Device to produce small spheres of water, i.e. the Leidenfrost effect.
The larger concave plate is pierced by regularly spaced small holes, while the other is a concave copper plate.

The Leidenfrost effect is produced by heating one of the dishes with a flame until it reaches incandescence. If a little water is dropped on the plate, it does not expand, but collects instead in small flattened globular drops, which are very mobile because they are not in contact with the plate, but separated from it by a layer of steam. If the perforated plate is used, the water does not pass through the holes.
The steam slows down the heating of the drops, and their temperature remains below boiling point. When the temperature of the plate decreases, the water from the drops evaporates violently.

Device on display.