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 Rx if the bridge is balanced

R1 Rx = R2 R3

We can calculate Vth and rth by the following:

* First Vth : Using voltage Kirchhoff law Vth =V1 − V2 = I1 R1 − I2 R2

=

seconded rth

Calculate the current flowing through Galvanometer in the figure bellow: