WHEAT-STONE BRIDGE
 
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.
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
 
 
RELATION AMONG RESISTANCES IN BALANCED CONDITION
 
R1/R2 = R3/R4
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
               I1R2 = I2R4------------(II)
Dividing equation (i) by equation (ii)
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
 
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