What is Voltage?

A capacitor is an electrical component that stores and releases energy in a circuit. It consists of two conductive plates separated by an insulator and is commonly used for filtering, power conditioning, and energy storage in electronic and electrical systems.

What is a Capacitor?

A capacitor is a key component in electronics and power systems. It temporarily stores electrical energy and is widely used in both AC and DC circuits.

✅ Stores and discharges electrical energy efficiently

✅ Used in filtering, timing, and power factor correction

✅ Found in electronics, motors, and power supplies

It is designed for energy storage and can store electric charges, which can be released when needed. In this article, we will delve into the fundamentals of capacitors, including their functions, types, and applications. To better understand how capacitors support overall system performance, explore our Power Quality overview covering the fundamentals of voltage stability and energy flow.

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A capacitor consists of two metallic plates separated by an insulating material known as the dielectric. The dielectric can be made from various materials, such as mica, paper, or ceramic. When voltage is applied across the plates, positive charges accumulate on one plate, while negative charges accumulate on the opposite plate. The amount of capacitor charge that can be stored depends on several factors, including plate area, plate separation, dielectric material, and voltage ratings. Capacitors are often used in capacitor banks to improve power factor and reduce energy losses in electrical systems.

How does a capacitor work? The primary function of a capacitor in an electronic circuit is to store electrical energy. Capacitors can be used for various purposes, such as filtering, timing, and coupling or decoupling signals. In addition, they play a crucial role in power supplies, ensuring that the output voltage remains stable even when there are fluctuations in the input voltage. Learn how capacitive loads influence circuit behavior and why they require precise capacitor selection for optimal performance.

A capacitor stores energy through the electrostatic field created between its plates. The stored energy can be calculated using the formula E = 0.5 * C * V^2, where E is the stored energy, C is the capacitance, and V is the voltage across the capacitor. Capacitance, measured in Farads, is a measure of a capacitor's ability to store charge. The capacitor voltage rating is crucial for ensuring safe operation and preventing dielectric breakdown during voltage spikes.

So, when I am asked what is a capacitor? I tell readers about several types of capacitors, each with unique applications. Common types include ceramic, electrolytic, film, and tantalum capacitors. Ceramic capacitors are widely used due to their low cost and small size. They are ideal for high-frequency applications and decoupling in power supply circuits. On the other hand, Electrolytic capacitors are popular for their high capacitance values and are commonly used in filtering and energy storage applications. Capacitors play a crucial role in power factor correction, enabling industrial systems to reduce demand charges and enhance energy efficiency.

Dielectric materials used in capacitors can be organic (such as paper) or inorganic (such as ceramic). The choice of dielectric material depends on factors like the desired capacitance value, voltage rating, and operating temperature range. Additionally, different dielectric materials exhibit varying properties, making them suitable for specific applications. For a deeper understanding of energy relationships, see how apparent power differs from real and reactive power in systems using capacitors.

A capacitor can be classified as polarized or non-polarized based on the presence or absence of polarity. Polarized capacitors, like electrolytic capacitors, have a positive and a negative terminal and must be connected correctly in a circuit to function properly. Non-polarized capacitors, like ceramic capacitors, do not have a specific polarity and can be connected in any orientation.

A Capacitor behaves differently in AC and DC voltage circuits. In DC circuits, once a capacitor is charged, it blocks the flow of current, essentially acting as an open circuit. However, in ac voltage circuits, capacitors allow the flow of alternating current. This phenomenon is known as displacement current, which occurs due to the continuous charging and discharging of charges.

So, what is a capacitor? Understanding what a capacitor is and how it works is essential for anyone interested in electronics. The Capacitor plays a vital role in a wide range of applications, from energy storage and filtering to signal coupling and decoupling. Understanding the various types of capacitors and their specific applications enables you to make informed decisions when designing or troubleshooting electronic circuits. Explore how an automatic power factor controller dynamically adjusts capacitor usage to maintain an efficient power factor in real-time.

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What is a voltmeter? A voltmeter is an electrical measuring instrument used to determine voltage across circuit points. Common in electronics, engineering, and power systems, it ensures accuracy, safety, and efficiency when monitoring current and diagnosing electrical performance.

What is a Voltmeter?

A Voltmeter provides a method to accurately measure voltage, which is the difference in electric potential between two points in a circuit, without changing the voltage in that circuit. It is an instrument used for measuring voltage drop.

