Fuel sending unit problem

[49studebaker]

Alright, I got another question. A co-worker has a 67 Chevy pickup. His fuel gauge didn't work so he replaced the printed circuit board and the sender. Still doesn't work. He's looking for power at the sender. I would think there wouldn't be 12 volts at the sending unit mounted behind the cab. I know from looking at the schematic last night that the power from the fuse block feeds all the gauges, switchs, etc. Can anyone explain to me how the varying resistance affects the fuel gauge? I can figure it out if I get some time looking and checking, but I'll bet some of you guys have the answers from past experience.