The classical electrostatic whirl is a rotor
with points bent to one side mounted over a pin, so it can rotate
easily. With the pin connected to a high-voltage power supply the rotor
turns, in reaction to the ionic wind blowing from the points. The
experiment typically requires quite high voltage (> 10 kV) and
several µA of current. It is usually attributed to Mr. Hamilton,
professor of Philosophy at the University of Dublin
(Hamilton's "mill", or "fly"), from an article in the Philosophical
Transactions by Benjamin Wilson relating the experiment in 1760 [1]. A
friction
electrostatic machine was used as power supply. It was known
too [2][3][4] that if the whirl is enclosed by a glass box the rotation
soon
stops, and is restored if the glass is touched.
A curious variation of the
electrostatic whirl can be obtained taking a square of aluminum foil or
paper folded at the diagonals (a "chi spinner") and mounting it over a
metal needle, so
it is balanced and can move easily. The assembly shall allow the needle
to be connected to a high-voltage supply and have space around for a
glass pot covering the rotor, with the glass insulated (by plastic,
that insulates better than glass) and not in
contact with the ground or with the power supply. The glass must be dry
and clean, presenting reasonably high insulation, but not too high. The
DC power supply
can be positive or negative, 5 to 10 kV. The required current is
much smaller than with a classical whirl. The following properties can
be
observed:
Without the glass pot the symmetrical terminal does not rotate.
With the glass pot covering the terminal, if the glass is touched
(or grounded in some way) the terminal rotates, even being perfectly
symmetrical.
If a lighted candle is placed close to the device the terminal
rotates. The ionized air generated by the flame is enough to ground the
glass pot.
If two devices, one connected to the power supply and other to
ground, are interconnected by a wire or even a wood stick placed over
the
glass pots, both rotate.
With two devices, one connected to the power supply and the other
with a wire connecting it to the glass pot of the first, if the second
has its glass pot touched or placed close to a flame, both rotate.
These two experiments just show different series connections of two
devices.
There is tendency for synchronization of two spinners if they are
close.
A candle placed over the glass pot
causes rotation.
A glass plate inserted between the device and a lighted candle
stops the rotation.
A metal screen around the terminal does not cause rotation,
grounded or with a flame close.
An insulated metal screen around the glass pot doesn't have any
effect. It
works as it were not present.
A metal screen around the candle reduces the rotation.
A small electrostatic machine operated nearby, even at more than
1 m away, causes rotation.
Too high voltage causes rotation even with the glass pot
insulated. High humidity in the air too. Direct leakage to the air is
then enough.
A spinner with curved points turns too, with much smaller power
required, and only if the pot is grounded.
A spinner made with straight pointed wires works in the same way.
The reason of the rotation is probably the charge deposited
inside the
pot by the points of the terminal, repelling the points. Any small
asymmetry
starts a rotation, and as the glass is not a perfect insulator, the
charge leaks away if the pot is not insulated, and the repulsion is
continuous. The synchronization of two spinners is probably consequence
of the rotating electric fields around the pots. The videos below show
some of these experiments, and others:
References:
[1] Benjamin Wilson, "Farther experiments in electricity,"
Philosophical Transactions of the Royal Society, Vol. 51, part II,
1761, pp.
896-906.
[2] Tiberius Cavallo, "A complete tratise of electricity," 4th ed. Vol.
I, Dilly, London, 1795, p. 297.
[3] Augustin Privat-Deschanel, "Elementary
treatise on natural philosophy," Blackie & Son, London,
1870, p. 579.
[4] Tomlinson, "Experiments on the electrical fly," Philosophical
Magazine, Vol. 27, No. 181, pp. 202-218, March 1864.
The classical electrostatic whirl is a rotor
with points bent to one side mounted over a pin, so it can rotate
easily. With the pin connected to a high-voltage power supply the rotor
turns, in reaction to the ionic wind blowing from the points. The
experiment typically requires quite high voltage (> 10 kV) and
several µA of current. It is usually attributed to Mr. Hamilton,
professor of Philosophy at the University of Dublin
(Hamilton's "mill", or "fly"), from an article in the Philosophical
Transactions by Benjamin Wilson relating the experiment in 1760 [1]. A
friction
electrostatic machine was used as power supply. It was known
too [2][3][4] that if the whirl is enclosed by a glass box the rotation
soon
stops, and is restored if the glass is touched.