The Water Battery - Lord Kelvin's Electrostatic Generator

Famous electrostatic machines include the Tesla Coil and the Van de Graaf generator as well as the Wimshurst machine. Less commonly known is Lord Kelvin's electrostatic generator, yet there is nothing more improbable or fantastic. The water battery is created by dripping water through two open-ended soup cans that are cross-connected with two collecting cans. This process gradually generates enough electrical potential (thousands of volts) for a spark to jump a sizable distance.

MIT Professor's Live Demonstration

The apparatus is aptly demonstrated in this six minute video by Walter Lewin of the Massachusetts Institute of Technology. Unfortunately, the good professor fails to explain how this machine works. He leaves that up to the students to discover. The answer is not only known, but is really pretty fascinating, and well demonstrates the electrical/physics property of "induction."

What is Induction?

Induction may be harder to define than to illustrate. If two objects are near each other but do not touch, and if one of the objects is charged positively or negatively and it causes the opposite charge to appear on the second object, it is said to have "induced" that opposite charge on the object. This is a commonly recognized phenomenon of physics.

Charge Buildup

In "Kelvin's Thunderstorm," the water streams passing through two open-ended cans induce the opposite charges onto those cans. The water then collects below. On the left side, the upper can and the lower can have opposite charges. The same can be said of the right two cans. By cross-linking the lower left can with the upper right one, positive feedback results, ever increasing the charge and induction effects. The same is true of the other side. The charge builds as more water is transferred, until so much collects that a spark of electricity jumps across a spark gap.

Unclear without a diagram as to what is being described above? Visit the references and resources below to compare diagrams with provided text. Allow us to suggest that rather than using the rings as shown in the diagrams, cans do better, as demonstrated by Professor Lewin.

References and Resources:

RMCybernetics - "Kelvins Thunderstorm"

Swedish Association for New Physics - "Kelvin's Thunderstorm"