Measurement of medium resister by the voltmeter and ammeter method.

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Aim:- Measurement of medium resister by the voltmeter and ammeter method.
Apparatus:- DC ammeter(0-500mA)
DC Voltmeter (0-5V)
Dc power supply (0-30V)
Variable Resistance -100 ohm.
Connecting wires.
Circuit Diagram:-
Observation Table:-
Volt(voltage) Current(Amp) Resistance
(calculated)
Resistance
(measured)
Theory: -
Two types of the connections are done one employed for the ammeter
voltmeter method as shown in the figure voltmeter and ammeter are connected in series,
where ammeter measures the total current flowing through the circuit and voltmeter
measures the voltage across the unknown resistance .The voltmeter should have ideally
infinite resistance and ammeter should have ideally zero resistance so that it will measure
total current flowing through the unknown resistance. But practically it is not possible
and measured value Rm of the resistance is the sum of resistance of ammeter and actual
resistance.
Rm=R1+Ra
Where
R1=Actual resistance.
Ra=resistance of the ammeter.
It is clear from the expression that the value of measured resistance is equal to actual
resistance when ammeter has zero resistance.
Procedure:-
1) Make the connections as per circuit diagram.
2) Switch on the supply and note down the readings of ammeter and voltmeter.
3) Calculate the value of the unknown resistance by ohms low.
4) Perform the procedure for the other case similarly.
Result: - Hence we study the measured and the actual vale of the unknown resistance is
found .
Viva Questions:-
1) What are the resistance values required for ammeter and voltmeter?
2) What are the disadvantages of this method?
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