I have a question for anyone who knows a little more about electricity
that I do (i.e., I know not to stick my finger in a light socket).

I have been working on a project in which I have a small computer and
digital camera suspended from a helium balloon (it's a big balloon). I
am connecting to the computer via wireless networking to directly
control the digital camera from the ground.

My problem is getting power to the camera in the gondola. The computer I
am using in the gondola (a Mini-ITX motherboard) uses an external AC/DC
converter, outputing 12V DC. I fashioned an extension cable to go
between the AC/DC converter and the gondola from 18 guage zip-cord. I
found, through trial and error, that the computer starts to fail if the
extension cord is greater than about 60 feet long. I assume this is due
to resistance in the cable. This cable also functions as the tether for
the balloon. I need to be able to fly the balloon higher than this -
preferably up to about 150 ft.

Does anyone know of a type of wire I could use instead of the 18 guage
zip cord that would be both lighter (as the weight of the wire impacts
how well the balloon fliers) and would have lower resitance? Or is there
a way to step up my power source to overcome the resitance?

I am exploring using some kind of on-board battery but I am dealing with
  similar weight problems.

Thanks for any imput!

-Eric Wolf