Current Measurements: How-To Guide - National Instruments Current Measurements: How-To Guide Publish Date: Nov 11, 2014 | 20 Ratings | 4.20 out of 5 | Print | Submit your review Overview This document is part of the How-To Guide for Most Common Measurements centralized resource portal. Table of Contents Current Webcasts, Tutorials, and Other How-To Resources 1. Current Overview Electric current is the flow of electric charge. The SI unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second. While there are several methods of measuring current, the most common method is to perform an indirect measurement by measuring the voltage across a precision resistor and using Ohm’s law to measure the current across the resistor. Current Fundamentals In solid conductive metal, a large population of electrons is either mobile or free. When a metal wire is connected across the two terminals of a DC voltage source such as a battery, the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward the positive terminal under the influence of this field.

The free electron is therefore the current carrier in a typical solid conductor. For an electric current of 1 ampere rate, 1 coulomb of electric charge (which consists of about 6.242 × 1018 electrons) drifts every second through the imaginary plane through which the conductor passes. Figure 1. Illustration of Current Flow Conventional current was defined early in the history of electrical science as a flow of positive charge. In solid metals, like wires, the positive charge carriers are immobile, and only the negatively charged electrons flow. Because the electron carries a negative charge, the electron current flows in the opposite direction of the conventional (or electric) current. When solving electrical circuits, the actual direction of current through a specific circuit element is usually unknown. Consequently, each circuit element is assigned a current variable with an arbitrarily chosen reference direction. When the circuit is solved, the circuit element currents may have positive or negative values. A negative value means that the actual direction of current through that circuit element is opposite that of the chosen reference direction. 2.

How to Make a Current Measurement Current Measurement Methods There are two main ways to measure current – one is based on electromagnetics and is associated with the early moving coil (d’Arsonval) meter, and the other is based on the main theory of electricity, Ohm’s law. D’Arsonval Meter/Galvanometer A d’Arsonval meter is a type of ammeter, which is an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil. The d’Arsonval form used today is constructed with a small pivoting coil of wire in the field of a permanent magnet. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny torsion spring pulls the coil and pointer to the zero position. When a direct current (DC) flows through the coil, the coil generates a magnetic field. This field acts against the permanent magnet. The coil twists, pushing against the spring, and moves the pointer. The hand points at a scale indicating the electric current.

Careful design of the pole pieces ensures that the magnetic field is uniform, so that the angular deflection of the pointer is proportional to the current. Other Ammeters Essentially, most of today’s ammeters are based on the fundamental theory of electricity, Ohm’s law. Modern ammeters are essentially voltmeters with a precision resistor, and using Ohm’s law, an accurate yet cost-effective measurement can be made. Ohm’s Law – Ohm’s law states that, in an electrical circuit, the current passing through a conductor between two points is directly proportional to the potential difference (in other words, voltage drop or voltage) across the two points, and inversely proportional to the resistance between them. The mathematical equation that describes this relationship is: I = V/R where I is the current in amperes, V is the potential difference between two points of interest in volts, and R is a circuit parameter, measured in ohms (which is equivalent to volts per ampere), called the resistance. Ammeter Operation – Today’s ammeters have an internal resistance to measure the current across the particular signal. However, when the internal resistance is not enough to measure larger currents, an external configuration is needed.

To measure larger currents, you can place a precision resistor called a shunt in parallel with the meter. Most of the current flows through the shunt, and only a small fraction flows through the meter. This allows the meter to measure larger currents. Any resistor is acceptable, as long as the maximum expected current multiplied by the resistance does not exceed the input range of the ammeter or data acquisition device. When measuring current in this fashion, you should use the smallest value resistor possible because this creates the smallest interference with the existing circuit. However, smaller resistances create smaller voltage drops, so you must make a compromise between resolution and circuit interference. Figure 2 shows a common schematic of current measurement across a shunt resistor. Figure 2. Connecting a Shunt Resistor to a Measurement Using this approach, the current is not actually directed into the ammeter/data acquisition board but instead through an external shunt resistor. The largest current you can measure is theoretically limitless, provided the voltage drop across the shunt resistor does not exceed the working voltage range of the ammeter/data acquisition board. Current Conventions Conventional Currents Conventional currents are the current measurements common in today’s electronics, electrical circuitry, transmission lines, and so on.

They do not conform to a transmission standard, and they can range from zero to large values of amperage. Current Loops/4-20 mA Convention Analog current loops are used for any purpose where a device needs to be either monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time. “Four to 20 milliamp current loop,” or 4-20 mA, is an analog electrical transmission standard for industrial instrumentation and communication. The signal is a current loop where 4 mA represents the zero percent signal and 20 mA represents the 100 percent signal.[1] The “mA” stands for milliampere, or 1/1000 of an ampere. The “live zero” at 4 mA allows the receiving instrumentation to distinguish between a zero signal and a broken wire or a dead instrument.[1] Developed in the 1950s, this standard is still widely used in industry today. Benefits of the 4-20 mA convention include wide use by manufacturers, relatively low implementation costs, and its ability to reject many forms of electrical noise.

Also, with the live zero, you can directly power low-power instruments from the loop, saving the cost of extra wires. Accuracy Considerations Placement of the shunt resistor in the circuit is important. If the external circuit shares a common ground with the computer containing the ammeter/data acquisition board, then you should place the shunt resistor as close to the ground leg of the circuit as possible. If not, the common-mode voltage produced by the shunt resistor might be outside the specification for the ammeter/data acquisition board, which could lead to inaccurate readings or even damage the board. Figure 3 shows the correct and incorrect placements of the shunt resistor. Figure 3. Shunt Resistor Placement Data Acquisition Device Measurements There are three different methods of measuring analog inputs. Please refer to the “How to Make a Voltage Measurement” article for additional information on each configuration. As an example, consider the NI CompactDAQ USB data acquisition system. Figure 4 shows the NI cDAQ-9178 chassis and an NI 9203 analog current input module. The NI 9203 does not require an external shunt resistor due to the presence of an internal precision resistor. Figure 4.

NI cDAQ-9178 Chassis and NI 9203 Analog Current Input Module Figure 5 shows the connection diagram for Reference Single-Ended (RSE) current measurements using an NI cDAQ-9178 chassis with an NI 9203 as well as the pin-outs for the module. In the figure, Pin 0 corresponds to the “Analog Input 0” channel and Pin 9 corresponds to the common ground. Figure 5. Current Measurement in RSE Configuration In addition to the NI 9203, general-purpose analog input modules, such as the NI 9205 , can provide current input functionality using an external shunt resistor. Getting to See Your Measurement: NI LabVIEW Once you have connected the sensor to the measurement instrument, you can use LabVIEW graphical programming software to visualize and analyze data as needed. Figure 6. LabVIEW Current Measurement References Return to Table of Contents Introduction Electrical measurements often come down to either measuring current or measuring voltage. Even if you are measuring frequency, you will be measuring the frequency of a current signal or a voltage signal and you will need to know how to measure either voltage or current.

