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CHAPTER

Measuring Devices

LET’S LOOK AT THE INSTRUMENTS THAT ENGINEERS USE TO MEASURE ELECTRICAL QUANTITIES. Some measuring devices work because electric and magnetic fields produce forces proportional to the field intensity. Some meters determine electric current by measuring the amount of heat produced as charge carriers move through a medium with a known resistance. Some meters have small motors whose speed depends on the measured quantity of charge carriers. Still other meters count electrical pulses or cycles.

Electromagnetic Deflection

Early experimenters noticed that an electric current produces a magnetic field. They could detect this field by placing a magnetic compass near a wire. The compass pointed toward magnetic north when the wire carried no current. But when the experimenters drove DC through the wire by connecting it to the terminals of a battery, the compass needle deflected toward the east or west. The extent of the deflection depended on the distance between the compass and the wire, and also on how much current the wire carried. When scientists first observed this effect, they called it electromagnetic deflection, and that’s still a good descriptive term today.

Experimenters tried various compass-and-wire arrangements to find out how much they could force the compass needle to rotate. The experimenters also wanted to make the device as sensitive as possible. When scientists wrapped the wire in a coil around the compass, as shown in Fig. 3-1, they got a device that could detect small currents. They called this effect galvanism, and they called the coil-around-a-compass device a galvanometer. The extent of any given galvanometer’s needle displacement increased with increasing current. The experimenters had almost reached their goal of building a meter that could quantitatively measure current, but one final challenge remained: calibrate the galvanometer somehow.

3-1   A simple galvanometer. The magnetic compass must lie flat.

You can make a galvanometer at home. Buy a cheap compass, about two feet of insulated bell wire, and a large 6-V lantern battery. Wind the wire around the compass four or five times, as shown in Fig. 3-1, and align the compass so that the needle points along the wire turns when the wire is disconnected from the battery. Make sure that the compass lies flat on a horizontal surface, such as a table or desk. Then connect one end of the wire to the negative (−) terminal of the battery. Touch the other end to the positive (+) terminal for a moment, and watch the compass needle. Don’t leave the wire connected to the battery for more than a few seconds at a time.

You can buy a resistor and a linear-taper potentiometer at an electronics retail outlet and set up an experiment that shows how galvanometers measure current. For a 6-V lantern battery, the fixed resistor should have a value of at least 330 ohms and should be rated to dissipate at least ¼ W of power. The potentiometer should have a maximum value of 10 k. Connect the resistor and the potentiometer in series between one end of the bell wire and one terminal of the battery, as shown in Fig. 3-2. Short-circuit the center contact of the potentiometer to one of its end contacts. Use the resulting two terminals in the circuit.

3-2   A circuit for demonstrating how a galvanometer indicates relative current. Resistances are in ohms; k indicates kilohms.

When you adjust the potentiometer, the compass needle should deflect more or less, depending on the current through the wire. As the resistance decreases, the current increases, and so does the number of degrees by which the needle deflects. You can vary the current by changing the potentiometer setting. You can reverse the direction of needle deflection by reversing the battery polarity. Early experimenters calibrated their galvanometers by referring to the “degrees” scale around the perimeter of the compass, and generating graphs of degrees versus amperes. They calculated the theoretical currents in amperes by dividing the known voltage by the known resistance, taking advantage of Ohm’s law, about which you’ll learn more in the next chapter.

Electrostatic Deflection

Electric fields produce forces, just as magnetic fields do. Have you noticed this effect when your hair stands on end in dry, cold weather? If you live in a place where the winters get severe, shuffle around on a rug while wearing hard-soled shoes next January, and see if you can get your hair to stand up. Have you heard that people’s hair stands up just before a lightning bolt hits nearby? Sometimes it does, but not always.

The electroscope is a common physics lab device for demonstrating electrostatic force. It consists of two foil leaves, attached to a conducting rod, and placed in a sealed container so that air currents can’t disturb the leaves (Fig. 3-3). When a charged object comes near, or touches, the contact at the top of the rod, the leaves stand apart from each other because the leaves become flooded with like electric charges—either an excess or a deficiency of electrons—and “like poles always repel.” The extent to which the leaves stand apart depends on the amount of electric charge. It’s difficult to measure this deflection and correlate it with charge quantity; electroscopes don’t make good meters. But electrostatic forces can operate against tension springs or magnets, allowing engineers to build sensitive, accurate electrostatic meters.

3-3   An electroscope can detect the presence of an electrostatic charge.

