Ammeter
Amp Meter - Ammeter Use and History
Most electricians use a clamp on type amp meter where the circuit conductor,
one at a time, is encircled by the jaws of the meter. A reading is them
displayed either with an analog needle or a digital readout.
When the current flow is large or small, there are attachments which can be
used on the conductor to either amplify or reduce thee induced current so a
reading can be obtained.
A simple way to make a low value reading with a small conductor is to pass
the circuit conductor through the jaws of the meter more than just the once
which is typical. Then the actual current flow is the reading divided by the
number of turns or passes the circuit conductor makes through the jaws.
There are also amp meters which allow for a reading by just placing the
circuit conductor in the notch at the end of the meter probe. This is a much
quicker way to take a reading when smaller conductors are to be measured.
Here are links to meters which are for sale online. You certainly do not
have to buy any of these items. But by clicking on these links, you will be able
to read about teach type. Specifications and price are included for your
information.
Another great article about ammeters. SOURCE
From Wikipedia SOURCE
ammeter
Wire carrying current to be measured
Spring providing restoring force
An ammeter is a measuring
instrument used to measure the flow of electric
current in a circuit.
Electric currents are measured in amperes,
hence the name. The word "ammeter" is commonly misspelled or
mispronounced as "ampmeter" by some.
The earliest design is the D'Arsonval galvanometer.
It uses magnetic
deflection, where current passing through a coil causes the coil to move in a magnetic
field. The voltage
drop across the coil is kept to a minimum to minimize resistance in any
circuit into which the meter is inserted.
A galvanometer can burn out if its tiny, delicate coil overheats. To
measure larger currents, a resistor
called a shunt
is placed in parallel
with the coil. Most of the current flows through the shunt, and only a small
fraction flows through the meter. With this solution, arbitrarily large
currents can be measured with a single meter. Traditionally, the meter used
with a shunt reaches full-scale deflection when a voltage of 50mV is placed
across its coil, so shunts are typically designed to produce a voltage drop of
50mV when carrying their full rated current.
Cruder ammeters simply use a moving piece of iron
(or a magnet) that is acted-upon by the electromagnetic force of fixed coil of
(usually heavy gauge
wire. At very high current ratings, such an ammeter can actually just clamp on
to an existing conductor (where the conductor acts as a single-turn coil);
this later example is sometimes used in automotive applications where it
clamps-on to the main battery
wire to show the charging and discharging of the battery.
More modern ammeter designs use an analog
to digital converter to measure the voltage across the shunt resistor. The
ADC is read by a microcomputer that performs the calculations to display the
current through the resistor.
One problem with the use of an ammeter is the need for the meter to be
inserted into the circuit and become part of it. In AC circuits, an inductive
coupling adapter converts the magnetic field around a conductor
into a small AC current that can be easily read by a meter. See clamp
meter. In a similar way, accurate DC non-contact ammeters have been
constructed using Hall
effect magnetic field sensors.
See also
This entry is from Wikipedia, the leading user-contributed encyclopedia. It
may not have been reviewed by professional editors (see full
disclaimer)
Click HERE
for more references from Google.
AMMETERS
Measurement of current being supplied to or from a component is
measured by an ammeter.
AMMETER SAFETY
PRECAUTIONS When you use an
ammeter, certain precautions must be observed to prevent injury to yourself or
others and to prevent damage to the ammeter or the
equipment on which you are working. The following list
contains the MINIMUM precautions to observe when using an ammeter
· Ammeters
must always be connected in series with the circuit under test. ·
Always start with the
highest range of an ammeter. ·
Deenergize and discharge
the circuit completely before you connect or disconnect the ammeter.
· In
dc ammeters, observe the proper circuit polarity to prevent the meter from being
damaged. ·
Never use a dc ammeter to
measure ac. ·
Observe the general safety
precautions of electrical and electronic devices.
Ammeters
Ammeter is an instrument for measuring either direct or
alternating electric current, in amperes. An ammeter can measure a wide range of
current values because at high values only a small portion of the current is
directed through the meter mechanism; a shunt in parallel with the meter carries
the major portion.
Ammeters vary in their operating principles and accuracies.
The D'Arsonval-movement ammeter measures direct current with accuracies
of from 0.1 to 2.0 percent. The electrodynamic ammeter uses a moving coil
rotating in the field produced by a fixed coil. It measures direct and
alternating current with accuracies of from 0.1 to 0.25 percent. In the thermal
ammeter, used primarily to measure alternating current with accuracies of from
0.5 to 3 percent, the measured current heats a thermoconverter (thermocouple);
the small voltage thus generated is used to power a millivoltmeter. Digital
ammeters, with no moving parts, use a circuit such as the dual slope integrator
to convert a measured analogue (continuous) current to its digital equivalent.
