Ammeter And Voltmeter working principle and it's types

 Ammeters and Voltmeters 

Ammeter

An ammeter is used to measure the flow of current in a circuit. It is thus connected in series with the circuit under test
(See Fig. ) so that current to be measured or a fraction of it passes through the instrument itself. The ammeter must
be capable of carrying this current without injury to itself and without abnormally increasing the resistance of the circuit into
which it is inserted. For this reason, an ammeter is designed to have low resistance.

 Voltmeter

A voltmeter is used to measure the potential difference between two points of a circuit. It is thus connected in parallel with the circuit or some part of the circuit as shown in Fig.
 The voltmet must have enough resistance so that it will not be injured by the current that flows through it, and so that it will not materially affect the current in the circuit to which it is connected. For this reason, a voltmeter is designed to have high resistance.

The basic principle of the ammeter and of the voltmeter is the same. Both are current operated devices , i.e. deflecting torque is produced when current flows through their operating coils.

 In the ammeter, the deflecting torque is produced by the current we wish to measure, or a certain fraction of that current.
In the voltmeter, the deflecting torque is produced by a current which is proportional to the potential difference we wish to measure.

Thus, the same instrument can be used as an ammeteror voltmeter with proper design.

The following types of instruments are used for making voltmeters and ammeters :

(i)   Permanent-magnet moving coil type
(ii)  Dynamometer type
(iii) Moving-iron type
(iv) Hot-wire type
(v)  Electrostatic type (for voltmeter only)
 (vi) Induction type.