Measurement of Unknown inductance of a coil by Anderson bridge.
Object: Measurement of Unknown inductance of a coil by Anderson bridge.
Apparatus Used Anderson bridge, connecting wires, Head phone.
Theory Anderson’s bridge is the most accurate bridge used for the measurement of self – inductance over a wide range of values, from a few microHenries to several Henries. In this method the unknown selfinductance is measured in terms of known capacitance and resistances, by comparison. It is a modification of Maxwell’s L – C bridge. In this bridge, double balance is obtained by the variation of resistances only, the value of capacitance being fixed.
Formula Used: The following formula is used for the determination of unknown inductance of
coil.
Since, S = R ; thus
If P=Q then S=R ; Hence, L=C [RQ + 2rR]
So
L=RC [Q+2r]
Where symbols have their usual meaning as shown in figure.
Circuit Diagram:
Fig (A): Bridge with DC source and galvanometer
Fig (B): Bridge with AC source and Head Phone
Procedure
 Make connections as shown in fig A. i.e. connect source point with DC source, connect galvanometer at detector point and inductor at unknown point in your given kit.
 Now vary R, keeping r equal to zero, so that deflection in galvanometer becomes zero. This is a DC balancing. Note this value of R.
 Now, make connections as shown in fig B. i.e. replace DC source with AC source, replace galvanometer with Head phone.
 Now vary r keeping the same value of R, so that no sound is heard in Headphone. This is known as AC balancing. Note the value of r.
 Calculate the value of L using above formula and given value of C &Q for your circuit.
 Repeat the Points 1 to 5 for the different inductors.
 Repeat the Points 1 to 6 selecting different value of C.
Observations
 P = ………..Ω
 Q =………..Ω
 Table for value of R and r when C =………µf
Sr. No. 
Inductor 
R(Ω) 
r(Ω) 
L(mH) (Inductance) L = RC[Q+2r] 
1 
First 


L1 = 
2 
Second 


L2 = 
3 
Third 


L3 = 
4. Table for value of R and r when C =……….. µf
Sr. No. 
Inductor 
R(Ω) 
r(Ω) 
L(mH) (Inductance) L = RC[Q+2r] 
1 
First 


1 = 
2 
Second 


2 = 
3 
Third 


3= 
Calculation:
Show calculation for all value of inductance (L).
Result:
 Inductance of I inductor = =…..…..mH
 Inductance of II inductor = =….…..mH
 Inductance of III inductor = =……..mH
Precaution:
 To avoid inductive effect short straight wires should be used.2. Movement in galvanometer should be free. The resistances should be high and noninductive.
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