Electrical Resistance and Resistivity | Definition and Factors V is the potential difference measured across the conductor (in volts) I is the current through the conductor (in amperes) R is the constant of proportionality called resistance (in ohms) The electrical resistance of a circuit is defined as the ratio between the voltage applied to the current flowing through it. Rearranging the above relation, The unit of electrical resistance is ohms. 1 ohm = \( \frac {1~ volt}{ 1 ~ampere}\) Electric charge flows easily through some materials than others. The electrical resistance measures how much the flow of this electric charge is restricted within the circuit. Electrical Resistance and Resistivity So what is resistivity? How is resistivity connected to electrical resistance? Imagine water flowing through a pipe. The ease of flow of this water will be related to certain things like the cross section of the pipe, the length of the pipe, even the material of the pipe. Resistance is similar to this. The flowing water is an analogy for flowing charge through a conductor.

Electrical resistance of a conductor is dependent on the following factors: Cross-sectional area of the conductor Length of the conductor Material of the conductor Temperature of the conducting material Electrical resistance is directly proportional to length (L) of the conductor and inversely proportional to the cross sectional area (A). It is given by the following relation. R = \( \frac{ρL}{A}\), where ρ is the resistivity of the material (measure in Ωm, ohm meter) Resistivity is a qualitative measurement of a material’s ability to resist flowing electric current. Obviously, insulators will have a higher value of resistivity than that of conductors. The resistivities of a few materials are given below for a comparison. Materials with a low value of resistivity conduct electricity very well. Silver – 1.00×10−8 Voltage, Current, Resistance, and Ohm's Law - learn.sparkfun.com Voltage, Current, Resistance, and Ohm's Law Voltage, Current, Resistance, and Ohm's Law Share on Twitter Share on Facebook Pin It Electricity Basics When beginning to explore the world of electricity and electronics, it is vital to start by understanding the basics of voltage, current, and resistance.

These are the three basic building blocks required to manipulate and utilize electricity. At first, these concepts can be difficult to understand because we cannot “see” them. One cannot see with the naked eye the energy flowing through a wire or the voltage of a battery sitting on a table. Even the lightning in the sky, while visible, is not truly the energy exchange happening from the clouds to the earth, but a reaction in the air to the energy passing through it. In order to detect this energy transfer, we must use measurement tools such as multimeters, spectrum analyzers, and oscilloscopes to visualize what is happening with the charge in a system. Fear not, however, this tutorial will give you the basic understanding of voltage, current, and resistance and how the three relate to each other. Georg Ohm How electrical charge relates to voltage, current, and resistance. What voltage, current, and resistance are. What Ohm’s Law is and how to use it to understand electricity. A simple experiment to demonstrate these concepts. Suggested Reading What is a Circuit? Electrical Charge Electricity is the movement of electrons. Electrons create charge, which we can harness to do work.

Your lightbulb, your stereo, your phone, etc., are all harnessing the movement of the electrons in order to do work. They all operate using the same basic power source: the movement of electrons. The three basic principles for this tutorial can be explained using electrons, or more specifically, the charge they create: Voltage is the difference in charge between two points. Current is the rate at which charge is flowing. Resistance is a material’s tendency to resist the flow of charge (current). So, when we talk about these values, we’re really describing the movement of charge, and thus, the behavior of electrons. A circuit is a closed loop that allows charge to move from one place to another. Components in the circuit allow us to control this charge and use it to do work. Georg Ohm was a Bavarian scientist who studied electricity. Ohm starts by describing a unit of resistance that is defined by current and voltage. So, let’s start with voltage and go from there. Voltage We define voltage as the amount of potential energy between two points on a circuit. One point has more charge than another. This difference in charge between the two points is called voltage.

It is measured in volts, which, technically, is the potential energy difference between two points that will impart one joule of energy per coulomb of charge that passes through it (don’t panic if this makes no sense, all will be explained). The unit “volt” is named after the Italian physicist Alessandro Volta who invented what is considered the first chemical battery. Voltage is represented in equations and schematics by the letter “V”. When describing voltage, current, and resistance, a common analogy is a water tank. In this analogy, charge is represented by the water amount, voltage is represented by the water pressure, and current is represented by the water flow. So for this analogy, remember: Water = Charge Pressure = Voltage Flow = Current Consider a water tank at a certain height above the ground. At the bottom of this tank there is a hose. The pressure at the end of the hose can represent voltage. The water in the tank represents charge. The more water in the tank, the higher the charge, the more pressure is measured at the end of the hose. We can think of this tank as a battery, a place where we store a certain amount of energy and then release it.

