An electrodynamic instrument is a moving-coil instrument in which the operating field is produced, not by a permanent magnet but by another fixed coil. When a current-carrying conductor is put inside a magnetic field, it is subjected to a mechanical force, which causes the conductor to deflect.
Electrodynamometer types of instruments are used as a.c. voltmeters and ammeters are both in the range of power frequencies and lower part of the audio power frequency range. They are used as watt-meters, and with some modifications as power factor meters and frequency meters.
Other types of voltmeters are:
Voltmeter – Symbol of Voltmeter
In circuit diagrams, a voltmeter is represented by the letter V in a circle, with two emerging lines representing the two points of measurement.
Voltmeter and Hydraulic Analogy
The hydraulic analogy, or the electric-fluid analogy, is a widely used analogy between hydraulics and electricity, which is a useful tool for teaching and for those who are struggling to understand how circuits work. it can also be applied to heat transfer problems.
Since electric current is invisible and the processes in play in electronics are often difficult to demonstrate, the various electronic components are represented by hydraulic equivalents. The relationship between voltage and current is defined (in ohmic devices like resistors) by Ohm’s law. Ohm’s Law is analogous to the Hagen–Poiseuille equation, as both are linear models relating flux and potential in their respective systems.
Electricity (as well as heat) was originally understood to be a kind of fluid, and the names of certain electric quantities (such as current) are derived from hydraulic equivalents.
Voltage is like the pressure difference that pushes water through the hose. It is measured in volts (V). This model assumes that the water is flowing horizontally so that the force of gravity can be ignored. So that voltmeter is equivalent to pressure difference measurement.