Linear Circuit Analysis


Multimeters

Voltmeters

A voltmeter is a device that measures the voltage across a component (or between any two points) in a circuit. It has a very high resistance, so it does not significantly affect the circuit when connected in parallel with the component being measured.

0510 V DC V0
Fig. 1. The voltmeter. The red wire denotes the positive wire of the voltmeter.

An ideal voltmeter has infinite resistance, meaning no current flows through it when it is connected in parallel with a component. Since the voltmeter has infinite resistance, the ideal voltmeter does not affect the circuit at all.

A real voltmeter has a very high but finite resistance, so it affects the circuit only slightly (often negligibly) when connected in parallel with a component. Any practical voltmeter is real; however, it can often be approximated as ideal.

Ammeters

An ammeter is a device that measures the current through a component (or a wire) in a circuit. Unlike a voltmeter, an ammeter has very low resistance, so it does not significantly affect the circuit when connected in series with the component being measured.

0 5 10 A DC I0
Fig. 2. The ammeter. The red wire denotes the positive wire of the ammeter (i.e. the wire through which current flows inside the ammeter).

An ideal ammeter has zero resistance, meaning all the current flows through it when it is connected in series with a component. Since the ammeter has zero resistance, the ideal ammeter does not affect the circuit at all.

A real ammeter has a very low but finite resistance, so it affects the circuit only slightly (often negligibly) when connected in series with a component. Any practical ammeter is real; however, it can often be approximated as ideal.

Ohmmeter

An ohmmeter is a device that measures the resistance of a component in a circuit. It works by applying a known voltage across the component (usually a resistor) and measuring the resulting current, then using Ohm's law ($R = \frac{V}{I}$) to calculate the resistance.

1k 100 10 0 Ω OHMS R0
Fig. 3. The ohmmeter.
Powermeter

A powermeter is a device that measures the power consumed by a component in a circuit. It works by measuring the voltage across the component and the current through it, then using the formula $P = VI$ to calculate the power.

  • Ideal voltmeters and ammeters
  • Real voltmeters and ammeters