If we drain our tank a certain amount, the pressure created at the end of the hose goes down. We can think of this as decreasing voltage, like when a flashlight gets dimmer as the batteries run down. There is also a decrease in the amount of water that will flow through the hose. Less pressure means less water is flowing, which brings us to current. Current We can think of the amount of water flowing through the hose from the tank as current. The higher the pressure, the higher the flow, and vice-versa. With water, we would measure the volume of the water flowing through the hose over a certain period of time. With electricity, we measure the amount of charge flowing through the circuit over a period of time. Current is measured in Amperes (usually just referred to as “Amps”). An ampere is defined as 6.241*1018 electrons (1 Coulomb) per second passing through a point in a circuit. Amps are represented in equations by the letter “I”. Let’s say now that we have two tanks, each with a hose coming from the bottom. Each tank has the exact same amount of water, but the hose on one tank is narrower than the hose on the other.

We measure the same amount of pressure at the end of either hose, but when the water begins to flow, the flow rate of the water in the tank with the narrower hose will be less than the flow rate of the water in the tank with the wider hose. In electrical terms, the current through the narrower hose is less than the current through the wider hose. If we want the flow to be the same through both hoses, we have to increase the amount of water (charge) in the tank with the narrower hose. This increases the pressure (voltage) at the end of the narrower hose, pushing more water through the tank. This is analogous to an increase in voltage that causes an increase in current. Now we’re starting to see the relationship between voltage and current. But there is a third factor to be considered here: the width of the hose. In this analogy, the width of the hose is the resistance.

This means we need to add another term to our model: Water = Charge (measured in Coulombs) Pressure = Voltage (measured in Volts) Flow = Current (measured in Amperes, or “Amps” for short) Hose Width = Resistance Resistance Consider again our two water tanks, one with a narrow pipe and one with a wide pipe. It stands to reason that we can’t fit as much volume through a narrow pipe than a wider one at the same pressure. This is resistance. The narrow pipe “resists” the flow of water through it even though the water is at the same pressure as the tank with the wider pipe. In electrical terms, this is represented by two circuits with equal voltages and different resistances. The circuit with the higher resistance will allow less charge to flow, meaning the circuit with higher resistance has less current flowing through it. This brings us back to Georg Ohm. Ohm defines the unit of resistance of “1 Ohm” as the resistance between two points in a conductor where the application of 1 volt will push 1 ampere, or 6.241×1018 electrons.

This value is usually represented in schematics with the greek letter “&ohm;”, which is called omega, and pronounced “ohm”. Ohm's Law Combining the elements of voltage, current, and resistance, Ohm developed the formula: Where I = Current in amps R = Resistance in ohms This is called Ohm’s law. Let’s say, for example, that we have a circuit with the potential of 1 volt, a current of 1 amp, and resistance of 1 ohm. Using Ohm’s Law we can say: Let’s say this represents our tank with a wide hose. The amount of water in the tank is defined as 1 volt and the “narrowness” (resistance to flow) of the hose is defined as 1 ohm. Using Ohms Law, this gives us a flow (current) of 1 amp. Using this analogy, let’s now look at the tank with the narrow hose. Because the hose is narrower, its resistance to flow is higher. Let’s define this resistance as 2 ohms.

The amount of water in the tank is the same as the other tank, so, using Ohm’s Law, our equation for the tank with the narrow hose is But what is the current? Because the resistance is greater, and the voltage is the same, this gives us a current value of 0.5 amps: So, the current is lower in the tank with higher resistance. Now we can see that if we know two of the values for Ohm’s law, we can solve for the third. Let’s demonstrate this with an experiment. An Ohm's Law Experiment For this experiment, we want to use a 9 volt battery to power an LED. LEDs are fragile and can only have a certain amount of current flowing through them before they burn out. In the documentation for an LED, there will always be a “current rating”. This is the maximum amount of current that can flow through the particular LED before it burns out. Materials Required An LED NOTE: LEDs are what’s known as a “non-ohmic” devices. This means that the equation for the current flowing through the LED itself is not as simple as V=IR.

The LED introduces something called a “voltage drop” into the circuit, thus changing the amount of current running through it. However, in this experiment we are simply trying to protect the LED from over-current, so we will neglect the current characteristics of the LED and choose the resistor value using Ohm’s Law in order to be sure that the current through the LED is safely under 20mA. For this example, we have a 9 volt battery and a red LED with a current rating of 20 milliamps, or 0.020 amps. To be safe, we’d rather not drive the LED at its maximum current but rather its suggested current, which is listed on its datasheet as 18mA, or 0.018 amps. If we simply connect the LED directly to the battery, the values for Ohm’s law look like this: therefore: and since we have no resistance yet: Dividing by zero gives us infinite current! Well, not infinite in practice, but as much current as the battery can deliver. Since we do NOT want that much current flowing through our LED, we’re going to need a resistor.

