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Voltmeters and Ammeters Null Measurements Voltmeters and Ammeters Circuits and Direct Currents > Voltmeters and Ammeters Free to share, print, make copies and changes. Get yours at
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Key terms galvanometer An analog measuring device, denoted by G, that measures current flow using a needle deflection caused by a magnetic field force acting upon a current-carrying wire. null measurements methods of measuring current and voltage more accurately by balancing the circuit so that no current flows through the measurement device potentiometer an instrument that measures a voltage by opposing it with a precise fraction of a known voltage, and without drawing current from the unknown source. shunt resistance a small resistance R placed in parallel with a galvanometer G to produce an ammeter; the larger the current to be measured, the smaller R must be; most of the current flowing through the meter is shunted through R to protect the galvanometer Wheatstone bridge An instrument used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Free to share, print, make copies and changes. Get yours at Circuits and Direct Currents
Voltmeter in Parallel (a) To measure the potential difference in this series circuit, the voltmeter (V) is placed in parallel with the voltage source or either of the resistors. Note that terminal voltage is measured between points a and b. It is not possible to connect the voltmeter directly across the EMF without including its internal resistance, r. (b) A digital voltmeter in use Free to share, print, make copies and changes. Get yours at OpenStax CNX. "OpenStax College, College Physics. October 25, 2012." CC BY View on Boundless.comCC BY 3.0http://cnx.org/content/m42360/latest/?collection=col11406/latestView on Boundless.com Circuits and Direct Currents
Voltmeters and Ammeters A brief introduction to voltmeters and ammeters for introductory physics students. Free to share, print, make copies and changes. Get yours at View on Boundless.com Circuits and Direct Currents
Ammeter in Series An ammeter (A) is placed in series to measure current. All of the current in this circuit flows through the meter. The ammeter would have the same reading if located between points d and e or between points f and a, as it does in the position shown. (Note that the script capital E stands for EMF, and r stands for the internal resistance of the source of potential difference. ) Free to share, print, make copies and changes. Get yours at OpenStax CNX. "OpenStax College, College Physics. October 25, 2012." CC BY View on Boundless.comCC BY 3.0http://cnx.org/content/m42360/latest/?collection=col11406/latestView on Boundless.com Circuits and Direct Currents
Voltmeter Demonstration voltmeter from a physics class Free to share, print, make copies and changes. Get yours at Wikipedia. "Voltmeters." CC BY View on Boundless.comCC BYhttp://en.wikipedia.org/wiki/VoltmetersView on Boundless.com Circuits and Direct Currents
Wheatstone Bridge The Wheatstone bridge is used to calculate unknown resistances. The variable resistance R3 is adjusted until the galvanometer reads zero with the switch closed. This simplifies the circuit, allowing Rx to be calculated based on the IR drops. Free to share, print, make copies and changes. Get yours at OpenStax CNX. "OpenStax College, College Physics. October 26, 2012." CC BY View on Boundless.comCC BY 3.0http://cnx.org/content/m42362/latest/?collection=col11406/latestView on Boundless.com Circuits and Direct Currents
Voltmeter Connected to Battery An analog voltmeter attached to a battery draws a small but nonzero current and measures a terminal voltage that differs from the EMF of the battery. (Note that the script capital E symbolizes electromotive force, or EMF. ) Since the internal resistance of the battery is not known precisely, it is not possible to calculate the EMF precisely. Free to share, print, make copies and changes. Get yours at OpenStax CNX. "OpenStax College, College Physics. October 26, 2012." CC BY View on Boundless.comCC BY 3.0http://cnx.org/content/m42362/latest/?collection=col11406/latestView on Boundless.com Circuits and Direct Currents
Potentiometer The potentiometer is a null measurement device. (a. ) A voltage source connected to a long wire resistor passes a constant current I through it. (b.) An unknown EMF (labeled script Ex) is connected as shown, and the point of contact along R is adjusted until the galvanometer reads zero. The segment of wire has a resistance Rx and script Ex=IRx, where I is unaffected by the connection, since no current flows through the galvanometer. The unknown EMF is thus proportional to the resistance of the wire segment. Free to share, print, make copies and changes. Get yours at OpenStax CNX. "OpenStax College, College Physics. October 26, 2012." CC BY View on Boundless.comCC BY 3.0http://cnx.org/content/m42362/latest/?collection=col11406/latestView on Boundless.com Circuits and Direct Currents
Attribution Boundless Learning. "Boundless." CC BY-SA BY-SA 3.0http:// Boundless Learning. "Boundless." CC BY-SA BY-SA 3.0http:// Wikipedia. "Ammeters." CC BY-SA BY-SA 3.0http://en.wikipedia.org/wiki/Ammeters OpenStax CNX. "OpenStax College, College Physics. September 18, 2013." CC BY BY Wikipedia. "Voltmeters." CC BY-SA BY-SA 3.0http://en.wikipedia.org/wiki/Voltmeters Wiktionary. "Wheatstone bridge." CC BY-SA BY-SA 3.0http://en.wiktionary.org/wiki/Wheatstone_bridge Wiktionary. "potentiometer." CC BY-SA BY-SA 3.0http://en.wiktionary.org/wiki/potentiometer Boundless Learning. "Boundless." CC BY-SA BY-SA 3.0http:// Wikipedia. "Potentiometer (measuring instrument)." CC BY-SA BY-SA Wikipedia. "Wheatstone bridge." CC BY-SA BY-SA 3.0http://en.wikipedia.org/wiki/Wheatstone_bridge OpenStax CNX. "OpenStax College, College Physics. September 18, 2013." CC BY BY Free to share, print, make copies and changes. Get yours at Circuits and Direct Currents