Voltmeter is used for measuring voltage potential of an electric circuit or the potential difference between two points. Technically specified, all types of voltmeters are Ammeters because they measure current rather than the voltage. A voltmeter measures voltage only when current is transmitted in a circuit through a resistance. Due to this reason, voltmeters are sometimes referred to as high resistance Ammeters too.They also measure differences of electric potential in volts or units that are fractions or multiples of volts.

A multipurpose voltmeter instrument usually comes combined with an ammeter and an ohmmeter. Most voltmeters are analog types and are based on a galvanometer.

Uses of Voltmeter

A voltemeter is used for the following purposes and applications

• Voltage Sensing
• Voltage Measurement
• Electrical Testing
• Educational Laboratory
• Electronics Industry

Working Principle of Voltmeter

A Voltmeter can be called a versatile instrument because it measures not only voltage but also current and resistance. a Multimeter. It can measure voltages of both types of electric currents- direct current (DC) or alternating electric current (AC). A typical voltmeter scale is graduated in volts, millivolts (1/1000 volt), or kilovolts (1,000 volts). A laboratory standard voltmeter instrument employs electromechanical mechanism for functioning and typically covers ranges within 1000 - 3000 volts (V).

• The voltmeter measures voltage by passing current through a resistance.
• It is designed in such a way so as to offer minimum disturbance to the circuit. This is made possible by using a sensitive ammeter in series with a high resistance.
• The moving coil galvanometer is a type of voltmeter working on this principle.
• The sensitivity of a voltmeter is usually specified in ohms/ volt.

Types of Voltmeter

Although Digital Voltmeter and Analog Voltmeters are the most popular types made by instrument manufacturers, there are many other types of voltmeters.

Digital Voltmeters

A Digital Voltmeter provides voltage readings that are composed of a group of digits. With the gaining popularity of digital electronic devices, more digital voltmeters are used in physics laboratories. These digital voltmeters can measure the current flow in amperes and then display the voltage digitally. The Digital Voltmeters provide accurate details about current draw and current continuity thus helping the users to troubleshoot erratic loads and trends. Modern Digital Voltmeters can have both positive and negative leads and feature extremely low internal resistance.

Analog Voltmeters

Analog voltmeters use a needle and a calibrated scale to indicate values. These instruments give voltage readings that vary over a continuous range as indicated by a scale and pointer. A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with the instrument. It uses a small coil of fine wire that is suspended in a strong magnetic field. When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current through the coil. For using it as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.

Electrostatic Voltmeters

Electrostatic Voltmeters can effectively measure voltages without any charge transfer. One of the important feature of an Electrostatic voltmeter is that it can precisely measure surface potential (voltage) on any type of material without the need of physical contact. As such there is no occurrence of charge transfer as well as loading of the voltage source. Electrostatic voltmeters are mostly suitable in cases where voltage measurements cannot be performed using conventional contacting voltmeters. While conventional contacting Voltmeters need charge transfer to the voltmeter leading to loading and adjustment of the source voltage, electrostatic voltmeter can be used without bothering about these issues.

Oscilloscope

An oscilloscope is an advanced type of voltmeter that can be used to follow the rapid change of voltage in an AC circuit, where the current changes directions many times per second. A regular voltmeter cannot react fast enough to follow rapid voltage changes. An oscilloscope has a cathode ray tube. Free electrons are created at the back and accelerated toward the front in this vacuum tube where they strike a screen coated with phosphorous, which then gives off light. Beam of electrons coming from the back of the tube is swept across the screen from left to right by a changing electric field. The vertical motion of the electron beam is determined by the voltage applied to the leads of the oscilloscope. If a constant (D.C.) voltage is applied to the leads, a spot shifts up by an amount proportional to the applied voltage. If an A.C. sine wave voltage is applied to the leads, the spot sweeps up and down as the sine wave voltage alternates between the maximum positive and maximum negative values. As the spot sweeps left to right at the same time, a sine wave shaped curve appears on the screen.

Microvoltmeter

Microvoltmeter is a multipurpose instrument that measures low DC voltage. Digital Microvoltmeters can measure voltage even down to 1µV. Some of the exceptional features that are offered by the latest range of Microvoltmeters manufactured include.

• low temperature drift
• Automatic polarity indication
• Recorder facility
• 100 percent overranging.
• Digital panel meter for better accuracy and convenience
• IC regulated internal power supplies.

Vacuum Tube Voltmeters (Electronic Voltmeter)

As electronic voltmeter or vacuum tube voltmeter (VTVM) employs a tube or transistor circuit for amplifying the input voltage. This type of voltmeter design facilitates two objectives- one, it gives rise to increased sensitivity and secondly it gives rise to increased input impedance. A Vacuum tube Voltmeter generally has an input resistance in the range of 10 to 20 megohms.

Solenoid Voltmeters

A Solenoid voltmeter is the most common type of voltmeter used by electricians for testing and debugging of electrical power circuits. Solenoid voltmeters are very tough and can be handled roughly as they do not get damaged easily even due to electrical overload.

Also read about Ohmmeter