Measurement

Tip

Exam Focus

Know how each meter connects and why: ammeter in series (low resistance), voltmeter in parallel (high resistance), wattmeter (series current coil + shunt pressure coil), energy meter (kWh), megger for insulation resistance, and the fall-of-potential method for earth resistance. Range is extended by a shunt (ammeter) or multiplier (voltmeter), and by CT/PT in AC.

Ammeter and Voltmeter

Note

Key Point

An ammeter carries the load current, so it is connected in series and made low-resistance (ideally $0$). A voltmeter samples potential difference, so it is connected in parallel and made high-resistance (ideally $\infty$). A wrong connection (ammeter across a source, or voltmeter in series) either shorts the circuit or reads nothing.

Ammeter in series with the load; voltmeter in parallel across it.

Caution

Key Formula — Range extension

$$\text{Ammeter shunt: } R_{sh}=\frac{I_gR_g}{I-I_g},\qquad \text{Voltmeter multiplier: } R_{se}=\frac{V}{I_g}-R_g,$$

where $I_g,R_g$ are the movement’s full-scale current and resistance.

Power, Energy and Other Meters

Instrument	Measures	Principle / connection
Ammeter	Current	Series, low $R$; PMMC/MI movement
Voltmeter	Voltage	Parallel, high $R$
Wattmeter	Real power	Series current coil + shunt pressure coil; $P=VI\cos\varphi$
Energy meter	Energy (kWh)	Induction disc; counts revolutions $\propto\int P\,dt$
Megger	Insulation resistance (M$\Omega$)	Hand/electronic 500 V–1000 V source
Earth tester	Earth resistance ($\Omega$)	Fall-of-potential (3-point) method

Common electrical measuring instruments.

Note

Key Point

In AC systems, meter range is extended with a current transformer (CT, series) and potential transformer (PT, parallel); standard secondaries are 5 A (CT) and 110 V (PT). Energy is billed in kilowatt-hours: $1\,\text{kWh}=3.6\,MJ$.