An electrostatic meter can quantify alternating (or AC) electric charges as well as direct (or DC) charges. This property gives electrostatic meters an advantage over electromagnetic meters, such as the galvanometer. If you connect a source of AC to the coil of the galvanometer device portrayed in Fig. 3-1, the current in one direction pulls the meter needle one way, the current in the other direction pushes the needle the opposite way, and the opposing forces alternate so fast that the needle doesn’t have time to deflect noticeably in either direction! But if you connect a source of AC to an electrostatic meter, the plates repel whether the charge is positive or negative at any given instant in time. The alternations make no difference in the direction of the force.

Most electroscopes aren’t sensitive enough to show much deflection with ordinary 117-V utility AC. Don’t try connecting 117 V to an electroscope anyway, however. That electricity can present an electrocution hazard if you bring it out to points where you can come into physical contact with it.

An electrostatic meter has another useful property. The device does not draw any current, except a tiny initial current needed to put a charge on the plates. Sometimes, an engineer or experimenter doesn’t want a measuring device connected to a significant amount of current because so-called current drain affects the behavior of the circuit under test. Galvanometers, by contrast, always need some current to produce an indication.

If you have access to a laboratory electroscope, try charging it up with a glass rod after rubbing the rod against a dry cloth. When you pull the rod away from the electroscope, the foil leaves will remain standing apart. The charge “sits there” on the foil, trapped! If the electroscope drew any current, the leaves would fall back together again, just as the galvanometer compass needle returns to magnetic north the instant you take the wire away from the battery.

Thermal Heating

Whenever current flows through a substance with a finite, nonzero resistance, the temperature of that substance rises. The extent of the temperature increase depends on the current; for any particular sample, more current generates more heat. If we choose a metal or alloy with known physical properties, tailor a wire from that alloy to a certain length and diameter, use a sensitive, accurate thermometer to measure the wire’s temperature, and place the entire assembly inside a thermally insulated package, we end up with a hot-wire meter. This device allows us to measure AC as well as DC because the heating doesn’t depend on the direction of current flow. Hot-wire meters allow for the measurement of AC at frequencies up to several gigahertz.

We can take advantage of a variation of the hot-wire principle by placing two different metals (called dissimilar metals) into direct contact with each other, forming a boundary called a junction. The junction heats up when current flows through it. Engineers call this effect the thermocouple principle. As with the hot-wire meter, we can use a thermometer to measure the extent of the heating. The thermocouple principle works in reverse as well. When we apply heat to a thermocouple, it generates DC, which we can measure with a galvanometer. This effect allows us to build an electronic thermometer.

Ammeters

A magnetic compass, surrounded by a coil of wire, makes an effective but temperamental current-measuring meter. The compass must lie flat on a horizontal surface. We must align the coil with the compass needle under no-current conditions. We must rotate the compass so that the needle points at the “N” on the scale (that is, 0° magnetic azimuth) under no-current conditions. All of these restrictions add up to quite an annoyance for experimenters working in labs among complex electronic systems. You’ll hardly ever see a compass galvanometer in a professional engineer’s or technician’s workshop.

The external magnetic field for a galvanometer need not come from the earth. A permanent magnet, placed near or inside the meter, can provide the necessary magnetic field. A nearby magnet supplies a far stronger magnetic force than does the earth’s magnetic field (or geomagnetic field), allowing for the construction of a meter that can detect much weaker currents than an old-fashioned galvanometer can. We can orient such a meter in any direction, and slant it any way we want, and it will always work the same way. We can attach the coil directly to the meter pointer, and suspend the pointer from a spring bearing in the field of the magnet. This type of metering scheme, called the D’Arsonval movement, has existed for more than a century. Some metering devices still employ it. Figure 3-4 illustrates the functional principle of a D’Arsonval current-measuring meter.

3-4   Functional drawing of a D’Arsonval movement for measuring current.

We can fabricate a variation of the D’Arsonval movement by attaching the meter needle to a permanent magnet, and winding the coil in a fixed form around the magnet. Current in the coil produces a magnetic field, which in turn generates a force if the coil and magnet line up correctly with respect to each other. This scheme works okay, but the mass of the permanent magnet results in a slower needle response, and the meter is more prone to overshoot than the true D’Arsonval movement. In overshoot, the inertia of the magnet’s mass, once overcome by the magnetic force, causes the needle to fly past the actual point for the current reading, and then to wag back and forth a couple of times before coming to rest in the right place.

