568
12 Electric Circuits
Fig. 12.43
This is balanced by 0.01 V due to the second cell:
∴
8
10 + R
= 0.01
∴
R = 790
12.38 Resistance across ac (Fig. 12.20) is
1
4 × 12, 000 = 3000 . The combined resistance of voltmeter (6000 ) in parallel with 3000 resistance is
6000 × 3000
6000 + 3000
= 2000 .
Resistance across bc is
3
4 × 12, 000 = 9000 . Effective resistance of the
circuit = 9000 + 2000 = 11, 000 .
P.D. across ac is
V =
220 × 2000
11, 000
= 40 V
Thus the voltmeter reads 40 V.
12.39 R
3.5
=
l
100 − l
=
30
100 − 30
=
3
7
∴ R = 1.5
12.40 (a) N =
i
i 1
=
100 mA
1 mA
= 100
S =
G
N − 1
=
80
100 − 1
= 0.808
A shunt of 0.808 should be provided for the moving coil meter.