Our circuit should look like this: We can use Ohm’s Law in the exact same way to determine the reistor value that will give us the desired current value: therefore: plugging in our values: solving for resistance: So, we need a resistor value of around 500 ohms to keep the current through the LED under the maximum current rating. 500 ohms is not a common value for off-the-shelf resistors, so this device uses a 560 ohm resistor in its place. Here’s what our device looks like all put together. Success! We’ve chosen a resistor value that is high enough to keep the current through the LED below its maximum rating, but low enough that the current is sufficient to keep the LED nice and bright. This LED/current-limiting resistor example is a common occurrence in hobby electronics. You’ll often need to use Ohm’s Law to change the amount of current flowing through the circuit. Another example of this implementation is seen in the LilyPad LED boards. With this setup, instead of having to choose the resistor for the LED, the resistor is already on-board with the LED so the current-limiting is accomplished without having to add a resistor by hand.

Current Limiting Before or After the LED? To make things a little more complicated, you can place the current limiting resistor on either side of the LED, and it will work just the same! Many folks learning electronics for the first time struggle with the idea that a current limiting resistor can live on either side of the LED and the circuit will still function as usual. Imagine a river in a continuous loop, an infinite, circular, flowing river. If we were to place a dam in it, the entire river would stop flowing, not just one side. Now imagine we place a water wheel in the river which slows the flow of the river. It wouldn’t matter where in the circle the water wheel is placed, it will still slow the flow on the entire river. This is an oversimplification, as the current limiting resistor cannot be placed anywhere in the circuit; it can be placed on either side of the LED to perform its function. For a more scientific answer, we turn to Kirchoff’s Voltage Law . It is because of this law that the current limiting resistor can go on either side of the LED and still have the same effect. For more info and some practice problems using KVL, visit this website .

Resources and Going Further Now you should understand the concepts of voltage, current, resistance, and how the three are related. Congratulations! The majority of equations and laws for analyzing circuits can be derived directly from Ohm’s Law. By knowing this simple law, you understand the concept that is the basis for the analysis of any electrical circuit! These concepts are just the tip of the iceberg. If you’re looking to study further into more complex applications of Ohm’s Law and the design of electrical circuits, be sure to check out the following tutorials. Potential Difference and Resistor Voltage Division Related Product: Resistor For electrical circuits, the earth or ground potential is usually taken to be at zero volts ( 0V ) and everything is referenced to that common point in a circuit. This is similar in theory to measuring height. We measure the height of hills in a similar way by saying that the sea level is at zero feet and then compare other points of the hill or mountain to that level. In a very similar way we can call the common point in a circuit zero volts and give it the name of ground, zero volts or earth, then all other voltage points in the circuit are compared or referenced to that ground point.

The use of a common ground or reference point in electrical schematic drawings allows the circuit to be drawn more simply as it is understood that all connections to this point have the same potential. For example: Potential Difference As the units of measure for Potential Difference are volts, potential difference is mainly called voltage. Individual voltages connected in series can be added together to give us a “total voltage” sum of the circuit as seen in the resistors in series tutorial. Voltages across components that are connected in parallel will always be of the same value as seen in the resistors in parallel tutorial, for example. For series connected voltages, For parallel connected voltages, Potential Difference Example No1 By using Ohm’s Law , the current flowing through a resistor can be calculated as follows: Calculate the current flowing through a 100Ω resistor that has one of its terminals connected to 50 volts and the other terminal connected to 30 volts. Voltage at terminal A is equal to 50v and the voltage at terminal B is equal to 30v.

Therefore, the voltage across the resistor is given as: VA = 50v, VB = 30v, therefore, VA – VB = 50 – 30 = 20v The voltage across the resistor is 20v, then the current flowing through the resistor is given as: I = VAB ÷ R = 20V ÷ 100Ω = 200mA Voltage Divider We know from the previous tutorials that by connecting together resistors in series across a potential difference we can produce a voltage divider circuit giving ratios of voltages with respect to the supply voltage across the series combination. This then produces a Voltage Divider network that only applies to resistors connected in series, because as we saw in the Resistors in Parallel tutorial, parallel resistors produce a current divider network. Consider the series circuit below. Related Products: Resistor Fixed Single-Through Hole Voltage Division The circuit shows the principle of a voltage divider circuit where the output voltage drops across each resistor within the series chain, with resistors R1, R2, R3 and R4 being referenced to some common reference point (usually zero volts).

