Bennet's doubler

Bennet's doubler [p88][4][37][56] is a clever device for the continuous doubling of an initial small charge through a sequence of operations with three plates. It was invented in 1787 by the Reverend Abraham Bennet, that used it for studies about the electric state of the air. The figure shows the original implementation, using Bennet's gold foil electroscope covered with a metal plate and two other metal plates with insulating handles. The top plate has its handle attached to its center, and the central plate has it attached to its side, as shown. The top and central plates have their undersides varnished for insulation

a) An initial charge is applied to the electroscope, with the central plate over it, grounded by a touch by the operator but insulated by the varnished surfaces. (This is the same initial operation of Volta's "condenser electroscope", and attracts to the electroscope much more charge than it's body only would attract.)

b) The central plate is moved away from the electroscope (causing an increase in the voltage between them, visible by the separation of the electroscope foils), and the top plate is used as an electrophorus over it. When the top plate, positioned over the central plate but insulated from it by the varnish layers, is touched by the operator, an inverted copy of the charge in the central plate appears on it.

c) The charge in the top plate is added to the electroscope's charge by conduction when they touch, attracted by the central plate, that is again placed over the electroscope and touched by the operator. The charges in the central plate and in the electroscope are doubled.

The process (b, c) can be repeated any number of times, doubling the electroscope's charge, or almost, at each step, as can be verified by the approximate doubling of the divergence at the electroscope. The multiplication ends only when sparks or corona start to drain the charges away.

The doubler is an instrument to measure small charges, turning them visible in the electroscope. A problem is that natural initial charges are easily multiplied in this way, and charges are generated by any rubbing of the plates, making the doubler imprecise for measurements requiring many doubling steps. In a few cycles, visible charges can be produced in the electroscope starting from apparently discharged plates. The idea can be used as basis for a self-starting electrostatic influence generator. Curiously, except for some simple direct mechanized versions, also designed as measuring instruments, as the revolving doublers developed by Nicholson and Bohnenberger, the principle was never used in more powerful influence machines.

A working model

In April 2000 I built a model of Bennet's doubler. I first made a simple electroscope using an acrylic box, with two light plastic foam balls suspended from thin wires as charge detector. The balls were painted with nankin (china) ink to become conductive, and the wires were suspended by small hooks from a small wire ring connected to a brass bar inserted in the top plate of the electroscope. I cut away the bottom of the plastic box, replacing it with a wooden base, and glued two aluminum foil disks to the internal sides of the box, to avoid charge accumulation inside the box. The top plate of the electroscope and the other two plates of the doubler were made from wood disks, with a light layer of varnish, insufficient for insulation. The wood plates are conductive enough for this purpose, but don't produce visible sparks. I glued two plastic disks to the central plate to serve as effective insulators, and omitted special insulation of the surfaces of the other plates. The two plates received handles turned from nylon (a bad choice. The handles become too conductive after some time, and I had to replace them by polished PVC tubes to make the doubler work again). The device is charged by a sequence of operations, as shown above. It can be operated with one hand, as I discovered while trying to take these pictures, holding the camera with the other hand, but with two hands the operation is more practical, and it's more easy to avoid mistakes in the sequence (It was said that Bennet's doubler "multiplies electricity at the cost of mental as well as mechanical energy" [37]). It never fails to charge from nothing to the saturation of the electroscope (~5 kV) in 10 to 20 operations. With dry conditions, once the set is charged, the top plate can be used as an electrophorus over the central plate, or the central plate can be used with the electroscope, producing charges of both polarities for other experiments. It's possible to revert the polarity of the electroscope by conduction, by discharging it and touching it with the charged central plate, or by influence, by touching it while the charged top plate is held close to it.

With wooden plates, the doubler can easily get charged by friction between the plates, specially if the central plate is touched and frictioned against the electroscope plate. Apparently, the plastic disks (polyethylene?) used produce good charge when frictioned against partially varnished wood. This is actually a convenient way of charging the electroscope, but is not the idea of the doubler. By 2002, I made another set using wood plates covered with aluminum foil and acrylic disks, that doesn't get charged by friction, but works in the same way and can produce small sparks. I made also another set with wood disks, but using a different varnish, that doesn't get charged by friction.

The picture above shows the voltage at the electroscope, as measured using a high-impedance voltmeter (Phywe elektrofeldmeter), for negative charging. The doubling at each cycle is incomplete due to losses and to the loading due to the meter capacitance. The operation was interrupted at -1200 V, the meter limit, but the doubling could continue to about -5000 V.

A document, in Portuguese, describing the construction of an improved Bennet's doubler. For automated versions of Bennet's doubler, see Nicholson's revolving doubler, and Bohnenberger's machine.

This is a video showing a Bennet's doubler made with wood disks covered with aluminum foil in operation. The lower plate is put over an insulated metal plate, and an electroscope measures the potential in the plate.
A similar video, in Youtube, with a wooden doubler. Several other videos with doublers can be seen in the same account too.

Created: 3 April 2000
Last update: 6 April 2012
Developed and maintained by Antonio Carlos M. de Queiroz.
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