Lenz's Law Explained

Ground electricity underpins grounding and earthing practices, managing earth potential and fault current. It ensures equipotential bonding, surge protection, and safety compliance per IEC/NEC standards for power systems, electronics, and lightning protection.

What Is Ground Electricity?

Ground electricity is the earth reference and return path that stabilizes systems and safely carries fault current.

✅ Provides low-impedance fault current paths to trip protection

✅ Uses earth electrodes, bonding, and grounding conductors

✅ Reduces touch voltage, EMI, and lightning surge damage

Understanding Ground Electricity 

Ground Electricity is a general term that is often used loosely to refer to “Bonding” (Safety Grounding) and “Earth Grounding”.  Personnel of major industries have often misinterpreted grounding to mean that anything in contact with the earth is considered to be grounded. For foundational context, see our overview on how electricity works to understand why grounding concepts are sometimes conflated.

Electrical Grounding is the reference zero for the electrical system.  When we talk about 120 Volts RMS, it represents 120 units above the zero electrical reference.  In residential settings, this reference is commonly tied to service equipment, as explained in our guide to home electricity and typical distribution practices.

Electrical Grounding is the reference zero for the electrical system and all the equipment should be at this same equipment potential plane. Maintaining that equipotential plane is central to electricity safety because it minimizes touch voltage hazards.

According to accident reports, most electrical accidents occur when the earth is used as the safety grounding system (Bonding).   These incidents underscore the broader dangers of electricity when protective paths are improperly designed.

Accident reports details the hazards of using earth as a safety grounding and the practice of electrical grounding the equipment through local grounds. (grounding each individual piece of equipment with a separate ground rod, driven into the earth).  Improper local grounds can also complicate fault clearing during a system electrical fault, increasing damage and downtime.

The best method for ensuring the safety of personnel is to use a “Bonding Conductor”. This conductor should be continuous, with sufficient ampacity and the lowest impedance path back to the circuit interrupting devices, which insures positive operation of the protective devices. A robust bonding network ensures rapid clearing of a ground fault by providing a low-impedance return path for protective devices.

Once a system that connects all the metallic frames of electrical equipment together has been established, then the grounding system must be placed at “Zero Reference Potential”.  The earth grounding electrode should have the lowest resistance value possible, preferably 5 ohms or less.   Coordinating equipment bonding with proper transformer grounding helps maintain stable reference potential across the installation.

This low resistance to earth ground electricity will limit the potential of the electrical system to earth (limiting the stresses placed on components such as switches, insulators and transformers).

It will also reduce the effects of static charges on the electrical system and will protect against lightning strokes, lightning induced voltages, and minimizes the effects of transient over-voltages.

Visit Our Electrical Grounding and Bonding Course 

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Ampere’s Law describes the relationship between magnetic fields and electric currents, a fundamental concept in electromagnetism. It explains how current produces a magnetic force, guiding the design of circuits, solenoids, coils, and transformers in electrical engineering.

What is Ampere’s Law?

Ampere's Law is a fundamental principle in electromagnetism that describes the relationship between electric current and the resulting magnetic field.

✅ It states that the magnetic field around a closed path is proportional to the electric current passing through it.

✅ It is a fundamental principle of electromagnetism, linking current and magnetic flux.

✅ Used in analyzing coils, solenoids, transformers, and magnetic circuits.

Named after the French physicist André-Marie Ampère, this powerful principle helps us understand the behaviour of magnetic fields generated by electric currents. It is crucial to develop the numerous technologies we use on a daily basis. Understanding Ampere's Law is easier when explored alongside related concepts in basic electricity, which provide the foundation for electrical theory.