So for any number of resistors connected together in series, dividing the supply voltage VS by the total resistance, RT will give the current flowing through the series branch as: I = VS/RT, (Ohm’s Law). Then the individual voltage drops across each resistor can be simply calculated as: V = IxR where R represents the resistance value. The voltage at each point, P1, P2, P3 etc. increases according to the sum of the voltages at each point up to the supply voltage, Vs and we can also calculate the individual voltage drops at any point without firstly calculating the circuit current by using the following formula. Voltage Divider Formula Where, V(x) is the voltage to be found, R(x) is the resistance producing the voltage, RT is the total series resistance and VS is the supply voltage. Example No2 In the circuit above, four resistors of values, R1 = 10Ω, R2 = 20Ω, R3 = 30Ω and R4 = 40Ω are connected across a 100 volts DC supply.

Using the formula above, calculated the voltage drops at points P1, P2, P3 and P4 and also the individual voltage drops across each resistor within the series chain. 1. The voltages at the various points are calculated as: 2. The individual voltage drops across each resistor are calculated as: Then by using this equation we can say that the voltage dropped across any resistor in a series circuit is proportional to the magnitude of the resistor and the total voltage dropped across all the resistors must equal the voltage source as defined by Kirchoff’s Voltage Law . So by using the Voltage Divider Equation, for any number of series resistors the voltage drop across any individual resistor can be found. Related Products: Resistor Networks and Arrays Thus far we have seen that voltage is applied to a resistor or circuit and that current flows through and around a circuit. But there is a third variable we can also apply to resistors and resistor networks. Power is a product of voltage and current and the basic unit of measurement of power is the watt.

In the next tutorial about Resistors, we will examine the power dissipated (consumed) by resistance in the form of heat and that the total power dissipated by a resistive circuit, whether it is series, parallel, or a combination of the two, we simply add the powers dissipated by each resistor. Current and Voltage Measurements in Circuits About Watch and Favorite Watch Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made. Favorite Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students. Current and Voltage Measurements in Circuits The electrical current is directly proportional to the voltage applied and inversely related to the resistance in a circuit. Learning Objective Describe the relationship between the electrical current, voltage, and resistance in a circuit Key Points A simple circuit consists of a voltage source and a resistor . Ohm 's law gives the relationship between current I, voltage V, and resistance R in a simple circuit: I = V/R.

The SI unit for measuring the rate of flow of electric charge is the ampere , which is equal to a charge flowing through some surface at the rate of one coulomb per second. Terms Register for FREE to remove ads and unlock more features! Learn more Full Text To understand how to measure current and voltage in a circuit, you must also have a general understanding of how a circuit works and how its electrical measurements are related. A simple electric circuit made up of a voltage source and a resistor According to Ohm's law, The electrical current I, or movement of charge, that flows through most substances is directly proportional to the voltage V applied to it. The electric property that impedes current (crudely similar to friction and air resistance) is called resistance R. Collisions of moving charges with atoms and molecules in a substance transfer energy to the substance and limit current. Resistance is inversely proportional to current. Ohm's law can therefore be written as follows: $I = V/R$ where I is the current through the conductor in amperes, V is the potential difference measured across the conductor in volts, and R is the resistance of the conductor in ohms (Ω).

More specifically, Ohm's law states that R in this relation is constant, independent of the current. Using this equation , we can calculate the current, voltage, or resistance in a given circuit. For example, if we had a 1.5V battery that was connected in a closed circuit to a lightbulb with a resistance of 5Ω, what is the current flowing through the circuit? To solve this problem, we would just substitute the given values into Ohm's law: I = 1.5V/5Ω; I = 0.3 amperes. If we know the current and the resistance, we can rearrange the Ohm's law equation and solve for voltage V: $V = IR$ Resistors and Ohm's Law RESISTORS AND OHM'S LAW Glossary Learning Outcomes After studying this section, you will (a) know how to measure currents, potential differences and resistances, (b) know the difference between resistors connected in series and in parallel, and (c) know Ohm's Law and be able to apply it to given electrical circuits. 1. Electric current An electric current is a flow of electric charge through a conductor.

