A voltmeter reads 5.0 volt at full scale deflection and is graded according to its resistance per volt at full-scale deflection as. How will you convert it into a voltmeter that reads at full-scale deflection.?

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75000 Ω should be connected in series

75000 Ω should be connected in parallel

85000 Ω should be connected in series

None of these

answer is A.

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Detailed Solution

Concept- Use the idea that a higher current system is not desirable compared to a lower current system for the same full scale deflection in this question. If we want to modify the full scale deflection with the same grade, we must add an external resistance later on in the answer.
Let R be the voltmeter resistance,
If the voltmeter has to read

at full scale deflection, we need an additional resistance

in series with it. Thus

since the voltmeter is graded as

for full scale deflection of

From (1)

So, to convert a given voltmeter (of range 0 to 5V) into a voltmeter (of range 0 to 20V) a resistance of 75000 Ω should be connected in series. With the given metre.
Hence, option 1 is correct.

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