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Substitution Method

9.2. Substitution Method. The connections for measurement of resistance by substitution shown in Fig. 32.19

Fig. 32. Measurement of resistance by substitution.

Let R be a variable resistance which can be changed in small steps, say of 0.1 Ω.

(i)                           Refer Fig. 32 (a). First resistance X is put in the circuit and the value of current noted.

Then resistance X is removed and it is substituted by a known variable resistance R which is varied so that the value of the current is same in both the cases. This value of R is equal to the unknown resistance.

(ii)                        If R is of fixed value note the readings of the ammeter for the following cases:

  1. For resistances X and R in series.
  2. When resistance X is removed.

Let the readings of the ammeter for these cases be I1 and I220

(iii) Refer Fig. 32 (b). The two-way switch first makes contact with 1 and then with 2 and let these readings be I1 and I221