✅ Ensures accurate voltage measurement for safety and performance

✅ Used in electrical engineering, electronics, and power systems

✅ Helps diagnose faults and maintain efficient operation

Electrical current consists of a flow of charge carriers. Voltage, also known as electromotive force (EMF) or potential difference, manifests as "electrical pressure" that enables current to flow. Given an electric circuit under test with a constant resistance, the current through the circuit varies directly in proportion to the voltage across the circuit. A voltmeter measures potential difference, which directly relates to Ohm’s Law, the fundamental equation connecting voltage, current, and resistance in circuits.

A voltmeter can take many forms, from the classic analog voltmeter with a moving needle to modern instruments like the digital voltmeter (DVM) or the versatile digital multimeter. These tools are essential for measuring electrical values in electronic devices, enabling technicians to measure voltage, current, and resistance with precision and accuracy. While analog units provide quick visual feedback, digital versions deliver more precise measurements across wider voltage ranges, making them indispensable for troubleshooting and maintaining today’s complex electrical systems.

A voltmeter can be tailored to have various full-scale ranges by switching different values of resistance in series with the microammeter, as shown in Fig. 3-6. A voltmeter exhibits high internal resistance because the resistors have large ohmic values. The greater the supply voltage, the larger the internal resistance of the voltmeter because the necessary series resistance increases as the voltage increases. To understand how a voltmeter works, it helps to first review basic electricity, as voltage, current, and resistance form the foundation of all electrical measurements.

Fig 3-6. A simple circuit using a microammeter (tA) to measure DC voltage.

A Voltmeter, whether digital or analog, should have high resistance, and the higher the better. You don't want the meter to draw a lot of current from the power source. (Ideally, it wouldn't draw any current at all.) The power-supply current should go, as much as possible, towards operating whatever circuit or system you want to use, not into getting a meter to tell you the voltage. A voltmeter is commonly used to measure voltage drop across conductors or devices, helping electricians ensure circuits operate efficiently and safely. For quick calculations, a voltage drop calculator provides accurate estimates of conductor losses based on length, size, and current. Understanding the voltage drop formula allows engineers and technicians to apply theoretical principles when designing or troubleshooting electrical systems.

Also, you might not want to keep the voltmeter constantly connected in parallel in the circuit. You may need the voltmeter for testing various circuits. You don't want the behavior of a circuit to be affected the moment you connect or disconnect the voltmeter. The less current a voltmeter draws, the less it affects the behavior of anything that operates from the power supply. Engineers often ask: What is a voltmeter?  They use a voltmeter in power system analysis, where accurate voltage readings are crucial for ensuring safety, reliability, and optimal performance.

Alternative types of voltmeters use electrostatic deflection, rather than electromagnetic deflection, to produce their readings. Remember that electric fields produce forces, just as magnetic fields do. Therefore, a pair of electrically charged plates attracts or repels each other. An electrostatic type utilizes the attractive force between two plates with opposite electric charges or a large potential difference. A voltmeter is used to measure the potential difference. Figure 3-7 portrays the functional mechanics of an electrostatic meter. It constitutes, in effect, a sensitive, calibrated electroscope. A voltmeter draws essentially no current from the power supply. Nothing but air exists between the plates, and air constitutes a nearly perfect electrical insulator. A properly designed electrostatic meter can measure both AC voltage and DC voltage. However, the meter construction tends to be fragile, and mechanical vibration can influence the reading.

Fig 3-7. Functional drawing of an electrostatic voltmeter movement.

It's always good when a voltmeter has a high internal resistance. The reason for this is that you don't want the voltmeter to draw a significant amount of current from the power source. This cur­rent should go, as much as possible, towards working whatever circuit is hooked up to the supply, and not just into getting a reading of the voltage. Additionally, you may not want or need to have the voltmeter constantly connected in the circuit; instead, you might need it for testing various circuits. You don't want the behavior of the circuit to be affected the instant you connect the voltmeter to the supply. The less current a voltmeter draws, the less it will affect the behavior of anything that is working from the power supply.

If you connect an ammeter directly across a source of voltage, a battery, the meter needle will deflect. In fact, a milliammeter needle will probably be "pinned" if you do this with it, and a microammeter might well be wrecked by the force of the needle striking the pin at the top of the scale. For this reason, you should never connect milli-ammeters or micro-ammeters directly across voltage sources. An ammeter, perhaps with a range of 0-10 A, may not deflect to full scale if it is placed across a battery; however, it's still a bad idea to do so, as it will rapidly drain the battery. Some batteries, such as automotive lead-acid cells, can explode under these conditions. This is because all ammeters have low internal resistance. They are designed that way deliberately. They are meant to be connected in series with other parts of a circuit, not right across the power supply. Because voltage is inseparable from current, learning what is current electricity provides deeper insight into why voltmeters are vital diagnostic tools.