The principle states that the line integral of a magnetic field (B) around a closed loop is equal to the product of the permeability of free space (μ₀) and the net electric current (I) passing through the loop. This can be mathematically represented as:

∮ B⋅dl = μ₀I

Ampere’s Law at a Glance

Ampere’s Law and Magnetism

The principle can be applied to determine the magnetic field around current-carrying wires and other conductive materials, as well as within various electrical systems. It is an essential part of Maxwell's equations, a set of four equations that serve as the foundation of classical electromagnetism. These equations relate electric and magnetic fields to their sources (electric charges and currents) and describe how they propagate through space. The connection between electricity and magnetism is central to electromagnetism, where Ampere’s Law works hand-in-hand with Faraday’s Law to explain induction.

It calculates magnetic fields through the Biot-Savart Law, a mathematical expression that relates the magnetic field produced by a steady electric current to the current's geometry. Both principles have specific applications, with the Biot-Savart Law being more suitable for cases with intricate current configurations. At the same time, it is typically employed when dealing with symmetrical setups.

Ampere’s Law has numerous real-life applications, especially in developing and understanding devices and systems that involve electromagnetism. For example, it is used in the design of transformers, inductors, and solenoids, as well as in various applications such as telecommunications systems, motors, and generators. By applying it, engineers can predict and control the magnetic fields generated in these devices, ensuring they function optimally and efficiently.

Gauss’ Law and Electric Fields Around a Circle of Radius

When studying electromagnetism, a common problem is analyzing the behavior of electric fields around a symmetric object, such as a circle of radius r or a sphere. Gauss’ Law is especially powerful in such cases, because it states that the electric flux through a closed surface is proportional to the net charge enclosed. This means the distribution of field lines can be calculated without solving complex integrals directly.

For a uniformly charged circle of radius r, the electric field at a point along the axis can be derived by considering the superposition of contributions from each charge element. The result reveals that the electric field depends on both the radius of the circle and the distance from the observation point. This demonstrates how Gauss’ Law simplifies problems with high symmetry.

Mathematically, the relationship is expressed as:

∮ E · dA = Q / ε₀

Here, E represents the vector field of the electric field, dA is the infinitesimal area vector on the closed surface, Q is the enclosed charge, and ε₀ is the permittivity of free space. By applying this principle, one can determine that electric fields radiate symmetrically outward from charges, with strength diminishing with distance according to the geometry of the surface considered.

The application of Gauss’ Law in analyzing a circle of radius r is connected to Ampere’s Law, as both emphasize symmetry and integration around closed paths. Where Ampere’s Law links magnetic fields to current, Gauss’ Law links electric fields to charge, and together they form part of Maxwell’s equations, the foundation of electromagnetism.

Relationship between Ampere's Law and Faraday's Law

The relationship between Ampere's Law and Faraday's Law of electromagnetic induction is apparent in the phenomenon of electromagnetic induction itself. When a changing magnetic field induces an electric current in a conductive material, the resulting magnetic field generated by this electric current, in turn, affects the overall magnetic field. It helps us understand how these interacting magnetic fields behave and influence each other.

Ampere's Law and its applications in various devices and systems enable numerous technological advancements. For instance, when designing motors and generators, engineers can utilize the principle to optimize the magnetic field within the device, resulting in higher efficiency and improved performance. Ampere’s Law is also linked to the behavior of capacitance and inductance, both of which are essential in circuits and energy storage systems.

In the telecommunications realm, it helps explain the propagation of electromagnetic waves in cables and antennas. It enables engineers to design systems that minimize signal loss and maximize data transfer rates, ensuring that our smartphones, computers, and other devices remain connected and up-to-date.

By understanding the magnetic field lines and the interaction between electric current and magnetic fields, Ampere's Law opens doors to scientific discovery and innovation in numerous fields. From determining the magnetic field at a distance to the thumb rule and hand rule applications, this fundamental principle plays a crucial role in shaping the world of electromagnetism and the technology that surrounds us.

