Determination of equivalent resistance of two resistors when connected in series


To determine the equivalent resistance of two resistors when connected in series.

Materials Required

  • Two resistors
  • A battery eliminator
  • Ammeter
  • Plug key
  • Connecting wires
  • A piece of sandpaper
  • Voltmeter
  • Rheostat


When two or more resistors are connected end to end to each other, then they are said to be connected in series.

In series combination,

  • Current flowing is same through each resistor.
  • The potential difference across each resistor is different.

The formula to calculate equivalent resistance in series is: Rs = R1+R2

Circuit Diagram


  1. Setup the devices as shown in the circuit diagram.
  2. Plug the key and adjust the rheostat by sliding its variable terminal till the ammeter and the voltmeter show a reading.
  3. Note the readings of voltmeter and ammeter. Take out the plug or switch off the battery eliminator for a moment.
  4. Repeat the steps 2 and 3 for the different values of current by varying the sliding terminal of rheostat.
  5. Calculate resistance using formula R = V/I.
  6. Perform these steps to calculate R1, R2 and Rs.


For R1 :

S. No.Voltmeter Reading (V)Ammeter Reading (I)Resistance (R = V/I)
1.0.02 V0.01 A2 Ω
2.0.04 V0.02 A2 Ω
3.0.08 V0.04 A2 Ω

Mean value of R1 = (2+2+2)/3 = 2 Ω

For R2

S. No.Voltmeter Reading (V)Ammeter Reading (I)Resistance (R = V/I)
1.0.01 V0.01 A1 Ω
2.0.02 V0.02 A1 Ω
3.0.04 V0.04 A1 Ω

Mean value of R2 = (1+1+1)/3 = 1 Ω

For Rs

S. No.Voltmeter Reading (V)Ammeter Reading (I)Resistance (R = V/I)
1.0.03 V0.01 A3 Ω
2.0.06 V0.02 A3 Ω
3.0.12 V0.04 A3 Ω

Mean value of Rs = (3+3+3)/3 = 3 Ω

*These observations may be different for different resistors.


  • The calculated value of equivalent resistance, Rs = R1+R2 = 3 Ω
  • The experimental value of equivalent resistance, Rs = 3 Ω
  • The equivalent resistance Ris more than the individual resistances.


  1. Thick copper wires should be used as connecting wires and using sandpaper, their insulation should be removed.
  2. The connections should be tight.
  3. Voltmeter should be connected in parallel with the resistor.
  4. To avoid unnecessary heating in the circuit, the current should be passed for a short time.

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