Yet another alternative avails itself: We can substitute an electromagnet in place of the permanent magnet in a D’Arsonval meter assembly. The electromagnet works with the same current that flows in the coil attached to the meter needle. This arrangement gets rid of the need for a massive, permanent magnet inside the meter. It also eliminates the possibility that the meter sensitivity will change over time if the strength of the permanent magnet deteriorates. Such a demise can result from exposure to heat, severe mechanical vibration, or the mere passage of years.

The sensitivity of any D’Arsonval type meter depends on the amount of current needed to produce a certain force inside the device. That force, in turn, depends on the strength of the permanent magnet (if the meter uses a permanent magnet) and the number of turns in the coil. As the strength of the magnet and/or the number of coil turns increases, the amount of current necessary to produce a given force goes down. In an electromagnet type D’Arsonval meter, the combined number of coil turns affects the sensitivity. If we hold the current constant, the force increases in direct proportion to the number of coil turns. The more magnetomotive force the coils produce, the greater the needle deflection for a given amount of current, and the less current it takes to cause a certain amount of needle movement. The most sensitive D’Arsonval current meters can detect a microampere or two. The amount of current for full scale deflection (the needle goes all the way up without banging against the stop pin) can be as little as about 50 μA in commonly available microammeters.

Sometimes we want an ammeter that will allow for a wide range of current measurements. We can’t easily change the full-scale deflection of a meter because that task would require altering the number of coil turns and/or the strength of the magnet inside the assembly. However, all ammeters have a certain amount of internal resistance (even though in a well-designed ammeter, the internal resistance is extremely low; in an ideal one, it would be zero). If we connect a resistor, having the same internal resistance as the meter, in parallel with the meter, the resistor will draw half the current while the meter draws the other half. Then it will take twice the current through the assembly to deflect the meter to full scale, as compared with the meter alone. By choosing a resistor of a specific value, we can increase the full-scale deflection of any ammeter by a fixed, convenient factor, such as 10, or 100, or 1000. The resistor must be able to carry the necessary current without overheating. The resistor might have to deal with practically all of the current flowing through the meter/resistor combination, leaving the meter to carry only 1/10, or 1/100, or 1/1000 of the current. We call a resistor in this application a shunt (Fig. 3-5).

3-5   We can connect a resistor, called a meter shunt, across a current-detecting meter to reduce the meter’s sensitivity.

Voltmeters

Current, as we have seen, consists of a flow of charge carriers. Voltage, also called electromotive force (EMF) or potential difference, is manifest as “electric pressure” that makes current possible. Given a circuit having a constant resistance, the current through the circuit varies in direct proportion to the voltage across the circuit.

Early experimenters saw that they could use ammeters to measure voltage indirectly. An ammeter acts as a constant-resistance circuit (although the resistance is low). If you connect an ammeter directly across a source of voltage such as a battery, the meter needle deflects. In fact, a milliammeter needle will probably “hit the pin” if you connect it right across a battery; the meter might even suffer permanent damage. (Never connect milliammeters or microammeters directly across voltage sources!) An ammeter, perhaps with a range of 0-10 A, might not deflect to full scale if you place it across a battery, but the meter coil will rapidly drain the battery. Some batteries, such as automotive lead-acid cells, can rupture or explode under these conditions.

Ammeters, as we’ve learned, have low internal resistance. That’s because they’re intended for connection in series with other parts of a circuit. A circuit under test should “see” an ammeter as a short circuit—ideally like a piece of copper wire—so the meter won’t affect the operation of the circuit. Ammeters aren’t meant to be connected directly across a source of voltage! However, if you place a large resistor in series with an ammeter, and then connect the combination across a battery or other type of power supply, you no longer have a short circuit. The ammeter will give an indication that varies in direct proportion to the source voltage. The smaller the full-scale reading of the ammeter, the larger the resistance needed to get a meaningful indication on the meter. With a microammeter and a gigantic resistance in series, you can construct a voltmeter that will draw only a little current from the source.

You can tailor a voltmeter to have various full-scale ranges by switching different values of resistance in series with the microammeter, as shown in Fig. 3-6. The meter exhibits high internal resistance because the resistors have large ohmic values. As the supply voltage increases, so does the meter’s internal resistance because the necessary series resistance increases as the voltage increases.

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

A voltmeter should have high internal resistance; the higher the better. (An ideal voltmeter would have infinite internal resistance.) You don’t want the meter to draw significant current from the power source; ideally it wouldn’t draw any current at all. You don’t want circuit behavior to change, even a tiny bit, when you connect or disconnect the meter. The less current a voltmeter draws, the less it affects the behavior of anything that operates from the power supply.