Electromagnetism

It is a cornerstone of electromagnetism that helps us understand the relationship between electric current and how it creates a magnetic field. It is a vital component of Maxwell's equations and intricately connected to other principles, such as Faraday's Law and Biot-Savart's Law. Ampere's Law has numerous applications in real-life scenarios and is essential for the functioning of many devices and systems that rely on electromagnetism. Its significance in the development of technology cannot be overstated, as it continues to drive scientific discovery and innovation. For students exploring fundamentals, the history of concepts like what is electricity and its evolution in electricity history provides valuable context to Ampere’s discoveries.

How does Ampere’s Law relate to Faraday's Law and Biot-Savart Law?

They are all essential principles in electromagnetism that describe various aspects of the interaction between electric currents and magnetic fields. Although each addresses different aspects, these are interrelated, forming a more comprehensive understanding of electromagnetism.

It describes the relationship between an electric current and its generated magnetic field. Mathematically, it states that the line integral of the magnetic field (B) around a closed loop is proportional to the net electric current (I) passing through the loop:

∮ B⋅dl = μ₀I

Ampere’s Law is useful for calculating magnetic fields in highly symmetrical situations, such as around straight wires, loops, or solenoids.

Faraday's Law: Faraday's Law of Electromagnetic Induction describes how a changing magnetic field induces an electromotive force (EMF) in a conductor. Mathematically, it states that the induced EMF is proportional to the rate of change of the magnetic flux (ΦB) through a closed loop formed by the conductor:

EMF = -dΦB/dt

Faraday's Law is fundamental to understanding the operation of devices such as generators, transformers, and induction motors, which rely on converting mechanical and electrical energy.

The Biot-Savart Law calculates the magnetic field at any point in space due to a specific current distribution. Mathematically, it can be expressed as:

dB = (μ₀ / 4π) * (Idl × r̂) / r²

The Biot-Savart Law is particularly useful for calculating magnetic fields in complex current configurations without symmetry.

Ampere's Law and Biot-Savart Law

Ampere's Law and Biot-Savart Law: Both deal with the magnetic field generated by an electric current. While the first is useful for calculating magnetic fields in symmetric situations, the Biot-Savart Law applies to a wider range of configurations, including those with intricate geometries. It can be derived from the Biot-Savart Law for specific symmetric situations.

Ampere's Law and Faraday's Law

Ampere's Law and Faraday's Law: These laws are related through Maxwell's equations, which connect electric and magnetic fields. While the first deals with the magnetic field generated by a steady electric current, Faraday's Law deals with the induced EMF resulting from a changing magnetic field. Both laws contribute to our understanding of electromagnetic phenomena and play a role in operating devices that rely on electromagnetism.

The Biot-Savart Law enables us to determine the magnetic field generated by a specific current distribution. Faraday's Law describes how a changing magnetic field can induce an EMF. In cases where the magnetic field changes due to a varying current, the Biot-Savart Law can be used to calculate the magnetic field, and then Faraday's Law can be applied to determine the induced EMF.

All three are interconnected principles in electromagnetism, each addressing a specific aspect of the interaction between the electric current and the electric field. Together, these form a more comprehensive understanding of electromagnetic phenomena and provide a basis for analyzing and designing various devices and systems that rely on electromagnetism.

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Electricity safety in electrical engineering prioritizes grounding, insulation coordination, overcurrent protection, arc-flash mitigation, lockout/tagout, and PPE to reduce shock, burn, and fire hazards in high-voltage and low-voltage installations through standards compliance and risk assessment.

What Is Electricity Safety?

Electricity safety is engineering practice to prevent shock, arc flash, and fire through design, controls, and standards.

✅ Implement grounding/earthing, bonding, and insulation coordination.

✅ Use overcurrent, residual current, and arc-fault protection.

✅ Apply NFPA 70E/IEC 60364, lockout/tagout, and arc-flash PPE.

Electricity Safety dangers involving voltages of greater than 50V are capable of producing heart fibrillation if they produce electric currents in body tissues which happen to pass through the chest area. The electrocution danger is mostly determined by the low conductivity of dry human skin. If skin is wet, or if there are wounds, or if the voltage is applied to electrodes which penetrate the skin, then even voltage sources below 40 V can be lethal if contacted. A DC circuit may be especially dangerous because it will cause muscles to lock around the wire. For a broader overview of common electrical hazards, the guide at dangers of electricity explains mechanisms of shock and injury.

