A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit , one leg of which includes the unknown component. The primary benefit of the circuit is its ability to provide extremely accurate measurements (in contrast with something like a simple voltage divider ). Its operation is similar to the original potentiometer .
15-468: The Wheatstone bridge was invented by Samuel Hunter Christie (sometimes spelled "Christy") in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. One of the Wheatstone bridge's initial uses was for soil analysis and comparison. In the figure, R x is the fixed, yet unknown, resistance to be measured. R 1 , R 2 , and R 3 are resistors of known resistance and
30-400: A combustible gas detector , operates on the principle of resistance proportional to heat—a wire is heated, and a sample of the gas is introduced to the hot wire. Combustible gases burn in the presence of the hot wire, thus increasing the resistance and disturbing a Wheatstone bridge , which gives the reading. A flashback arrestor is installed in the device to avoid the explosimeter igniting
45-417: A voltage level off a meter than to adjust a resistance to zero the voltage. At the point of balance, both the voltage and the current between the two midpoints (B and D) are zero. Therefore, I 1 = I 2 , I 3 = I x , V D = V B . Because of V D = V B , then V DC = V BC and V AD = V AB . Dividing the last two equations by members and using
60-607: Is his name, rather than Christie's, that is now associated with the device. Christie taught mathematics at the Royal Military Academy, Woolwich , from 1838 until his retirement in 1854. He died at Twickenham , on 24 January 1865. A portrait photograph of Christie in 1865 by Ernest Edwards is held by the National Portrait Gallery . He had ten children (five with each wife), of whom eight survived him. His eldest son with his second wife
75-428: Is the fundamental bridge, but there are other modifications that can be made to measure various kinds of resistances when the fundamental Wheatstone bridge is not suitable. Some of the modifications are: Samuel Hunter Christie Samuel Hunter Christie FRS (22 March 1784 – 24 January 1865) was a British physicist and mathematician . He studied mathematics at Trinity College, Cambridge , where he won
90-617: The Smith's Prize and was second wrangler . He was particularly interested in magnetism , studying the Earth's magnetic field and designing improvements to the magnetic compass . Some of his magnetic research was done in collaboration with Peter Barlow . He became a Fellow of the Royal Society in 1826, delivered their Bakerian Lecture in 1833 and served as their Secretary from 1837 to 1853. In 1833 he published his 'diamond' method,
105-574: The above currents equalities, then First, Kirchhoff's first law is used to find the currents in junctions B and D: Then, Kirchhoff's second law is used for finding the voltage in the loops ABDA and BCDB: When the bridge is balanced, then I G = 0 , so the second set of equations can be rewritten as: Then, equation (1) is divided by equation (2) and the resulting equation is rearranged, giving: Due to I 3 = I x and I 1 = I 2 being proportional from Kirchhoff's First Law, I 3 I 2 / I 1 I x cancels out of
120-452: The above equation. The desired value of R x is now known to be given as: On the other hand, if the resistance of the galvanometer is high enough that I G is negligible, it is possible to compute R x from the three other resistor values and the supply voltage ( V S ), or the supply voltage from all four resistor values. To do so, one has to work out the voltage from each potential divider and subtract one from
135-412: The balance and are readily detected. Alternatively, if R 1 , R 2 , and R 3 are known, but R 2 is not adjustable, the voltage difference across or current flow through the meter can be used to calculate the value of R x , using Kirchhoff's circuit laws . This setup is frequently used in strain gauge and resistance thermometer measurements, as it is usually faster to read
150-488: The forerunner of the Wheatstone bridge , in a paper on the magnetic and electrical properties of metals , as a method for comparing the resistances of wires of different thicknesses. However, the method went unrecognised until 1843, when Charles Wheatstone proposed it, in another paper for the Royal Society, for measuring resistance in electrical circuits. Although Wheatstone presented it as Christie's invention, it
165-434: The other. The equations for this are: where V G is the voltage of node D relative to node B. The Wheatstone bridge illustrates the concept of a difference measurement, which can be extremely accurate. Variations on the Wheatstone bridge can be used to measure capacitance , inductance , impedance and other quantities, such as the amount of combustible gases in a sample, with an explosimeter . The Kelvin bridge
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#1732772150094180-410: The resistance of R 2 is adjustable. The resistance R 2 is adjusted until the bridge is "balanced" and no current flows through the galvanometer V g . At this point, the potential difference between the two midpoints (B and D) will be zero. Therefore the ratio of the two resistances in the known leg ( R 2 / R 1 ) is equal to the ratio of the two resistances in
195-453: The unknown leg ( R x / R 3 ) . If the bridge is unbalanced, the direction of the current indicates whether R 2 is too high or too low. At the point of balance, Detecting zero current with a galvanometer can be done to extremely high precision. Therefore, if R 1 , R 2 , and R 3 are known to high precision, then R x can be measured to high precision. Very small changes in R x disrupt
210-639: Was specially adapted from the Wheatstone bridge for measuring very low resistances. In many cases, the significance of measuring the unknown resistance is related to measuring the impact of some physical phenomenon (such as force, temperature, pressure, etc.) which thereby allows the use of Wheatstone bridge in measuring those elements indirectly. The concept was extended to alternating current measurements by James Clerk Maxwell in 1865 and further improved as Blumlein bridge by Alan Blumlein in British Patent no. 323,037, 1928. The Wheatstone bridge
225-415: Was the astronomer William Henry Mahoney Christie (1845–1922). Samuel Christie is the son of one James Christie Explosimeter An explosimeter is a gas detector which is used to measure the amount of combustible gases present in a sample. When a percentage of the lower explosive limit (LEL) of an atmosphere is exceeded, an alarm signal on the instrument is activated. The device, also called
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