But if you place a large resistor in series with an ammeter, and then connect the ammeter across a battery or other type of power supply, you no longer have a short cir­cuit. The ammeter will give an indication that is directly proportional to the voltage of the supply. The smaller the full-scale reading of the ammeter, the larger the resistance needed to get a meaningful indication on the meter. Using a microammeter and a very large resistor in series, it can be devised that draws only a small current from the source.

So, What is a Voltmeter? In summary, a voltmeter is a fundamental instrument for electrical work, allowing professionals and students to accurately measure voltage and understand circuit behaviour. Whether using an analog or digital design, voltmeters and multimeters provide precise insights that support safety, efficiency, and reliable performance in electrical systems.

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A potentiometer is a variable resistor that adjusts voltage in a circuit. It’s used for controlling electrical devices like volume knobs, sensors, and dimmers. Potentiometers regulate current flow by varying resistance, making them essential in analog electronic applications.

What is a Potentiometer?

A potentiometer is a type of adjustable resistor used to control voltage or current in an electrical circuit.

✅ Adjusts resistance to control voltage in circuits

✅ Commonly used in audio controls and sensors

✅ Essential for analog signal tuning and regulation

At its core, a potentiometer functions as a variable resistor. By moving the wiper (a movable terminal) across a resistive element, the device varies the output voltage. Depending on the position of the wiper, varying amounts of resistance are introduced into the circuit, thereby adjusting the current flow.

When the wiper moves along the resistive track, it adjusts the total resistance in the circuit, which controls the flow of current. To learn more, see our guide on Electrical Resistance.

A potentiometer acts as an adjustable Voltage divider, splitting the input voltage proportionally between two output terminals based on the wiper’s position.

This relationship is governed by Ohm’s Law Formula, which states that voltage equals current multiplied by resistance (V = IR).

As shown in Figure 6-8, the basic construction of a potentiometer includes:

• A resistive track (usually carbon, cermet, or wire wound)

• A movable wiper

• Three terminals (two fixed, one connected to the wiper)

This setup enables the potentiometer to function as both a voltage divider and a simple variable resistor.

Fig. 6-8 Construction geometry of a potentiometer

Types of Potentiometers

Potentiometers come in several forms, each designed for specific applications:

A potentiometer is considered a variable resistor, one of several important types covered in our guide to Types of Resistors.

Rotary Potentiometer

The most common type, rotary potentiometers, adjust resistance through the rotation of a knob. These are frequently found in volume controls, light dimmers, and measuring instruments. The resistive track inside a potentiometer is made from materials that partially conduct electricity, such as carbon or cermet. For more on conductive materials, see Conductor of Electricity.

Figure 6-9 illustrates the typical circuit symbol for a rotary potentiometer.

Linear Potentiometer (Slide Potentiometer)

Instead of rotating, a linear potentiometer, often referred to as a slide potentiometer, adjusts by sliding a control lever. These are widely used in audio mixers and precision instruments where fine, linear adjustments are needed.

Audio Taper Potentiometer

In audio equipment, human hearing sensitivity is non-linear. Audio taper potentiometers adjust resistance logarithmically to provide a natural, smooth volume change that matches human perception.

Note: If you use a linear-taper potentiometer for audio volume control, the sound may seem to jump suddenly instead of increasing smoothly.

Digital Potentiometer

Digital potentiometers, also known as "digipots," are electronically controlled rather than manually adjusted. They find use in automatic tuning circuits, programmable amplifiers, and microcontroller applications.

Rheostat (Variable Resistor)

Although technically a type of potentiometer, a rheostat uses only two terminals: one fixed terminal and the wiper. It is optimized to control current rather than voltage. Rheostats are commonly used in applications like motor speed control and light dimming.

Practical Applications of Potentiometers

Potentiometers are found in a wide range of everyday and industrial applications:

• Audio Equipment: Volume and tone controls on stereos and guitars

• Automobiles: Throttle position sensors, dashboard dimmers

• Industrial Controls: Machinery speed adjustments

• Consumer Electronics: Game controller joysticks

• Laboratory Equipment: Calibration and fine adjustments

Potentiometers are versatile components used in both AC and DC electrical systems, from audio controls to automotive sensors.

Their ability to fine-tune voltage and resistance makes them essential in both analog and digital systems.