In this short lesson, we will examine those two measurements - starting with measuring voltage. However, first we should note a few common characteristics of the meters you use for those measurements. Many times you will use a digital multimeter - a DMM - to measure either voltage or current. Actually, a DMM will also usually measure frequency (of a voltage signal) and resistance. You should note the following about typical DMMs. Polarity is important. Usually the terminals of the DMM will be coded to indicate polarity. Often that polarity is indicated by a red terminal (positive) and a black terminal (negative). In other cases, the polarity could be indicated by printed notes on the terminals. Often one of the terminals on the DMM may be connected to the ground. That would normally be the black terminal, or it may be indicated with a ground symbol. With that in mind, let's get on to measuring voltage. Measuring Voltage Voltage is one of the most common quantities measured. That's because many other variables - like temperature, for example - are measured by generating a voltage with a sensor. So, even if you want to measure temperature you might end up having to measure a voltage and convert that reading into the temperature reading you wanted.

Voltage is measured with a voltmeter. However, digital multimeters (DMMs) - which can function as voltmeters - often have considerably more capability and can measure current, resistance and frequency. And, there are other instruments - like oscilloscopes - that measure voltage and should be thought of as voltmeters. No matter what the instrument is, if it measures voltage you have to treat the instrument as a voltmeter. When you measure voltage you have to remember that voltage is an across variable. When you measure voltage you have to connect the voltmeter to the two points in a circuit where you want to measure voltage. Here is a circuit with a voltmeter connected to measure the voltage across element #4. Note the following about this measurement. Notice that the voltmeter measures the voltage across element #4, +V4. (And, the plus sign is important. Remember the polarity issue.) Notice the polarity definitions for V4, and notice how the red terminal is connected to the "+" end of element #4. If you reversed the leads, by connecting the red lead to the "-" terminal on element #4 and the black lead to the "+" end of element #4, you would be measuring -V4.

And, remember this as well. When you measure voltage, the voltmeter should not disturb the circuit where you are attempting to measure the voltage. In the circuit above, that disturbance is the current drawn by the voltmeter. You want that current to be as close to zero as it can possibly be. That means that you need to have the resistance of the voltmeter as large as possible. There's more discussion of that effect in the lesson on measuring voltage . Ideally, the resistance of a voltmeter would be infinite. There are numerous different instruments, and we have a separate lesson just on measuring voltage that discusses some of those instrument, and you can get to that lesson by clicking here . Measuring Current Current is measured with an ammeter. While voltage is a more common measurement, it is often necessary to measure current. When measuring current, it is important to remember that current is a flow variable. Current flows through electrical elements, and if you want to measure current you have to get it to flow through the ammeter. Here's the same circuit we used in the example above. Consider what we would have to do to measure the current flowing through element #4.

If we want to measure the current through element #4, we have to get that current to flow through the ammeter. Here's a way to insert an ammeter into the circuit to measure that current. However, this doesn't give the whole picture. Remember that polarity is important. In the circuit the polarity for the voltage across element #4 is defined, but the current polarity is not defined. In the diagram below, we have defined the direction of that current, and given it an algebraic name, Im. As with the voltmeter, you need to pay attention to the polarity, and you also want to remember this. When you measure current, the ammeter should not disturb the circuit where you are attempting to measure the current. In the circuit above, that disturbance is the voltage across the ammeter. You want that voltage to be as close to zero as it can possibly be. That means that you need to have the resistance of the voltmeter as small as possible. Ideally, the resistance of an ammeter would be zero. Links to Related Lessons Amperage Amperage After reading this section you will be able to do the following: Define amperes and name the instrument that is used to measures amperage.

Construct an experiment to determine the amount of amps flowing in a circuit. It is very important to have a way to measure and quantify the flow of electrical current. When current flow is controlled it can be used to do useful work. Electricity can be very dangerous and it is important to know something about it in order to work with it safely. The flow of electrons is measured in units called amperes. The term amps is often used for short. An amp is the amount of electrical current that exists when a number of electrons, having one coulomb (ku`-lum) of charge, move past a given point in one second. A coulomb is the charge carried by 6.25 x 10^18 electrons. 6.25 x 10^18 is scientific notation for 6,250,000,000,000,000,000. That is a lot of electrons moving past a given point in one second! Since we cannot count this fast and we cannot even see the electrons, we need an instrument to measure the flow of electrons.

An ammeter is this instrument and it is used to indicate how many amps of current are flowing in an electrical circuit. Review Amperage is a term used to describe the number of electrons moving past a fixed point in a conductor in one second. Current is measured in units called amperes or amps. Voltmeters and Ammeters Voltmeters and Ammeters 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. Voltmeters and Ammeters Voltmeters and ammeters are used to measure voltage and current, respectively. Learning Objective Compare circuit connection of an ammeter and a voltmeter Key Points A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit . An ammeter is a measuring device used to measure the electric current in a circuit.

A voltmeter is connected in parallel with a device to measure its voltage , while an ammeter is connected in series with a device to measure its current. At the heart of most analog meters is a galvanometer , an instrument that measures current flow using the movement, or deflection, of a needle. The needle deflection is produced by a magnetic force acting on a current-carrying wire. Terms shunt resistance a small resistance R placed in parallel with a galvanometer G to produce an ammeter; the larger the current to be measured, the smaller R must be; most of the current flowing through the meter is shunted through R to protect the galvanometer Register for FREE to remove ads and unlock more features! Learn more Full Text Voltmeters and ammeters measure the voltage and current, respectively, of a circuit. Some meters in automobile dashboards, digital cameras, cell phones, and tuner-amplifiers are voltmeters or ammeters. A brief introduction to voltmeters and ammeters for introductory physics students. Voltmeters A voltmeter is an instrument that measures the difference in electrical potential between two points in an electric circuit.

An analog voltmeter moves a pointer across a scale in proportion to the circuit's voltage; a digital voltmeter provides a numerical display. Any measurement that can be converted to voltage can be displayed on a meter that is properly calibrated; such measurements include pressure , temperature, and flow. (a) To measure the potential difference in this series circuit, the voltmeter (V) is placed in parallel with the voltage source or either of the resistors. Note that terminal voltage is measured between points a and b. It is not possible to connect the voltmeter directly across the EMF without including its internal resistance, r. (b) A digital voltmeter in use Ammeters An ammeter measures the electric current in a circuit. The name is derived from the name for the SI unit for electric current, amperes (A). In order for an ammeter to measure a device's current, it must be connected in series to that device . This is necessary because objects in series experience the same current. They must not be connected to a voltage source -- ammeters are designed to work under a minimal burden, (which refers to the voltage drop across the ammeter, typically a small fraction of a volt).

An ammeter (A) is placed in series to measure current. All of the current in this circuit flows through the meter. The ammeter would have the same reading if located between points d and e or between points f and a, as it does in the position shown. (Note that the script capital E stands for EMF, and r stands for the internal resistance of the source of potential difference. ) Galvanometers (Analog Meters) Analog meters have needles that swivel to point at numbers on a scale, as opposed to digital meters, which have numerical readouts.The heart of most analog meters is a device called a galvanometer, denoted by G. Current flow through a galvanometer, IG, produces a proportional movement, or deflection, of the needle. The two crucial characteristics of any galvanometer are its resistance and its current sensitivity. Current sensitivity is the current that gives a full-scale deflection of the galvanometer's needle -- in other words, the maximum current that the instrument can measure.

