A Rotating Bennet's doubler

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 Bohnenberger, 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 is fixed, and plates B and C are mounted on rotating insulating supports that rotate simultaneously, placing the plates B and C at the positions (a) and (b) as seen. In position (a), the charges in plates A and C, 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 and B 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 and C, 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 and C.  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". Plate A is a wood block permanently connected to a metal brush that touches the sectors C of the disk seen at left. A grounded brush (just connected to the wood structure) touches the same sectors C where the disks overlap.

Rotating doubler drawing

Rotating doubler drawing

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.

Rotaring doubler photo

Rotating doubler photo

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 [4]. 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.

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