Many digital ammeters have accuracies better than 0.1 percent.
Pictures below show some examples of ammeters used in the end of 19th
- first half of 20th centuries:
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Figure 1: Ammeter "WOOD"AN-Meter
Made by Fort Wayne Electric Works. Late 19th
century.
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 0-50 Amp
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0-400 Amp, 1901
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Figure 2: Weston Ammeter
Made by Weston Electrical Instrument Company, Newark, NJ, U.S.A. Beginning
of 20th century (two samples).
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 Figure
3: Westinghouse Ammeter, Style No. 35120
Made Westinghouse Electrical Instruments,
U.S.A.
20th century, in
a wooden case with lid.
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Figure 4: Ammmeter, Model PY-5
Made by Westinghouse Electrical Instruments,
U.S.A.
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Figure 5: Ammeter
Unidentified model. First half of 20th
century.
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Figure 7: Weston A.C. Ammeter, Model 155
Weston Electrical Instrument Corp., Newark, N.J. ca. 1929.
(see acknowledgment)
Instruments such as this were used by trained
technical / scientific staff for precision laboratory measurements of
electrical current, and as secondary standards for calibrating other
meters. The mirrored scale makes it easier to read the instrument to a
given accuracy by enabling the user to eleiminate parallax errors. This
particular instrument was specified to be accurate to ±0.5%. The
instrument in housed in a 71/8 x 6 3/4 x 3 1/4" walnut case with a
leather handle. All hardware is nickel-plated brass. The front panel has
a window cut into it to view the meter face. The meter has a mirrored
scale. The instrument has four rubber feet on the bottom for storage and
four more feet on the back for use during operation. Beneath the window
is a brass escutcheon labeled "0 CORRECTION" giving access to
the zero correction screw.
Company
description of this instrument.
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Figure 8: Weston
A.C. Ammeter, Model 433
Weston Electrical Instrument Corp., Newark, N.J. ca. 1960.
(see acknowledgment)
The meter has a molded black bakelite case 3
1/4" x 5" x 5 1/4". There is a black leather carrying
strap. The meter has a mirrored scale and knife-edge pointer. This model
has a double range (0-150 & 0-300 mA) chosen by binding posts on the
top of the meter. Model 433 meters have a rated accuracy of 0.75% of
full scale when used in the horizontal position.
Company
description of this instrument.
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Figure 9: Weston D.C. Milliammeter, Model
931
Weston Electrical Instrument Corp., Newark, N.J. ca. 1960.
(see acknowledgment)
For laboratory, production and general testing of DC
currents. These are top-of-the-line, hand-calibrated, meters for regular
use, and may function as secondary standards for routine applications.
Note the mirrored scales and knife edged needles designed to eliminate
parallax reading errors. For highest accuracy they would be used
horizontally, and thus have feet on the back as well on the bottom of
the case. The meter has a molded black bakelite case 3 1/4" x
5" x 5 1/4". There is a black leather carrying strap. The
meter has a mirrored scale and knife-edge pointer. This model has a
triple range (0-0.15; 0-1.5, & 0-15 mA) chosen by binding posts on
the top of the meter. Model 931 meters have a rated accuracy of 0.5% of
full scale when used in the horizontal position.
Company
description of this instrument.
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Figure 10: DC Micro Ammeter, Model 622
Weston Electrical Instrument Corp., Newark, N.J.
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Figure 11: Recording AC Ammeter, Model R53
Made by AMP Probe
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 Acknowledgment: I would
like to thank Professor Robert A. Paselk, Scientific Instrument Museum, Humboldt
State University, for the kind permission to use pictures (#7,8,9) and the
related texts adopted from his web
site.
Here is more on the history of the ammeter amp meter SOURCE
Ammeters
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The analogue ammeter is a basic meter movement with
a shunt placed in parallel across it. The movement goes full scale
with only a milliampere or so of current through it, and the shunt
passes the extra current around the meter movement. The fraction of
the overall current which passes through the movement is a function
of the resistance of the movement and the much lower resistance of
the shunt. The basic mechanism is that developed by Edward Weston in
the last few years of the 19th century.
That being said, almost all 20th century ammeters
look alike. Here are three meters which have unique cases. At the
right is a Current Indicator made by Whitney Electrical Instrument
Company of New Hampshire; the earliest patent date is May 16, 1893.
This instrument is in the Kenyon College collection.
The two ammeters below are in private collections
and date from the early years of the 20th century..
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Richard Zitto
Thomas Greenslade
And then there is the Volt-Ammeter. This probably has
an internal shunt for use as an ammeter and an internal multiplier for
use as a voltmeter.
