Here is the "Electricity  Basic Navy Training Courses" (NAVPERS 10622) in its
entirety. It should provide one of the Internet's best resources for people
seeking a basic electricity course  complete with examples worked out. See
copyright. See
Table of Contents.
¶ U.S. GOVERNMENT PRINTING OFFICE; 1945  618779
CHAPTER 18
ELECTRICAL
METERS
WHY METERS?
What is the most important fact to know when you are filling an
automobile tire? The air pressure within the tire, of course. And, how
is this air pressure measured? By a "tire gage"  a simple airpressure
METER. Try to fill a tire without a gage. Nine times out of ten the
tire is too hard or too soft. The airpressure meter is your "eye" to
"see" and measure air pressure. In a sense, all meters are "eyes" specially
developed to see and measure invisible forces and quantities.
The
speed of a ship is measured by a taffrail log or a tachometer  both
are meters. Boiler pressure is read directly from a meter. In gunnery,
both range and elevation are calculated by meters. All meters collect
data INSIDE a system and deliver it to the "OUTSIDE where it can be
used. Meters answer the question, "how much?"
In an electrical
system, "how much?" is asked about four quantities  CURRENT, VOLTAGE,
RESISTANCE, and POWER. And four kinds of meters measure these quantities
 ammeters, voltmeters, ohmmeters, and wattmeters. Each meter's name
indicates its use.
Every conductor, every motor, every circuit
of any kind has a RATED current load. Exceed this rated load and you
ask for trouble  heat develops, connections melt, and insulation burns.
AMMETERS tell you exactly how much current is flowing  they forewarn
you of overload.
Each insulation has a voltage rating. And 120volt
insulation will not stand 240 volts. It's like trying to put 70 pounds
of air pressure in a bicycle tire rated at 35 pounds. Something lets
loose. Motors are built to operate at a definite voltage of 110, 220,
or 440 volts. If you try to operate a motor above or below its rated
voltage, it either "burns out" or gives such poor service that it's
useless. VOLTMETERS keep you informed of the voltage of every line and
every generator aboard ship. Resistance and power can always be calculated
from ammeter and voltmeter readings. R = E/I and P = EI. However, OHMMETERS
and WATTMETERS, which read these values directly, are handier  and
they are built to do your dividing and multiplying for you.
WHAT METERS DO
Meters have been compared to your eyes  remember they are just.
about as delicate. A good meter costs much more than a good watch. Every
meter is built to measure quantities within a definite range. And, if
you want your meters to stay accurate, don't drop them and don't overload
them. An outboard motor is a good LITTLE engine, but, it won't drive
a destroyer. Neither will a 10ampere ammeter measure 100 amperes.
One thing that makes the study of meters easy is the fact that
ALL METERS MEASURE CURRENT. Yes, the voltmeter, ohmmeter, and wattmeter
all measure current  because the voltage, the resistance, and the power
are ALL PROPORTIONAL TO THE CURRENT. The CURRENT in a voltmeter gives
an accurate picture of how much VOLTAGE is pushing that current. The
CURRENT in an ohmmeter tells you exactly how much RESISTANCE is holding
that current back. Power is the product of E and I, and the amount of
CURRENT, therefore, controls the amount of power.
Although all meters
measure currentthey are not all calibrated (scaled) in amperes. Each
meter has a scale to read the units it is meant to measure. Ammeters
read directly in amperes, voltmeters read volts, ohmmeters read ohms,
and wattmeters read watts.
HOW THEY DO THE JOB
Current produces three effects on an electrical circuit. All three
effects are used in meter construction. The three current effects are
 HEAT, MAGNETISM, and MOTOR ACTION. The STRENGTH of each one of these
effects depends on the amount of current. There are three types of meters
built  each type measures one of the current effects. And although
it is the effect which is measured, the STRENGTH OF THE EFFECT is an
accurate gage of the STRENGTH OF THE CURRENT.
HOT WIRE METERS
The HOT WIRE METERS use the heatproducing effect of current. As
current flows in a conductor, the friction produces heat. The stronger
the current, the greater the heat. This type meter uses a high resistance
wire that expands a great deal when it is heated. When the wire cools,
it shrinks back to normal length.
Figure 187 shows a hot wire
meter. Notice that the high resistance wire has become heated and expanded
by the current flowing in it. The slack in the wire allows the spring
to pull the indicator needle to the right. In this way, the needle is
measuring the expansion of the wire. (Get the idea  heat effect is
measured.) And this expansion is proportional to the SQUARE of the current
in the Wire.
Figure 187.  Hot wire meter.
