Question

A micrometre has a resistance of 100$\Omega$ and a full-scale range of 50$\mu$A. It can be used as a
voltmeter or a higher range ammeter provided resistance is added to it. Pick the correct
range and resistance combination(s).

• A. $50V$ range with $10k\Omega$ resistance in series.
• B. $10V$ range with $200k\Omega$ resistance in series.
• C. $5mA$ range with $1\Omega$ resistance in parallel.
• D. $10mA$ range with $1k\Omega$ resistance in parallel.

Answer 1

Answer: (B), (C)

The correct options are
B $10V$ range with $200k\Omega$ resistance in series.
C $5mA$ range with $1\Omega$ resistance in parallel.

It act as voltmeter when it is in series.

$V=i_g(R+S)$

Put V= 10v

$10=5\times {10}^{-5}(S+100)$

$2\times {10}^5=S+100$

$S\approx 2K\Omega$

It act as ammeter when it is in parallel.

Put $I=5mA$

$I\times S=i_g(R+S)$

$5\times {10}^{-3}\times S=5\times {10}^{-5}(R+S)$

$100S=R+S$

$S\approx 1$

Hence, Option $B$ and $C$ is answer