Notesale: Turn your study into money

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3

CHAPTER

Measuring Devices
NOW THAT YOU’RE FAMILIAR WITH THE PRIMARY UNITS COMMON IN ELECTRICITY AND ELECTRONICS,
let’s look at the instruments that are employed to measure these quantities
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Such meters work by means of electromagnetic deflection or electrostatic deflection
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Such meters work by
thermal heating principles
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The rotation rate, or the number of rotations in a given time, can be measured or counted
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These are electronic counters
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When a magnetic compass is placed near a wire carrying a direct electric current, the
compass doesn’t point toward magnetic north
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The extent of the displacement depends on how close the compass is brought to the wire, and also on how much current the
wire is carrying
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An attempt was
made to obtain the greatest possible current-detecting sensitivity
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3-1)
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Once this device
was made, the scientists saw that the extent of the needle displacement increased with increasing
current
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You can make your own galvanometer
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Set it up as shown in Fig
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Wrap the wire around the compass four
or five times, and align the compass so that the needle points along the wire turns while the wire is
disconnected from the battery
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Click here for terms of use
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The compass must lie flat
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Touch the other end to the positive (+) terminal for a second or two, and watch the compass
needle
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You can buy a resistor and a potentiometer at a place like RadioShack, and set up an experiment
that shows how galvanometers measure current
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The potentiometer should have
a maximum value of 10 kΩ
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3-2
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When you adjust the potentiometer, the compass needle should deflect more or less, depending on the current through the wire
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3-2 A circuit for
demonstrating how a
galvanometer indicates
relative current
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You have noticed this when your hair feels
like it’s standing on end in very dry or cold weather
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(This is no myth!)
The most common device for demonstrating electrostatic forces is the electroscope
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3-3)
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This is because the two leaves become
charged with like electric poles—either an excess or a deficiency of electrons—and like poles always
repel
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It is difficult to measure this deflection and correlate it with charge quantity; electroscopes do not make very
good meters
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An electrostatic meter can quantify alternating (or ac) electric charges as well as direct (or dc)
charges
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If you connect a source of ac to the coil of the galvanometer device in Fig
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3-2, the compass needle will not give a clear deflection; current in one direction pulls the meter
needle one way, and current in the other direction pushes the needle the opposite way
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Most electroscopes aren’t sensitive enough to show much deflection with ordinary 117-V utility ac
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It can present an electrocution hazard
if you bring it out to points where you can easily come into physical contact with it
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This is the fact that the device does not draw any current, except a tiny initial current
needed to put a charge on the plates
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Galvanometers, by contrast, always need some current to produce an indication
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Ammeters 39

If you have access to a laboratory electroscope, try charging it up with a glass rod that has been
rubbed against a cloth
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The charge just sits there! 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 from the battery
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All
conductors have some resistance; none are perfect
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By choosing the right metal or alloy, and by making the wire a certain length and diameter, and
by employing a sensitive thermometer, and by putting the entire assembly inside a thermally insulating package, a hot-wire meter can be made
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A variation of the hot-wire principle can be used to advantage by placing two different metals
into contact with each other
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This is called the thermocouple principle
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But there is also another effect
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This dc can be measured with a galvanometer
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The hot-wire and thermocouple effects are sometimes used to measure ac at high frequencies,
in the range of hundreds of kilohertz up to tens of gigahertz
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It has to be lying flat, and the coil has
to be aligned with the compass needle when there is no current
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It can be provided by a permanent magnet
near or inside the meter
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Such a meter
can be turned in any direction, and its operation is not affected
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This type of metering scheme, called the D’Arsonval movement, has been around since the earliest days of electricity,
but it is still used in some metering devices today
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3-4
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A variation of the D’Arsonval movement can be obtained by attaching the meter needle to a
permanent magnet, and winding the coil in a fixed form around the magnet
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This works all right, but the mass of the permanent magnet causes
a slower needle response
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40 Measuring Devices

3-4 A functional drawing of
a D’Arsonval meter
movement (spring
bearing not shown)
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This electromagnet can be operated by the same current that flows in the coil attached to the meter
needle
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It also eliminates
the possibility that the meter sensitivity will change in case the strength of the permanent magnet
deteriorates (such as might be caused by heat, or by severe mechanical vibration)
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The sensitivity of the D’Arsonval-type meter, and of similar designs, depends on several factors
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Second is the
number of turns in the coil
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If the meter is of the electromagnet type, the combined number of coil turns affects the sensitivity
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For a given current (number of amperes), the
force increases in direct proportion to the number of coil turns
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The most sensitive ammeters can detect currents of just a microampere or two
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Sometimes, it is desirable to have an ammeter that will allow for a wide range of current measurements
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But all ammeters have a certain amount of internal resistance
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Then it will
take twice the current through the assembly to deflect the meter to full scale, as compared with the
meter alone
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This resistor must be capable of carrying
the current without burning up
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This is called
a shunt resistance or meter shunt (Fig
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Meter shunts are used when it is necessary to measure
very large currents, such as hundreds of amperes
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Voltmeters 41