If the slack in the wire allows the needle to move onehalf
inch for the first ampere of current, then the scale behind the needle
cannot be calibrated in equal spaces. In this particular meter, the
scale is calibrated in unequal divisions. But, because the heat produced
is proportional to the square of the amount of current, each division
stands for the same amount of current. A hot wire meter would have much
larger amounts of current calibrated on the righthand end of the scale.
A particular scale might read something like this  1 ampere, 2 amperes,
3 amperes, 14 amperes, all spaced further apart. The lefthand end is
called a CRAMPED or COMPRESSED scale.
A.C. and D.C. produce
equal amounts of heat per ampere, therefore, the HOT WIRE METERS CAN
BE USED ON EITHER A.C. OR D.C.
MAGNETIC METERS
MAGNETIC METERS make use of the magnetic flux effect of a current
traveling in a conductor. There are two general types of magnetic meter
 the D'ARSONVAL or MOVABLE COIL type and the MOVABLE IRON type.
Figure 188.  D'Arsonval type meter.
The D'Arsonval type consists of a strong Vshaped, permanent
magnet mounted in a stationary frame. Notice, in figure 188, that the
pole pieces of this magnet are circular. This shape insures an even
distribution of flux from the permanent magnet. In the center of the
space between the pole pieces is mounted an iron core  also stationary.
This iron core is small so that a small movable coil can turn in the
air space between the core and the permanent magnet.
The movable
coil carries the current' of the circuit, and has the indicator needle
attached. When current flows in the coil, it acts like the armature
of a motor. The field of the coil reacts with the field of the permanent
magnet, producing torque. The action is illustrated in figure 189. Note
that the iron core preserves much of the flux of the permanent magnet.
In this drawing, the coil is represented by only one turn of wire. Actually
the coil contains many turns  20 or more.
Figure 189.  D'Arsonval action.
The feeders for the movable coil are the two spiral springs
shown in figure 188. They do two jobs  conduct the current to the coil
and ACT AGAINST THE COIL'S TORQUE. Thus, the amount of turning produced
is proportional to the combined effect of the torque AND the opposition
of these springs. As the current increases; the torque becomes stronger
and the amount of turn is greater. The indicator needle is carried across
the scale as the coil turns.
The scale is a LINEAR " calibration.
That is, the divisions are of equal size and represent equal amounts
of current. For example, they might read 1, 2, 3, 4, 5 amperes. A linear
scale can be used because the amount of magnetic effect or torque produced
is DIRECTLY proportional to the strength of the current.
The
D'Arsonval type meter is extremely sensitive. In fact, it is the best
action to use in the sensitive galvanometer. In order to make full use
of the sensitivity and accuracy of this meter, the shaft holding the
coil runs in jeweled bearings. Also, the weight of the indicator needle
is counterbalanced by a weight threaded on the needle's other end. This
cancels any gravitational torque which might be added to the coil's
torque.
Since this meter depends on motor action  current in
one direction  it can be used ONLY ON D.C.
The MOVABLE IRON
type of meter has one marked advantage over the D'Arsonval type. It
can be used on EITHER D.C. or A.C.
Follow the movable iron action
through each diagram of figure 190. A shows two pieces of iron held
in a "dead" coil. No action! B shows what happens when the coil is energized
by d.c. The Coil Hand Rule tells you that polarity is induced in the
iron as shown. Like poles are produced on adjacent ends of the iron
pieces. Repulsion throws the two pieces of iron away from each other.
C shows that A.C. produces the same effect  either norths or souths
repel with equal force. D is a duplicate of A except that one piece
of iron is permanently attached to the inside of the coil. When the
coil is energized with a.c. or d.c. as in E, the remaining piece of
iron " free to moveis repelled to the opposite side of the coil.
F is the developed meter as it actually is. The movable piece
of iron and the indicator needle are mounted on a jeweled shaft. This
shaft permits the movable iron to move only rotationally. The amount
of current in the coil determines the amount of repulsion and the amount
of rotation. And the amount of rotation controls the indicator needle's.
The movable iron meter is a honey  it works on A.C. or D.C.
It is almost as sensitive as the D'Arsonval type. And like the D'Arsonval
type, the movable iron type has a spiral spring to oppose torque and
to restore the needle to the zero point.
Figure 190.  Movable iron action.
DYNAMOMETER METERS
The DYNAMOMETER type meter uses the motor action of two fields for
measuring current. The first field is set up by two large stationary
coils (the heavy coils of figure 191). The second field is set up by
a small movable coil pivoted in the center of the large coils. Notice,
in figure 191, that this light coil has opposing spiral springs and
carries the needle indicator on its shaft.