For example, a galvanometer with a current sensitivity of 50 μA has a maximum deflection of its needle when 50 μA flows through it, is at the scale's halfway point when 25 μA flows through it, and so on. If such a galvanometer has a 25-Ω resistance, then a voltage of only V = IR = (50 μA)(25 Ω) = 1.25 mV produces a full-scale reading. By connecting resistors to this galvanometer in different ways, you can use it as either a voltmeter or ammeter to measure a broad range of voltages or currents. Galvanometers as Voltmeters A galvanometer can function as a voltmeter when it is connected in series with a large resistance R. The value of R is determined by the maximum voltage that will be measured. Suppose you want 10 V to produce a full-scale deflection of a voltmeter containing a 25-Ω galvanometer with a 50-μA sensitivity. Then 10 V applied to the meter must produce a current of 50 μA.

The total resistance must be: $R_{tot} = R + r = \frac{V}{I} = \frac{10V}{50\mu A} = 200 k\Omega,$ or: $R = R_{tot} - r = 200 k\Omega - 25 \Omega \approx 200 k \Omega.$ (R is so large that the galvanometer resistance, r, is nearly negligible. ) Note that 5 V applied to this voltmeter produces a half-scale deflection by sending a 25-μA current through the meter, and so the voltmeter's reading is proportional to voltage, as desired. This voltmeter would not be useful for voltages less than about half a volt, because the meter deflection would be too small to read accurately. For other voltage ranges, other resistances are placed in series with the galvanometer. Many meters allow a choice of scales, which involves switching an appropriate resistance into series with the galvanometer. Galvanometers as Ammeters The same galvanometer can also function as an ammeter when it is placed in parallel with a small resistance R, often called the shunt resistance .

Since the shunt resistance is small, most of the current passes through it, allowing an ammeter to measure currents much greater than those that would produce a full-scale deflection of the galvanometer. Suppose, for example, we need an ammeter that gives a full-scale deflection for 1.0 A and that contains the same 25-Ω galvanometer with 50-μA sensitivity. Since R and r are in parallel, the voltage across them is the same.

These IR drops are: IR = IGr so that: $IR = \frac{I_G}{I} = \frac{R}{r}.$ Solving for R, and noting that IG is 50 μA and I is 0.999950 A, we have: $R = r \frac{I_G}{I} = (25 \Omega) \frac{50 \mu A}{0.999950 A} = 1.25 \times 10^{-3} \Omega.$ How to Measure Current on an Electronic Circuit - dummies How to Measure Current on an Electronic Circuit How to Measure Current on an Electronic Circuit How to Measure Current on an Electronic Circuit By Doug Lowe Electric current is measured in amperes, but actually in most electronics work, you’ll measure current in milliamps, or mA. To measure current, you must connect the two leads of the ammeter in the circuit so that the current flows through the ammeter. In other words, the ammeter must become a part of the circuit itself. The only way to measure the current flowing through a simple circuit is to insert your ammeter into the circuit.

Here, the ammeter is inserted into the circuit between the LED and the resistor. Note that it doesn’t matter where in this circuit you insert the ammeter. You’ll get the same current reading whether you insert the ammeter between the LED and the resistor, between the resistor and the battery, or between the LED and the battery. To measure the current in the LED circuit, follow these steps: Set your multimeter’s range selector to a DC milliamp range of at least 20 mA. This circuit uses direct current (DC), so you need to make sure the multimeter is set to a DC current range. Remove the jumper wire that connects the two terminal strips. The LED should go dark, as removing the jumper wire breaks the circuit. Touch the black lead from the multimeter to the LED lead that connects to the terminal strip (not the bus strip). Touch the red lead from the multimeter to the resistor lead that connects to the terminal strip (not the bus strip). The LED should light up again, as the ammeter is now a part of the circuit, and current can flow. Read the number on the multimeter display. It should read between 12 and 13 mA.

(The exact reading will depend on the exact resistance value of the resistor. Resistor values aren’t exact, so even though you’re using a 470 Ω resistor in this circuit, the actual resistance of the resistor may be anywhere from 420 to 520 Ω. Congratulate yourself! You have made your first official current measurement. After a suitable celebration, replace the jumper wire you removed in Step 2. If you forget to replace the jumper wire, you won’t be able to take other measurements successfully. There are two places in this circuit that you should not connect the ammeter. First, don’t connect the ammeter directly across the two battery terminals. This effectively shorts out the battery. It will get real hot, real fast. Second, don’t connect one lead of the ammeter to the positive battery terminal and the other directly to the LED lead. That will bypass the resistor, which will probably blow out the LED. If you want to experiment a little more, try measuring the current at other places in the circuit. For example, remove the battery snap connector from the battery, and then reconnect it so that just the negative battery terminal is connected.

Then, touch the red meter lead to the positive battery terminal and the black lead to the lead of the resistor that’s connected to the bus strip. This measures the current by inserting the ammeter between the resistor and the battery. You should get the same value that you got when you measured between the LED and the resistor. You can use a similar method to measure the current between the LED and the negative battery terminal. Again, the result should be the same. Moving Coil Meters Moving Coil Meters The design of a voltmeter , ammeter or ohmmeter begins with a current-sensitive element. Though most modern meters have solid state digital readouts, the physics is more readily demonstrated with a moving coil current detector called a galvanometer . Since the modifications of the current sensor are compact, it is practical to have all three functions in a single instrument with multiple ranges of sensitivity. Schematically, a single range "multimeter" might be designed as illustrated. Go Back Voltmeter A voltmeter measures the change in voltage between two points in an electric circuit and therefore must be connected in parallel with the portion of the circuit on which the measurement is made. By contrast, an ammeter must be connected in series.

In analogy with a water circuit , a voltmeter is like a meter designed to measure pressure difference. It is necessary for the voltmeter to have a very high resistance so that it does not have an appreciable affect on the current or voltage associated with the measured circuit. Modern solid-state meters have digital readouts, but the principles of operation can be better appreciated by examining the older moving coil meters based on galvanometer sensors. Go Back Ammeter An ammeter is an instrument for measuring the electric current in amperes in a branch of an electric circuit. It must be placed in series with the measured branch, and must have very low resistance to avoid significant alteration of the current it is to measure. By contrast, an voltmeter must be connected in parallel. The analogy with an in-line flowmeter in a water circuit can help visualize why an ammeter must have a low resistance, and why connecting an ammeter in parallel can damage the meter. Modern solid-state meters have digital readouts, but the principles of operation can be better appreciated by examining the older moving coil meters based on galvanometer sensors.

