What Are Analogue Instruments Their Classification and Operating Principles？

Definition of Analogue Instruments

An analogue instrument is defined as a device where the output is a continuous function of time, maintaining a fixed relationship with the input. Physical quantities such as voltage, current, power, and energy are measured using analogue instruments. Most analogue instruments utilize a pointer or dial to indicate the magnitude of the measured quantity.

Classification of Analogue Instruments

The classification of analogue instruments is based on the type of physical quantity they measure. For instance, an instrument used to measure current is called an ammeter, while a voltmeter measures voltage. A wattmeter and a frequency meter are used to measure power and frequency, respectively.

Classification of Analogue Instruments

Analogue instruments can be classified based on the type of current they measure, yielding three main categories:

• Direct Current (DC) Analog Instruments

• Alternating Current (AC) Analog Instruments

• Instruments for Both Direct and Alternating Current

They can also be classified by how they present the measured quantity, including:

1. Indicating Instruments

These instruments display the magnitude of the measured quantity using a dial and pointer. Examples include ammeters and voltmeters. They are further divided into:

• Electromechanical Instruments

• Electronic Instruments

2. Recording Instruments

These provide continuous readings over a specified period, with quantity variations recorded on paper.

3. Integrating Instruments

These measure the total sum of an electrical quantity over a given time period.

Another classification is based on the method used to compare the measured quantity:

• Direct Measuring Instruments
  These directly convert the measurand into energy to activate the device, enabling direct measurement of unknown quantities. Examples include ammeters, voltmeters, wattmeters, and energy meters.

• Comparison Instruments
  These determine unknown quantities by comparing them against standard values. AC and DC bridges are typical examples.

Analogue instruments may also be classified by their accuracy levels.

Principles of Operation

Analogue instruments can be categorized by their operational principles, with many relying on the following effects:

Magnetic Effect

When current flows through a conductor, it induces a magnetic field around the conductor. For instance, if the conductor is coiled, the combined magnetic fields of the coil turns act like an imaginary magnet.

Thermal Effect

When the measured current passes through heating elements, it raises their temperature. A thermocouple attached to these elements converts this temperature change into an electromotive force (emf). This conversion of current to emf via temperature is known as the thermal effect.

Electrostatic Effect

Electrostatic force acts between two charged plates, causing one plate to displace. Instruments operating on this principle are called electrostatic devices.

Induction Effect

A non-magnetic conducting disc placed in a magnetic field (induced by an electromagnet excited by alternating current) generates an electromotive force (emf). This emf induces current in the disc, and the interaction between the induced current and the magnetic field causes the disc to move. This effect is primarily utilized in induction-type instruments.

Hall Effect

When a material carries an electric current in the presence of a transverse magnetic field, a voltage is generated between the two edges of the conductor. The magnitude of this voltage depends on the current, magnetic flux density, and the material properties of the conductor.