The current to be
measured flows in BOTH coils. Since the current in BOTH the stationary
and movable coils is the SAME CURRENT, the POLARITIES OF THE COILS ARE
ALWAYS THE SAME. Repulsion is produced and the movable coil turns.
This meter can be used on d.c.  it acts just like a motor.
It also can be used on a.c. since the polarities of both its coils REVERSE
TOGETHER on A.C.
Figure 191.  Dynamometer action.
THERMOCOUPLE METERS
A fourth type of meter makes use of the voltage produced by a thermocouple
 really a heat  type meter. This type meter is illustrated in figure
192. The joined end of the thermocouple T, is wrapped in a coil of high
resistance wire C. Then the circuit current is sent through this coil.
The resulting heat generates a voltage in the thermocouple proportional
to the current which produced the heat.
A regular type meter
 usually the D'Arsonval type  measures the current produced by the
generated voltage. The scale is adjusted and calibrated to read NOT
the GENERATED voltage or current but the current of the heating COIL.
This meter seems complicated and tricky. It is, but it permits
the use of a sensitive D'Arsonval type for measuring a.c.  and that
makes it worthwhile.
Figure 192.  Thermocouple meter.
WHICH METER WHERE?
Why all these types of meters? For the same reason that there are
both tug boats and battle . ships. Each can do some one job best.
The D'Arsonval is extremely sensitive but it can be used only
on d.c.
The movable iron is not as sensitive as the D'Arsonval
but it can be used on d.c. or a.c.
The hot wire is not particularly
sensitive and it takes time to heat the high resistance wire. But it
can be used on all d.c. or a.c. circuits including radio circuits.
The thermocouple is complicated but it can be used on d.c. or
a.c. Also it is a good meter for radio work. It is fairly accurate and
it adopts the sensitive D'Arsonval movement to the measurement of a.c.
The dynamometer is the most sensitive a.c.d.c. meter. However,
it cannot be used on the high frequencies of radio circuits.
As the various types of meters have been pictured, they are operating
as galvanometerssensitive, and carrying the full current of the line.
In practical use, any type of meter can be CONNECTED to act as an ammeter,
voltmeter, or ohmmeter. The whole problem is to connect the meter to
do its job, BUT not "burn out" the meter.
AMMETERS
Ammeters measure the CURRENT IN A LINE. Therefore, they must be
in series with the line and carryall the line current or a DEFINITE
FRACTION SHUNT of the total line current. If the line current is small
 a fraction of an ampere, then the meter can usually handle all of
it. But seldom do you deal with such small currents. Except in some
radio circuits, you'll have to measure currents far too heavy for the
light coils of meters. To reduce the current to a safe value in the
meter coils a shunt is connected around the meter. This shunt is simply
a parallel path carrying most of the current. In this way, the meter
carries only a DEFINITE SMALL FRACTION of the total current. Figure
193 shows an ammeter with an INTERNAL shunt.
Figure 193.  Ammeter with internal shunt.
Notice that there are two paths for current within the meter.
One through the meter coils, the other through the shunt. Say that the
resistance of the meter is 1 ohm, the resistance of the shunt is 0.01
ohm, and the total current is 101 amperes. The ratio of current in the
meter to current in the shunt is 100 to 1. That is, the meter handles
1 ampere while the shunt carries 100 amperes. The total circuit  meter
and shunt  carries the line's current of 101 amperes. Now  just what
does the meter read  1 ampere, 100 amperes, or 101 amperes? It will
read 101 amperes BECAUSE the scale has been calibrated for a 101 MULTIPLIER.
The ratio of the TOTAL current to the METER current is the multiplier.
Internal shunts  the shunts inside a meter case  are fixed.
Regardless of what circuit the meter is used in, the shunt is a part
of that circuit, and the meter scale is calibrated to read the actual
meter current times, the multiplier.
EXTERNAL shunts are shunts
connected OUTSIDE the meter box  on the back of switchboards, on the
outside of the meter box  or, if portable, any place convenient. Figure
194 shows three types of shunt. A is a switchboard shunt. B is it shunt
for mounting on meters. And C is a portable and adjustable shunt.
If the shunts are outside the meters, they are easily changed
and the meter's range can be increased by merely changing a shunt.
For example, an ammeter with a range of 010 amperes has a resistance
of 0.1 ohm. The range is to be increased to 0100 amperes. What size
shunt is required?
Just looking at the problem tells you that
the multiplier is 10  because you are increasing the capacity TENFOLD.