Lessons In Electric Circuits -- Volume VI (Experiments) - Chapter 2 Lessons In Electric Circuits -- Volume VI Chapter 2 One light-emitting diode (Radio Shack catalog # 276-026 or equivalent) Small "hobby" motor, permanent-magnet type (Radio Shack catalog # 273-223 or equivalent) Two jumper wires with "alligator clip" ends (Radio Shack catalog # 278-1156, 278-1157, or equivalent) A multimeter is an electrical instrument capable of measuring voltage, current, and resistance. Digital multimeters have numerical displays, like digital clocks, for indicating the quantity of voltage, current, or resistance. Analog multimeters indicate these quantities by means of a moving pointer over a printed scale. Analog multimeters tend to be less expensive than digital multimeters, and more beneficial as learning tools for the first-time student of electricity. I strongly recommend purchasing an analog multimeter before purchasing a digital multimeter, but to eventually have both in your tool kit for these experiments.

CROSS-REFERENCES Lessons In Electric Circuits, Volume 1, chapter 1: "Basic Concepts of Electricity" Lessons In Electric Circuits, Volume 1, chapter 8: "DC Metering Circuits" LEARNING OBJECTIVES Characteristics of voltage: existing between two points Selection of proper meter range ILLUSTRATION INSTRUCTIONS In all the experiments in this book, you will be using some sort of test equipment to measure aspects of electricity you cannot directly see, feel, hear, taste, or smell. Electricity -- at least in small, safe quantities -- is insensible by our human bodies. Your most fundamental "eyes" in the world of electricity and electronics will be a device called a multimeter. Multimeters indicate the presence of, and measure the quantity of, electrical properties such as voltage, current, and resistance. In this experiment, you will familiarize yourself with the measurement of voltage. Voltage is the measure of electrical "push" ready to motivate electrons to move through a conductor. In scientific terms, it is the specific energy per unit charge, mathematically defined as joules per coulomb.

It is analogous to pressure in a fluid system: the force that moves fluid through a pipe, and is measured in the unit of the Volt (V). Your multimeter should come with some basic instructions. Read them well! If your multimeter is digital, it will require a small battery to operate. If it is analog, it does not need a battery to measure voltage. Some digital multimeters are autoranging. An autoranging meter has only a few selector switch (dial) positions. Manual-ranging meters have several different selector positions for each basic quantity: several for voltage, several for current, and several for resistance. Autoranging is usually found on only the more expensive digital meters, and is to manual ranging as an automatic transmission is to a manual transmission in a car. An autoranging meter "shifts gears" automatically to find the best measurement range to display the particular quantity being measured. Set your multimeter's selector switch to the highest-value "DC volt" position available. Autoranging multimeters may only have a single position for DC voltage, in which case you need to set the switch to that one position.

Touch the red test probe to the positive (+) side of a battery, and the black test probe to the negative (-) side of the same battery. The meter should now provide you with some sort of indication. Reverse the test probe connections to the battery if the meter's indication is negative (on an analog meter, a negative value is indicated by the pointer deflecting left instead of right). If your meter is a manual-range type, and the selector switch has been set to a high-range position, the indication will be small. Move the selector switch to the next lower DC voltage range setting and reconnect to the battery. The indication should be stronger now, as indicated by a greater deflection of the analog meter pointer (needle), or more active digits on the digital meter display. For the best results, move the selector switch to the lowest-range setting that does not "over-range" the meter. An over-ranged analog meter is said to be "pegged," as the needle will be forced all the way to the right-hand side of the scale, past the full-range scale value. An over-ranged digital meter sometimes displays the letters "OL", or a series of dashed lines. This indication is manufacturer-specific.

What happens if you only touch one meter test probe to one end of a battery? How does the meter have to connect to the battery in order to provide an indication? What does this tell us about voltmeter use and the nature of voltage? Is there such a thing as voltage "at" a single point? Be sure to measure more than one size of battery, and learn how to select the best voltage range on the multimeter to give you maximum indication without over-ranging. Now switch your multimeter to the lowest DC voltage range available, and touch the meter's test probes to the terminals (wire leads) of the light-emitting diode (LED). An LED is designed to produce light when powered by a small amount of electricity, but LEDs also happen to generate DC voltage when exposed to light, somewhat like a solar cell. Point the LED toward a bright source of light with your multimeter connected to it, and note the meter's indication: Batteries develop electrical voltage through chemical reactions. When a battery "dies," it has exhausted its original store of chemical "fuel." The LED, however, does not rely on an internal "fuel" to generate voltage; rather, it converts optical energy into electrical energy.

So long as there is light to illuminate the LED, it will produce voltage. Another source of voltage through energy conversion a generator. The small electric motor specified in the "Parts and Materials" list functions as an electrical generator if its shaft is turned by a mechanical force. Connect your voltmeter (your multimeter, set to the "volt" function) to the motor's terminals just as you connected it to the LED's terminals, and spin the shaft with your fingers. The meter should indicate voltage by means of needle deflection (analog) or numerical readout (digital). If you find it difficult to maintain both meter test probes in connection with the motor's terminals while simultaneously spinning the shaft with your fingers, you may use alligator clip "jumper" wires like this: Determine the relationship between voltage and generator shaft speed? Reverse the generator's direction of rotation and note the change in meter indication. When you reverse shaft rotation, you change the polarity of the voltage created by the generator. The voltmeter indicates polarity by direction of needle direction (analog) or sign of numerical indication (digital). When the red test lead is positive (+) and the black test lead negative (-), the meter will register voltage in the normal direction.

If the applied voltage is of the reverse polarity (negative on red and positive on black), the meter will indicate "backwards." Glass of water Table salt This experiment describes how to measure the electrical resistance of several objects. You need not possess all items listed above in order to effectively learn about resistance. Conversely, you need not limit your experiments to these items. However, be sure to never measure the resistance of any electrically "live" object or circuit. In other words, do not attempt to measure the resistance of a battery or any other source of substantial voltage using a multimeter set to the resistance ("ohms") function. Failing to heed this warning will likely result in meter damage and even personal injury. CROSS-REFERENCES Lessons In Electric Circuits, Volume 1, chapter 1: "Basic Concepts of Electricity" Lessons In Electric Circuits, Volume 1, chapter 8: "DC Metering Circuits" LEARNING OBJECTIVES Characteristics of resistance: existing between two points Selection of proper meter range Relative conductivity of various components and materials ILLUSTRATION INSTRUCTIONS Resistance is the measure of electrical "friction" as electrons move through a conductor.

It is measured in the unit of the "Ohm," that unit symbolized by the capital Greek letter omega (Ω). Set your multimeter to the highest resistance range available. The resistance function is usually denoted by the unit symbol for resistance: the Greek letter omega (Ω), or sometimes by the word "ohms." Touch the two test probes of your meter together. When you do, the meter should register 0 ohms of resistance. If you are using an analog meter, you will notice the needle deflect full-scale when the probes are touched together, and return to its resting position when the probes are pulled apart. The resistance scale on an analog multimeter is reverse-printed from the other scales: zero resistance in indicated at the far right-hand side of the scale, and infinite resistance is indicated at the far left-hand side. There should also be a small adjustment knob or "wheel" on the analog multimeter to calibrate it for "zero" ohms of resistance. Touch the test probes together and move this adjustment until the needle exactly points to zero at the right-hand end of the scale.

