators can be monitored with voltmeters and frequency meters. The output frequency will be proportional to the speed which can also be monitored with a tachometer.
The most critical parameter of a generator’s performance is whether it is being overstressed which can lead to overheating and damage to the device.  The instruments that may be used to indicate whether a generator may overheat are the wattmeter, ammeter, or a stator thermocouple.  The wattmeter and ammeter show whether the generator’s ratings are being exceeded, which would lead to overheating while the thermocouple measurement shows directly when this occurs.


Fasteners
Machine screws are used for fastening to metal devices. 
They have cylindrical bodies with threads that match those in the hole in the material.  A bolt is a similar device but it passes through the material and is held by a nut on the far side.  Lock nuts are sometimes used to keep the connection secure.
Wood screws are tapered and are tightened into unthreaded holes in wood.
There are screws made for specific materials like sheet metal screws and drywall screws.
Toggle bolts are used in hollow drywall.  A large hole is drilled and the nut on the bolt toggles after it is inserted.  The nuts grip the inside of the drywall as the bolt is tightened. Hollow wall anchors are also used for walls and doors.   They expand as the screws are tightened.  All these devices are more or less permanent.
Expansion bolts are used to fasten to existing concrete or masonry.  They consist of a shield that expands when the screw is threaded into it and tightened.

AC Power Distribution and Usage
Electric power is almost exclusively generated and distributed as 3-phase alternating current (AC) because this is more efficient.  The voltage and current are sinusoidal and reverse at regular intervals.  Each cycle requires 1/60 second to complete; ½ cycle takes 1/120 second.
The standard commercial electrical service for power and lighting is 60 hertz (cycles/sec) 3 phase 4 wire.  Each phase is separated by 120 degrees (1/3 of a cycle) from the others.
The standard residential service is 60 hertz 3-wire single phase 240 volts where each line supplies 120 VAC to neutral.  This is also sometimes called 2-phase AC.  The full 240 volt potential is used for higher current devices like air conditioners and ovens.  Using both lines results in less current and therefore lower line voltage drops.

Effects of Resistance
Power is dissipated by the resistance in loads, wires, or components.  In a resistive load such as a heater or light bulb power in the form of heat and light is put to good use, but in the other cases it is beneficial to minimize these losses.  When current flows through a wire, voltage drops occur and the wire heats up.  This lost energy is not available to the circuit loads.  The greater the resistance and the current, the larger is the loss.  Voltage drops in conductors can be reduced by decreasing the resistance by using larger gauge wires.

Series and Parallel Load Connections
Typically, loads are connected in parallel so they can operate independently and at full voltage.  Electrical outlets and lighting circuits are always wired in this way.
In some special cases, devices are connected in series.  An electrical timer must be wired in series with the loads it supplies in order to control them.  The individual lamps of some Christmas lights are wired in series so each lamp receives the proper voltage (only a fraction of the 120 volts).
The heating elements of a stove are wired in series parallel combinations that are controlled by a switch to provide varying current and heat to the burners.
Multiple start buttons on a motor start circuit are connected in parallel.