Ammeter is an electrical instrument that has generally a low resistance and is connected in series with an electrical circuit whose current is to be measured.
The voltmeter is an electrical instrument which has generally a high resistance and is connected across the electrical circuit whose voltage is to be measured.
The most commonly used ammeters and voltmeters are classified into three categories:
i. Moving Iron Type: It is used in both AC and DC. It has a cramped scale. The symbol used in this instrument are given below:
ii. Permanent Magnet Moving Coil Type (PMMC): It is used only in DC. It has a long uniform scale. The symbol used in this instrument is given below:
iii. Dynamometer Type: It can be applied in both AC and DC. It has a non-uniform scale. The symbol used in this instrument are shown below:
Moving Instrument (Ammeter & Voltmeter):
The operation of the moving iron instrument depends upon the interaction between current-carrying fixed coils and movable soft iron vanes mounted on the Spindle of the Instrument in the moving system.
When the pointer of the instrument moves to start from zero position due to repulsive force between fixed coils and movable iron vanes said to be a repulsion type moving iron instrument and when the pointer of the instrument moves from zero position due to attractive force between fixed coils and movable iron vanes then it is said to attraction type iron instrument.