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Question
What is the sensitivity of the galvanometer (that is, what current gives a full-scale deflection) inside a voltmeter that has a $25.0 \textrm{ k}\Omega$ resistance on its 100-V scale?

$4.00 \textrm{ mA}$

Solution Video

# OpenStax College Physics Solution, Chapter 21, Problem 43 (Problems & Exercises) (1:08)

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This is College Physics Answers with Shaun Dychko. The galvanometer is a component in a voltmeter which is in series with some resistance that's put inside the voltmeter. The potential difference across this entire thing from one end to the other is going to be the current through it multiplied by the total resistance this external resistor, external to the galvanometer but internal to the voltmeter, added to the internal resistance of the galvanometer. So we can solve for the current here because when there's a question asking what is the sensitivity of the galvanometer what that's asking is what current causes full scale deflection. So we're solving for current in this case then. We divide both sides by capital <i>R</i> plus little <i>r</i> and that equals the voltage at which full scale deflection occurs divided by the total resistance. So that's 100 volts divided by 25 times ten to the three ohms and this gives 4.00 milliamps.