How to Test a Potentiometer

Testing a potentiometer is straightforward:

1. Disconnect power to the circuit.

2. Use a multimeter set to measure resistance (ohms).

3. Connect the multimeter probes to the outer two terminals to measure total resistance.

4. Measure between the wiper and one outer terminal; adjust the control and observe the changing resistance.

Consistent, smooth changes confirm proper operation. Jumps or dead spots may indicate a worn or faulty potentiometer.

A potentiometer is a simple but versatile component that provides adjustable control over voltage or resistance in a circuit. Whether used in audio systems, automotive sensors, or industrial machinery, its importance in electronic design and control systems is undeniable.

Understanding the various types and practical applications of potentiometers can help in selecting the appropriate device for a specific task.

For readers seeking a broader understanding of basic electrical principles, visit our overview of Electricity Fundamentals.

Frequently Asked Questions

What is the difference between a potentiometer and a rheostat?

A potentiometer typically acts as a voltage divider with three terminals, while a rheostat uses only two terminals to control current.

Where are potentiometers commonly used?

Potentiometers are used in volume controls, sensors, gaming controllers, industrial equipment, and calibration tools.

How does a potentiometer adjust voltage?

By moving the wiper across the resistive track, a potentiometer divides the input voltage proportionally between the two output terminals, adjusting the output voltage.

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How to save electricity? Improve energy efficiency, switch to LED lighting, manage standby power, use smart thermostats, insulate homes, schedule heavy appliances off-peak, and conduct energy audits to cut power consumption and lower bills.

How to Save Electricity?

Reduce power consumption with LEDs, smart thermostats, insulation, and off-peak usage to lower energy bills.

✅ Upgrade to LED lighting and ENERGY STAR appliances

✅ Program smart thermostats; set HVAC to efficient ranges

✅ Eliminate standby power with advanced power strips

How to Save Electricity is a popular question. It involves energy conservation and lessens real dollars and preserves a public resource. Here are some ways to cut energy costs without compromising your lifestyle too much. For a deeper primer on cutting household consumption, explore this saving electricity guide for actionable steps.

Control heating and cooling costs

In some climates, heating and cooling represent the largest part of household energy use. In many climates, running your air conditioner at 78 instead of 72 will earn 40% of your cooling bill. You don't have to freeze or roast to death in order to use less energy and earn money. In some climates, heating and cooling represent the largest part of household energy use. In many climates, running your air conditioner at 78 instead of 72 will earn 40% of your cooling bill. You don't have to freeze or roast to death in order to use less energy and earn money.In some climates, heating and cooling represent the largest part of household energy use. In many climates, running your air conditioner at 78 instead of 72 will earn 40% of your cooling bill. You don't have to freeze or roast to death in order to use less energy and earn money. Here are a few tips:

Understanding how HVAC loads draw electric power can help you schedule operation during cooler periods for efficiency.

• How to Save Electricity? Make sure your filters are clean. Check with the manufacturer of your equipment or with your utility company to see if filters on your units need to be cleaned.
• Don't heat or cool when no one is home. If you are going to be gone for more than a half an hour, you can turn your heating or cooling off or down. Don't turn off the heat in a cold climate because that may result in the pipes breaking.
• Supplement your main unit with portable units Running a fan can help you use less air conditioning. Using portable heaters when you are asleep or otherwise staying in one room can mean less use of heat if it means that you don't have to use the main unit.
• Try setting your thermostat to run less frequently Turn your air conditioner up a degree or two or your heater down a degree or two and see if you still can be comfortable.
• Time your opening and closing of windows and drapes to reduce heading and cooling costs. On cold, sunny days, opening curtains and drapes while leaving windows closed will help you heat your home. Opening the windows on summer nights helps cool your home. Buying storm windows in some climates reduces heating costs.
• Check for holes in your roof and in your pipes. This can help reduce up to 10 per cent of your heating and cooling costs.

Track your improvements by reading your electricity meter regularly and noting weekday versus weekend patterns.

Get energy saving ideas from your utility company 

Utility companies are among the few businesses who hope that you use less of their product. Most power companies are anxious to postpone construction of new power plants, so they strongly encourage customers to use less power. 
Utility companies offer energy audits, tips, and other help for customers who want to reduce energy consumption. Call your local utility or log onto their website to see what they have to offer. Many utilities explain how electricity cost components like demand and time-of-use rates affect bills.

