Ammeters & Voltmeters
Ammeters & Voltmeters
- A meter becomes part of the circuit. Its resistance can change the quantity it is trying to measure.
- Good meter design makes this change as small as possible.
Choose the connection from the job
- An ammeter must carry the current being measured, so it is connected in series and should have very low resistance.
- A voltmeter compares two points, so it is connected in parallel and should have very high resistance.
- Connecting an ammeter directly across a source creates a very low-resistance path and may produce a dangerously large current.
Real meters slightly change the circuit
- A real ammeter adds a small series resistance, so the circuit current becomes slightly smaller.
- A real voltmeter draws a small parallel current. Its finite resistance can lower the resistance of the branch it measures.
- Treat a meter as ideal only when the question says its resistance is negligible or very large.
Your turn9702-style
Explain why an ammeter should have low resistance and a voltmeter should have high resistance. [2]
Show worked answer
A low-resistance ammeter produces little extra p.d. in series. A high-resistance voltmeter draws very little current from the branch across which it is connected.