For practical purposes, we imagine an electric current to be a flow of positive charges (we call this current the CONVENTIONAL CURRENT, but the reality is that current is carried by the negative charges on electrons within the conductor. Conventional current flows from positive to negative, whereas the electrons flow in the other direction. The unit of current is the AMPERE, A, named in honour of the French scientist, Amp�re . "One ampere is the current in a conductor when a charge of one coulomb passes through a cross section of the conductor each second." Note that this is consistent with the formal SI definition of the ampere, namely, "The ampere is the constant current which, when flowing in each of two parallel conductors of infinite length and 1m apart in a vacuum, produces between them a force of 2 x 10-7 N per metre of length". In order for a current to flow through a conductor, two prerequisites must be present: The electrons must be able to move through a closed circuit. In other words, the electrons move round and round the circuit. A break in the circuit will stop the flow of electrons and hence the current. There must be a source of energy to drive the electrons round the circuit.

This is normally provided by an energy source that sets up an electrical potential difference between two points in the circuit. Electrons are repelled by the point of low potential (the negative terminal of a battery, for example) and attracted by the point of high potential (the positive terminal). The conventional current flows the other way. Current is measured using an instrument called an AMMETER. Ammeters are connected in series with the part of the circuit through which one wishes to measure the current, I, and they HAVE NEGLIGIBLE RESISTANCE. 2. Potential difference Current flows in a circuit as a result of a DIFFERENCE IN POTENTIAL between two points in the circuit. The potential difference between two points in a conductor is the work per unit charge done by the charge in moving from a point of higher potential to a point of lower potential. The unit of potential difference is called the VOLT, V, after the Italian scientist Alessandro Volta . One volt of potential difference exists between two points if one joule of work is done by each coulomb of charge in moving between them. Potential difference is measured by an instrument called a VOLTMETER. Voltmeters are connected in parallel to the component across which one wishes to measure the potential difference.

They have a resistance which is several orders of magnitude higher than the resistance of the component. THE CURRENT WHICH FLOWS THROUGH A VOLTMETER IS NEGLIGIBLE. 3. Resistance It can be shown that for any conductor, the ratio of the potential difference across the conductor and the current flowing through it, is constant. This constant is called the RESISTANCE OF THE CONDUCTOR, R. We can write: The unit of resistance is the OHM, Ω named after Georg Ohm , who discovered this relationship, which is called Ohm's law (See below). "A conductor has a resistance of one ohm if on application of a potential difference of one volt, a current of one ampere flows through it." Resistance is a property of a particular conductor and depends on: The material of which the conductor is made. The length, L, of the conductor (R ∝ L). The cross-sectional area, A, of the conductor (R ∝ A-1) The temperature of the conductor. (Resistance increases non-linearly with temperature). 4.

Ohm's law Ohm's Law states that Ohm's Law "For any particular conductor at a constant temperature, the current that flows through it is directly proportional to the potential difference applied across it." A conductor that has a significant resistance is called a RESISTOR. Resistors are normally depicted in circuit diagrams in one of two ways, as shown on the right. The symbol (a) is currently used, but in older textbooks the symbol (b) is used. Conductors made of metals or carbon (graphite) obey Ohms' Law. 5. Resistors in series Consider a circuit in which three resistors R1, R2 and R3 are connected series, thus The total, or equivalent, resistance of a set of resistors connected in series (in this case three), is the sum of the resistances of the individual resistors: 6. Resistors in parallel Consider a circuit in which three resistors R1, R2 and R3 are connected PARALLEL, thus The total or equivalent, resistance of a set of resistors connected in parallel (in this case three) is given by the equation (See the derivation ) 7.

Network of resistors Resistors can be connected in networks in which some are arranged in series and others in parallel, as shown in the example on the left. In analyzing such a network, it is important to simplify the circuit by recognizing which resistors are in series or parallel, and replacing them by a single EQUIVALENT RESISTANCE, as shown in the worked example. Also remember that for resistors in series, The current flowing through each resistor is the same. The potential difference, Vi, across each resistor Ri is given by Vi = RiV/R, where V is the potential drop across the whole circuit, and R is the resistance of the circuit. The total potential difference V is the sum of the potential differences across each of the resistors. See the conventions used in drawing circuit diagrams . What is a "voltage divider"? (Click here for a discussion) 8. Problems involving light bulbs In many problems that are encountered, the resistors are light bulbs, which are represented as shown here on the right. Light bulbs are considered to have a constant resistance, and therefore obey Ohm's law. This is not strictly true as the resistance will in fact increase as the temperature of the filament rises - however it is a useful simplification.