Accidental contact with high voltage will usually result in severe injury or death. This can occur as a person's body provides a path for current flow causing tissue damage and heart failure. Other injuries can include burns from the arc generated by the accidental contact. These can be especially dangerous if the victims airways are affected. Injuries may also be suffered as a result of the physical forces exerted as people may fall fall from height or be thrown considerable distance. For context on regulatory and industry thresholds, see what is considered high voltage and how those limits are defined.

Electricity Safety

Depending on the short circuit current available at a switchgear line-up, a hazard is presented to maintenance and operating personnel due to the possibility of a high-intensity electric arc. Maximum temperature of an arc can exceed 10,000 kelvin, and the radiant heat, expanding hot air, and explosive vaporization of metal and insulation material can cause severe injury to unprotected workers. Such switchgear line-ups and high-energy arc sources are commonly present in electric power utility substations and generating stations and power lines, industrial plants and large commercial buildings. In the United States the National Fire Protection Association, has published a guideline standard NFPA 70E for evaluating and calculating arc flash hazard, and provides standards for the protective clothing required for electrical workers exposed to such hazards in the workplace. And even then, workers must still be careful. Understanding fault energy begins with the basics of a short circuit and how fault paths escalate incident energy.

While arc flash and arc blast are distinct, an arc fault can originate from insulation breakdown or wiring damage and quickly intensify conditions.

Explosion hazard

Even voltages insufficient to break down air can be associated with enough energy to ignite atmospheres containing flammable gases or vapours, or suspended dust. For example, air containing hydrogen gas or natural gas or gasoline vapor can be ignited by sparks produced by electrical apparatus fed by electricity lines or electricity power lines. Examples of industrial facilities with hazardous areas are petrochemical refineries, chemical plants, grain elevators, and some kinds of coal mines. In many environments, seemingly minor static electricity discharges are sufficient to ignite vapors if controls are inadequate.

Measures taken to prevent such explosions include:

• Intrinsic safety, which is apparatus designed to not accumulate enough stored energy to touch off an explosion
• Increased safety, which applies to devices using measures such as oil-filled enclosures to prevent contact between sparking apparatus and an explosive atmosphere
• Explosion-proof enclosures, which are designed so that an explosion within the enclosure cannot escape and touch off the surrounding atmosphere (this designation does not imply that the apparatus will survive an internal or external explosion).

In recent years standards for explosion hazard protection have become more uniform between European and North American practice. The "zone" system of classification is now used in modified form in U.S. National Electrical Code and in the Canadian electrical code. Intrinsic safety apparatus is now approved for use in North American applications, though the explosion-proof enclosures used in North America are still uncommon in Europe. For clarity on terminology used across these standards, consult this electricity terms glossary for consistent definitions.

The Electric Safety Home is a home in which the outlet plugs, electric cords, and each electricity plug and cord plug near sinks and water sources are GFCI protected. That makes them safe. Understanding the nature of a ground fault helps explain why GFCIs trip quickly to prevent shock.

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Home electricity powers lighting, appliances, and electronics using energy from the grid or renewable sources like solar. Understanding usage, safety, and efficiency helps reduce costs and improve performance in daily life.

What is Home Electricity?

Home electricity refers to the power used to operate household systems and devices.

✅ Powers lights, appliances, HVAC, and electronics

✅ Supplied by utilities or solar energy systems

✅ Measured in kilowatt-hours and billed monthly

How Electricity Enters and Flows Through Your Home

Electricity originates from a distant power station, fueled by coal, natural gas, nuclear power, or renewable sources, and is transformed into high-voltage energy for long-distance transmission. Along the way, transformers step this voltage down to safe, usable levels. When it reaches your home, the electricity flows through a service line to a main panel, where circuit breakers distribute it to branch circuits that power every room and outlet.