Other Electricity Saving tips:

• Consider how to increase your energy efficiency when buying appliances Appliances are labeled with energy usage comparison tables. Look at these figures. Also, you may be able to buy an adapter to make your older appliances more energy efficient.
• Turn off appliances that no one is using Turning off TV's, lamps, computers, VCR's, ovens, and other appliances that no one is using can reduce electric energy consumption. Even turning them off for short periods can produce noticeable results.
• See if an alternative energy company is available In California, electrical deregulation is beginning to result in alternative providers of electric power. This is a trend that should increase nationally. See if that option is available to you.
• Install low-flow heads in your water closets. Also, check your water heater's temperature and set it for 120 degrees. Putting insulation over your water heater and pipes can also help.
• Wash and dry only full loads of clothing or dishes

Stay aware of regional electricity prices so you can plan high-use activities for lower-cost periods.

You may not make you rich, but it will help you cut down some on your costs and will help prevent blackouts and power shortages in your community.

Are your company's profits being eroded by escalating energy costs? Can you gamble that your energy costs will go down in the future? What are you doing about preparing and protecting your company from spiraling energy costs?

If you operate in competitive markets, review how electricity deregulation shapes supplier choices and contract terms.

Do you have the knowledge of the dynamic market with the right equipment that will make the 'smart decisions' and make an effective difference in your energy consumption and see your energy bill go down?

Facilities with inductive loads can cut charges by improving power factor through correction equipment and better motor controls.

Do you know about the latest technologies that you can implement now?

These questions can be answered by our new Industrial, Commercial & Institutional Energy Efficiency Handbook. This 100+ page book is jammed with practical information on reducing energy consumption, increase Energy efficiency by using modern technologies such as variable frequency drives, high efficiency electrical devices, energy efficient lighting, energy management control systems, metering and management systems, as well as fan, pump and blower efficiency improvements. We will also list Federal, State, utility and Canadian government programs that will reduce your energy consumption and increase your energy efficiency.

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Electricity works by moving electrons through a conductor, creating an electric current. Power stations generate electricity, which travels through wires to homes and businesses. This flow powers devices, lights, and machines, making modern life possible through electric energy and circuits.

Explain How Electricity Works

✅ Electrons move through conductors to create electric current

✅ Power plants convert energy into usable electricity

✅ Electrical systems distribute power to homes, industries, and devices

What Is Electricity and Where Does It Come From?

Electricity energy is as common to us as running water in many areas, especially in industrialized countries. Despite this, there is a great deal of ignorance about this mysterious force and its origin. 

• The concept of voltage is central to how electricity flows, as it represents the electrical pressure that pushes electrons through a circuit.

• Understanding alternating current is essential, as it's the form of electricity most commonly delivered to homes and businesses.

Atomic Structure and the Nature of Electric Charge

If you can picture an atom as a sphere, imagine in the nucleus, in the centre, that contains at least one proton and at least one neutron. The proton is positively charged. In orbit around the nucleus is at least one electron, which is negatively charged. The reason they have these opposite charges takes us deep into the realm of quantum physics. We know that the neutron is made up of quarks and the electron is an elementary particle (it is not made up of anything and is a particle in its own right), but the reason why they have opposite charges is a matter beyond my meagre capabilities and, in any case, this area is at the fringes of human knowledge.

Electron Movement and Free Charge in Conductive Materials

Atoms may contain several protons and electrons. This variation is what distinguishes one element from another. Although described as sub-atomic particles, electrons have the properties of both particles and waves when it comes to fields of magnetism in electric circuits. In theory, at least, they could be both at the same time. If you want to know what materials conduct electricity well, see our overview of conductors, which explains how they allow electrons to move freely.

If an atom has no electric charge, i.e. it is neutral, then it contains the same number of protons as electrons. In some materials, most notably metals, the electrons' orbits around the nucleus are quite loose, allowing them to spin away from the atom. When this happens, the atom becomes positively charged because protons are in the majority within the atom. A free electron can join another atom. When this occurs, then the new host atom becomes negatively charged because the electrons are in the majority (assuming the atom was neutral in the first place). Devices like ammeters and multimeters are essential for measuring electrical current and diagnosing circuit performance.

Potential Difference and the Creation of Electric Current

There are many views about the subject. If you ask science experts on YouTube to show how static electricity works, they will report that opposites attract. The greater the difference between the number of electrons and protons, the greater the attraction will be. This is called a potential difference. If we can therefore manage to produce a negative charge at one end of a copper wire and a positive charge at the other end, free electrons would move towards the positive end. As electrons leave those atoms nearest the positive end, they leave behind positively charged atoms. Electrons from neighbouring atoms will be attracted towards these positive atoms, thus creating yet more positive atoms in their wake. This continuing transfer of electrons is called current. The greater the potential difference, or voltage, measured in its unit, the greater the force of the flow of electrons, or current.