3-5 A resistor, called a meter
shunt, can be connected
across a currentdetecting meter to
reduce the sensitivity
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Voltage, or electromotive force
(EMF), or potential difference, is the “pressure” that makes current possible
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Early electrical experimenters recognized that an ammeter could be used to measure voltage, because an ammeter is a form of constant-resistance circuit
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In fact, a milliammeter needle will probably be “pinned” if you do this with it, and a microammeter might well be
wrecked by the force of the needle striking the pin at the top of the scale
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An ammeter, perhaps with a range of 0 to 10 A, might not deflect to full scale if it is placed across a battery, but it’s
still a bad idea to do this, because it will rapidly drain the battery
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Ammeters have low internal resistance
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They are meant
to be connected in series with other parts of a circuit, not right across a power supply
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The ammeter will give an indication
that is directly proportional to the voltage of the supply
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Using a
microammeter and a very large value of resistance in series, a voltmeter can be devised that will draw
only a little current from the source
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3-6)
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The greater the supply voltage, the larger
the internal resistance of the meter, because the necessary series resistance increases as the voltage
increases
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This current should
go, as much as possible, toward operating whatever circuit is hooked up to the power supply, and
not into getting a reading of the voltage
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You don’t want the behavior of a circuit to be affected the instant you connect the voltmeter
to the supply
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42 Measuring Devices

3-6 A simple circuit using a
microammeter (µA) to
measure dc voltage
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Remember that electric fields produce forces, just as do magnetic fields
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The electrostatic voltmeter takes
advantage of the attractive force between two plates having opposite electric charge, or having a large
potential difference
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It draws almost no current from the power supply
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The electrostatic meter can indicate ac voltage as well as dc voltage
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3-7 A functional drawing of
an electrostatic
voltmeter movement
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This
provides us with a means for measuring resistance
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3-8)
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The zero point on the milliammeter or microammeter is assigned the value of infinity ohms, meaning a perfect insulator
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An ohmmeter must be calibrated at the factory where it is made, or in an electronics lab
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Therefore, precise tolerances are needed for these resistors
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It is also necessary that the battery provide exactly the right voltage
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That means the graduations are not of the same width
everywhere on the meter scale
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Because of this, it is difficult to interpolate for high values of resistance unless the
appropriate meter range is selected
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Then they switch the range down until the meter needle is in a part of
the scale that is easy to read
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Figure 3-9 shows an ohmmeter reading
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7, but the range switch says 1 kΩ
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7 kΩ, or 4700 Ω
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This is because such a voltage either adds to, or subtracts from, the ohmmeter’s own battery voltage
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44 Measuring Devices

3-9 An example of an
ohmmeter reading
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7 × 1 kΩ = 4
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“more than infinity” ohms! The needle will hit the pin at the left end of the scale
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The best way to do this is to switch off the equipment in question
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The volt-ohm-milliammeter (VOM) is the most often
used
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You
should not have trouble envisioning how a single milliammeter can be used for measuring voltage,
current, and resistance
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Commercially available multimeters have certain limits in the values they can measure
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The measurement of larger voltages requires special probes and
heavily insulated wires, as well as other safety precautions
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The maximum measurable resistance is on the order of several
megohms or tens of megohms
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1 to 1 Ω
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Besides the electrostatic-type voltmeter, there is another way to get
high internal resistance
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When a minuscule current is drawn from a circuit, the equivalent resistance is always extremely
high
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(Don’t worry about how such

Wattmeters 45

amplifiers work right now; you’ll learn all about that later in this book
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It has extremely high input resistance, along with good sensitivity and amplification
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Remember that in a dc circuit, the power (P ) in watts is the product of the voltage (E ) in
volts and the current (I ) in amperes
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In fact, watts are sometimes called volt-amperes
in dc circuits
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For most dc circuits, this is an excellent way to measure power, as shown in Fig
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Sometimes, it’s simpler yet
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Utility power is a good example
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Although it’s ac, and not dc, power in most utility circuits can be measured in the same way as power
is measured in dc circuits: by means of an ammeter connected in series with the circuit, and calibrated so that the multiplication (times 117) has already been done
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If the meter reading is 300
W, the current is I = P/E = 300/117 = 2
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An electric iron might consume 1000 W, or a current
of 1000/117 = 8
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A large heating unit might gobble up 2000 W, requiring a current of
2000/117 = 17
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You should not be surprised if this blows a fuse or trips a circuit breaker, because these devices are often rated for 15 A
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But almost all of these meters, whatever the associated circuitry, use simple ammeters, milliammeters, or
microammeters as their indicating devices
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46 Measuring Devices

