|Time standard cell.
Rubidium and cesium, which are in the same column of the periodic table and share many of the same chemical properties, are also both used as very high accuracy time standards. A collection of cesium clocks is the current international standard for time. Rubidium clocks are not as accurate, but they are a whole lot cheaper: Functional units can be had for just a hundred dollars or so on eBay.
This is the smallest currently available rubidium cell from inside such a time standard. You can just barely see the drop of rubidium inside the square glass cell (which is only about 1/4" on edge, to give you an idea of how small this thing is). The picture is focused such that the rubidium dot is in focus on the back face of the cell: See the 3D version to get a better view of the unit as a whole. In operation the cylinder next to the cell heats up (you can see the electrical contacts on the back in the 3D view) to the point where the rubidium is vaporized, the coils wrapped around the unit as a whole sets up a magnetic field, and it is the frequency of an electronic transition of the vaporized rubidium atoms in this field that is used as the time standard.
Source: eBay seller pablof_azul
Contributor: Theodore Gray
Acquired: 20 June, 2003