An alternative type of voltmeter uses electrostatic deflection, rather than electromagnetic deflection, to produce its readings. Remember that electric fields produce forces, just as magnetic fields do. Therefore, a pair of electrically charged plates attract or repel each other. An electrostatic voltmeter takes advantage of the attractive force between two plates having opposite electric charge, or having a large potential difference. Figure 3-7 portrays the functional mechanics of an electrostatic voltmeter. It constitutes, in effect, a sensitive, calibrated electroscope. The device draws essentially no current from the power supply. Nothing but air exists between the plates, and air constitutes a nearly perfect electrical insulator. A properly designed electrostatic meter can indicate AC voltage as well as DC voltage. However, the construction tends to be fragile, and mechanical vibration can influence the reading.

3-7   Functional drawing of an electrostatic voltmeter movement.

Ohmmeters

The current through a circuit depends on the resistance, as long as we hold all other factors constant. This principle provides us with a way to measure the DC resistance of a component, device, or circuit.

We can construct an ohmmeter by placing a milliammeter or microammeter in series with a set of fixed, switchable resistances and a battery that provides a known, constant voltage, as shown in Fig. 3-8. If we select the resistances carefully, we can get the meter to give us indications in ohms over practically any metering range we want. A typical ohmmeter can quantify resistances from less than 1 ohm to several tens of megohms. We assign the zero point on the meter scale the value of infinity ohms, theoretically describing a perfect insulator. The value of the series resistance sets the full-scale meter point to a certain minimum resistance, such as 1 ohm, 10 ohms, 100 ohms, 1 k, 10 k, and so on.

3-8   A circuit using a milliammeter (mA) to measure DC resistance.

An analog ohmmeter with a D’Arsonval meter “reads backwards.” The maximum resistance corresponds to the left-hand end of the meter scale, and the resistance decreases as we move toward the right on the scale. Engineers must calibrate an ohmmeter at the site of manufacture, or else in a well-equipped electronics lab. A slight error in the values of the series resistors can cause gigantic errors in measured resistance. The resistors must have precise tolerances. In other words, they must exhibit values close to what the manufacturer claims, to within a fraction of 1 percent if possible. For an ohmmeter to work right, its internal battery must provide a precise, constant, and predictable voltage.

An ohmmeter built from a milliammeter or microammeter always has a nonlinear scale. The increments vary in size, depending on where you look on the scale. In most ohmmeters, the graduations “squash together” towards the “infinity” end of the scale. Because of this nonlinearity, you might find it difficult to read the meter for high values of resistance unless you select the optimum meter range by switching the appropriate resistance in series with the meter device.

A technician will usually connect an ohmmeter in a circuit with the meter set for the highest resistance range first. Then she’ll switch the range down until the meter needle comes to rest in a readable part of the scale. After taking down the actual meter reading from the scale, the technician multiplies that number by the appropriate amount, as indicated on the range switch. Figure 3-9 shows an ohmmeter reading. The meter itself indicates approximately 4.7, and the range switch says 1 k. This combination indicates a resistance of about 4.7 k, or 4700 ohms.

3-9   An ohmmeter, in this case showing about 4.7 × 1k, or 4700 ohms.

Ohmmeters give inaccurate readings if a potential difference exists between the points in the circuit to which the meter is connected. The external voltage either adds to, or subtracts from, the ohmmeter’s internal battery voltage. Sometimes, in this type of situation, an ohmmeter might tell you that a circuit has “more than infinity” ohms! The needle will hit the pin at the left end of the scale. When you use an ohmmeter to measure DC resistance, you must always make certain that no voltage exists between the points where you intend to connect the meter terminals. You can easily check for such “distracting voltage” using a voltmeter before you use the ohmmeter. If you observe a voltage between those points, you’ll probably have to power-down the entire circuit before you try to measure the resistance.

Multimeters

In every good electronics lab, you’ll find a test instrument called a multimeter, which combines several measuring devices into a single unit. The volt-ohm-milliammeter (VOM) is the most often-used type of multimeter. As its name implies, the VOM combines voltage, resistance, and current measuring capabilities. You’ve learned how a single current meter can determine voltage and resistance. You should have no trouble, therefore, imagining the necessary resistors, a multi-position switch, a battery, and a milliammeter or microammeter in a single box!

Commercially available VOMs can measure current, voltage, and resistance within reasonable limits, but not all the way from “zero to infinity.” The maximum limit for voltage typically ranges from 1000 V to 2000 V. The measurement of larger voltages requires special insulated probes and cables, as well as other safety precautions, to prevent the user from receiving a lethal shock. A common VOM can measure currents up to around 10 A. The maximum measurable resistance is several tens of megohms, while the lower limit is a fraction of 1 ohm.