The two instruments below have an unusual upright configuration on a
horizontal base. The one at the left, by the Ziegler Electric Co. of
Boston is in the museum room at the Physics Department of Washington and
Lee University; the example by Knott on the right appeared in a eBay
auction.
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The small meter at the left is in author's
collection. He bought it at a yard sale near Boston ca.
1985 for only a dollar or two.
The 1916 catalogue of the L. E. Knott
Apparatus Co., Boston, describes this as a "Horizontal
Galvanometer, D'Arsonval movement, jeweled bearing,
0 center. This is a commercial type of instrument in
horizontal or laboratory form. The range is such as to
make it of the greatest value in general laboratory
practice. Quick action, quick reading, adapted to a wide
range of experiments, such as Induction, Polarization of
Cells, Measurements of the Wheatstone Bridge, where an
accuracy equal to ½ millimeter on the bridge is
considered sufficient. Owing to the form of the pole
pieces the scale is proportional to the amount of
current going through, thus giving the instrument a
range of usefulness far grater than its sensibility
would indicate .....................................$7.50"
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This little galvanometer in the
Greenslade Collection is only 10 cm high. It was
made by the "Thompson-Levering Co., Makers of
Scientific Instruments, Philadelphia, Pa." and
is marked on the top, in ink, "2 µA/div",
with 10 divisions on either side of zero.
A very similar instrument was sold
by Leeds and Northrup of Philadelphia. In their 1907
catalogue this is listed as a portable d'Arsonval
galvanometer and priced at $20.00. The
sensitivity is the same as the Thompson-Levering
instrument.
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The range of a basic ammeter movement may be extended to
lower values by adding low-resistance shunts across it.
These shunts by Weston Electric probably date from the
nineteen twenties. The current leads are the heavy
connections at the top, and the ammeter leads are the black
ones at the bottom. These shunts were probably used with the
ammeter below.
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The large Weston laboratory
standard ammeter at the left was probably used
with the shunts shown above. This meter includes
a thermometer for temperature corrections, a
mirrored scale and a bubble level.
It is retired from the
laboratories at the University of Texas in
Austin.
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This second example of a
"Weston Direct Reading Laboratory
Standard Milli-Volt Meter" is at
Westminster College in western Pennsylvania.
It has an 1890 patent date,
is 40 cm square, and is serial number 621,
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This early ammeter,
dating from the very beginning of the
twentieth century, is included because
its wood has acquired a very nice patina
over the years. The case is marked
"Keystone Electrical Instrument
Co., Philadelphia, Pa, DC
Milliammeter" and the writing on
the scale says "Made for the
Central Scientific Co., Chicago,
Ill."
It is in the collection
of historical instruments at Kenyon
College in Gambier, Ohio.
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The meter at
the left has a basic galvanometer
movement. On the front panel are
shunts to enable the meter to
measure currents up to 25 A and
multipliers to allow potentials up
to 125 V to be measured.
The 1916 catalogue
of the C.H. Stoelting Co. of Chicago
notes that "this instrument is
designed for the lecture table, but
is equally well adapted for
students' use, having all its
mechanism, internal and external,
exposed to view. The movement
employed in it is of the well known
Weston Standard patented movable
coil type."
A damping resistor
must be permanently connected, as
the pointer will come to rest
without oscillations.
This instrument is
in the Greenslade Collection.
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This massive
ammeter, with a thick,
cast-brass front, has a
full-scale reading of 44
Amperes. The lower portion of
the scale is non-linear,
suggesting that it was used to
measure alternating current. On
the front is cast "Fort
Wayne [Indiana] Electric
Works", "'Wood'
Am-Meter" and "Pat'd
Oct 8, 1889; May 22,
1894".
It is in the
Greenslade Collection.
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The handsome
ammeter at the left was made
by the American Instrument
Company of Newark, New Jersey.
It bears patent dates from
1906 and 1907, and is in the
collection of Westminster
College in western
Pennsylvania.
Its twin is
on the voltmeter
page.
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The device
at the left, made by the
Brush Electric Company of
Cleveland, Ohio, is a
completely different
approach to the problem of
measuring electric current.
Here, the
current passes through a
pair of coils. The magnetic
field, and hence the
magnetic force of attraction
for the pair of soft-iron
rods, is proportional to the
magnitude of the current.
The
apparatus is at Case Western
Reserve University in
Cleveland.
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At the
left is another unusual
ammeter from Case Western
Reserve University. This
was made by the Edison
General Electric Company
of Schenectady, New York.
The curved iron
wire is drawn up into the
curved solenoid when
current passes through the
coil. The needle is
attached to the wire at
the same point as the
wire's pivot. The scale is
reasonably linear.
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