The brightness of a light bulb increases as more current flows through it, and decreases as the current flowing though it decreases. 9. The Wheatstone bridge The Wheatstone bridge is a circuit specially designed to give an accurate value of unknown resistances. Referring to the diagram on the right, X is an unknown resistance, while P, Q and R are resistances of known value. G is a sensitive galvanometer, able to detect very small currents. The resistances P, Q and R are varied until the reading on G is zero, a condition that obtains when the potential difference between the points B and D is zero. This means that the potential differences across X and Q are the same, and the current through X (let's call it I1) must equal the current through P. Using the same argument, the current through Q must equal the current through R (let's call it I2). We can then write Derivation of series resistors formula The current, I, which flows through the circuit, must flow through each of the resistors The total resistance between the points A and B, call it R, will produce a potential drop between the test points, where V = IR.

(1) The potential difference between each of the individual resistors will be The total potential drop between A and B will be the sum of the individual potential drops across each resistor. Thus, Derivation of parallel resistors formula At the point A, the current, I, divides into three smaller currents I1, I2, and I3, in such a way that the total current is given by I = I1 + I2 + I3 (1) If this were not so, some current would disappear or accumulate somewhere! The potential difference across the points A and B will be V, and will be equal to the current I multiplied by the total resistance of the parallel resistors, R. Thus, V = IR. Since each resistor is connected across the points A and B, the potential difference across each is the same, namely, V. The currents through each resistor in turn will be I1 = V/R1, I2 = V/R2 and I3 = V/R3. Some conventions used in drawing circuit diagrams Circuit diagrams can be extraordinarily complex. In particular, lines that designate conducting wires may cross other lines, and be or may not be connected at the crossing point.

The following are the conventions that are used: Basic Techniques for Accurate Resistance Measurement | CAS Basic Techniques for Accurate Resistance Measurement Date Published: January 20, 2014 Application: White Papers Download the PDF version Reduce Measurement Errors in your Application At CAS DataLoggers we often receive calls from users working in resistance measurement applications, for example using string pots to measure movement, measuring thermistors/RTDs for temperature, measuring the resistance on test samples, and many more applications. Some of our callers are surprised to learn that there’s a variety of different techniques to use to obtain more accurate measurements. We also talk with callers who report getting weird readings, for example: “With the logger I’m using, I’m seeing numbers that don’t make sense.” This is usually resolved by realizing that resistance measurements span many different ranges which require the use of different measurement techniques. In this white paper, we’ll cover a few simple ways to reduce error and improve accuracy at the low, mid- and high resistance ranges. Use the Right Measurement Technique for Your Range: Resistance measurements are represented using the ohm unit (Ω).

1 ohm represents the resistance between 2 points of a conductor when a constant potential difference of 1 volt, applied to these points, produces a current of 1 ampere in the conductor, given that the conductor does not produce any electromotive force on its own. Resistance is one of the few values in electronics that in common applications varies over such a large range (over 12 orders of magnitude), and many users don’t take this into account when collecting data. For most applications values less than 100 ohms can be considered as the low value of resistance measurement, while 100 ohms up to a million ohms (megohm) is an intermediate range. High resistance ranges go up from the megohm range, and we have had a few calls from users measuring way up in the gigaohm range (1 billion ohms). When callers ask us, “I need to measure resistance—what logger do you recommend?” our Application Specialists help them narrow it down by asking, “What’s the resistance range you’re trying to measure? Milliohm, kilohm?” Before starting your particular application, it’s important to consider that each of these ranges require the use of different measurement techniques.

In fact there’s no single technique for measuring every value of resistance, and you can easily get inaccurate results by using the wrong technique for your range. For example, without matching your technique to your range, your data may only be within 5% of the actual value. In some applications this isn’t a serious problem, but in other cases your measurements need to be more accurate, for example when measuring at the milliohm level or when measuring the much larger gigaohm values where the capacitance and insulation resistance of your cabling can make a big difference in how accurate your readings will be. Of the three, mid-range measurement is the most straightforward, while very low- and high-range measurements introduce problems which you’ll see in the form of measurement errors and lowered accuracy. Ohm’s Law: The foundation of resistance measurement, Ohm's Law states that the ratio of potential difference (V) across the ends of a conductor of resistance (R) to the current (I) flowing in that conductor will be constant, provided that the temperature also remains constant.

