History and Types of Ammeters
– Relation between electric current, magnetic fields, and physical forces observed by Hans Christian Ørsted in 1820
– Tangent galvanometer used to measure currents using this effect
– Multipliers used to increase sensitivity of the instrument
– Word ‘rheoscope’ coined by Sir Charles Wheatstone in 1840 to describe a detector of electrical currents
– Early ammeters relied on Earth’s magnetic field for operation
– Panel meters designed to be mounted on control panels
– Flat, horizontal, or vertical type often called an ‘edgewise meter’
– Milliammeters and microammeters used to measure smaller currents
– Rheostat, a device used to adjust current in a circuit, similar to a makeup
– Makeup is a historical term for a variable resistance
Moving-coil Ammeters
– D’Arsonval galvanometer is a moving coil ammeter
– Magnetic deflection causes the coil to move when current passes through it
– Edward Weston developed the modern form of this instrument
– Linear scales used for basic meter movements
– Moving coil meters only usable directly for DC, not AC
Moving magnet Ammeters
– Moving magnet ammeters operate on the same principle as moving coil meters
– Coil mounted in the meter case, and a permanent magnet moves the needle
– Can carry larger currents than moving coil instruments
– Thicker wire used for the coil, and the current does not have to be carried by hairsprings
– Some moving magnet ammeters use a fixed permanent magnet for the restoring force
Electrodynamic Ammeters
– Electrodynamic ammeters use an electromagnet instead of a permanent magnet
– Can respond to both alternating and direct current
– Indicates true RMS for AC
– Used as a wattmeter for alternative applications
– Similar principle to moving coil meters, but with an electromagnet instead of a permanent magnet
Ammeter Application and Design
– Ammeters must be connected in series with the circuit to be measured.
– For small currents, an ammeter may pass the whole circuit current.
– For larger direct currents, a shunt resistor carries most of the circuit current.
– For alternating current circuits, a current transformer may be used to provide a small current to drive an instrument.
– The use of a shunt or current transformer allows convenient location of the indicating meter.
– Ammeters must not be connected directly across a voltage source.
– Ammeters are designed for a low voltage drop across their terminals.
– Shunts are used to measure larger currents.
– Shunts have resistances in the milliohm range.
– Ayrton shunt or universal shunt can be used for multi-range ammeters.
– Zero-center ammeters are used for measuring current with both polarities.
– They are commonly placed in series with a battery.
– Charging the battery deflects the needle to one side of the scale.
– Discharging the battery deflects the needle to the other side.
– Zero-center ammeters for testing high currents use a pivoted bar magnet.
Additional Information:
– See also: Clamp meter, Class of accuracy in electrical measurements, Electric circuit, Electrical measurements, Multimeter
– References: Geddes, L.A. (Feb–Mar 1996). Looking back: How measuring electric current has improved through the ages. Bowers, Brian (2001) [1975]. Sir Charles Wheatstone FRS: 1802–1875. Spitzer, Frank; Howarth, Barry (1972). Principles of Modern Instrumentation. Fragebogen aus der Personenmappe Friedrich Drexler (1858–1945) Questionnaire from the Friedrich Drexler personal folder (in German). Permanent magnet radiation hardness tests at the 100MeV Linac: Preliminary results (PDF). Source: https://en.wikipedia.org/wiki/Ammeter
An ammeter (abbreviation of Ampere meter) is an instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. For direct measurement, the ammeter is connected in series with the circuit in which the current is to be measured. An ammeter usually has low resistance so that it does not cause a significant voltage drop in the circuit being measured.
Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. Early ammeters were laboratory instruments that relied on the Earth's magnetic field for operation. By the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems. It is generally represented by letter 'A' in a circuit.