FET Voltmeters

A good voltmeter disturbs the circuit under test as little as possible, and this capability requires that the meter have high internal resistance. Besides the electrostatic scheme described earlier in this chapter, there’s another way to get high internal resistance: sample a tiny current, far too small for any meter to directly indicate, and then amplify this current so that a conventional milliammeter or microammeter can display it. When we draw a vanishingly small amount of current from an electrical circuit, the equivalent meter has extremely high resistance.

A device called a field-effect transistor (FET) provides an effective way to amplify currents on the order of picoamperes, or trillionths of an ampere, where a trillionth represents 0.000000000001 or 10⁻¹². (Don’t worry about how FET amplifiers work right now. You’ll learn all about amplifiers later in this book.) A voltmeter that uses a FET amplifier to minimize the current drain from the circuit under test is called a FET voltmeter (FETVM). Besides exhibiting “practically infinite” input resistance, a well-designed FETVM lets us accurately measure currents smaller than any conventional VOM can even detect.

Wattmeters

We can measure the DC electrical power that a component, circuit, or system dissipates by simultaneously measuring the voltage across it and the current through it. Remember that in a DC circuit, the power (P) in watts equals the product of the voltage (E) in volts and the current (I) in amperes, as follows:

P = EI

When we calculate DC power as a product of voltage and current, we call the resulting quantity volt-amperes (VA) or volt-ampere power (VA power). The VA figure provides an excellent indication of the actual power (or true power) that a component, circuit, or system consumes, as long as the circuit isn’t too complicated, and as long as it works with pure DC.

Figure 3-10 illustrates a method of measuring the DC VA power consumed by a common lantern bulb. We connect a DC voltmeter in parallel with the bulb, thereby getting a reading of the voltage across it. We hook up a DC ammeter in series to get a reading of the current through the bulb. We take the voltage and current readings simultaneously, and then multiply volts times amperes to get the number of DC watts consumed by the bulb.

3-10   In a DC circuit, we can measure power with a voltmeter and an ammeter, connected as shown here.

If we want to measure AC power, we must use specialized wattmeters having more sophisticated designs than the simple VA power meter just described. The same requirement obtains if we want to measure the peak audio power in a high-fidelity amplifier, or the average power produced or consumed over a period of time by any system.

Watt-Hour Meters

In small-scale, everyday systems, we can measure electrical energy in watt-hours (Wh) or kilowatt-hours (kWh). Not surprisingly, we call a metering device for these units a watt-hour meter or a kilowatt-hour meter.

A traditional (older) electrical energy meter employs a small motor, the speed of which depends on the current, and thereby, on the power at a constant voltage. The number of turns of the motor shaft, in a given length of time, varies in direct proportion to the number of watt-hours or kilowatt-hours consumed. We connect the motor at the point where the utility wires enter the building where the voltage is nominally 234 V AC. At this point the utility system splits into one group of circuits providing 234 V AC for heavy-duty appliances, such as the range, oven, washer, and dryer, and a second group of circuits providing 117 V AC for small appliances, such as lamps, clock radios, and television sets.

If you’ve observed this type of kilowatt-hour meter, you’ve seen a disk spinning, sometimes fast, and at other times slowly. Its speed depends on the power being used at any given time. The total number of times the disk rotates, hour after hour, day after day, during the course of each month determines the size of the bill that you get from the power company (which is also a function of the cost per kilowatt-hour, of course).

Kilowatt-hour meters count the number of disk turns by means of geared, rotary drums, or pointers. The drum-type meter gives a direct digital readout. The pointer type has several scales calibrated from 0 to 9 in circles, some going clockwise and others going counterclockwise. To read a pointer type device, you must “make your mind’s eye go” in whatever direction (clockwise or counterclockwise) the scale goes for each individual meter. Figure 3-11 shows an example. You should read the meters from left to right. For each meter scale, take down the number that the pointer has most recently passed. Write down the rest as you go. This particular meter display shows a little more than 3875 kWh.

3-11   A utility meter with four rotary analog dials. In this example, the reading is a little more than 3875 kWh.

Digital Readout Meters

The display of a numeric digital meter tells you the size of a quantity in plain numerals. Anybody who can read numerals can read this type of meter without errors caused by guesswork, as can occur when people imprecisely read old-fashioned “needle-and-scale” analog meters. You’ll find numeric digital readouts in utility power meters, clocks, and (in some situations) ammeters, voltmeters, and wattmeters. Numeric digital meters work well when the value of the measured quantity does not change often or fast.

