pods, cell phones, laptops, and coffeemakers; these are all items you use everyday that rely on circuits to power them. But do the batteries in your graphing calculator use the same amount of current as the milk-foamer in your espresso machine? I think not! Different circuits require different amounts of potential difference to maintain the flow of current. This potential difference is measured by a device known as a voltmeter. Before getting into the specifics about the voltmeter itself, it is important to realize what it actually measures. Electric potential can be defined as the electric potential energy per unit of charge. Since electric potential energy is measured in Joules and charge is measured in Coulombs, it can be said that 1 Volt= 1 Joule/Coulomb.
So now we can look more closely at what a voltmeter is. The original principals behind voltmeters were established by a Danish physicist named Hans Christian Oersted (1777-1851) in 1820, when he discovered that an electric current in a wire produced a magnetic field around it.The first ammeter, which is actually just an extremely sensitive and nonresistent voltmeter, was used by physicist Andre Ampere as early as 1820 to measure current. But almost all types of voltmeters are founded off of the models in which the pointers were attached to moving coils, which were developed by French physicist Jacques-Arsene d'Arsonval(1851-1940) in 1882. Since then, their measuring capacity has increased, and some modern day models can measure up to 20,000 volts.
A voltmeter is a galvanometer that has been modified to measure the potential difference between two points in a circuit, series or parallel. It is often represented in schematic circuits as You may be wondering, what is a galvanometer? First created by French physicist Jacques-Arsene d'Arsonval(1851-1940) in 1882, a galvanometer is simply a fancy-looking word for a device created by putting coiled wire into a permanent magnetic field and attaching it to a spring and calibrated dial. In layman's terms, it's that box with the red needle that functions as a voltmeter. Let's have a look at the basic galvanometer.
A galvanometer can also be modified to become an ammeter which is used to measure larger currents. But when using such a device to measure voltage, the galvanometer turned voltmeter is connected in parallel with the area it is measuring from, like so:
Because the voltmeter must be attached to the circuit in parallel, it needs to be constructed in such a way that it has very high resistance. In a circuit, current will always choose the "path of least resistance". When measuring the potential difference in a given part of a circuit, it is important to change that part as little as possible while the measurements are being taken. If a voltmeter had equal or lesser resistance than the resistor itself, the same or even more current might choose to travel through the voltmeter instead of the resistor, altering the results. In order to keep the readings as accurate as possible, large resistors are connected in series with the coiled wire inside the voltmeter. On the display, most voltmeters give graduated readings in either volts, millivolts(.001 volt), or kilovolts(1,000 volts). The most commonly used type is the DC, or Direct Current voltmeter. Since too much current traveling through the voltmeter would damage its setup, a shunt is constructed so that only a previously established percent of the current can be read. The voltmeter can then use this information to give a reading on all of the current.
Application of Voltage ReadingsEdit
Measurements of voltage are applicable to many situations, but perhaps the most prevalent is it's part in Ohm's Law, which states: V=IR when V=voltage[V], I=current[A], and R=resistance[Ohms]
Using this relationship, voltage readings can be used along with another variable to determine another unknown, or the knowledge of the current and resistance can produce the voltage(or potential difference) without the use of a voltmeter.
In this situation, one can use the equation V=IR to solve for V. It is important to realize that in this kind of circuit, resistors in series may have different values for voltage because in series circuits, Vtot=V1+V2+V3+... The total voltage remains the same but each individual resistor may produce a different value of V.
So for example, if the resistor being measured above by the voltmeter was a 6-Ω resistor and the current was .5 amperes, how would one go about finding the potential difference? V=IR I=.5A R=6-Ω
Using the established values of current and resistance allows you to determine that the potential difference is 3 volts.
This situation is very different than that of a series circuit. When the voltmeter is placed here, the readings should remain constant because in a parallel circuit, Vtot=V1=V2=V3=...
Wolfram Research http://scienceworld.wolfram.com/biography/Kelvin.html
Barron's Review Course Series: Physics- The Physical Setting by Miriam A. Lazar Barron's Educational Series, Inc. 2004
Physics: Principles and Problems by Paul W. Zitzewitz The McGraw-Hill Companies, Inc. 1999
Jacques-Arsene d'Arsonval http://chem.ch.huji.ac.il/~eugeniik/history/arsonval.html
"Galvanometers" The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2006, Columbia University Press. All rights reserved.
Voltmeter picture http://www.sciencekit.com/category.asp_Q_c_E_440361