Although your multimeter is capable of providing quantitative values of measured resistance, it is also useful for qualitative tests of continuity: whether or not there is a continuous electrical connection from one point to another. You can, for instance, test the continuity of a piece of wire by connecting the meter probes to opposite ends of the wire and checking to see the the needle moves full-scale. What would we say about a piece of wire if the ohmmeter needle didn't move at all when the probes were connected to opposite ends? Digital multimeters set to the "resistance" mode indicate non-continuity by displaying some non-numerical indication on the display. Some models say "OL" (Open-Loop), while others display dashed lines. Use your meter to determine continuity between the holes on a breadboard: a device used for temporary construction of circuits, where component terminals are inserted into holes on a plastic grid, metal spring clips underneath each hole connecting certain holes to others.

Use small pieces of 22-gauge solid copper wire, inserted into the holes of the breadboard, to connect the meter to these spring clips so that you can test for continuity: An important concept in electricity, closely related to electrical continuity, is that of points being electrically common to each other. Electrically common points are points of contact on a device or in a circuit that have negligible (extremely small) resistance between them. We could say, then, that points within a breadboard column (vertical in the illustrations) are electrically common to each other, because there is electrical continuity between them. Conversely, breadboard points within a row (horizontal in the illustrations) are not electrically common, because there is no continuity between them. Continuity describes what is between points of contact, while commonality describes how the points themselves relate to each other. Like continuity, commonality is a qualitative assessment, based on a relative comparison of resistance between other points in a circuit. It is an important concept to grasp, because there are certain facts regarding voltage in relation to electrically common points that are valuable in circuit analysis and troubleshooting, the first one being that there will never be substantial voltage dropped between points that are electrically common to each other.

Select a 10,000 ohm (10 kΩ) resistor from your parts assortment. This resistance value is indicated by a series of color bands: Brown, Black, Orange, and then another color representing the precision of the resistor, Gold (+/- 5%) or Silver (+/- 10%). Some resistors have no color for precision, which marks them as +/- 20%. Other resistors use five color bands to denote their value and precision, in which case the colors for a 10 kΩ resistor will be Brown, Black, Black, Red, and a fifth color for precision. Connect the meter's test probes across the resistor as such, and note its indication on the resistance scale: If the needle points very close to zero, you need to select a lower resistance range on the meter, just as you needed to select an appropriate voltage range when reading the voltage of a battery. If you are using a digital multimeter, you should see a numerical figure close to 10 shown on the display, with a small "k" symbol on the right-hand side denoting the metric prefix for "kilo" (thousand).

Some digital meters are manually-ranged, and require appropriate range selection just as the analog meter. If yours is like this, experiment with different range switch positions and see which one gives you the best indication. Try reversing the test probe connections on the resistor. Does this change the meter's indication at all? What does this tell us about the resistance of a resistor? What happens when you only touch one probe to the resistor? What does this tell us about the nature of resistance, and how it is measured? How does this compare with voltage measurement, and what happened when we tried to measure battery voltage by touching only one probe to the battery? When you touch the meter probes to the resistor terminals, try not to touch both probe tips to your fingers. If you do, you will be measuring the parallel combination of the resistor and your own body, which will tend to make the meter indication lower than it should be! When measuring a 10 kΩ resistor, this error will be minimal, but it may be more severe when measuring other values of resistor. You may safely measure the resistance of your own body by holding one probe tip with the fingers of one hand, and the other probe tip with the fingers of the other hand.

Note: be very careful with the probes, as they are often sharpened to a needle-point. Hold the probe tips along their length, not at the very points! You may need to adjust the meter range again after measuring the 10 kΩ resistor, as your body resistance tends to be greater than 10,000 ohms hand-to-hand. Try wetting your fingers with water and re-measuring resistance with the meter. What impact does this have on the indication? Try wetting your fingers with saltwater prepared using the glass of water and table salt, and re-measuring resistance. What impact does this have on your body's resistance as measured by the meter? Resistance is the measure of friction to electron flow through an object. The less resistance there is between two points, the harder it is for electrons to move (flow) between those two points. Given that electric shock is caused by a large flow of electrons through a person's body, and increased body resistance acts as a safeguard by making it more difficult for electrons to flow through us, what can we ascertain about electrical safety from the resistance readings obtained with wet fingers? Does water increase or decrease shock hazard to people? Measure the resistance of a rectifying diode with an analog meter.

Try reversing the test probe connections to the diode and re-measure resistance. What strikes you as being remarkable about the diode, especially in contrast to the resistor? Take a piece of paper and draw a very heavy black mark on it with a pencil (not a pen!). Measure resistance on the black strip with your meter, placing the probe tips at each end of the mark like this: Move the probe tips closer together on the black mark and note the change in resistance value. Does it increase or decrease with decreased probe spacing? If the results are inconsistent, you need to redraw the mark with more and heavier pencil strokes, so that it is consistent in its density. What does this teach you about resistance versus length of a conductive material? Connect your meter to the terminals of a cadmium-sulphide (CdS) photocell and measure the change in resistance created by differences in light exposure.

Just as with the light-emitting diode (LED) of the voltmeter experiment, you may want to use alligator-clip jumper wires to make connection with the component, leaving your hands free to hold the photocell to a light source and/or change meter ranges: Experiment with measuring the resistance of several different types of materials, just be sure not to try measure anything that produces substantial voltage, like a battery. Suggestions for materials to measure are: fabric, plastic, wood, metal, clean water, dirty water, salt water, glass, diamond (on a diamond ring or other piece of jewelry), paper, rubber, and oil. ILLUSTRATION INSTRUCTIONS This is the simplest complete circuit in this collection of experiments: a battery and an incandescent lamp. Connect the lamp to the battery as shown in the illustration, and the lamp should light, assuming the battery and lamp are both in good condition and they are matched to one another in terms of voltage. If there is a "break" (discontinuity) anywhere in the circuit, the lamp will fail to light. It does not matter where such a break occurs!

Many students assume that because electrons leave the negative (-) side of the battery and continue through the circuit to the positive (+) side, that the wire connecting the negative terminal of the battery to the lamp is more important to circuit operation than the other wire providing a return path for electrons back to the battery. This is not true! Using your multimeter set to the appropriate "DC volt" range, measure voltage across the battery, across the lamp, and across each jumper wire. Familiarize yourself with the normal voltages in a functioning circuit. Now, "break" the circuit at one point and re-measure voltage between the same sets of points, additionally measuring voltage across the break like this: What voltages measure the same as before? What voltages are different since introducing the break? How much voltage is manifest, or dropped across the break? What is the polarity of the voltage drop across the break, as indicated by the meter? Re-connect the jumper wire to the lamp, and break the circuit in another place. Measure all voltage "drops" again, familiarizing yourself with the voltages of an "open" circuit.