In some applications, numeric digital meters can cause frustration. Consider, for example, the signal-strength indicator in a “shortwave” radio receiver. The intensity of a “shortwave” radio signal typically varies from moment to moment. In a situation like this, a numeric digital meter will display a constantly changing, meaningless jumble of numerals. Numeric digital meters require a certain length of time to “lock in” to the current or voltage under scrutiny. If this quantity never settles at any fixed value for a long enough time, the meter can never “lock in.”

Analog meters, in contrast to numeric digital meters, work well in situations where the measured quantity fluctuates all the time. Analog meters give the user an intuitive sense of the “whereabouts” of the measured quantity relative to values slightly larger or smaller. Analog meters can follow along when a quantity changes from moment to moment, although not always with great accuracy. Some engineers and technicians, especially those who got their training in the 1970s or earlier, prefer analog meters even in situations where digital meters work as well.

With any metering device, you’d better know where the decimal point belongs! If you read the numerals in a digital meter correctly but you put the decimal point to the right or left by one digit, you’ll get a reading that’s off by a factor of 10. You must also be sure that you know the correct units. For example, a frequency indicator might read out in megahertz, but if you think it’s telling you kilohertz, you’ll be off by a factor of 1000.

Frequency Counters

Digital metering works well to measure the energy that a home or business uses over a period of time. Everyone finds a digital kilowatt-hour meter easier to read than the pointer-type meter. Digital metering also works well for measuring the frequencies of radio signals, but for a different reason.

A frequency counter measures the frequency of an AC wave by counting pulses or cycles, in a manner similar to the way a utility meter counts the number of turns of a motor. The frequency counter works electronically, without any moving parts. It can keep track of thousands, millions, or billions of pulses per second, and it shows the rate on a numeric digital display.

The accuracy of the frequency counter depends on the lock-in time, which can vary from a fraction of a second to several seconds. A typical frequency counter allows the user to select from lock-in times of 0.1 second, 1 second, or 10 seconds. Increasing the lock-in time by a factor of 10 causes the accuracy to increase by one additional digit. Modern frequency counters can provide readings accurate to six, seven, or eight digits. Sophisticated lab devices can show frequency to 10 digits or more.

Other Meter Types

Following are brief descriptions of some less common types of meters that you’ll occasionally encounter in electricity and electronics.

Loudness Meters

High-fidelity equipment, especially the more sophisticated amplifiers (“amps”), sometimes have loudness meters built-in. The meters express sound intensity in decibels, a unit that you will often have to use (and interpret) in reference to electronic signal levels. The decibel actually represents a specific difference in the intensity between two signals: an increase or decrease that you can just barely detect when you expect it. You’ll learn more about decibels in Chap. 26.

Engineers sometimes measure audio loudness with a volume-unit (VU) meter. The typical VU meter scale has a zero marker with a heavy line or curve (red in some cases) to the right and a thinner, black line or curve to the left. The meter scale is calibrated in negative decibels (dB) below the zero marker and positive decibels above the zero marker. The zero marker itself corresponds to an audio power level of 2.51 mW. Figure 3-12 shows an example.

3-12   A VU (volume-unit) meter. The thick-line part of the scale (to the right of 0) indicates a high risk of audio distortion.

When music or a voice comes through a high-fidelity audio system, the VU meter needle kicks up. A competent audio engineer keeps the gain (that is, the volume control) set low enough so that the meter doesn’t go past the zero mark and into the red range (shown in Fig. 3-12 as a thick black arc). If the meter does kick up into the red part of the scale, it means that the amplifier will likely produce distortion in the audio output, degrading the quality of the sound.

We can measure sound level in a more general way using a sound-level meter, calibrated in decibels and connected through a semiconductor diode to the output of a precision amplifier with a microphone of known sensitivity, as shown in Fig. 3-13. The diode “chops off” the negative-polarity part of every AC audio wave cycle, leaving only the positive part so that a DC meter can quantify it. Has anyone ever told you that a vacuum cleaner will produce “80 dB” of sound, and a large truck going by will subject your ears to “90 dB”? Acoustic engineers determine these figures using sound-level meters, and define the intensity levels relative to a faint sound at the threshold of hearing. That’s the weakest sound that a person with good ears can hear in a room with excellent acoustic insulation to keep out stray noise.

3-13   A simple device for measuring sound levels. The diode converts the AC audio signal to DC that the milliammeter can detect. We calibrate the meter scale in decibels relative to a pre-determined reference level.