By doing a little reasoning you know that 1/10th of the current will
go through the meter and 9/10ths through the shunt. Therefore, the shunt
must have 1/9th as much resistance as the meter  it carries 9 times
as much current. The resistance of the shunt is 1/9th of 0.1 ohm or

0.1 x 1/9 = 0.1/9 = 0.011 ohms.
Shunt sizes can be reasoned out this way  just remember that
you have a simple twoway parallel circuit. One branch is the meter
 the resistance is fixed. The other branch is the shunt  you set this
resistance to fit the job. The resistance must below enough so that
most of the current will be kept away from the meter. But the resistance
of the shunt must also be an EASY FRACTION of the total resistancebecause
an easy fraction gives you a handy multiplier.
Figure 194.  External shunts.
Here is the formula for calculating shunt resistance 
R
_{s} = R
_{m}/(N1)
When
R
_{s} =
the resistance of the shunt;
R
_{m} = the resistance of the meter;
N = the multiplier.
Using this formula for the solution of the
example above 
R
_{s}
= R
_{m}/(N1)
R
_{s}
= 0.1/(101) = 0.1/9 = 0.011 ohm.
Let's say this ammeter is
used to measure an 80 ampere circuit. Here is what happens  80 amperes
are in the line and passing through the meter AND shunt. Because the
shunt has only 9/10th the resistance of the meter, the shunt carries
9 times the current of the meter, or 9/10ths of the total current. Hence,
the shunt will carry 9/10ths x 80 = 72 amperes and the meter only 8
amperes. And the meter reads only 8 amperes. BUT the multiplier is 10
 so the TOTAL current is the reading (8 amperes) times the multiplier
(10) or 8 x 10 = 80 amperes.
Remember that ammeters and their
shunts must carry the ENTIRE line current. Wherever this current is
reasonably large  1 ampere or more  a shunt must be used to protect
the meter coils. It's true that an ammeter coil could be built to carry
the full line current  but it would have to be made of heavy wire.
Such a coil would weigh too much  the meter would be cumbersome and
not very sensitive. The only ammeter which can carry the full line current
is the hot wire type  no coils are used.
You will run across
ammeters with multiple scales. These are meters using more than one
shunt.
Figure 195 shows an ammeter with two shunts. This meter
has three different scales. The first scale is 01 ampere  no shunt
is used. The second scale is 010 amperes  the first shunt is used.
And the third scale is 0100 amperes  the second shunt is used.
Figure 195.  Multiple shunt ammeter.
The resistances of the shunts are shown in the figure. You can
see that multipliers of 10 and 100 were used in calculating the resistance
of each. Use the formula for finding shunt resistance  prove the values
given in figure 195.
CAUTION
Ammeters are extremely LOW RESISTANCE instruments. They are designed
to be connected IN SERIES WITH THE LINE AND ITS LOAD. If you should
connect an ammeter across the line, or in parallel with a load, the
full line voltage will force a tremendous surge of current through,
the delicate coils. The burning action is so fast that line fuses cannot
melt in time to save the meter.
REMEMBER  AMMETERS ARE ALWAYS
CONNECTED IN SERIES WITH A LOAD.
VOLTMETERS
Look at this formula: E = I/R. It describes the voltage necessary
to force a certain current through a certain resistance. Suppose, for
a given circuit, the RESISTANCE is CONSTANT. You put an ammeter in the
circuit. Every time the CURRENT INCREASES  you know the VOLTAGE must
have INCREASED. Every time the CURRENT DECREASES  you know the VOLTAGE
must have DECREASED. Remember  the resistance is constant. Exactly
what is the ammeter doing? IT IS MEASURING VOLTAGE AS WELL AS CURRENT.
And if the scale of the meter reads volts instead of amperes then you
have a voltmeter. But wait a minute  it's not quite that simple. In
practical circuits, the resistance is seldom constant. However, you
can add a resistance that is constant ALWAYS CONSTANTby making it
a fixed part of the meter. Such a meter is a VOLTMETER.
Figure 196.  Voltmeter connection.
Look at figure 196  notice that the meter is in parallel with
the load. Now changes in LOAD resistances have no effect on the METER
resistance.
This is how the voltmeter works  there is a potential
difference between points A and B of figure 196  so much voltage. This
voltage forces a SMALL current through the high resistance, R, and then
through the meter. The meter acts on this small current.
