Electronic devices use adaptors to convert the AC current supplied by wall outlets to DC power that electronic devices can use. Once converted to DC, the current flows in one direction, hence the term direct current. One wire is positive, which takes current to the device, and the other is negative, which completes the circuit. The direction of flow is called polarity and is depicted on chargers by a diagram. If the diagram isn't present on your Nokia charger, you need to test the polarity to determine if the charger is safe for your device.
Plug the charger into a wall outlet.
Turn on the multimeter and set the control to direct current voltage. Most multimeters label this setting as VDC.
Insert the tip of the positive red probe into the hole in the tip of the charger.
Touch the tip of the black negative probe to the outer metal part of the charger tip.
Look at the reading on the multimeter. You should see either a positive or negative number depicting the DC output voltage of the charger. If the number is positive, the polarity of the inner part of the charger tip is positive. If the number is negative, the polarity of the inner part is negative.