Light Meters

Photographers commonly measure the intensity of visible light rays using a light meter, technically called an illuminometer. You can make an illuminometer by connecting a microammeter to a solar cell (technically called a photovoltaic cell), using a potentiometer (variable resistor) to control the meter sensitivity, as shown in Fig. 3-14. More sophisticated devices use DC amplifiers, similar to the type found in a FETVM, to enhance the sensitivity and to allow for several different ranges of readings.

3-14   A simple light meter with a potentiometer to adjust the sensitivity.

Solar cells don’t respond to light at exactly the same wavelengths as human eyes. An engineer would say that the sensitivity-versus-wavelength function of a typical solar cell differs from the sensitivity-versus-wavelength function of the human eye. We can overcome this problem by placing a specially designed color filter in front of the solar cell, so that the solar cell becomes sensitive to the same wavelengths, in the same proportions, as our eyes. Illuminometer manufacturers calibrate their products at the factory so that the meter displays visible-light intensity in standard illumination units, such as lumens or candela.

With appropriate modification, we can use a meter such as the one diagrammed in Fig. 3-14 to roughly determine the intensity of infrared (IR) or ultraviolet (UV) rays. Various specialized photovoltaic cells exhibit their greatest sensitivity at non-visible wavelengths, including IR and UV.

Pen Recorders

A D’Arsonval meter, equipped with a marking device at the needle’s tip to keep a graphic record of the level of some quantity with respect to time, gives us a pen recorder. A flat sheet of graph paper, with a calibrated scale, is taped around a cylindrical drum. The drum is connected to a slowly rotating motor-driven shaft. The drum speed can vary; it might rotate at one revolution per minute, one revolution per hour, one revolution a day, or even one revolution a week! Figure 3-15 illustrates the principle.

3-15   Functional drawing of a pen recorder.

You can use a pen recorder, along with a wattmeter, to get a reading of the power consumed by your household at various times during the day. In this way, you can find out when you use the most power, and at what particular times you might consume too much.

Oscilloscopes

Another graphic metering device, popular with electronics engineers, is the oscilloscope, which measures and records quantities that oscillate (vary periodically) at rates of hundreds, thousands, or millions of times per second.

An old-fashioned oscilloscope created a “graph” by throwing a beam of electrons at a phosphor screen. A cathode-ray tube (CRT), similar to the kind in a television set, was employed. More modern oscilloscopes have electronic conversion circuits that allow for the use of a solid-state liquid-crystal display (LCD).

Oscilloscopes are useful for observing and analyzing the shapes of signal waveforms, and also for measuring peak signal levels (rather than only the effective levels). We can use an oscilloscope to indirectly measure the frequency of a waveform. The horizontal scale of an oscilloscope shows time, and the vertical scale shows the instantaneous signal voltage. We can also use an oscilloscope to indirectly measure power or current, by placing a known value of resistance across the input terminals.

Technicians and engineers develop a sense of what a signal waveform “ought to look like.” Then they can often ascertain, by observing the oscilloscope display, whether or not the circuit under test is behaving the way it should.

Bar-Graph Meters

A cheap, simple kind of meter comprises a string of light-emitting diodes (LEDs) or an LCD along with a digital scale to indicate approximate levels of current, voltage, or power. This type of meter, like a digital meter, has no moving parts. To some extent, it offers the relative-reading feeling you get with an analog meter. Figure 3-16 shows a bar-graph meter designed to indicate the power output, in kilowatts, for a radio transmitter. This meter can follow along fairly well with fluctuations in the reading. In this example, the meter indicates about 0.8 kW, or 800 W.

3-16   A bar-graph meter. In this case, the indication is about 80 percent of full-scale, representing 0.8 kW or 800 W.

The chief drawback of bar-graph meters is the fact that most of them don’t give precise readings; they can only approximate. For this reason, engineers and technicians rarely use bar-graph meters in a laboratory environment. In addition, the individual LEDs or LCDs in some bar-graph meters flicker intermittently on and off when the signal level “falls between” two values given by the bars. Viewers find this phenomenon distracting or irritating.

Quiz

Refer to the text in this chapter if necessary. A good score is 18 out of 20 correct. Answers are in the back of the book.

1.  You can use an oscilloscope to

(a)  see the shape of an AC wave.

(b)  detect an electrostatic charge.

(c)  measure an extremely high resistance.

(d)  measure electrical power.

2.  An advantage of a meter that relies on electrostatic deflection rather than electromagnetic deflection is the fact that the electrostatic meter can measure

(a)  the frequency of an AC wave.

(b)  AC voltage as well as DC voltage.

