I am new to electrical engineering.

I cut the barrel jack off the end of a 24V DC power adapter, exposing a red and white wire. I'm trying to determine the polarity of the red and white wires with a multimeter. Here's what I've tried, with my multimeter ends connected like so: black to COM, red to V/mA/uA/omega/Hz/Temp.

 1. Set multimeter to VDC mode (as opposed to VAC)
 2. Touch Black tip to white wire, red tip to red wire.
 3. No reading at any range (including mV)

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 1. Set multimeter to VDC mode
 2. Touch black tip to white or red wire
 3. Touch red tip to wall jack side of the adapter
 4. No reading

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 1. Set multimeter to VAC mode
 2. Touch black tip to white or red wire
 3. Touch red tip to the other wire
 4. Steady reading of some random low voltage

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Why aren't I getting a voltage reading from my power adapter? The multimeter gives accurate voltage readings when tested with a battery. Note that I haven't plugged the adapter into a power source as I read this is dangerous, so I don't quite understand how I'm supposed to get a reading?