In the experimental set up for determination of resistance of a given resistor, it is advised to take out the plug from the key when the observatlions are not being taken. Why?
If key is not unplugged then a current will continue to flow through the resistor and in sometime resistor will be heated up. On heating its resistance will increase and hence, current reading in the circuit will fall and our observation will not be correct. To avoid this, the key is unplugged.