Measurement of the medium resistance by using whetstone bridge method.

Aim: - Measurement of the medium resistance by using whetstone bridge method.
Apparatus: - Power supply,
Resistor: - 10K_-1no,5K_-1no,11K_-1no
Unknown resistor=100_,
Pot =1K-1no.
Wheat stone bridge kit.
Digital multimeter-1no,
Patch codes.
Circuit Diagram:-
Observation Table: -
P (ratio arm
resistor)
Q (ratio arm
resistor)
Standard resistor
S
R measured
value
R actual
Theory-
A very important device used in the measurement of medium resistances is the
Wheat stones bridge .it is an accurate and reliable instrument .The wheat stone bridge is
an instrument based on the principle of null indication and comparison measurements .
The basic circuit of a wheat stone bridge is shown in fig . it has four resistive
arms, consisting of resistances P,Q,R and S together with a source of emf and a null
detector , usually a galvanometer G or other sensitive current meter is used. The current
through the galvanometer depends on the potential difference between point’s b and d
.The bridge is said to be balanced when there is on current through the galvanometer or
when the potential difference across the galvanometer is zero. this occurs when the
voltage from point ‘b’ to point ‘a’ equals the voltage from point ‘d’ to point ‘a’ or by
referring to other battery terminal , when the voltage from point ‘d’ to point ‘c’ equals the
voltage from point ‘b’ to point ‘c’.
For bridge balance;
I1P=I2 R ……………. (.1)
I1=I3=E/P+Q (2)
I2=I4=E/R+S (3)
E=emf of battery.
Combining equ (1) and (2) we get
P/P+Q=R/R+S (4)
QR=PS (5)
Equ . (5) Shows the balance condition of wheat stone bridge. If three of the resistances
are known then fourth may be determined by formula…
R=S*P/Q
Where R is the unknown resistance, S is called the standard arm resistor and P and Q are
called the ratio arms.
Procedure: -
1) Connect the patch cord as per the circuit diagram.
2) Note the resistance of P and Q using multimeter.
3) Adjust the resistance of P, Q, R, S
4) Switch on the power supply and adjust the resistance S such that galvanometer
shows the zero deflection.
5) Now calculate R, R=P*S/Q
Result:- Hence we have studied the low resistance by using whetstone bridge.
Viva Questions: -
1) What are the other methods used for measurement of medium resistance?
2) Why we use this method for measurement of medium resistance?

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