3-11 A utility meter with four rotary analog dials
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Watt-Hour Meters
Electrical energy, as you now know, is measured in watt-hours or kilowatt-hours (kWh)
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The most often used means of measuring electrical energy is by using a small electric motor, the
speed of which depends on the current, and thereby on the power at a constant voltage
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The motor is placed at the point where the utility wires enter the building
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At this point the circuit is split into some
circuits with 234 V (for heavy-duty appliances such as the oven, washer, and dryer) and general
household circuits at 117 V (for smaller appliances such as lamps, clock radios, and television sets)
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Its speed depends on the power being used at any given time
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Kilowatt-hour meters count the number of disk turns by means of geared rotary drums or
pointers
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The pointer type has several scales calibrated from 0 to 9 in circles, some going clockwise and others going counterclockwise
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An example of a pointer-type utility meter is illustrated in
Fig
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Read from left to right
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Write down the rest as you go
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Digital Readout Meters
Increasingly, metering devices are being designed so that they provide a direct readout
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It’s that simple
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The main advantage of a digital meter is the fact that it’s easy for anybody to read, and there is no
chance for interpolation errors
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It works well when the value of the quantity does not change often or fast
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One good example is the
signal-strength indicator in a radio receiver
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A digital meter will show nothing
but a constantly changing, meaningless set of numerals
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If this quantity
never settles at any one value for a long enough time, the meter can never lock in
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Their main advantages are that they
allow interpolation, they give the operator a sense of the quantity relative to other possible values,
and they follow along when a quantity changes
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One potential hang-up with digital meters is being certain of where the decimal point goes
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Also, you need to be sure you
know what the units are
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That’s a mistake by a factor of
1000! Of course, this latter type of error can happen with analog meters, too
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A digital kilowatt-hour meter is easier to read than the pointer-type meter
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A frequency counter measures the frequency of an ac wave by actually counting pulses, in a manner similar to the way the utility meter counts the number of turns of a motor
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It can keep track of thousands, millions, or
billions of pulses per second, and it shows the rate on a digital display that is as easy to read as a digital watch
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Lock-in is usually done
in 0
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Increasing the lock-in time by a factor of 10 will cause the
accuracy to increase by one additional digit
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Other Meter Types
Here are a few of the less common types of meters that you will occasionally encounter in electrical
and electronics applications
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These are calibrated in decibels, a unit that you will often have to use, and interpret,
in reference to electronic signal levels
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Audio loudness is given in volume units (VU), and the meter that indicates it is called a VU
meter
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3-12)
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As music is played

48 Measuring Devices

3-12 A VU (volume-unit)
meter
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through the system, or as a voice comes over it, the VU meter needle kicks up
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If
the meter does kick up into the red scale, it means that distortion is taking place within the amplifier circuit
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3-13)
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Light Meters
The intensity of visible light is measured by means of a light meter or illumination meter
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As things work out, this is a good way to construct an inexpensive light meter
(Fig
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More sophisticated devices use dc amplifiers, similar to the type found in a FETVM, to
enhance sensitivity and to allow for several different ranges of readings
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The output of the audio amplifier is
rectified to produce dc that the meter can detect
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A
microammeter can be
substituted for the
milliammeter if greater
sensitivity is required
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This can be overcome by placing a colored filter in front of the solar
cell, so that the solar cell becomes sensitive to the same wavelengths, in the same proportions, as
human eyes
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This must usually be done at the factory, in
standard illumination units such as lumens or candela
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3-14 can be used to measure infrared (IR) or ultraviolet (UV) intensity
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Pen Recorders
A meter movement can be equipped with a marking device to keep a graphic record of the level of
some quantity with respect to time
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The paper, with a calibrated scale, is taped to a rotating drum
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A simplified drawing of a pen
recorder is shown in Fig
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3-15 A functional drawing of a pen recorder
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In this way you can find out when
you use the most power, and at what particular times you might be using too much
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This measures and records quantities that vary
rapidly, at rates of hundreds, thousands, or millions of times per second
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A cathode-ray tube, similar to the kind in a television set, is employed
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Oscilloscopes are useful for observing and analyzing the shapes of signal waveforms, and also
for measuring peak signal levels (rather than just the effective levels)
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The horizontal scale of an oscilloscope shows time, and the vertical scale shows the instantaneous signal voltage
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Technicians and engineers develop a sense of what a signal waveform should look like, and then
they can often tell, by observing the oscilloscope display, whether or not the circuit under test is behaving the way it should
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Bar-Graph Meters
A cheap, simple kind of meter can be made using a string of light-emitting diodes (LEDs) or an
LCD along with a digital scale to indicate approximate levels of current, voltage, or power
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To some extent, it offers the relativereading feeling you get with an analog meter
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This meter can follow along
quite well with rapid fluctuations in the reading
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8 kW,
or 800 W
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For this reason it is not
generally used in laboratory testing
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This creates an illusion of circuit instability
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3-16 A bar-graph meter
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8 kW,
or 800 W

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