and the resulting equation is rearranged, giving: Due to and being proportional from Kirchhoff's First Law, cancels out of the above equation. The desired value of is now known to be given as: On the other hand, if the resistance of the galvanometer is high enough that is negligible, it is possible to compute from the three other resistor values and the supply voltage (), 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 the other. The equations for this are: where is the voltage of node D relative to node B. Significance 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 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 by Alan Blumlein in British Patent no. 323,037, 1928. Modifications of the fundamental bridge The Wheatstone bridge 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: Carey Foster bridge, for measuring small resistances Kelvin bridge, for measuring small four-terminal resistances Maxwell bridge, and Wien bridge for measuring reactive components Anderson's bridge, for measuring the self-inductance of the circuit, an advanced form of Maxwell’s bridge See also Diode bridge, product mixer – diode bridges Phantom circuit – a circuit using a balanced bridge Post office box (electricity) Potentiometer (measuring instrument) Potential divider Ohmmeter Resistance thermometer Strain gauge References External links DC Metering Circuits chapter from Lessons In Electric Circuits Vol 1 DC free ebook and Lessons In Electric Circuits series. Test Set I-49 Electrical meters Bridge circuits Measuring instruments English inventions Impedance measurements pl:Mostek (elektronika)#Mostek Wheatstone'a