The
degree of action depends on the amount of current. And the AMOUNT OF
CURRENT IS EXACTLY PROPORTIONAL TO THE VOLTAGE. Therefore, the DEGREE
OF ACTION IS PROPORTIONAL TO THE VOLTAGE. And the meter can be calibrated
in volts instead of amperes. Notice that the voltmeter must always be
connected ACROSS THE LINE. If it were connected in series with any load,
the resistance of the load would cut down the current and give a reading
too low.
Figure out this voltmeter problem  A meter of 50 ohms
resistance will give full scale deflection with 0.0005 ampere of current
through its coils. How much resistance must be added to the meter circuit
in order to use this meter as a 0500 volt voltmeter? From the 0500
volt range, you know that full scale deflection must be produced by
500 volts. Also, you know that 0.0005 ampere in the meter coils will
give full scale deflection. You can say it either way  both mean the
same  full scale deflection by 500 VOLTS or by 0.0005 AMPERE. These
two items, line voltage and meter current, tell you how much resistance
is needed. It's a simple Ohm's law problem 
R = E/I = 500/0.0005 = 1,000,000 ohms.
The 1,000,000 ohms is
the TOTAL resistance. You need only 1,000,000 minus 50 or 999,950 OHMS
ADDITIONAL RESISTANCE.
With 1,000,000 ohms resistance, the meter
will pass 0.0005 ampere at 500 volts  full deflection. But if the voltage
is less  say 200 volts  only 0.0002 ampere passes through the meter
coil because 
I = E/R = 200/1,000,000
= 0.0002 amp.
And 0.0002 ampere is exactly twofifths of full deflection
current (0.000.5 ampere). Therefore, the meter pointer deflects exactly
twofifths of full scale. And on a scale of 0500 volts, 200 volts would
be twofifths of the way to 500 volts. Every voltage produces its corresponding
current. And every current causes a deflection  and reading  corresponding
to the voltage which produced it.
Figure 197.  Voltmeter with series resistance.
The complete meter circuit looks like figure 197. The large
resistor, R, is usually included INSIDE the voltmeter and the. range
is labeled 0500 volts.
For MULTIPLE SCALE VOLTMETERS, more
than one resistor is connected inside the meter. Look at the connections
in figure 198. If the meter is connected from + to 250, both resistors
are in series with the meter coil and the range is 0250 volts.
If it is connected from + to 125, only one resistor is in series
with the meter coil and the range is 0125 volts.
CAUTION
The voltmeter is more rugged than the ammeter  the voltmeter has
a high resistance to' protect it. Nevertheless, you can "cook" the windings
by trying to measure a voltage higher than the meter's range.
BEFORE you connect ANY meter, make certain your instrument has a
range high enough to handle the current or voltage of the circuit. When
in doubt  USE THE HIGHEST SCALE.
Figure 198.  Multiple scale voltmeter.
VOLTMETERAMMETER METHODS
There are four electrical factors in every circuit  current, voltage,
resistance and power. There is a special meter to measure each  BUT
 you don't need all four meters to know all four facts.
With
only the ammeter and voltmeter readings, you can CALCULATE both the
resistance and the dc power. Use Ohm's law for resistance 
R = E/I
and use the Power
Equation for power 
P = EI
Wattmeters and ohmmeters are simply instruments which do the
dividing or multiplying for you. Remember that all you need to
measure in any dc circuit are voltage and current. But for ac power
measurement, you must use a wattmeter.
WATTMETERS
Power is composed of two thingsvoltage and current. A change in
either one changes the power. Therefore, the wattmeter must measure
both the current and the voltage. How one meter measures both quantities
is shown in figure 199.
Figure 199.  Wattmeter connections.
The two coils marked A are stationarythey are the AMMETER part.
Notice, they are in series with the load. Coil B is movable and carries
the indicator needle  it is the VOLTMETER part. Notice that B is across
the line and in series with the resistor, R.
The wattmeter is
like the other meters except. that there are TWO coils. The strength
of coil is determined by the line current. The strength of coil B is
determined by the line voltage. Two flux fields are set up  A and B.
They exert a force on each other. Coil B is the only coil free to move,
and both forces are concentrated on it. Coil B turns, carrying the needle
with it. THE AMOUNT IT TURNS IS PROPORTIONAL TO BOTH THE FORCES ACTING
ON IT. Therefore, it reads the product of both forces  POWER.
Wattmeters having multiple scales usually have their multipliers
inside the meter case. It's a tricky job to add external multipliers
and then calculate the correct power. If you don't have a correct range
wattmeter, the best thing to do is measure current and voltage separately.
Then multiply the two readings for the power.
THE OHMMETER
The OHMMETER is a voltmeter! But  with a very special arrangement
of a resistor and a battery.
