[header] How to test resistors [title] Remove power from the circuit containing the resistor. [step] This can be done by unplugging it from the mains or by removing the batteries if it is a portable device. Keep in mind that some devices still can be charged with a potentially harmful voltage until minutes after removing its power! [title] Isolate the resistor from the circuit. 
[step] An attempt to measure a resistor that is still connected to the circuit can yield an incorrect calculation, as part of the circuit might also be measured. [substeps] Disconnect one end of the resistor from the circuit.