Float from a rotameter, salvaged from a piece of equipment made in 1970* that we finally replaced with the latest model... from the same manufacturer.

A rotameter is a piece of equipment that gives a visual indication of amount of liquid (often water) flowing through a pipe. It works by having a slightly conical shaped section of clear tube mounted vertically, with the float placed in this section. The force of the liquid flowing pushes the float up the tube until it reaches a balance position where the level of liquid flowing can pass by the float. Increasing the flow causes the float to move higher up the tube. So this solid lump of stainless steel, that's about 35mm diameter at the top, does appear to float, despite its heft. Some of the floats are designed to spin/rotate in the tube. Cheaper and lower flow rate versions are mostly plastic and may use a ball-bearing for the float. This very old one was salvaged from our ancient educational flow rig, and its tube section is made of thick glass. Had to clean it up a bit before this photo.

* - recently came across the original manufacturer's test certificates for the old flow rig. They were on old yellowed printed paper, with the results written with a fountain pen, and dated 1970... when I was in primary school. So I was a couple of years out, when I'd suggested replacing the ~50 year old flow rig with the newest version. The former Mechanical Engineering Technician and I had been attempting to keep the disintegrating, fragile, and leaky old rig running for years.