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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.  Hot wire meter.
Figure 188.  D'Arsonval type meter.
Figure 189.  D'Arsonval action.
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.
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.
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.
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.
Figure 190.  Movable iron action.
Figure 191.  Dynamometer action.
Figure 192.  Thermocouple meter.
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.
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.
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.
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
Figure 193.  Ammeter with internal shunt.
Figure 194.  External shunts.
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.
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.
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.  Multiple shunt ammeter.
Figure 196.  Voltmeter connection.
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.
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
CONSTANT  by making it a fixed part of the meter. Such a meter is a VOLTMETER.
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.
TThe 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.
Figure 197.  Voltmeter with series resistance.
Figure 198.  Multiple scale voltmeter.
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.
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.
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.
Figure 200.  Series ohmmeter.
Figure 201.  Shunt ohmmeter.
Figure 202.  Megger 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.
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.
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.
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.
Figure 203.  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.
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)
Posted April 30, 2024 (updated from original post
on March 15, 2014)
