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Power Supply adapter question

I have a power supply like this:

Is it okay to assume that the left wire is negative and the right wire is positive?
I don't have a connection in the end as shown in the picture. I need them open to connect to a terminal block. Thus, I need to know which is positive and which is negative.

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If there's no tip on it anymore the only way to be SURE which is which is by hooking a meter up to it, if it comes up as negative voltage then you've got it backwards.  If you don't have a meter I suppose you could hook it up to an LED with a resistor (any diode would work, but an LED has a built in "it's working!" light :) )

You can also look at the wire...."wall wart" wiring:

Black = Negative

Black with white stripe = Positive

In the picture you linked to the polarity is shown just below the model number . The center of the plug is positive and the outer sleeve is negative . You should test it with a multimeter if you have one . This will tell you the polarity as well as the actual voltage . It's a good ideal to check the voltage coming out of any power supply before you use it .