Electricity 1: Meters

What is the main purpose of a meter?

What are the three most common analog meters?

Voltmeter, ammeter, and ohmmeter

What physical principle do all analog meters operate on?

Electromagnetism — interaction between magnetic fields created by current.

What is the basic principle of meter movement?

The pointer deflects proportionally to the current passing through the coil — more current causes greater deflection.

What causes the meter needle to move in a D’Arsenval meter movement this type of meter?

The magnetic field interaction between a permanent magnet and a rotating coil.

What is meter sensitivity?

The amount of current required to drive the coil to full-scale deflection.

What are the typical characteristics of a D’Arsonval meter movement?

Internal resistance of 50 ohms and full-scale deflection current of 1 mA (0.001 A).

How should an ammeter be connected in a circuit?

In series with the element whose current is being measured.

Why should an ammeter never be connected across a voltage source?

Because its low resistance would cause a high current, possibly damaging the meter.

What should you do to prevent overloading an ammeter?

Start with the highest range and reduce as needed; avoid exceeding full-scale deflection.

What happens if polarity is reversed when connecting an ammeter?

The needle may deflect backward or bend, damaging the meter.

How is a voltmeter connected in a circuit?

In parallel with the circuit component being measured.

What is a “multiplier resistance” (RM)?

A series resistor added to a voltmeter to limit current and allow higher voltage measurement ranges.

Define voltmeter sensitivity.

Resistance per volt (Ω/V).

Why should a voltmeter have high resistance?

To prevent it from drawing current and altering the circuit’s behavior (minimizing shunting effect).

What is the function of an ohmmeter?

To measure electrical resistance.

What additional components does an ohmmeter include internally?

A battery and a variable resistor in series with the meter movement.

What does full-scale deflection on an ohmmeter indicate when leads are open?

Infinite resistance (open circuit).

What does zero deflection indicate when leads are shorted?

Zero resistance (short circuit).

What is the purpose of the variable resistor in an ohmmeter?

To adjust zero reading and compensate for changes in battery voltage.

Why is the ohmmeter scale called a “back-off scale”?

Because resistance values decrease from left to right on the scale.

What does the setting R×1, R×10, R×100, etc., mean on an ohmmeter?

Multiply the reading on the scale by the selected range factor to obtain actual resistance.

How are different resistance ranges achieved in an ohmmeter?

By switching different shunt resistors into the circuit.

What is a megger used for?

Measuring high resistance values, such as insulation resistance and continuity tests in electrical systems.

How does a megger differ from a standard ohmmeter?

It can apply a high “breakdown” voltage to test insulation under stress.

What replaces the mechanical movement in digital meters?

Microprocessors and LED/LCD displays.

List three advantages of digital meters over analog ones.

They are more accurate, lighter, and less expensive with greater durability.

How does a digital meter measure current?

By measuring the voltage drop across a series resistor and converting it via microprocessor.

How does a digital meter measure voltage?

The input signal is converted from analog to digital and compared to an internal reference voltage.

How does it measure resistance?

It applies a reference voltage, measures resulting current, and calculates resistance using Ohm’s Law.