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How can we check DC polarity using multimeter?

If the DC cable (the positive and negative end) is connected in the cable rack (terminal block) how can one identify which cable is positive and which cable is negative?

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I just measured a Lithium battery with unknown polarity with a digital multimeter and got the following measurement. The same results should apply for other DC sources:

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I hypothesized that the negative result may have meant that I'd attached the positive (red) lead of the multimeter to the negative lead of the battery. To confirm this hypothesis I flipped the battery leads around and found further evidence this may be true:

enter image description here

So it appears that if you connect a multimeter postive lead to positive, and negative to negative on the circuit under test the meter will give a positive measurement. If the connections are reversed you will get a negative voltage reading.

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    \$\begingroup\$ +1 for using the scientific method passive aggressively, an opportunity which rarely presents itself. \$\endgroup\$ – Matt Mar 27 '14 at 18:39

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