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AMMETER VOLTMETER AND MEGGER

AMMETER

An ammeter is required to measure the current. Therefore, it must be connected in series to the load of which the current is to be measured. As it is connected in series, it must have low resistance, otherwise the resistance of the instrument will appreciably change the current through the load. For connecting it in series with the load, open the circuit at one place and connect the meter in the circuit as shown in Figure.
Note : An ammeter should never be connected in parallel with the load otherwise it can get damaged.

VOLTMETER

A voltmeter is used to measure the potential difference. Therefore it must be connected in parallel to the circuit of which the potential difference is to be measured. A voltmeter must have high resistance otherwise it will take heavy current. When a voltmeter is connected in series, it measures the voltage drop across its own terminals. For connecting the voltmeter in parallel to the circuit, the meter should be connected across the load resistance as shown in Figure.
Wattmeter :

 It is used to measure power. It has two coils. A current coil is connected in series with the circuit and a pressure coil is connected in parallel with the circuit. The deflection is due to the interaction of the two and is proportional to the power in the circuit.

 MEGGER :

 It is the simplest and most popular instrument used for testing and measuring the resistance of the insulators and conductors. It is also used to trace out faults of the electrical circuits such as open circuit, short circuit, earth and continuity.
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