(c)  magnetic field strength as well as electric field strength.

(d)  All of the above

3.  An electronic thermometer works by measuring the DC output of

(a)  a solar cell.

(b)  an electroscope.

(c)  an illuminometer.

(d)  a thermocouple.

4.  Which of the following voltages would produce the bar-graph-meter indication shown in Fig. 3-17?

3-17   Illustration for Quiz Question 4.

(a)  0.040 mV

(b)  0.40 mV

(c)  4.0 mV

(d)  40 mV

5.  Numeric digital meters work best when the measured quantity

(a)  is extremely large.

(b)  is current or voltage, but not power or resistance.

(c)  does not fluctuate rapidly.

(d)  constantly changes.

6.  Which of the following phenomena can you use an oscilloscope to measure or observe?

(a)  The waveform of an AC signal

(b)  The frequency of an AC signal

(c)  The peak-to-peak voltage of an AC signal

(d)  Any of the above

7.  An electric utility meter measures, on a monthly-use basis,

(a)  energy.

(b)  voltage.

(c)  current.

(d)  power.

8.  You place a 12-V battery in series with a resistor and a galvanometer. The resulting current causes the compass needle to deflect 20 degrees toward the west. How can you get the needle to deflect 30 degrees toward the west?

(a)  Maintain the battery polarity and decrease the resistance.

(b)  Maintain the battery polarity and increase the resistance.

(c)  Reverse the battery polarity and decrease the resistance.

(d)  Reverse the battery polarity and increase the resistance.

9.  Electrostatic force can directly cause

(a)  two objects having opposite electric charges to repel.

(b)  two objects having like electric charges to repel.

(c)  electric current to stop flowing in a conductor if the voltage is too high.

(d)  a compass needle to veer to the right or left, depending on the polarity.

10.  The VU meter shown in Fig. 3-18, assuming that we see the highest level to which the audio peaks ever get, tells us that

3-18   Illustration for Quiz Question 10.

(a)  those peaks are about 6 dB too high to avoid distortion.

(b)  those peaks are about 6 dB too low to avoid distortion.

(c)  those peaks are about 6 dB below the point where distortion is likely.

(d)  nothing useful at all.

11.  You want to test a 330-ohm resistor to ensure that its actual resistance comes close to the specified value. You have an analog ohmmeter with a nonlinear scale that runs from “infinity” (at the far left) to 1 (at the far right) with 6 roughly in the middle (such as in Fig. 3-9), and that has range switches with six settings marked “x 1” to “x 100 k” in powers of 10. Which range switch will provide the most accurate reading? You can use Fig. 3-9 as a visual aid.

(a)  x 1

(b)  x 100

(c)  x 10 k

(d)  x 100 k

12.  An ideal ammeter would have

(a)  infinite internal resistance.

(b)  moderate internal resistance.

(c)  low internal resistance.

(d)  zero internal resistance.

13.  Where would you place a DC voltmeter if you wanted to directly measure the voltage of a battery connected to an electrical circuit?

(a)  Between either battery pole and electrical ground

(b)  Between the negative battery pole and the circuit input

(c)  Between the positive battery pole and the circuit input

(d)  Between the negative battery pole and the positive battery pole

14.  An ideal voltmeter would have

(a)  infinite internal resistance.

(b)  moderate internal resistance.

(c)  low internal resistance.

(d)  zero internal resistance.

15.  Why should a voltmeter have a high internal resistance?

(a)  To maximize the current that the meter draws from the circuit under test

(b)  To minimize the risk of electric shock to technicians who use the meter

(c)  To minimize the extent to which the meter disturbs the circuit under test

(d)  To minimize the risk of the meter burning out

16.  A factor called lock-in time determines the accuracy of

(a)  an oscilloscope.

(b)  a frequency counter.

(c)  an analog voltmeter.

(d)  a VU meter.

17.  In a general sense, bar-graph meters lack

(a)  useful range.

(b)  sensitivity.

(c)  precision.

(d)  physical ruggedness.

18.  An analog ohmmeter has

(a)  a nonlinear scale.

(b)  a high current requirement.

(c)  a bar-graph display.

(d)  an AC power source.

19.  You might find a D’Arsonval movement in an analog

(a)  voltmeter.

(b)  ammeter.

(c)  ohmmeter.

(d)  Any of the above

20.  The ohmmeter shown in Fig. 3-19 displays

3-19   Illustration for Quiz Question 20.

(a)  4.7 ohms.

(b)  47 ohms.

(c)  470 ohms.

(d)  4700 ohms.