What? Is it time to wake up?
Oh yeah, Dell Latitude CPX,http://www.amazon.com/Dell-Latitude-Notebook-650-MHz-Pentium/dp/B00005UJYF
AC adapter example for above,http://www.prontocharger.com/ProductDetails.asp?ProductCode=d1161
DC Power Jack example,http://www.all-power-jack.com/Dell/Dell-Latitude-CPx.html
I see the dilemma. The plug from the AC adapter has 3 socket holes, and the DC Power Jack has 3 prongs.
DC electricity has a Positive connection, and a Negative connection.
To find out which socket holes are for Positive, and Negative, let's look again at the DC Power Jack,http://www.all-power-jack.com/Dell/Dell-Latitude-CPx.html
The Top view shows the jack upside down, and turned around. (In relation to how it mounts on the motherboard )
The bottom left view shows the jack primarily how it sits, (Mounted), on the motherboard.
The bottom right view shows the jack turned over, laying on it's front.
Also laying over to it's right side.
Note the three prongs. (Flat bladed pins )
The one in the middle is Positive.
The two outer prongs are Negative.
Looking at the bottom Left view, the Top prong is Positive, and the two bottom prongs are Negative.
Match it up to the plug on the AC adapter cable.
Going to test the Voltage coming out? (20 Volts)
The red Positive probe lead goes into the top socket hole, in the plug.
The black Negative lead goes in either bottom socket hole.
What if I'm wrong?
A Digital multimeter will just show a negative value. -20
An Analog multimeter's needle will go down towards the 0, instead of towards 20 Volts. (DC)
Just switch the probe leads around.
For additional questions, or if I am way off base, post back in a Comment.