21.4 DC Voltmeters and Ammeters

Digital meters are able to detect smaller currents than analog meters employing galvanometers. How does this explain their ability to measure voltage and current more accurately than analog meters?

What is the sensitivity of the galvanometer (that is, what current gives a full-scale deflection) inside a voltmeter that has a $1 . \text{00} -\text{M} \Omega$ resistance on its 30.0-V scale?

Find the resistance that must be placed in series with a $\text{25} . 0-Ω$ galvanometer having a $\text{50}.\text{0} -μA$ sensitivity (the same as the one discussed in the text) to allow it to be used as a voltmeter with a 0.100-V full-scale reading.

Find the resistance that must be placed in parallel with a $\text{25} . 0-Ω$ galvanometer having a $\text{50} . 0-μA$ sensitivity (the same as the one discussed in the text) to allow it to be used as an ammeter with a 10.0-A full-scale reading. Include a circuit diagram with your solution.

Find the resistance that must be placed in series with a $\text{10} . 0-Ω$ galvanometer having a $\text{100}-\mu\text{A}$ sensitivity to allow it to be used as a voltmeter with: (a) a 300-V full-scale reading, and (b) a 0.300-V full-scale reading.

A certain ammeter has a resistance of $5 . \text{00} \times \text{10}^{- 5} \Omega$ on its 3.00-A scale and contains a $\text{10} . 0-Ω$ galvanometer. What is the sensitivity of the galvanometer?

(a) What resistance would you put in parallel with a $\text{40} . 0-Ω$ galvanometer having a $\text{25}. 0-μA$ sensitivity to allow it to be used as an ammeter that has a full-scale deflection for $\text{10} . 0-μA$? (b) What is unreasonable about this result? (c) Which assumptions are responsible?