In the United States, most residential circuits provide about 120 volts of alternating current (AC) at 60 Hz. Voltage measures the energy per unit charge: 120 volts means 120 joules of energy are transported by each coulomb of charge. One coulomb per second equals one ampere, so an appliance drawing 1 ampere at 120 volts consumes energy at 120 watts—120 joules per second.

How Appliances Use Electrical Energy

When you switch on an appliance, it allows 120 volts of potential to push current through the device—similar to using water pressure to move water through a pipe. As electric charge flows, energy is transformed into useful work like heating, lighting, or mechanical motion. Voltage gradually drops to nearly zero by the time the current exits the appliance.

This “spent” charge is carried back to your electrical panel via the neutral wire and ultimately to the ground through the grounding system. The ground serves as a vast reservoir, stabilizing voltages and protecting users in the event of a fault.

Understanding Home Wiring and Safety

In a standard polarized receptacle, the smaller prong connects to the hot wire (120 volts), and the wider prong to the neutral (0 volts). The hot wire delivers the current; the neutral completes the circuit back to the panel. A separate ground wire exists for safety—it’s not part of the working circuit but protects against electric shock if a fault occurs.

Most wall outlets contain two receptacles (a duplex) wired in parallel. This means two appliances plugged in will receive the same 120-volt supply but can draw different amounts of current. Parallel wiring is standard throughout the home, allowing consistent voltage distribution while accommodating various electrical loads.

Where the Energy Comes From and Where It Goes

Electricity used at home ultimately comes from generators that elevate the voltage of electric charge before it travels across the grid. After powering your home, the now low-energy charge returns to the ground. This closed-loop system—from the earth to the generator and back—enables efficient energy use and the safe disposal of spent charge.

Whether your electricity comes from traditional utilities or rooftop solar panels, the underlying principles remain the same: raise voltage at the source, deliver it efficiently, use it safely, and return it to ground. Understanding these concepts helps homeowners stay informed, reduce waste, and use electricity more safely and effectively.

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Open circuit voltage is the potential difference measured across the terminals of a device when no external load is applied. Common in batteries, solar cells, and electrical circuits, it helps evaluate performance, efficiency, and voltage characteristics.

What is Open Circuit Voltage?

It is the maximum voltage measured across terminals when no current flows in the circuit, providing a baseline for performance evaluation.

✅ Indicates battery and solar cell efficiency

✅ Helps assess electrical circuit performance

✅ Defines voltage without current flow

What is open circuit voltage? Often abbreviated as OCV, is an essential concept within electrical engineering, particularly relevant to professionals handling electrical systems or devices. Defined as the electrical potential difference between two points in a circuit when no current flows, OCV represents the maximum voltage achievable without applying a load. For electrical workers, understanding OCV is crucial, as it enables the evaluation of power sources and the identification of potential issues within a circuit before engaging with it under load. Knowledge of OCV benefits electrical workers by providing insights into system readiness, ensuring operational safety, and facilitating troubleshooting for optimal equipment performance. Understanding basic electricity is the foundation for grasping what open circuit voltage means, since it defines how voltage behaves when no current flows.

Determining Open Circuit Voltage

OCV can be measured using instruments like digital multimeters, which provide readings of the maximum electrical potential in the circuit. When conducting a test, it’s essential to measure the resistance between two terminals with no current flow. For instance, if a circuit is connected to a 12-volt battery with no load, the multimeter will display the OCV, which typically matches the battery’s maximum voltage. Similarly, in a solar cell, the OCV provides an indication of the maximum power it can generate when fully charged. Such measurements are helpful in evaluating the state of charge and operational status, providing valuable data to maintain system health. A solid grasp of electrical resistance is also critical, as resistance affects how potential differences are measured when a circuit is open.

