Function of the 3rd pin on newer 20V power brick

[frankiepankie]

Hi guys,

Bought a lead with power connector (DC side) from eBay, to put on my car adapter (i am quite handy with a soldering iron).

But the lead has 3 wires:
1 blue (ground)
1 brown (positive terminal)
1 thin black (???)

The laptop works fine if 20V is applied to the blue and brown wires, but what is the use of the thin black wire? It is connected to the small center pin on the connector.

The laptop recognises it as an 90W AC adapter if i connect 20V to the both wires. It does the same with the original adapter, so this is good But i am still wondering what the use is of the 3rd wire (thin black one)

(I bought the wire here: http://cgi.ebay.nl/Power-Cord-Plug-for- ... _964wt_939 )

[visionviper]

Isn't the thin black one supposed to be the ground?