10.
A multimeter has an internal resistance of 0.5 Ohms while another has an internal
resistance of 5×106 Ohms. What use can the two multimeters be best put to?
a. The first as an ammeter and the second as a voltmeter
b. The second as an ammeter and the first as a voltmeter
c. Both can be used as ammeters but not as voltmeters
d. Both can be used as voltmeters but not as ammeters

Multimeter Internal Resistance: Ammeter vs Voltmeter Uses Explained

The correct answer is option a: The first as an ammeter and the second as a voltmeter. A multimeter with 0.5 Ω internal resistance suits ammeter use due to minimal circuit impact, while the 5×10⁶ Ω one fits voltmeter use by drawing negligible current.

Option Analysis

Option a (Correct): Ammeters connect in series and require near-zero resistance (ideally 0 Ω) to measure true current without voltage drop. The 0.5 Ω multimeter approximates this well. Voltmeters connect in parallel and need infinite resistance (ideally >> circuit resistance) to avoid shunting current; 5 MΩ achieves this effectively.

Option b (Incorrect): Reversing roles fails: 5 MΩ in series drops excessive voltage, distorting current readings. The 0.5 Ω in parallel bypasses most voltage, yielding inaccurate measurements.

Option c (Incorrect): High-resistance (5 MΩ) multimeters cannot serve as ammeters due to large voltage drops altering circuit current.

Option d (Incorrect): Low-resistance (0.5 Ω) multimeters cannot act as voltmeters as they draw significant current, reducing measured voltage.

Multimeter internal resistance ammeter voltmeter selection determines measurement accuracy in circuits. For the query on a multimeter with 0.5 Ohms versus 5×10⁶ Ohms (5 MΩ), option a prevails as low resistance suits series current measurement while high resistance fits parallel voltage tasks.

Core Principles

Ammeter function demands minimal internal resistance to pass full current without altering circuit behavior, as any drop (via Ohm’s law, V=IR) skews readings. Voltmeter function requires maximal resistance to draw near-zero current (<<1% of circuit current) when paralleled.

Practical Implications

• 0.5 Ω Multimeter (Ammeter): Negligible drop in low-current circuits; typical real ammeters range 0.01-1 Ω.

• 5×10⁶ Ω Multimeter (Voltmeter): Standard digital meters offer 10 MΩ; this suffices for most applications.

In JGEEBILS-style exams, such questions test meter theory fundamentals. Always verify range settings to optimize resistance characteristics.