A Rotating Bennet's doubler
is a simple
electrostatic device where three insulated
plates are used for charge multiplication. It can be built for manual
operation, as originally described in 1787, or in a variety of
automated forms, as the rotating versions designed by Nicholson
, and the back-and-forth
versions designed by Bohnenberger
In all these forms, two plates and one or more contacts move. A version
of Bennet's doubler where only two plates
move, and all the connections are made by fixed contacts, can be built
as shown below. Plate A
fixed, and plates B
are mounted on rotating insulating
supports that rotate simultaneously, placing the plates B
positions (a) and (b) as seen. In position (a), the charges in plates A
, interconnected, are concentrated
in plate A
, while an inverted
copy is generated on plate B
grounded. In position (b), An
inverted copy of the charge in plate B
is generated in plate C
grounded. With the return to position
(a), the charges in plates A
are doubled, with
exactly the same operations of Bennet's doubler. Note that it's
possible to use more than one set of plates B
, as shown in (c).
This leads to the idea of using disks with sectors, as in a Wimshurst
machine, with the sectors providing sets of plates B
. With several sectors, it's
not even necessary to ensure synchronous movement.
The picture below shows my plans for the construction of this machine.
The two rotating disks are mounted side by
side on two parallel axles, with an overlap, and moved by a crank and
pulleys mechanism. The mechanical structure is similar to the one that
I used in the "half Wimshurst machine
a wood block permanently connected to a metal brush that touches the
of the disk seen at
left. A grounded brush (just connected to the wood structure) touches
the same sectors C
At the other side, seen above,
another grounded brush touches the sectors B of the other disk, when they are
in front of plate A. The
connection between plate A and
the brush serves as output terminal for the machine, an a ball
electrometer is mounted on it.
The machine was built in 2006, with
acrylic disks having 17 cm of diameter, sectors made of adhesive
aluminum tape, insulating bars made of acrylic, structure in wood,
small pulleys and other details in nylon, and brass rods. Plastic beads
are used in interconnections and to hold the brushes (made of thin
nickel-chrome wire). Brass screws fix everything in place.
The machine self-excites easily and gets fully charged with a
single turn of the crank, or five of the disks. If turned for some
time, it reverts polarity, certainly due to charge accumulation at the
back sides of the disks. The machine was built without a proper set of
output terminals. A pair of terminals could be added taking charge from
the disks at the top or bottom of the machine trough brushes of combs,
what would not affect much the operation of the doubler.
Note that this machine is quite similar to the "symmetrical
Toepler machine", but the charge collector and inductor assembly at
one of the sides is replaced by direct induction (or "influence")
between the disks. The
other collector/inductor assembly corresponds to the fixed "accumulator
plate" A and the brush
connected to it. It's possible to add a second
accumulator plate between the disks, and then have the Toepler machine
built with a different structure, that is electrically symmetrical but
physically asymmetrical. A machine assembled as a doubler but with this
second accumulator was described by Wilson in 1804 .
The symmetrical machine, and the half Wimshurst machine, as I built
them, are more powerful than this machine, using similar disks. This is
a sign that there is room for improvements on the machine.
Created: 19 February 2007.
Last update: 6 April 2012.
Developed and maintained by Antonio Carlos
M. de Queiroz.
Return to Electrostatic Machines