For most applications you can use a basic Ohm’s Law equation of I = V/R where I is the current through the conductor (given in amperes), V is the potential difference measured across the conductor (given in volts), and R is the resistance of the conductor (here R is a constant given in ohms). It’s easy to work with Ohm’s Law to find any of these values. For example it’s also true that Resistance equals Voltage divided by Current (R=V/I), and also that Voltage = Current multiplied by Resistance (V=I*R). In this way you can derive any missing variable if you know the other two. Using Data Loggers for Resistance Measurement: In addition to using multimeters, users can also measure resistance using a properly-configured universal input datalogger. However, these dataloggers have limited measurement ranges so it helps to keep this in mind. Universal analog inputs allow dataloggers to record voltage or current measurement, calculate resistance values and save the data. In addition to the particular measurement device you use, there are subtleties about resistance measurement that you need to understand to increase accuracy.

Low-Resistance Measurement: First let’s consider the cables which are connecting the device to be measured to the instrument: If you’re measuring resistance near to the source, you’ll get different readings than when you’re taking measurements from a sensor placed 200 ft away. As an example, say we have a resistor which we want to measure. We have to connect to it somehow, so we connect a wire to a 1-ohm resistor. But the wire isn’t a perfect conductor either—it has some resistance in it too, as with any piece of wire. If this resistance is 1 ohm per 100 ft. (typical for 20 gauge wire), and we have 3 ft. of cabling going to the device and coming back (6 ft. total) we may expect to see a resistance reading of 1 ohm, but what we’ll actually see is a reading of 1.06 ohms. When measuring small resistances, a common technique is to force a known current and then measure the voltage across the device under test (DUT--see Figure 1 below). This is in accordance with Ohm’s Law since you’re using the current and voltage to determine the resistance.

Let’s assume you have a precision current source (for example of 2 milliamps or 200 microamps), and you’ve got a highly accurate voltmeter. Figure 1--Forcing a Known Current You run 2 wires on either side of the resistor, then force the current through both sets of leads. However this creates an error in your measurements because the voltage measured at the end of the leads isn’t the same as the voltage across the resistor, as it also includes the voltage drop across the wires between the meter and DUT. Therefore in this case you can reduce the error by taking a 4-wire measurement (see Figure 2 below) where you use one set of leads to carry the sourced current and a second set of leads which is only used to measure the voltage that you see across the resistor. This method provides a much more accurate result by getting around the additional voltage drop caused by the current source through the leads. In this case it is assumed that the voltmeter draws negligible current which is usually not a problem.

Figure 2--Typical 4-Wire Measurement At CAS DataLoggers, our dataTaker and Grant Squirrel data loggers have the ability to take 4-wire measurements for RTDs or other sensors. These devices both force a current through one pair of wires and then measure the voltage through the other pair of wires. However there is a limit to how large a resistance you can measure. For example at the 10,000 ohm range, the smallest current the data logger can force is 200μamps which results in a voltage of 2 volts; at higher resistances (say 1 million ohms) this same current would result in a voltage of 200 volts which is way beyond the capabilities of the loggers. You can’t use 4-wire measurements to measure high resistances with these types of data loggers because you need to either source a very small current or measure a large voltage. For low resistance measurements it is possible to use an alternate to the 4-wire measurement by eliminating one of the conductors and performing a 3-wire measurement.

In this technique you measure two voltages: the voltage across the resistor as well as the voltage across the conductor that carries the test current. After taking these measurements, you can determine the error due to the voltage drop in one of the leads. When you’ve determined the error, simply double it and subtract it from your measurements to get a more accurate reading. Many data loggers can perform 3-wire measurements including dataTaker data loggers and the Grant Squirrel series. With a 3-wire measurement you save a piece of wire, but this setup assumes that the voltage drop in the two leads is identical—this is often but not always the case. If the voltage drop is uneven, you will encounter errors when using this technique. While most users are usually measuring between the 0 and 100,000 ohm range, specialized sensors such as conductivity or test samples fall within very high resistance ranges, so you may have to use another technique at high resistances: High Resistance Measurement: For these measurements you can use a reliable technique which is the inverse of the one given above for measuring at low resistances--here we force a voltage and measure the current to calculate the resistance (again in accordance with Ohm’s Law).

There are a few ways you can choose to do this: The first method requires a high-precision instrument designed to measure very low currents. Assuming that you have a voltage source and a current meter with negligible resistance, you can simply force 5 volts through the meter connected in series with the DUT and measure the current. As an example, if the resistance value is a million ohms (1 megohm), the current here is reasonably small at 5μA. An alternate technique to measure large resistances is to use a voltage source in series with a known test resistance to stimulate the unknown resistance and then to measure the voltage across the test resistance (see Figure 3 below). Knowing the value of the test voltage source, the known resistance and the voltage across this resistance--plus Ohm’s Law and a little algebra--allows you to calculate the value of the unknown resistance: Figure 3--Sourcing Voltage Through a Known Resistor For this technique to work well, the value of the test resistance should be similar to the value of the unknown resistance (within 1 to 2 orders of magnitude). Here again, your voltmeter has to have good accuracy, otherwise it’ll introduce an error into your measurements.