The first type ohmmeter is a voltmeter
with a resistance and a battery in SERIES with the meter coil. Figure
200 is a schematic of the series type ohmmeter.
Figure 200.  Series ohmmeter.
It works like this  the resistor, R, and the resistance of
the meter, R
_{m}, are in series. The battery ( a small dry cell)
furnishes just enough voltage to cause full scale deflection of the
meter. That is full scale deflection when the test prods are touched
together. When any resistance is placed between the test prods, it ADDS
RESISTANCE TO THE WHOLE CIRCUIT. Current is reduced and the meter will
read LESS than full deflection.
Figure 201.  Shunt ohmmeter.
The amount of the reading, less than full deflection, is a measure
of the size of the unknown resistance placed between the prods. Thus,
this meter reads from right to left. And it must be calibrated that
way.
The other type ohmmeter also has the battery and resistance
in series with a voltmeter. The battery provides just enough voltage
for full scale deflection. But in this type  the SHUNT OHMMETER  the
unknown resistance is connected in PARALLEL with the meter coil.
The best way to get the workings of this meter straight is to follow
the circuit in figure 201. The current path through the resistor, R,
the meter and the battery is shown by solid arrows. The current path
through the resistor; R, the unknown resistance, Rx, andthe battery
is shown by broken arrows. Two parallel paths. Apparently they have
no effect on each other. But R
_{x} DOES affect the meter circuit
and here's the reason. All batteries have internal resistance  therefore,
there is a voltage drop INSIDE the battery. Any increase in the internal
voltage drop of the battery (IR) is subtracted from the voltage at the
battery terminals, thus making the current through the meter less. When
R
_{x} is included between the test prods  current through the
battery increases  internal IR drop increases  terminal voltage DECREASES
and the METER GETS LESS CURRENT. The indicator reads less than full
deflection. The lower the R
_{x}, the more current it draws,
and the less the deflection.
This type ohmmeter reads from left
to right. And that's easily proved  if the prods are touched together
R
_{x} is practically zero. Almost all of the current is following
the broken arrows. And practically no current is left for the meter
 it reads almost zero. Which it should  the prods, themselves, have
practically zero resistance.
The shunt ohmmeter can be explained
another way, although it's NOT quite correct. Imagine that the battery
can put out just so much current. This current can go two ways  through
the meter or through R
_{x}. If R
_{x} is high, most of
the current goes throughthe meterhigh deflection. If Rx is low, most
of the current goes through R
_{x } low deflection.
This explanation is not entirely correct BECAUSE the amount of current
put out by a battery depends on the total resistance of the circuit
AND the terminal voltage. This explanation ignores terminal voltage
changes.
Both series and shunt ohmmeters use primary cells and
these cells lose capacity as they are used. This is the reason for the
variable resistance in the ohmmeter circuits. Look at figures 200 and
201 again. Notice the rheostat in series with the resistor and meter
coil. This rheostat is used to compensate for any decrease in battery
capacity. By increasing or decreasing the rheostat resistance, the meter
is set at zero reading before each use.
MEGGER
The MEGGER is a special instrument for measuring resistance. It's
more than just a meter  it contains a small hand generator and a sensitive
voltmeter. By following the circuits in figure 202, you will find that
the megger is connected very much like a shunt ohmmeter. But with this
big difference  the voltage is furnished by the hand generator at the
right, not by batteries. This little generator uses permanent magnets
for a field. Whenever permanent magnets provide the field for a generator,
the machine is called a MAGNETO. The armature is turned by a crank mounted
on the side of the megger case.
Figure 202.  Megger connections.
Now "why a megger instead of one of the ohmmeters?" Because
the megger has a constant voltage  no batteries to wear down. And the
megger magneto can be designed to produce, EMFs from 100 volts to 1,000
volts. It will measure a much higher resistance than the ohmmeter. In
fact, the megger's name comes from the units it measures  MEGOHMS millions
of ohms.
Navy men use meggers constantly to check circuits and
cables for grounds. The two megger prods are connected from conductor
to ground. This puts the conductor's insulation between the terminals
of the megger. Depending on the type of circuit, the insulation should
test between 200,000 and 5,000,000 ohms  1/5 to 5 megohms.
TOUGH ONES
When you use meters you're going to run into some tough problems.
And at the same time, you're going to run into the special meters built
to solve these problems. The catch is this  the meters are pretty complicated!
In fact, they're too complicated for this book. But you want to be up
on new things  and even if you can't understand all the inner workings,
you'll want to know what these special meters WILL DO. SO first a few
of the tough questions and then the meters which answer the questions.
