Voltmeter measure the difference in the electrical potential between two places, such as two points within a circuit. A voltmeter is two terminals device, one connected to wires another to leads.
When we place the one tip of lead at one of the points to be tested and the other tip at the other point and the meter shows you the difference. A voltmeter is an instrument used by electrical engineers to diagnose and troubleshoot electrical circuits.
How does a voltmeter work
Working of Voltmeter
Measurement of the voltage between two points of an electrical current can be done with an instrument called a voltmeter. The first type of voltmeter is a direct-current voltmeter. When we take a voltage reading the voltmeter is placed across the portion of the circuit that is to be measured.
The direct-current voltmeter has a horseshoe-shaped magnet, with a semicircular piece of soft iron attached to each end of the magnet. The iron is also magnetized. The iron ends of the magnet serve to direct the magnetic field in the direction of a small iron cylinder that is positioned between the ends (or poles) of the magnet.
Taking advantage of the soft iron’s characteristic to become highly magnetized, the iron cylinder focuses the magnetic field.
Surrounding the cylinder is a rectangular frame with a copper wire coil, with the ends of the wire attached to small spiral springs. Attached to the coil is a needle. The coil carries the electrical current, causing the needle to move. When the needle moves, it points to reading on a dial which represents voltage.
Reading
When the voltmeter is not in use the needle will point to zero on the dial. The needle starts to move when a current moves through the coil, then the magnetic field creates a force on the coil.
The force, caused by the electrical current running through the coil and the magnetic field of the magnet, causes the coil to turn. The springs attached to the ends of the coil oppose the coil’s motion, which serves to adjust the position of the needle to indicate the correct voltage.
Ideal Voltmeter
An ideal voltmeter is a two terminals device and it senses the voltage between the terminals where they may be connected to a circuit. Ideally, the voltmeter has infinite resistance. It is polarized and indicates polarity as well as the magnitude of the voltage.
Early meters were analog, based upon galvanometers with needles that were electromagnetically deflected linearly with a small amount of current. A 1-volt voltmeter was made with a galvanometer of say 1 milliamp sensitivity and a 1 K resistor (so that 1 mA flowed when 1 V was applied).
Those early instruments have been replaced by meters with all-electronic amplifiers, digital to analog converters, and digital displays that have a voltmeter input impedance of 10’s of meg-ohms or more and ammeter series resistances that are very low.
They are usually combined into one unit, a multimeter that may also make other measurements (resistance, temperature). Often they are called a digital Multimeter (DMM) and can come in handheld versions that are light and inexpensive and have .1% or better accuracy and more expensive versions for lab benchtops that have 5 and 6 digits of accuracy.
AC meters to measure AC volts and amps are made by using circuits called AC-to-DC converters that translate AC volts to a DC voltage that is proportional to the AC RMS content, a statistical averaging method that translates to equivalent heating power.