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
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 air-pressure METER. Try to fill a tire without
a gage. Nine times out of ten the tire is too hard or too soft. The air-pressure
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
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 120-volt 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 10-ampere 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 current-they 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
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 heat-producing 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 one-half 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 right-hand 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 left-hand end is called a CRAMPED or COMPRESSED
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 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 V-shaped, 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 move-is 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
Figure 190. - Movable iron action.
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.
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
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 galvanometers-sensitive,
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 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 in-creased by merely changing a shunt. For example, an ammeter with
a range of 0-10 amperes has a resistance of 0.1 ohm. The range is to be increased
to 0-100 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
two-way 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 resistance-because
an easy fraction gives you a handy multiplier.
Figure 194. - External shunts.
Here is the formula for calculating shunt resistance -
Rs = Rm/(N-1)
Rs = the resistance of the shunt;
Rm = the resistance of the meter;
N = the multiplier.
Using this formula for the solution of the example above -
Rs = Rm/(N-1)
Rs = 0.1/(10-1) = 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 cur-rent 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 0-1 ampere - no shunt is used. The second scale is 0-10
amperes - the first shunt is used. And the third scale is 0-100 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.
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.
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.
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
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 0-500
volt voltmeter? From the 0-500 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
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 two-fifths of full deflection current (0.000.5
ampere). Therefore, the meter pointer deflects exactly two-fifths of full scale.
And on a scale of 0-500 volts, 200 volts would be two-fifths of the way to 500 volts.
Every voltage produces its corresponding current. And every cur-rent 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 0-500 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 0-250 volts.
If it is connected from + to 125, only one resistor is in series with the meter
coil and the range is 0-125 volts.
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
Figure 198. - Multiple scale voltmeter.
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 d-c 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 d-c circuit are voltage
and current. But for a-c power measurement, you must use a wattmeter.
Power is composed of two things-voltage 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 stationary-they 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
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
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, Rm,
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, and-the battery is shown by broken arrows. Two parallel paths. Apparently
they have no effect on each other. But Rx 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 Rx 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 Rx, 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 Rx 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
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 Rx. If Rx is high, most
of the current goes through-the meter-high deflection. If Rx is low, most of the
current goes through Rx - 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.
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 ohm-meters?" 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.
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.
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?
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 0-150 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 0-10 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 0-infinite
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
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 jobs-all
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.
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 0-100 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 one-half milliampere. In short - you have
a reading that's far from right - only one-half of what it should be.
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 coil-a 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 low-down on the
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.
How can you use a d-c D'Arsonval meter on a.c.?
This sounds like a good idea-because the D'Arsonval movement is sensitive and
it reacts much faster than the heat-effect a-c meters.
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 1-1/2 inches
in diameter, bolted together. This unit is inserted in the a-c 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 d-c meter with a rectifier. But you'll
find that many of your a-c meters are actually d-c 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.
How can you get an actual picture of the a-c 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.
An oscilloscope is the answer. This instrument uses a vacuum tube and a fluorescent
screen to re-produce 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
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 re-design 'em - keep out of their innards. These special jobs cost
plenty, and you'll really be SNAFU if you foul one up!
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.
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
Chapter 18 Quiz