Construct the same circuit on a breadboard, taking care to place the lamp and wires into the breadboard in such a way that continuity will be maintained. The example shown here is only that: an example, not the only way to build a circuit on a breadboard: Experiment with different configurations on the breadboard, plugging the lamp into different holes. If you encounter a situation where the lamp refuses to light up and the connecting wires are getting warm, you probably have a situation known as a short circuit, where a lower-resistance path than the lamp bypasses current around the lamp, preventing enough voltage from being dropped across the lamp to light it up. Here is an example of a short circuit made on a breadboard: Here is an example of an accidental short circuit of the type typically made by students unfamiliar with breadboard usage: Here there is no "shorting" wire present on the breadboard, yet there is a short circuit, and the lamp refuses to light. Based on your understanding of breadboard hole connections, can you determine where the "short" is in this circuit? Short circuits are generally to be avoided, as they result in very high rates of electron flow, causing wires to heat up and battery power sources to deplete.

If the power source is substantial enough, a short circuit may cause heat of explosive proportions to manifest, causing equipment damage and hazard to nearby personnel. This is what happens when a tree limb "shorts" across wires on a power line: the limb -- being composed of wet wood -- acts as a low-resistance path to electric current, resulting in heat and sparks. You may also build the battery/lamp circuit on a terminal strip: a length of insulating material with metal bars and screws to attach wires and component terminals to.

Here is an example of how this circuit might be constructed on a terminal strip: 6-volt battery 6-volt incandescent lamp Basic circuit construction components such as breadboard, terminal strip, and jumper wires are also assumed to be available from now on, leaving only components and materials unique to the project listed under "Parts and Materials." CROSS-REFERENCES Lessons In Electric Circuits, Volume 1, chapter 1: "Basic Concepts of Electricity" Lessons In Electric Circuits, Volume 1, chapter 8: "DC Metering Circuits" LEARNING OBJECTIVES How to measure current with a multimeter How to check a multimeter's internal fuse Selection of proper meter range SCHEMATIC DIAGRAM ILLUSTRATION INSTRUCTIONS Current is the measure of the rate of electron "flow" in a circuit. It is measured in the unit of the Ampere, simply called "Amp," (A). The most common way to measure current in a circuit is to break the circuit open and insert an "ammeter" in series (in-line) with the circuit so that all electrons flowing through the circuit also have to go through the meter.

Because measuring current in this manner requires the meter be made part of the circuit, it is a more difficult type of measurement to make than either voltage or resistance. Some digital meters, like the unit shown in the illustration, have a separate jack to insert the red test lead plug when measuring current. Other meters, like most inexpensive analog meters, use the same jacks for measuring voltage, resistance, and current. Consult your owner's manual on the particular model of meter you own for details on measuring current. When an ammeter is placed in series with a circuit, it ideally drops no voltage as current goes through it. In other words, it acts very much like a piece of wire, with very little resistance from one test probe to the other. Consequently, an ammeter will act as a short circuit if placed in parallel (across the terminals of) a substantial source of voltage. If this is done, a surge in current will result, potentially damaging the meter: Ammeters are generally protected from excessive current by means of a small fuse located inside the meter housing.

If the ammeter is accidently connected across a substantial voltage source, the resultant surge in current will "blow" the fuse and render the meter incapable of measuring current until the fuse is replaced. Be very careful to avoid this scenario! You may test the condition of a multimeter's fuse by switching it to the resistance mode and measuring continuity through the test leads (and through the fuse). On a meter where the same test lead jacks are used for both resistance and current measurement, simply leave the test lead plugs where they are and touch the two probes together. On a meter where different jacks are used, this is how you insert the test lead plugs to check the fuse: Build the one-battery, one-lamp circuit using jumper wires to connect the battery to the lamp, and verify that the lamp lights up before connecting the meter in series with it. Then, break the circuit open at any point and connect the meter's test probes to the two points of the break to measure current. As usual, if your meter is manually-ranged, begin by selecting the highest range for current, then move the selector switch to lower range positions until the strongest indication is obtained on the meter display without over-ranging it.

If the meter indication is "backwards," (left motion on analog needle, or negative reading on a digital display), then reverse the test probe connections and try again. When the ammeter indicates a normal reading (not "backwards"), electrons are entering the black test lead and exiting the red. This is how you determine direction of current using a meter. For a 6-volt battery and a small lamp, the circuit current will be in the range of thousandths of an amp, or milliamps. Digital meters often show a small letter "m" in the right-hand side of the display to indicate this metric prefix. Try breaking the circuit at some other point and inserting the meter there instead. What do you notice about the amount of current measured? Why do you think this is? Re-construct the circuit on a breadboard like this: Students often get confused when connecting an ammeter to a breadboard circuit. How can the meter be connected so as to intercept all the circuit's current and not create a short circuit? One easy method that guarantees success is this: Identify what wire or component terminal you wish to measure current through. Pull that wire or terminal out of the breadboard hole. Leave it hanging in mid-air.

Insert a spare piece of wire into the hole you just pulled the other wire or terminal out of. Leave the other end of this wire hanging in mid-air. Connect the ammeter between the two unconnected wire ends (the two that were hanging in mid-air). You are now assured of measuring current through the wire or terminal initially identified. Again, measure current through different wires in this circuit, following the same connection procedure outlined above. What do you notice about these current measurements? The results in the breadboard circuit should be the same as the results in the free-form (no breadboard) circuit. Building the same circuit on a terminal strip should also yield similar results: The current figure of 24.70 milliamps (24.70 mA) shown in the illustrations is an arbitrary quantity, reasonable for a small incandescent lamp. If the current for your circuit is a different value, that is okay, so long as the lamp is functioning when the meter is connected. If the lamp refuses to light when the meter is connected to the circuit, and the meter registers a much greater reading, you probably have a short-circuit condition through the meter.

If your lamp refuses to light when the meter is connected in the circuit, and the meter registers zero current, you've probably blown the fuse inside the meter. Check the condition of your meter's fuse as described previously in this section and replace the fuse if necessary. Calculator (or pencil and paper for doing arithmetic) 6-volt battery Assortment of resistors between 1 KΩ and 100 kΩ in value I'm purposely restricting the resistance values between 1 kΩ and 100 kΩ for the sake of obtaining accurate voltage and current readings with your meter. With very low resistance values, the internal resistance of the ammeter has a significant impact on measurement accuracy. Very high resistance values can cause problems for voltage measurement, the internal resistance of the voltmeter substantially changing circuit resistance when it is connected in parallel with a high-value resistor. At the recommended resistance values, there will still be a small amount of measurement error due to the "impact" of the meter, but not enough to cause serious disagreement with calculated values.

CROSS-REFERENCES Lessons In Electric Circuits, Volume 1, chapter 2: "Ohm's Law" LEARNING OBJECTIVES ILLUSTRATION INSTRUCTIONS Select a resistor from the assortment, and measure its resistance with your multimeter set to the appropriate resistance range. Be sure not to hold the resistor terminals when measuring resistance, or else your hand-to-hand body resistance will influence the measurement! Record this resistance value for future use. Build a one-battery, one-resistor circuit. A terminal strip is shown in the illustration, but any form of circuit construction is okay. Set your multimeter to the appropriate voltage range and measure voltage across the resistor as it is being powered by the battery. Record this voltage value along with the resistance value previously measured. Set your multimeter to the highest current range available. Break the circuit and connect the ammeter within that break, so it becomes a part of the circuit, in series with the battery and resistor. Select the best current range: whichever one gives the strongest meter indication without over-ranging the meter. If your multimeter is autoranging, of course, you need not bother with setting ranges. Record this current value along with the resistance and voltage values previously recorded.

