# Direct Current Bridges.  A Wheatstone bridge can be used to measure resistance by comparing unknown resistor against precision resistors of known value,

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Direct Current Bridges

 A Wheatstone bridge can be used to measure resistance by comparing unknown resistor against precision resistors of known value,

 When the voltage between A and B is equal zero.  Then the voltage for R1 is equal to the voltage for R2 I1 R1 = I 2 R 2 (1) I3 R3 = I4 R4 (2)

 I Galvanometer is equal zero then I1 = I3, I2= I4 (3) Divide (1)/(I2 2) or

 Calculate the value of R x if the bridge is balanced

 R1 Rx = R2 R3

 We can calculate V th and r th by the following: * First V th : Using voltage Kirchhoff law Vth =V1 − V2 = I1 R1 − I2 R2 =

 seconded r th

 Calculate the current flowing through Galvanometer in the figure bellow:

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