# Ammeter Working Principle and Types of Ammeter

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### What is an Ammeter

#### When the current in a circuit is substantially less than an ampere, its value is determined by the use of a milliammeter or micro-ammeter, which measures full-scale currents in milli-amperes or microamperes, respectively. The symbol for the ammeter is a circle with the enclosed letter A, as shown in the following figure. ### Types of Ammeter

#### It is obvious that a coil used in a basic meter mechanism has a certain resistance, Rm, called meter resistance, as shown in Fig. #### By Ohm’s law, when a current Im, sufficient to produce full-scale deflection, flows through the mechanism, a voltage drop, Vm is produced across the meter mechanism. ### Solution #### In order to use a meter mechanism to measure the current larger than the rating of the mechanism, a large part of the current, being monitored, must be bypassed or shunted around the mechanism as shown in the following figure. #### The value of shunt resistance, Rsh, needed to increase the current range, is calculated by using the full-scale value. With reference to above figure, if a meter with a full-scale rating of  is to indicate a full-scale deflection for a current  (>), then the shunted value of current is ### Solution #### Multiple scale ammeters are constructed either by using switch arrangement or multiple terminals. 