1. Direct Current Bridges

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

4.  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)

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

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

7.  R1 Rx = R2 R3

9.  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 =

10.  seconded r th

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