A galvannometer having a resistance of 20 Ω and 30 divisions on both sides has figure of merit 0.005 ampere/division. The resistance that should be connected in series such that it can be used as a voltmeter upto 15 volt is
The series resistance R to convert the galvanometer to a voltmeter is given by:
R = V/Ig – Rg
Substituting V = 15, Ig = 30 × 0.005 A. Rg = 20 Ω:
R = 15/0.15 – 20 = 100 Ω
Answer: (A) 100 Ω.
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The voltage range of a voltmeter is given by:
V = (n × k) × (RG + R)
Where:
n = 30 divisions
k = 0.005 A/division (figure of merit)
Rg = 20 Ω (galvanometer resistance)
V = 15 volts
15 = (30 × 0.005) × (20 + R)
15 = 0.15 × (20 + R)
20 + R = 15/0.15 = 100
R = 80Ω
Answer: (C) 80 Ω
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