Open Circuit Voltage Test

The open-circuit voltage test, also known as the no-load test, is a standard procedure in electrical engineering for assessing a power source's condition when it is not under load. In this test, an engineer connects a voltmeter to the terminals of a circuit to measure the OCV. This process is valuable for detecting issues such as short circuits, high resistance, or compromised wiring, which can lead to performance problems. The results from this test enable electrical professionals to detect weak points in a circuit before it operates under load, ensuring smoother and safer functionality. Open-circuit voltage is directly related to capacitance, as capacitors store electrical potential that can be measured under no-load conditions.

Applications of Open Circuit Voltage 

In practical applications, open circuit voltage is not just a measurement but a vital diagnostic tool. For example, in renewable energy systems, engineers often assess solar cell efficiency by examining its OCV. A solar cell’s OCV indicates its potential output, enabling accurate calculations of energy capacity and state of charge. Understanding OCV also aids in selecting voltage levels appropriate for different components, especially in high-voltage systems where matching component capacity is essential. In this way, OCV serves as a baseline for electrical potential, enabling engineers to optimize systems for both performance and safety. Engineers often compare OCV with direct current behavior, where stable voltages are easier to measure without the influence of alternating loads.

The concept of OCV has safety implications. By knowing the maximum potential voltage in a circuit before activating it, engineers can implement safeguards to avoid overloads or shorts that might occur under load. In electrical troubleshooting, measuring OCV allows for the identification of circuits that aren’t performing optimally, pinpointing faults or abnormal resistance that could lead to hazards. Hence, for electrical workers, mastering OCV measurement is not only about system performance but also about adhering to safety standards that protect both personnel and equipment.

Frequently Asked Questions

What is Open Circuit Voltage?

Open circuit voltage refers to the electrical potential, or maximum voltage, present between two conductors in a circuit when there is no active current flowing. This concept is applicable to both direct current (DC) and alternating current (AC) circuits. In DC systems, the OCV remains stable at a maximum level when no load is connected. In AC circuits, OCV may vary depending on factors such as load fluctuations and circuit design. The measurement of OCV is crucial for determining the performance of various devices, including solar cells, where the state of charge can be observed by checking the OCV. Electrical engineers and technicians can use this information to diagnose issues and assess the readiness of systems for operation. In 3-phase electricity systems, knowing the open circuit voltage helps engineers ensure balance and reliability before load conditions are applied.

Why Open Circuit Voltage Matters

For anyone working in electrical engineering, understanding open-circuit voltage is essential for designing and troubleshooting systems. OCV indicates the maximum voltage a circuit can sustain, helping engineers select compatible components and design for peak efficiency. For instance, when assessing a solar cell, the OCV helps identify the electrical potential it can generate without applying any load. In this way, OCV is a guide to the expected performance under load-free conditions, ensuring that devices will perform within specified limits when placed in actual operation. The concept also closely relates to active power, as OCV provides a baseline for calculating the amount of real power a system can deliver once current begins to flow.

Does open circuit voltage change with temperature?

Yes, temperature can affect open circuit voltage. For example, solar cells typically show a decrease in OCV as temperature rises, which impacts efficiency and energy output.

Is the open circuit voltage always equal to the source voltage?

Not always. While OCV often matches the nominal source voltage, internal resistance, aging, or chemical changes in a battery can cause the measured value to differ slightly.

Can open circuit voltage predict battery health?

OCV can give an indication of a battery’s state of charge, but it is not a complete measure of health. Additional tests, such as load testing, are needed to assess the overall condition.

How does open circuit voltage relate to safety testing?

Measuring OCV before energizing equipment enables engineers to confirm expected voltage levels and prevent hazardous conditions that may arise under load.

Is open circuit voltage used in AC systems as well as DC?

Yes, OCV applies to both AC and DC systems. In AC circuits, variations may occur depending on the design and frequency, whereas DC systems typically provide a stable maximum value.