Also the voltmeter used to measure Vtest should not load the circuit, which is to say that its input impedance should be 100 to 1000 times greater than Rtest. Another problem when measuring high resistance ranges is that even with insulated cables, no insulation is perfect—there’s always leakage current. For example there may actually be 10 million ohms of resistance between the center of the wire and whatever it’s lying on, so the leakage from the cables can adversely affect your measurement. To alleviate this you can use a technique called guarding by using shielded cable along with a separate voltage source going to the shields and around the cables. You’ll use the separate voltage source to apply the same voltage to the shield. This way even though there’s resistance between the central conductor and the shield, because they’re maintained at the same voltage, there’s no leakage current because you used the separate voltage source to guard it. This method works well but requires a meter designed to provide a guard voltage or a second voltage source. When you measure high-range resistance you’ll find you can significantly reduce your measurement errors by using one of the above two techniques.

Intermediate Resistance Measurements: Within these ranges, the technique you’ll use largely depends on what measuring device you’re using—there’s no one method you can always rely on. Fortunately in this range errors are less common and measurement is more direct. Summary: At less than 10,000 ohms, you can use the techniques given above for the low resistance ranges—when you get to 100 ohms and lower, you’ll certainly need it to obtain accurate readings. Likewise above 100,000 ohms it’s best to use the force voltage technique described above for high ohm ranges. Essentially when taking resistance measurements you’ll need to recognize when you’re getting into high and low ranges and apply the appropriate techniques, and this will eliminate substantial errors and give you much higher accuracy in your measured values. Résistance | Article about Résistance by The Free Dictionary Résistance | Article about Résistance by The Free Dictionary resistance, in biology: see immunity immunity, ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity. ..... Click the link for more information. .

resistance, in psychiatry: see psychoanalysis psychoanalysis, name given by Sigmund Freud to a system of interpretation and therapeutic treatment of psychological disorders. Psychoanalysis began after Freud studied (1885–86) with the French neurologist J. M. ..... Click the link for more information. . resistance, property of an electric conductor by which it opposes a flow of electricity and dissipates electrical energy away from the circuit, usually as heat. Optimum resistance is provided by a conductor that is long, small in cross section, and of a material that conducts poorly. Resistance is basically the same for alternating and direct current circuits (see impedance impedance, in electricity, measure in ohms of the degree to which an electric circuit resists the flow of electric current when a voltage is impressed across its terminals. ..... Click the link for more information. ). However, an alternating current of high frequency tends to travel near the surface of a conductor. Since such a current uses less of the available cross section of the conductor than a direct current, it meets with more resistance than a direct current. In circuit analysis an ideal resistor resistor, two-terminal electric circuit component that offers opposition to an electric current.

Resistors are normally designed and operated so that, with varying levels of current, variations of their resistance values are negligible (see resistance). ..... Click the link for more information. , i.e., a circuit component whose only property is resistance, is called a resistance. The phenomenon of resistance arises from the interactions of electrons with ions in the conductor. The unit of resistance is the ohm ohm [for G. S. Ohm], symbol Ω, unit of electrical resistance, defined as the resistance in a circuit in which a potential difference of one volt creates a current of one ampere; hence, 1 ohm equals 1 volt/ampere. The megohm (1,000,000 ohms) and the milliohm (. ..... Click the link for more information. . See superconductivity superconductivity, abnormally high electrical conductivity of certain substances. The phenomenon was discovered in 1911 by Heike Kamerlingh Onnes, who found that the resistance of mercury dropped suddenly to zero at a temperature of about 4. ..... Click the link for more information. ; Ohm's law Ohm's law [for G. S.

Ohm], law stating that the electric current i flowing through a given resistance r is equal to the applied voltage v divided by the resistance, or i=v/r. ..... Click the link for more information. ; conduction conduction, transfer of heat or electricity through a substance, resulting from a difference in temperature between different parts of the substance, in the case of heat, or from a difference in electric potential, in the case of electricity. ..... Click the link for more information. . Resistance The physical property of a material to resist or impede the conduction of electrical current, measured in ohms. High resistance means poor conductivity and vice versa. resistance

Question: What name is given to the potential difference measured across a conductor of 1 ohm resistance when current of 1 ampere is flowing?
Answer: Volt