Verification Of Ohm’s Law Using Voltmeter And Ammeter

Current Electricity of Class 12


A voltmeter (V) and an ammeter (A) are connected in a circuit, along with a resistance R as shown in the figure, with a battery B and a rheostat Rh.  Simultaneous readings of the current i and the potential drop V are taken by changing the resistance in the rheostat (Rh). A graph of V versus i is plotted and it is found to be linear (within errors).

The magnitude of R is determined by :

(a) taking the ratio V/i,  (b) fitting to a straight line: V = iR, and is determining the slope R.


Systematic errors in this experiment arise from the current flowing through V (finite resistance of the voltmeter), the Joule heating effect in the circuit and the resistance of the connecting wires/ connections of the resistance. The effect of Joule heating may be minimsed by switching on the circuit for a short while only, while the effect of finite resistance of the voltmeter can be overcome by using a high resistance instrument or a potentiometer. The lengths of connecting wires should be minimised as much as possible.

Error analysis: The error in computing the ratio R = V/i is given by  

where δV and δi are of the order of the least counts of the instruments used.

Exercise 10:The following measurements were taken to measure a resistance R in an experiment involving Ohm’s law.


1.0 V

1.5 V

1.4 V


50 mA

75 mA

70 mA

Calculate the value of R and estimate the error in R.

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