He played a cello bow on a gong.

A cello bow dragged across a gong.

That’s the weird instrument on “Set the Controls for the Heart of the Sun.” Roger Waters was the bassist and occasional sound-effects guy, and he liked making things creepy. Rubbing a bow on a gong gives that eerie, sustained metallic whine — not a crash, not a hit, just an uneasy hum. It’s the kind of thing you only do because you’re a Pink Floyd bassist in 1968 and you’ve got nothing to lose.

I think it’s one of those studio gimmicks that actually works. Most people don’t notice it’s a gong at all — they just feel uncomfortable. That’s the point.