QUESTION ONE
Suppose you have ONE meter  it doesn't matter whether it's an ammeter,
a voltmeter, or an ohmmeter. But, say that it's a D'Arsonval VOLTMETER.
And you want to use it as an AMMETER or as an OHMMETER. How do you change
the meter to fit your purpose?
THE ANSWER
You can reconnect the meter to fit your purpose. If you strip the
voltmeter of its series resistance  then it's no longer a voltmeter.
It's just a plain D'Arsonval meter. Then by RECONNECTING resistances
and batteries, you can make it an AMMETER, a VOLTMETER, or an OHMMETER.
And it's not a very hard thing to do  because meters are built with
STANDARD RATINGS. This one has a standard full scale deflection with
0.001 ampere through its coil. It's called a "1 milliampere meter."
And it requires one volt to force this one milliampere of current through
the coil. Now, you can find the resistance of the coil  Ohm's law 
I = E/R = 1/0.001 = 1,000 ohms.
Since it requires 1 volt to produce full scale deflection of
the 1,000 ohm coil, it's a "1,000 ohms per volt" meter. That's TWO STANDARDS
for this meter  1 MILLIAMPERE and 1,000 OHMS PER VOLT. You know everything
there is to know about this meter! Full scale deflection is produced
by 0.001 ampere through the coil. And 0.001 ampere requires 1 volt because
the coil's resistance is 1,000 ohms.
To make it a VOLTMETER
with a 0150 volt range  add enough resistance to limit the current
to 0.001 ampere at 150 volts. Ohm's law again 
R = 150/0.001 = 150,000 Ω
You already have 1,000 ohms in the
meter's coil, so just add 149,000 ohms in series to get the 150,00 ohms
total. There's another way to find the required resistance. Remember
this meter has 1,000 ohms per volt. So 150 volts will require 150 x
1,000 = 150,000 ohms. Again just add 149,000 ohms in series with the
coil.
To make the meter an AMMETER with a 010 ampere range,
add a shunt heavy enough to carry 9.999 amperes. The shunt will leave
0.001 ampere of the full 10 amperes for the meter's coil. No strain
on figuring out the shunt resistance. The current is 9.999 amperes,
and since the shunt is in parallel with the coil, the shunts voltage
is the same as the coil's voltage 1 volt. Ohm's law again 
R = E/I = 1/9.999 = 0.100 Ω
The shunt's resistance is 0.1 ohm. To make the meter an OHMMETER
with a 0infinite ohm range, connect a resistance and a battery in series
so that full deflection is produced with no resistance across the terminals.
You would use a 1.5 volt flashlight battery. The current must be 0.001
ampere and the voltage is 1.5 volts. Ohm's law again 
R = E/I = 1.5/0.001 = 1,500 Ω
You already have 1,000 ohms in the
meter's coil  so just add 500 ohms in series with the coil to get the
1,500 ohms total. Usually this 500 ohms (or at least a part of it) is
in the form of a rheostat. By using a rheostat instead of a fixed resistance,
you can adjust the meter to a zero reading when the battery gets weak.
That's THREE meters  out of ONE. And all made by proper resistance
and battery connections! You'll see commercial jobs built like this.
They're smooth jobsall the resistances and batteries are inside the
meter case. And all you have to do is turn a selector switch to the
kind of meter you want. Then read the values off the correct scale.
QUESTION TWO
What are you going to do when you have to test the voltage drop
across a low current circuit? Say the voltage drop across a radio current
limiting resistor. The resistance is 100,000 ohms. The current is 1
milliampere so the voltage is 
E = IR = 0.001 x 100,000 = 100 volts
If you used the "1,000
ohms per volt" meter just studied, this is what would happen. You would
add 99,000 ohms to the meter to give it a 0100 volt. range. Then you'd
connect it across the resistor to measure the voltage. Now you have
two parallel paths. Each has 100,000 ohms resistance. One is the meter,
the other is the current limiting resistor. And you only have one milliampere
of current for both. The one milliampere splits up  half to each path.
And THE METER READS ONLY 50 volts, because it's only getting onehalf
milliampere. In short  you have a reading that's far from right  only
onehalf of what it should be.
THE ANSWER
There are two answers. The first is  use a more sensitive meter.
Use a meter with "20,000 ohms per volt." That means much finer and more
expensive winding on the coila much better meter. Then the meter, instead
of needing 1 milliampere for full scale deflection, would need only
0.05 milliampere for full scale deflection. ALWAYS USE A METER WITH
A SENSITIVITY HIGH ENOUGH FOR THE JOB.
