Meter bridge Post office box Potentiometer Carey Foster's bridge Callendar bridge Griffith's bridge,EXPRESSION FOR BALANCED BRIDGE

1. GALVANOMETER
Umar Jamil Qasmi
International Islamic University Pakistan

2. WHEAT-STONE BRIDGE
 Wheat stone bridge is an electrical circuit .
In wheat-stone bridge four resistance R1, R2,
R3 and R4 are connected end to end with
each other to form a closed loop. A sensitive
galvanometer "G" is connected between their
junctions as shown . The circuit is provided
with two keys ‘K1’ and ‘K2’.
Generally wheat-stone bridge is used to
determine unknown resistances.

3. BALANCED WHEAT-STONE BRIDGE
 When ‘K1’is connected, no current passes
through the galvanometer because K2 is
disconnected. When ‘K1’ and ‘K2’ are connected
and no current flows through the galvanometer,
the condition of bridge is called "Balanced
condition". It is possible only when the potential
difference between the terminals of
galvanometer is zero or potential of point ‘B’ =
potential of point ‘D’ i.e.VB – VD = 0
or
VB = VD

4. EXPRESSION FOR BALANCED BRIDGE
 FROM FIGURE: R1 & R2 are connected in
series. Reason: (only one path for the flow of
current)
R3 & R4 are connected in series.
R1 & R3 are connected in parallel. Reason: (two
paths for the flow of current)
R2 & R4 are connected in parallel.
Let current I1 flows through R1 & R2 and I2 through R3 &
R4.
When bridge is balanced,potential of point ‘B’ =
potential of point ‘D’
 Therefore, VAB = VAD but[V =
IR]
 IR1 = I2R3--------------(I)
 Similarly,VBC = VCD
or
I R = I R

5. RELATIONSHIP BY OHM’S LAW

6. APPLICATIONS OF WHEATSTONE BRIDGE
 A number of resistance measuring devices
have been devised on the principle of
wheatstone bridge.For example :
Meter bridge
Post office box
Potentiometer
Carey Foster's bridge
Callendar bridge
Griffith's bridge