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Circuit diagram
  1. Insert a voltmeter across the ends \(X\) and \(Y\) of the series combination of three resistors, as shown in figure.
  2. Plug the key in the circuit and note the voltmeter reading. It gives the potential difference across the series combination of resistors. Let it be \(V\). Now measure the potential difference across the two terminals of the battery. Compare the two values.
  3. Take out the plug key and disconnect the voltmeter. Now insert the voltmeter across the ends \(X\) and \(P\) of the first resistor, as shown in figure.
  4. Plug the key and measure the potential difference across the first resistor. Let it be \(V_1\).
  5. Similarly, measure the potential difference across the other two resistors, separately. Let these values be \(V_2\) and \(V_3\), respectively.
Deduce a relationship between \(V\), \(V_1\), \(V_2\) and \(V_3\).
 
According to Ohm's law, the potential differences \(V_1\), \(V_2\) and \(V_3\) across \(R_1\), \(R_2\) and \(R_3\), respectively, are given by
 
ii=iiiii=iiiii=iii
 
The sum of the potential differences across the ends of each resistor is given by
 
i=ii+ii+ii
 
It is deduced that the current throughout the series circuit. The current in every point or junction of the circuit is the .