Voltmeter and Ammeter

Measuring Voltage

Measuring the voltage between two points of an electrical current can be done with an instrument called a voltmeter. One type of voltmeter is a direct-current voltmeter. When taking a voltage reading the instrument is placed across the portion of the circuit that is to be measured.

Direct-Current Voltmeter

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.

The Electrical Current

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.

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When the needle moves, it points to a reading on a dial which represents voltage.

The Reading

The needle will point to zero on the dial when the voltmeter is not in use. When a current moves through the coil, the magnetic field creates a force on the coil, resulting in needle movement. 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.

What is an Ammeter?

Ammeters are used to measure the current in electricity in amperes. Named after French scientist Andre-Marie Ampere, amperes are a unit of measurement for determining the amount of electricity moving through a circuit. Ampere's Law simply states that the magnetic field within a closed loop is proportional to the electric current in that loop.

How Ammeters Work

Ammeters can thus be used to measure this current by having electric current move through a set of coils. In moving-coil ammeters, this movement results from the fixed magnets that are set opposite to the current. The movement then turns a centrally located armature that is attached to an indicator dial. This dial is set above a graduated scale that lets the operator know how much current is moving through a closed circuit, as stated by writers at the Ammeter Gauge website.

Types of Ammeters

There are various types of ammeters. One called the d'Arsonval ammeter, which uses the fixed magnet discussed above, also uses a galvanometer through which an electric current passes. Ammeters used to measure high frequency currents utilize the heat created by the electrical current that moves through the coils. The heat is detected by thermocouples, which helps drive a moving-coil indicator.

Uses for Ammeters

Ammeters have various applications where they can be used to read alternating current, AC or direct current, DC. They are often used in cars where they measure DC. It should be noted that ammeters and voltmeters are often produced as a single instrument and may digitally display current with very high accuracy, as mentioned by the writers at World of Invention.