What is open circuit voltage? Open circuit voltage is more than just a technical measurement; it is a vital reference point for understanding the behavior of batteries, solar cells, and electrical circuits under no-load conditions. By measuring OCV, electrical professionals gain valuable insights into efficiency, reliability, and safety before current flows, ensuring systems are prepared for real-world operation. Whether applied in renewable energy, troubleshooting, or equipment testing, open circuit voltage provides the foundation for sound engineering decisions and safer electrical practices.

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Electrical resistance definition explains how materials oppose current flow in circuits, measured in ohms, linked to voltage, resistivity, conductor geometry, temperature, and impedance, governed by Ohm's law and SI units in electronics.

What Is Electrical Resistance Definition?

It is the measure of how a material opposes electric current, equal to voltage divided by current and measured in ohms.

✅ Measured in ohms; per Ohm's law, resistance R equals voltage V over current I.

✅ Depends on material resistivity, length, cross-sectional area, and temperature.

✅ Key in circuit analysis, power dissipation, signal integrity, and safety.

Electrical Resistance Definition: ER occurs in an electrical circuit when current-carrying charged particles collide with fixed particles that make up the structure of the conductors. Resistance is measured in ohm. Resistance is the ohm w. Resistance occurs in every part of a circuit, including wires and especially power transmission lines. For a concise overview, see this introduction to electrical resistance to reinforce key definitions.

Dissipation of electric energy in the form of heat affects the amount of driving voltage required to produce a given current through the circuit. In fact, volts are mesured across a circuit divided by the current I (amperes) through that circuit defines quantitatively the amount of electrical resistance R. The ohm is the common unit of electrical resistance, equivalent to one volt per ampere and represented by the capital Greek letter omega, Ω. The electrical resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area. Resistance also depends on the material of the conductor. For instance, the resistance of a conductor generally increases with increasing temperature the resistivity. This is why some conductors have almost zero resistance when cooled to extremely low temperatures, as is the case with superconductors, because of the relative resistivity of metals. There is a temperature coefficient of resistivity. If you need a refresher on potential difference and its role in circuits, review this explanation of voltage to connect the concepts.

Alternating-current resistors for current measurement require further design consideration. For example, if the resistor is to be used for current-transformer calibration, its ac resistance must be identical with its dc resistance within 1/100th% or better, and the applied voltage difference between its voltage terminals must be in phase with the current through it within a few tenths of a minute. Thin strips or tubes of resistance material are used to limit eddy currents and minimize "skin" effect, the current circuit must be arranged to have small self-inductance, and the leads from the voltage taps to the potential terminals should be arranged so that, as nearly as possible, the mutual inductance between the voltage and current circuits opposes and cancels the effect of the self-inductance of the current circuit. In (a) a metal strip has been folded into a very narrow U; in (b) the current circuit consists of coaxial tubes soldered together at one end to terminal blocks at the other end; in (c) a straight tube is used as the current circuit, and the potential leads are snugly fitting coaxial tubes soldered to the resistor tube at the desired separation and terminating at the center. These design choices are also easier to contextualize by comparing common types of resistors used for precise AC measurements.

Electrical Resistance coils consist of insulated resistance copper wire wound on a bobbin or winding form, hard-soldered at the ends to copper terminal wires. Metal tubes are widely used as winding form for dc resistors because they dissipate heat more readily than insulating bobbins, but if the resistor is to be used in ac measurements, a ceramic winding form is greatly to be preferred because it contributes less to the phase-defect angle of the resistor. The resistance wire ordinarily is folded into a narrow loop and wound bifilar onto the form to minimize inductance. This construction results in considerable associated capacitance of high-resistance coils, for which the wire is quite long, and an alternative construction is to wind the coil inductively on a thin mica or plastic card. The capacitive effect is greatly reduced, and the inductance is still quite small if the card is thin. When specifying coil assemblies, it helps to recall the standardized unit of electrical resistance so ratings and tolerances are interpreted consistently.

