(c) Total current through the galvanometer is given by I=
V
Reff
=
2
1970+30
=
2
2000
= 10−3A As this current provides full scale deflection (i.e. 20 div). To get the deflection of 10 divisions, valueof resistance needed to connect can be obtained as θ=
nIAB
K
(i.e.θ∝I)
θ1
θ2
=
I1
I2
=2 I2=
I1
2
=
10−3
2
= 5×10−4A Now, I=
V
Reff+Rs
⇒ Rs=
V
I
−Reff =
2
5×10−4
−2000 =4×103−2000 = 2000 Ω So, the resistance of 1970 Ω is to be replaced by 1970 + 2000 = 3970 Ω.