Taking the measured figures for voltage and resistance, use the Ohm's Law equation to calculate circuit current. Compare this calculated figure with the measured figure for circuit current: Taking the measured figures for voltage and current, use the Ohm's Law equation to calculate circuit resistance. Compare this calculated figure with the measured figure for circuit resistance: Finally, taking the measured figures for resistance and current, use the Ohm's Law equation to calculate circuit voltage. Compare this calculated figure with the measured figure for circuit voltage: There should be close agreement between all measured and all calculated figures. Any differences in respective quantities of voltage, current, or resistance are most likely due to meter inaccuracies. These differences should be rather small, no more than several percent. Some meters, of course, are more accurate than others! Substitute different resistors in the circuit and re-take all resistance, voltage, and current measurements. Re-calculate these figures and check for agreement with the experimental data (measured quantities). Also note the simple mathematical relationship between changes in resistor value and changes in circuit current. Voltage should remain approximately the same for any resistor size inserted into the circuit, because it is the nature of a battery to maintain voltage at a constant level.

ILLUSTRATION INSTRUCTIONS Measure the resistance of the lamp with your multimeter. This resistance figure is due to the thin metal "filament" inside the lamp. It has substantially more resistance than a jumper wire, but less than any of the resistors from the last experiment. Record this resistance value for future use. Build a one-battery, one-lamp circuit. Set your multimeter to the appropriate voltage range and measure voltage across the lamp as it is energized (lit). Record this voltage value along with the resistance value previously measured. Set your multimeter to the highest current range available. Break the circuit and connect the ammeter within that break, so it becomes a part of the circuit, in series with the battery and lamp. Select the best current range: whichever one gives the strongest meter indication without over-ranging the meter. If your multimeter is autoranging, of course, you need not bother with setting ranges. Record this current value along with the resistance and voltage values previously recorded. Taking the measured figures for voltage and resistance, use the Ohm's Law equation to calculate circuit current.

Compare this calculated figure with the measured figure for circuit current: What you should find is a marked difference between measured current and calculated current: the calculated figure is much greater. Why is this? To make things more interesting, try measuring the lamp's resistance again, this time using a different model of meter. You will need to disconnect the lamp from the battery circuit in order to obtain a resistance reading, because voltages outside of the meter interfere with resistance measurement. This is a general rule that should be remembered: measure resistance only on an unpowered component! Using a different ohmmeter, the lamp will probably register as a different value of resistance. Usually, analog meters give higher lamp resistance readings than digital meters. This behavior is very different from that of the resistors in the last experiment. Why? What factor(s) might influence the resistance of the lamp filament, and how might those factors be different between conditions of lit and unlit, or between resistance measurements taken with different types of meters? This problem is a good test case for the application of scientific method. Once you've thought of a possible reason for the lamp's resistance changing between lit and unlit conditions, try to duplicate that cause by some other means.

For example, if you think the lamp resistance might change as it is exposed to light (its own light, when lit), and that this accounts for the difference between the measured and calculated circuit currents, try exposing the lamp to an external source of light while measuring its resistance. If you measure substantial resistance change as a result of light exposure, then your hypothesis has some evidential support. If not, then your hypothesis has been falsified, and another cause must be responsible for the change in circuit current. Two 1/4 watt resistors: 10 Ω and 330 Ω. Small thermometer The resistor values need not be exact, but within five percent of the figures specified (+/- 0.5 Ω for the 10 Ω resistor; +/- 16.5 Ω for the 330 Ω resistor). Color codes for 5% tolerance 10 Ω and 330 Ω resistors are as follows: Brown, Black, Black, Gold (10, +/- 5%), and Orange, Orange, Brown, Gold (330, +/- 5%). Do not use any battery size other than 6 volts for this experiment.

The thermometer should be as small as possible, to facilitate rapid detection of heat produced by the resistor. I recommend a medical thermometer, the type used to take body temperature. CROSS-REFERENCES Lessons In Electric Circuits, Volume 1, chapter 2: "Ohm's Law" LEARNING OBJECTIVES Importance of component power ratings Significance of electrically common points SCHEMATIC DIAGRAM ILLUSTRATION INSTRUCTIONS Measure each resistor's resistance with your ohmmeter, noting the exact values on a piece of paper for later reference. Connect the 330 Ω resistor to the 6 volt battery using a pair of jumper wires as shown in the illustration. Connect the jumper wires to the resistor terminals before connecting the other ends to the battery. This will ensure your fingers are not touching the resistor when battery power is applied. You might be wondering why I advise no bodily contact with the powered resistor. This is because it will become hot when powered by the battery. You will use the thermometer to measure the temperature of each resistor when powered. With the 330 Ω resistor connected to the battery, measure voltage with a voltmeter. In measuring voltage, there is more than one way to obtain a proper reading.

Voltage may be measured directly across the battery, or directly across the resistor. Battery voltage is the same as resistor voltage in this circuit, since those two components share the same set of electrically common points: one side of the resistor is directly connected to one side of the battery, and the other side of the resistor is directly connected to the other side of the battery. All points of contact along the upper wire in the illustration (colored red) are electrically common to each other. All points of contact along the lower wire (colored black) are likewise electrically common to each other. Voltage measured between any point on the upper wire and any point on the lower wire should be the same. Voltage measured between any two common points, however, should be zero. Using an ammeter, measure current through the circuit. Again, there is no one "correct" way to measure current, so long as the ammeter is placed within the flow-path of electrons through the resistor and not across a source of voltage. To do this, make a break in the circuit, and place the ammeter within that break: connect the two test probes to the two wire or terminal ends left open from the break.

One viable option is shown in the following illustration: Now that you've measured and recorded resistor resistance, circuit voltage, and circuit current, you are ready to calculate power dissipation. Whereas voltage is the measure of electrical "push" motivating electrons to move through a circuit, and current is the measure of electron flow rate, power is the measure of work-rate: how fast work is being done in the circuit. It takes a certain amount of work to push electrons through a resistance, and power is a description of how rapidly that work is taking place. In mathematical equations, power is symbolized by the letter "P" and measured in the unit of the Watt (W). Power may be calculated by any one of three equations -- collectively referred to as Joule's Law -- given any two out of three quantities of voltage, current, and resistance: Try calculating power in this circuit, using the three measured values of voltage, current, and resistance. Any way you calculate it, the power dissipation figure should be roughly the same.

Assuming a battery with 6.000 volts and a resistor of exactly 330 Ω, the power dissipation will be 0.1090909 watts, or 109.0909 milli-watts (mW), to use a metric prefix. Since the resistor has a power rating of 1/4 watt (0.25 watts, or 250 mW), it is more than capable of sustaining this level of power dissipation. Because the actual power level is almost half the rated power, the resistor should become noticeably warm but it should not overheat. Touch the thermometer end to the middle of the resistor and see how warm it gets. The power rating of any electrical component does not tell us how much power it will dissipate, but simply how much power it may dissipate without sustaining damage. If the actual amount of dissipated power exceeds a component's power rating, that component will increase temperature to the point of damage. To illustrate, disconnect the 330 Ω resistor and replace it with the 10 Ω resistor. Again, avoid touching the resistor once the circuit is complete, as it will heat up rapidly.

