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EXPERIMENT 4：
W H E AT S TO N B R I D G E
NAME
STUDENT ID
Scores
ROOM：Fundamental teaching Building
DATE:
1106
TABLE NUMBER:
PURPOSE: To experimentally verify the operation of a typical bridge circuit and to
measure the resistance of an unknown resistor.
THEORY:
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical
resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown
component. It was invented by Samuel Hunter Christie in 1833 and improved and
popularized by Sir Charles Wheatstone in 1843. One of the Wheatstone bridge's initial uses
was for the purpose of soils analysis and comparison.
In its simplest form, the bridge circuit consists of four resistors, a battery and a sensitive
galvanometer. The values of R1, R2 and R3 are all known, and Rx is the unknown
resistance.
Figure.1
When the bridge is balanced by adjusting the standard resistance R2, until the
galvanometer shows no deflection. (indicating no current flow through the galvanometer
branch) Then points B and D in the circuit are at the same potential; current I1 flows
through both R1 and Rx, and current I2 flows through both R3 and R2. Also, the voltage
drop VAB across R1 is equal to the voltage drop across R3, VAD, for a zero galvanometer
deflection:
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W H E AT S TO N B R I D G E
Similarly,
By ohm's law
Hence, when the bridge is balanced, the unknown resistance Rx can be found in terms of
the standard resistance R1, R2 and R3.
Equipment
rheostat, Double Pole Double Throw switch(which can change the current flow in the
circuit), a galvanometer , resistance box(0~9999.9Ω)
,wires.
PROCEDURE: 1. Connect the circuit shown in Figure 2. Take the DC power as 1
volts；Moving the point B near to the middle of the rheostat,
prevent excessive current damage the galvanometer.
is a protective resistor to
is a resistance box. We adjust
until the bridge balance. (Galvanometer point 0) But the problem is that R is a
rheostat. We do not know the value of R1 and R2. Now let us consider the function of the
switch.
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W H E AT S TO N B R I D G E
Figure
2
connect with AB
Figure 3
connect with CD
Figure 4
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W H E AT S TO N B R I D G E
3. When the switch is connected with AB, as shown in Figure 3, adjust R3 to let the Wheastone
bridge balance. There will be：
Record the value of R3.
When the switch is connected with CD. As shown in Figure 4. The role of switch is exchanged
positions Rx and R3. Then adjust R3 to let the Wheastone bridge balance again. In order to distinguish
the R3 in Figure 3 and Figure 4. The R3 in Figure 4 we call it
.
There will be：
We will get :
Note: Because the galvanometer is very sensitive, so we use a resistance box(
) to protect
the galvanometer.
You should first adjust R’ to a high value to lower the bridge’s sensitivity, then adjust R3 to
let the galvanometer return to zero.
Reduce R’ to a lower value to higher the bridge’s sensitivity, adjust R3 to let the
galvanometer return to zero again.
Keep doing the uper steps, until
is zero and the galvanometer is zero too.
DATA REPORT：
R0
R0
Rx
QUESTION：
1. How to judge the Wheastone bridge is under balance state.
2. What’s the advantage of using the exchange method to get the unknown resistor?
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