Understanding Direct and Alternating Current (DC vs AC)

Electric power can be supplied as direct current (e.g. from car batteries for lighting) or as alternating current (e.g. household mains). To explore the differences between current types, read our guide on the difference between AC and DC, which explains why each type is used in different applications.

How Transformers Adjust Voltage for Power Distribution

Often, an electrical product requires a different voltage from the one supplied by the mains electric power. In these cases, a transformer rating is required. The use of transformers is very common along power lines and in electrical devices. In addition to the step-up transformers that increase voltage, transformers can also reduce voltage. These step-down transformers can be found at utility substations where the very high voltages required to push electrons through long transmission wires are reduced for local consumption.

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Electrical resistance is the measure of how much a material opposes the flow of electric current. Measured in ohms (Ω), it affects voltage, limits current, and plays a vital role in circuit function, energy loss, and power distribution.

What is Electrical Resistance?

Electrical resistance is a key concept in electronics that limits the flow of electricity through a conductor.

✅ Measured in ohms (Ω) to indicate opposition to current flow

✅ Affects voltage, current, and overall power efficiency

✅ Essential in designing safe and effective electrical circuits

Electrical Resistance is an electrical quantity that measures how a device or material reduces the flow of electric current through it. The resistance is measured in units of ohms (Ω). If we make an analogy to water flow in pipes, the resistance is greater when the pipe is thinner, so the water flow is decreased.

Electrical Resistance is a measure of the opposition that a circuit offers to the flow of electric current. You might compare it to the diameter of a hose. In fact, for metal wire, this is an excellent analogy: small-diameter wire has high resistance (a lot of opposition to current flow), while large-diameter wire has low resistance (relatively little opposition to electric currents). Of course, the type of metal makes a difference, too. Iron wire has higher resistance for a given diameter than copper wire. Nichrome wire has still more resistance.

Electrical resistance is the property of a material that opposes the flow of electric current. The resistance of a conductor depends on factors such as the conducting material and its cross-sectional area. A larger cross-sectional area allows more current to flow, reducing resistance, while a smaller area increases it. The unit of electrical resistance is the ohm (Ω), which measures the degree to which a material impedes the flow of electric charge. Conductors with low resistance are essential for efficient electrical systems.

What causes electrical resistance?

An electric current flows when electrons move through a conductor, such as a metal wire. The moving electrons can collide with the ions in the metal. This makes it more difficult for the current to flow, and causes resistance.

Why is electrical resistance important?

Therefore, it is sometimes useful to add components called resistors into an electrical circuit to restrict the flow of electricity and protect the components in the circuit. Resistance is also beneficial because it allows us to shield ourselves from the harmful effects of electricity.

The standard unit of resistance is the ohm. This is sometimes abbreviated by the upper-case Greek letter omega, resembling an upside-down capital U (Ω). In this article, we'll write it out as "ohm" or "ohms."

You'll sometimes hear about kilohms, where 1 kilohm = 1,000 ohms, or about megohms, where 1 megohm = 1,000 kilohms = 1,000,000 ohms.

Electric wire is sometimes rated for resistivity. The standard unit for this purpose is the ohm per foot (ohm/ft) or the ohm per meter (ohm/m). You may also encounter the unit of ohms per kilometre (ohm/km). 

When an IV is placed across 1 ohm of resistance, assuming the power supply can deliver an unlimited number of charge carriers, there will be a current of 1 A. If the re­sistance is doubled, the current is halved. If the resistance is cut in half, the current doubles. Therefore, the current flow, for a constant voltage, is inversely proportional to the resistance. 

Typically, four-terminal resistors are used to measure current by measuring the voltage drop between the voltage terminals with current flowing through the current terminals. These standards, designed for use with potentiometers for precision current measurement, correspond in structure to the shunts used with millivoltmeters for current measurement with indicating instruments. Current standards must be designed to dissipate the heat they develop at rated current, with only a small temperature rise. They may be oil- or air-cooled; the latter design has a much greater surface area, as heat transfer to still air is less efficient than to oil. An air-cooled current standard with a 20 μω resistance and 2000 A capacity has an accuracy of 0.04%. Very low-resistance oil-cooled standards are mounted in individual oil-filled containers, provided with copper coils through which cooling water is circulated and with propellers to provide continuous oil motion.

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