1. A structure that serves as a model for an electron-tube system or an aerodynamic system, in which voltages are applied to test electron-tube design or to aid in computing ideal fluid flow.
2. A tank in which voltages are applied to an enlarged scale model of an electron-tube system or a reduced scale model of an aerodynamic system immersed in a poorly conducting liquid.

The equipotential lines between electrodes are traced with measuring probes, as an aid to electron-tube design.

It is also used as an aid to electron-tube design or in computing ideal fluid flow.