MOTION MOUNTAIN

60 1 electricity and fields
Challenge 48 e
F I G U R E 27 A dangerous hobby,
here demonstrated by Robert
Krampf (© Wikimedia).
the hand to the sphere changes the electric potential and this also the shape of the discharges.Ifyouapproachafluorescenttubetosuchaset-up,itwill
startglowing;andby
moving your finger on the tube, you can ‘magically’ change the glow region.The internet
is full of information on plasma globes.
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A high voltage can lead to current flow through air, because air becomes conductive in
high electric fields. In such discharges, air molecules are put in motion. As a result, one
can make objects that are attached to a pulsed high tension source lift up in the air, if
one optimizes this air motion so that it points downwards everywhere. The high tension
is thus effectively used to accelerate ionized air in one direction and, as a result,
an object will move in the opposite direction, using the same principle as a rocket. An
example is shown in Figure 29, using the power supply of a PC monitor. (Watch out:
danger!) Numerous websites explain how to build these so-calledliftersathome;inFigure
29, the bottle and the candle are used as high voltage insulator to keep one of the
two thin high voltage wires (not visible in the photograph) high enough in the air, in
order to avoid discharges to the environment or to interfere with the lifter’s motion. Unfortunately,
the majority of websites – not all – give incorrect or confused explanations
of the phenomenon. These websites thus provide a good challenge for one to learn to
distinguish fact from speculation.
∗∗
The electric effects produced by friction and by liquid flow are usually small. However, in
the 1990s, a number oil tankers disappeared suddenly.The sailors had washed out the oil
tanks by hosing sea water onto the tank walls. The spraying led to charging of the tank;
a discharge then led to the oil fumes in the tank igniting. This led to an explosion and
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net