Step 4: Solder the Socket Electrode
We're going to make the ground electrode be a washer on the outside of the Thinkpad, but the existing ground contact is way down in the socket. We're going to stick an electrode down in there. Again, we're using a strip of copper flashing, and we're going to solder it to a washer. The first picture is the finished product.
The strip should be slightly narrower than the ground contact inside the Thinkpad power socket, otherwise it won't have such a good connection. Do some test fittings on the laptop and bend a right angle into the strip so you know where to solder it. The strip should go all the way to the bottom of the socket. You can see that I folded over the end so that it fits more tightly.
Then, fire up your soldering iron and solder the strip to the washer. I found that the best soldering strategy is to heat the washer with the iron on one side of the strip (use a small blob of solder to increase heat conductivity), and have fresh solder ready to flow under the strip on the other side.