Crazy Gas Gauge

I just finished assembling a '66 Barracuda. I put in a new gas tank, fuel filter & sending unit. When I filled the tank, the handle clicked off and the gauge read Full. I drove it about 40 miles & it read Empty. I put in 3 gallons & it read Full again, only to drop to Empty in 40 or so miles. I knew I had gas in it, so I drove it 50+ miles with the gauge on "E" - which is a bit unsettling. I put in a second new sending unit & it is doing the same thing. The gauge itself tests fine. I was wondering what the gizmo that looks like a Silver Salute with a black wick (pictured below) does. I believe it has something to do with the gauge & maybe that's the problem - or.....anyone?
Thanks,
Mark

Gas gauge part.jpg