Physics 9702 · Paper 2

Ammeters & Voltmeters

A meter must not change the thing it is measuring, and that one idea decides where each meter goes.

Where each meter goes

An ammeter joins the line: the current it measures goes through it. A voltmeter is connected across a component: it compares the two ends.

An ammeter measures current, and current is a flow through something. So the ammeter joins the line: the current it measures passes through it.
A voltmeter measures p.d., and p.d. is a difference between two points. So the voltmeter is connected across a component, touching both ends.

An ammeter adds its own resistance to the line. Any resistance in the ammeter makes the current smaller. But that current is exactly what it is trying to measure. Ideal ammeter resistance: zero.
A voltmeter opens a second path between two points. Any current it takes is current that no longer goes through the component. Ideal voltmeter resistance: infinite.
Real digital voltmeters have a resistance of about 1 MΩ or more. A 10 MΩ voltmeter reading 2.5 V takes only $2.5 \times 10^{-7}$ A from the circuit.
A common exam question asks what a real voltmeter does to a reading. The worked numbers are on the effect-of-change page later in this chapter.

A meter must not disturb what it measures: ammeters in series with nearly zero resistance, voltmeters in parallel with nearly infinite resistance.

Ammeters & Voltmeters

A meter must not change the thing it is measuring, and that one idea decides where each meter goes.

An ammeter joins the line: the current it measures goes through it. A voltmeter is connected across a component: it compares the two ends.

An ammeter measures current, and current is a flow through something. So the ammeter joins the line: the current it measures passes through it.
A voltmeter measures p.d., and p.d. is a difference between two points. So the voltmeter is connected across a component, touching both ends.

An ammeter adds its own resistance to the line. Any resistance in the ammeter makes the current smaller. But that current is exactly what it is trying to measure. Ideal ammeter resistance: zero.
A voltmeter opens a second path between two points. Any current it takes is current that no longer goes through the component. Ideal voltmeter resistance: infinite.
Real digital voltmeters have a resistance of about 1 MΩ or more. A 10 MΩ voltmeter reading 2.5 V takes only $2.5 \times 10^{-7}$ A from the circuit.
A common exam question asks what a real voltmeter does to a reading. The worked numbers are on the effect-of-change page later in this chapter.

A meter must not disturb what it measures: ammeters in series with nearly zero resistance, voltmeters in parallel with nearly infinite resistance.