Made by Fort Wayne Electric Works. Late 19th century.
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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0-400 Amp, 1901 |
Figure 2: Weston Ammeter
Made by Weston Electrical Instrument Company, Newark, NJ, U.S.A. Beginning of 20th century (two samples). |
 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 6: Weston
A.C. Milliammeter, Model 45
Weston Electrical Instrument Corp., Newark, N.J., 1926. Additional pictures of this instrument showing its details. |
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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.
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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.
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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.
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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 |
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.