The second answer is
 use a vacuum tube meter. These vacuum tube meters are tops! They only
draw millionths of an ampere from the tested circuit. And they have
internal resistances and batteries for connecting as a voltmeter, a
milliammeter, or an ohmmeter. All you have to do is adjust a selector
switch for the kind of meter you want. Then adjust another selector
switch for the range.
And right here is one of the best things
about a vacuum tube meter  IT'S PRACTICALLY IMPOSSIBLE TO BURN 'EM
UP. They're designed so that you can't overload them more than 20 percent
which won't hurt the meter. Each meter has an instruction book for its
particular connections. Reading this book will give you the lowdown
on the instrument.
Another good thing about the vacuum tube meter
 it has no frequency error up to about 20,000 cycles. You'll find that
ordinary 60 cycle meters start to cut up and give you foul readings
on anything over 80 or 85 cycles.
QUESTION THREE
How can you use a dc D'Arsonval meter on a.c.?
This sounds
like a good ideabecause the D'Arsonval movement is sensitive and it
reacts much faster than the heateffect ac meters.
THE ANSWER
Rectify the a.c. so it can be fed as d.c. into the meter.
You already know about one kind of rectifier  the commutator. But
a different kind of rectifier is used for meters. It's the COPPER OXIDE
RECTIFIER. Here is the way it works. Two disks are bolted together 
one is copper, coated with copper oxide, and the other is lead. The
unit looks like two washers about 11/2 inches in diameter, bolted together.
This unit is inserted in the ac line to the meter  it becomes part
of the circuit. When a.c. flows, ONLY THE CURRENT FROM COPPER TO COPPER
OXIDE IS PASSED. The current in the opposite direction, from oxide to
copper, is stopped  eliminated. Thus the rectifier passes all the current
in one direction, but it stops all the current in the opposite direction.
It works like the check valve in a water intake pipe.
The meter
gets current in only one direction. So its torque is in only one direction.
However, the d.c. fed to the meter is PULSATING  the meter must have
a SPECIAL SCALE. That means that you can't just use ANY dc meter with
a rectifier. But you'll find that many of your ac meters are actually
dc jobs with copper oxide rectifiers and special scales.
The
SELENIUM OXIDE RECTIFIER is another type of rectifier. You'll find that
it works the same way as the copper oxide type. Construction and rectifying
effects are the same for both rectifiers.
QUESTION FOUR
How can you get an actual picture of the ac wave form? You'll need
that picture when you're paralleling alternators, when you're testing
and tuning radio circuits, and when you're testing a.c. circuits for
power.
THE ANSWER
An oscilloscope is the answer. This instrument uses a vacuum tube
and a fluorescent screen to reproduce the wave form of a.c. By proper
connections, the oscilloscope screen will give you the curves of voltage
and current for any circuit. The curves tell you the maximum voltage,
or any instantaneous value. And by connecting TWO circuits you can get
the phasing between TWO voltages or between a voltage and its current.
You'll learn a lot more about these special meters as you get
into the books for your own rating. But now, at least, you know something
about what they can do. Don't try to redesign 'em  keep out of their
innards. These special jobs cost plenty, and you'll really be SNAFU
if you foul one up!
READING METERS
Meters are tops in sensitivity and accuracy  but all the accuracy
of a good meter can be wasted by reading it wrong. There is only one
way to read a meter right. And that's by facing the scale squarely.
If your eyes are off to one side, you see the needle at an angle to
the scale. The needle seems to be where it isn't!
Figure 203
shows incorrect meter reading.
Imagine your eyes at an angle
above these meters. First, far to the right, then only a little to the
right, and finally to the left of the needle. In each position an error
is introduced  as much as 10 volts. There is no sense in using a good
meter if CARELESS READING gives an INACCURATE MEASUREMENT.
Figure 203.  Incorrect meter reading.
Here is another way to foul up a good meter  stand it on its
head! Every meter is built to be used in ONE position  either upright,
flat, or mounted in a switchboard. The meter's indicator needle is balanced
for its correct position. If you use it in an incorrect position, the
needle's weight will pull it off the correct reading. Here's a good
tip  an UNCONNECTED METER will read ZERO when it's in the correct POSITION
for reading.
PROFICIENCY
SAVER
FINALLY, here is a repeat  but it's worth it both to YOU and to
the NAVY. CONNECT YOUR METERS PROPERLY! USE METERS OF THE CORRECT RANGE!
Know how to connect properly ALL the meters in a circuit. Keep your
connections in mind  it may save a 4.0 for you!
Chapter 18 Quiz
(click
here)