Why are tennis rackets generally tire-shaped

Ball lightning from the water cup

During the first experiments, Fußmann and colleagues held paper in the way of the rising balls. Since it did not ignite when touched, the scientists suspected that the spheres are relatively cool. That turned out to be a mistake. In the past two years, the two professors, together with two diploma students, have tackled the phenomenon according to all the rules of the art of physical measurement - and have discovered all sorts of amazing things.

Spider web made of electrons

For example, high-speed cameras show how, shortly after the voltage discharge, a spider-like, luminous web spreads around the upper electrode within a few thousandths of a second on the surface of the water. The reason for this is the high voltage between the lower electrode and the water surface, on which electrical charge carriers collect. This also creates a high field between the surface and the upper electrode, to which the charge carriers flow from all directions.

A ball lightning in the making: In steps of a few milliseconds, the researchers record how the plasma ball grows. © Max Planck Institute for Plasma Physics

This current heats the water droplet on the upper electrode so much that it evaporates and is partially converted into a plasma, i.e. an ionized gas. The diameter of the ball, which rises at about one meter per second, increases from about eight to 20 centimeters. Then the ball deforms into a hoop-shaped structure and extinguishes. The plasma bubble only contains about one to three tenths of a gram of gas.

Heat melt gauge

Experiments with thermocouples provided the first knowledge about the temperature. In this case, these are two extremely thin wires made of different metals that are welded together at both ends. If there is a temperature difference between the ends, a current flows that can be easily measured. The strength of the current provides a measure of the temperature at the end of the thermocouple that is in contact with the measurement object. During these experiments, the physicists held it in the plasma balls. They measured around 1,300 degrees Celsius - then the wire melted.

Start with 5,000 degrees

Only non-contact measurements could help them. In such cases, spectroscopy is always the method of choice. The graduates Alex Versteegh from the Technical University of Eindhoven and Stefan Noack from the University of Leipzig, who had heard of Fußmann's experiments and wanted to do research on them, undertook these experiments. They had to analyze the light from the plasma sphere with a spectrograph at short intervals.

In their investigations, the researchers identified characteristic radiation lines from calcium hydroxide and other molecules as well as from atoms and ions. From this, the physicists conclude that the temperature in the spheres initially amounts to 5,000 degrees, which then drops to around half within the first tenth of a second. That is a very hot plasma in which electrons and positive ions from sodium, calcium or copper, for example, exist. While the first two substances are contained in tap water in the form of salts, the copper comes from the electrode.

Thomas Bührke / MaxPlanckResearch
Status: 08/22/2008