The safest way to do this is to disconnect one jumper wire from a battery terminal, then disconnect the 330 Ω resistor from the two alligator clips, then connect the 10 Ω resistor between the two clips, and finally reconnect the jumper wire back to the battery terminal. Caution: keep the 10 Ω resistor away from any flammable materials when it is powered by the battery! You may not have enough time to take voltage and current measurements before the resistor begins to smoke. At the first sign of distress, disconnect one of the jumper wires from a battery terminal to interrupt circuit current, and give the resistor a few moments to cool down. With power still disconnected, measure the resistor's resistance with an ohmmeter and note any substantial deviation from its original value. If the resistor still measures within +/- 5% of its advertised value (between 9.5 and 10.5 Ω), re-connect the jumper wire and let it smoke a bit more. What trend do you notice with the resistor's value as it is damaged more and more by overpowering? It is typical of resistors to fail with a greater-than-normal resistance when overheated.

This is often a self-protective mode of failure, as an increased resistance results in less current and (generally) less power dissipation, cooling it down again. However, the resistor's normal resistance value will not return if sufficiently damaged. Performing some Joule's Law calculations for resistor power again, we find that a 10 Ω resistor connected to a 6 volt battery dissipates about 3.6 watts of power, about 14.4 times its rated power dissipation. Little wonder it smokes so quickly after connection to the battery! Low-voltage incandescent lamp (Radio Shack catalog # 272-1130 or equivalent) Long lengths of wire, 22-gauge or larger Household light switch (these are readily available at any hardware store) Household light switches are a bargain for students of basic electricity. They are readily available, very inexpensive, and almost impossible to damage with battery power. Do not get "dimmer" switches, just the simple on-off "toggle" variety used for ordinary household wall-mounted light controls.

CROSS-REFERENCES Lessons In Electric Circuits, Volume 1, chapter 1: "Basic Concepts of Electricity" LEARNING OBJECTIVES Using an ohmmeter to check switch action SCHEMATIC DIAGRAM ILLUSTRATION INSTRUCTIONS Build a one-battery, one-switch, one-lamp circuit as shown in the schematic diagram and in the illustration. This circuit is most impressive when the wires are long, as it shows how the switch is able to control circuit current no matter how physically large the circuit may be. Measure voltage across the battery, across the switch (measure from one screw terminal to another with the voltmeter), and across the lamp with the switch in both positions. When the switch is turned off, it is said to be open, and the lamp will go out just the same as if a wire were pulled loose from a terminal. As before, any break in the circuit at any location causes the lamp to immediately de-energize (darken). Spool of 28-gauge magnet wire Large bolt, nail, or steel rod Electrical tape Magnet wire is a term for thin-gauge copper wire with enamel insulation instead of rubber or plastic insulation.

Its small size and very thin insulation allow for many "turns" to be wound in a compact coil. You will need enough magnet wire to wrap hundreds of turns around the bolt, nail, or other rod-shaped steel form. Be sure to select a bolt, nail, or rod that is magnetic. Stainless steel, for example, is non-magnetic and will not function for the purpose of an electromagnet coil! The ideal material for this experiment is soft iron, but any commonly available steel will suffice. CROSS-REFERENCES Lessons In Electric Circuits, Volume 1, chapter 14: "Magnetism and Electromagnetism" LEARNING OBJECTIVES ILLUSTRATION INSTRUCTIONS Wrap a single layer of electrical tape around the steel bar (or bolt, or mail) to protect the wire from abrasion. Proceed to wrap several hundred turns of wire around the steel bar, making the coil as even as possible. It is okay to overlap wire, and it is okay to wrap in the same style that a fishing reel wraps line around the spool. The only rule you must follow is that all turns must be wrapped around the bar in the same direction (no reversing from clockwise to counter-clockwise!).

I find that a drill press works as a great tool for coil winding: clamp the rod in the drill's chuck as if it were a drill bit, then turn the drill motor on at a slow speed and let it do the wrapping! This allows you to feed wire onto the rod in a very steady, even manner. After you've wrapped several hundred turns of wire around the rod, wrap a layer or two of electrical tape over the wire coil to secure the wire in place. Scrape the enamel insulation off the ends of the coil wires for connection to jumper leads, then connect the coil to a battery. When electric current goes through the coil, it will produce a strong magnetic field: one "pole" at each end of the rod. This phenomenon is known as electromagnetism. The magnetic compass is used to identify the "North" and "South" poles of the electromagnet. With the electromagnet energized (connected to the battery), place a permanent magnet near one pole and note whether there is an attractive or repulsive force. Reverse the orientation of the permanent magnet and note the difference in force.

Electromagnetism has many applications, including relays, electric motors, solenoids, doorbells, buzzers, computer printer mechanisms, and magnetic media "write" heads (tape recorders, disk drives). You might notice a significant spark whenever the battery is disconnected from the electromagnet coil: much greater than the spark produced if the battery is simply short-circuited. This spark is the result of a high-voltage surge created whenever current is suddenly interrupted through the coil. The effect is known as inductive "kickback" and is capable of delivering a small but harmless electric shock! To avoid receiving this shock, do not place your body across the break in the circuit when de-energizing! Use one hand at a time when un-powering the coil and you'll be perfectly safe. This phenomenon will be explored in greater detail in the next chapter (DC Circuits). See previous experiment for instructions on electromagnet construction.

CROSS-REFERENCES Lessons In Electric Circuits, Volume 1, chapter 14: "Magnetism and Electromagnetism" LEARNING OBJECTIVES Relationship between magnetic field strength and induced voltage SCHEMATIC DIAGRAM ILLUSTRATION INSTRUCTIONS Electromagnetic induction is the complementary phenomenon to electromagnetism. Instead of producing a magnetic field from electricity, we produce electricity from a magnetic field. There is one important difference, though: whereas electromagnetism produces a steady magnetic field from a steady electric current, electromagnetic induction requires motion between the magnet and the coil to produce a voltage. Connect the multimeter to the coil, and set it to the most sensitive DC voltage range available. Move the magnet slowly to and from one end of the electromagnet, noting the polarity and magnitude of the induced voltage. Experiment with moving the magnet, and discover for yourself what factor(s) determine the amount of voltage induced. Try the other end of the coil and compare results. Try the other end of the permanent magnet and compare. If using an analog multimeter, be sure to use long jumper wires and locate the meter far away from the coil, as the magnetic field from the permanent magnet may affect the meter's operation and produce false readings.

Digital meters are unaffected by magnetic fields. Lessons In Electric Circuits copyright (C) 2002-2015 Tony R. Kuphaldt, under the terms and conditions of the Design Science License .

Question: Which instrument measures the electrical current in a circuit?
Answer: AMMETER