Question E4B02:
What is the significance of voltmeter sensitivity expressed in ohms per volt?  

Choices:  
A: The full scale reading of the voltmeter multiplied by its ohms per volt rating is the input impedance of the voltmeter  
B: The reading in volts multiplied by the ohms per volt rating will determine the power drawn by the device under test  
C: The reading in ohms divided by the ohms per volt rating will determine the voltage applied to the circuit  
D: The full scale reading in amps divided by ohms per volt rating will determine the size of shunt needed    

Correct Answer is  A: The full scale reading of the voltmeter multiplied by its ohms per volt rating is the input impedance of the voltmeter  

To calculate the voltmeter’s input impedance for a given range:
$$R_{in} = (\text{ohms per volt}) \times (\text{full-scale voltage of that range})$$

Why the correct answer is correct:
- “Ohms per volt” is the sensitivity of an analog voltmeter (VOM). It tells you how many ohms of input resistance the meter presents per volt of full-scale range.
- Therefore, on a selected range with full-scale voltage $V_{fs}$, the input resistance is:
  $$R_{in} = S \times V_{fs}$$
  where $S$ is the sensitivity in $\Omega/\text{V}$.

Example:
- A 20 kΩ/V meter on the 10 V range has:
  $$R_{in} = 20{,}000 \,\Omega/\text{V} \times 10 \,\text{V} = 200{,}000 \,\Omega$$

Why the other choices are incorrect:
- B: “Volts × ohms per volt = ohms,” not power. Power would require $P = V^2/R$ or $P = VI$.
- C: “Reading in ohms ÷ ohms per volt” mixes unrelated quantities; it does not yield voltage and has no basis in meter specs.
- D: “Full-scale reading in amps ÷ ohms per volt → shunt size” is irrelevant. Ohms per volt relates to input resistance vs. voltage range, not current ranges or shunt sizing.

Useful formulas and tips:
- Sensitivity–movement relation:
  $$S = \frac{R_{in}}{V_{fs}} = \frac{1}{I_{fs}}$$
  Example: a 50 μA movement ⇒ $S = 1/(50 \times 10^{-6}) = 20\,\text{k}\Omega/\text{V}$.
- Loading rule of thumb: to keep voltage measurement error small, aim for:
  - $R_{in} \ge 10 \times R_{source}$ for ~≤10% error.
  - $R_{in} \ge 100 \times R_{source}$ for ~≤1% error.
- Digital multimeters typically have fixed high $R_{in}$ (e.g., 10 MΩ), not expressed in ohms per volt.