A galvanometer has an internal resistance RG = 294 and full-scale deflection for

Question

A galvanometer has an internal resistance RG = 294 and full-scale deflection for a current of 21 µA through its coil. It is connected to a series resistor RS as indicated in the figure below to form a voltmeter.

(a) Find the value of RS and the voltmeter's equivalent resistance if the voltmeter is to measure voltages in the range 0 to 19 V.
Correct Answer is 9.04x105 Ohms

(b) When the voltmeter reads 9.5 V, what is the current through the galvanometer?
Correct Answer is 10.5 mA

What are the voltage drops across RG and RS?

across RG = Answer is 3.09 mV

across RS = Answer is 9.5 V

Explanation / Answer

current, I=21uA

RG=294 ohms

a)

if V=19 Volt

use,

V=I*Rnet

19=21*10^-6*(294+RS)

===> RS=9.04*10^5 ohms

b)

if V=9.5 Volt

use,

V=I*Rnet

9.5=I*(294+9.04*10^5)

==> I=10.5*10^-6 A or I=10.5 uA

C)

voltage drop across RG is,

VG=I*RG

VG=10.5*10^-6*294

VG=3.09*10^-3 Volt or V=3.09mV

and

voltage drop across RS is,

VS=I*RS

VS=10.5*10^-6*9.04*10^5

VS=9.5 Volt

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