Balancing the Wheatstone Bridge




Goal: An understanding of how values of unknown resistances can be discovered using a variable resistance and a Wheatstone Bridge.

Setup: The setup of the experiment consists of a Wheatstone Bridge which is part of a simple circuit run by a known emf source. There is also a galvanometer as depicted in the figure. In this experiment the resistors R4; and R2 have the following fixed values
R4 = 10ohms
R2 = 20ohms
Rx is variable

Background: The Wheatstone bridge is used to determine an unknown resistance (R1 in our case) by adjusting a known resistance (Rx) so that the measured current through the galvanometer is zero. The potential across the galvanometer is hence, the same. This implies that the potential drop across resistors R2 and Rx  are equal. The potential drop across R4 is also equal to that across the resistor R1. This gives us the formula
for Rx.

                                                                                                Rx=(R2/R1)*R4.

The participant will need to calculate the value of the unknown resistance R1 by varying the variable resistance Rx. When a balanced state is reached as indicated in the applet, a point corresponding to a 0 reading on the galvanometer, the participant uses the above formula to manually calculate the value of R1 and then verify the result.