Resistors in which the wire forms the warp of a woven ribbon have lower time constants than either the simple bifilar- or card-wound types. Manganin is the resistance material most generally employed, but Evanohm and similar alloys are beginning to be extensively used for very high resistance coils. Enamel or silk is used to insulate the wire, and the finished coil is ordinarily coated with shellac or varnish to protect the wire from the atmosphere. Such coatings do not completely exclude moisture, and dimensional changes of insulation with humidity will result in small resistance changes, particularly in high resistances where fine wire is used. Material behavior, moisture effects, and long term stability are discussed further in this broader overview of electrical resistance for additional context.

Electrical Resistance boxes usually have two to four decades of resistance so that with reasonable precision they cover a considerable range of resistance, adjustable in small steps. For convenience of connection, terminals of the individual resistors are brought to copper blocks or studs, which are connected into the circuit by means of plugs or of dial switches using rotary laminated brushes; clean, well-fitted plugs probably have lower resistance than dial switches but are much less convenient to use. The residual inductance of decade groups of coils due to switch wiring, and the capacitance of connected but inactive coils, will probably exceed the residuals of the coils themselves, and it is to be expected that the time constant of an assembly of coils in a decade box will be considerably greater than that of the individual coils. Understanding how series and parallel combinations set the equivalent resistance will inform how decade boxes are deployed in complex networks.

Measurement of resistance is accomplished by a variety of methods, depending on the magnitude of the resistor and the accuracy required. Over the range from a few ohms to a megohm or more, an ohmmeter may be used for an accuracy of a few percent. A simple ohmmeter may consist of a milliammeter, dry cell, and resistor in a series circuit, the instrument scale being marked in resistance units, if you obey ohm law. For a better value, the voltage drop is measured across the resistor for a measured or known current through it. Here, accuracy is limited by the instrument scales unless a potentiometer is used for the current and voltage measurements. The approach is also taken in the wide variety of digital multimeters now in common use. Their manufacturers' specifications indicate a range of accuracies from a few percent to 10 ppm (0.001%) or better from the simplest to the most precise meters. Bridge methods can have the highest accuracy, both because they are null methods in which two or more ratios can be brought to equality and because the measurements can be made by comparison with accurately known standards. For two-terminal resistors, a Wheatstone bridge can be used; for four-terminal measurements, a Kelvin bridge or a current comparator bridge can be used. Bridges for either two- or four-terminal measurements also may be based on resistive dividers. Because of their extremely high input impedance, digital voltmeters that be used with standard resistors in unbalanced bridge circuits of high accuracy. For quick reference during test planning, the fundamental resistance formula clarifies how R, V, and I are related under Ohm law.

Digital multi meters are frequently used to make low-power measurements of resistors in the range between a few ohms and a hundred megohms or so. Resolution of such instruments varies from 1% of full scale to a part per million of full scale. These meters generally use a constant-current source with a known current controlled by comparing the voltage drop on an internal "standard" resistor to the EMF produced by a Zener diode. The current is set at such a level as to make the meter direct-reading in terms of the displayed voltage; that is, the number displayed by the meter reflects the voltage drop across the resistor, but the decimal point is moved and the scale descriptor is displayed as appropriate. Multimeters typically use three or more fixed currents and several voltage ranges to produce seven or more decade ranges with the full-scale reading from 1.4 to 3.9 times the range. For example, on the 1000-0 range, full scale may be 3,999.999 Q. Power dissipated in the measured resistor generally does not exceed 30 mW and reaches that level only in the lowest ranges where resistors are usually designed to handle many times that power. The most accurate multimeters have a resolution of 1 to 10 ppm of range on all ranges above the 10-0 range. Their sensitivity, linearity, and short-term stability make it possible to compare nominally equal resistors by substitution with an uncertainty 2 to 3 times the least count of the meter. This permits their use in making very accurate measurements, up to 10 ppm, or resistors whose values are close to those of standards at hand. Many less expensive multimeters have only two leads or terminals to use to make measurements. In those cases, the leads from the meter to the resistor to be measured become part of the measured

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