Some final observations:


If you search the Internet for electrostatic machines, and high voltage devices of other kinds, you will find some very weird material here and there. There are claims that machines can be built that extract energy from nothing, or that can counteract gravity, for example. In most cases without any practical, reproducible, demonstration, or with incorrect interpretation of simple phenomena as the "electric wind", or showing results obtained with plainly wrong measurement techniques. This is in part consequence of a role of these machines in the past, when the then not yet well understood phenomena of electricity were used for all kinds of strange purposes.

The theory that explains the working of these devices is now well understood, and there is nothing mysterious about them. They produce some effects commonly seen in mad-scientist movies, certainly, but they are nothing more than light, noise, electromagnetic fields, and ionization.


Small electrostatic machines are harmless, because the current that they produce is very small, in the order of microamperes. You can touch the terminals of a 50 kV machine and receive less current than if you hold a 9 V battery. This is not true if large Leyden jars are connected. There is no historical record of persons injured by the electrical output of an electrostatic machine, but stories about strong shocks taken due to careless manipulation of charged high-voltage capacitors are common. A 1 nF capacitance (a rather large Leyden jar) charged to 50 kV stores 1.25 Joules (E=0.5*C*V^2). To have an idea of how much is this, rise a 1 kg weight 12.7 cm above your little finger in a table and let it fall (E=M*G*H). A discharge with 10 Joules of energy is considered very dangerous.

Always try to operate electrostatic devices at a safe distance from electronic devices. RF interference may be significant, and an accidental direct spark can cause extensive damage. (Actually, I usually make my experiments in the same table where I keep my computer, and never had any problem with the computer turned off, but had some crashes with it turned on.)

Persons wearing implanted electronic devices, as pacemakers, insulin pumps, etc., shall avoid close proximity of high-voltage generators. The danger is small, however, because the high conductivity of the interior of the human body effectively shields electric fields, and modern devices are adequately protected.

The output current in a disk machine is proportional to the disk area and to the rotation speed. A Bonetti machine with 30 cm disks produces around 20 microamperes, what is still harmless. A machine with 1 m disks would produce 220 microamperes if rotated at the same speed, what starts to be dangerous if you take a shock. Note that a large machine will probably not be able to turn as fast as a small one. Assuming then that the rotation speed decreases in inverse proportion with the disk size, a 1 m machine will only produce 67 microamperes.

Note that there is always capacitance between you and the machine conductors, what causes disruptive discharges (spark sequences) with much higher peak current, what may cause burns and other damage. It is wise to avoid electric shocks of any intensity that you can feel.

A note about measurements: It is possible to measure the current output of these machines with a conventional microamperimeter. It's safer if a capacitor (0.1 uF) is connected across the input terminals, and the meter shall be kept highly insulated from the ground in differential measurements. Polished wood sticks with rounded ends can be used as resistors in series with the meter, for measurements of the output current with some output voltage too. Measuring the current that flows from points connected to the meter (corona), held close to the terminals of the machine, is also effective. In most types of bipolar machines, the maximum short-circuit output current can be measured between one of the terminals and the ground, or the neutralizer circuit. The output voltage is too high and the output impedance too high too for most conventional voltmeters, making the measurement meaningless. Digital meters can be easily damaged by an attempt of this kind of measurement.

The application of high voltage to vacuum devices can produce X rays, that are very dangerous. Do not try to connect electronic vacuum tubes to these machines, for example (unless you know -exactly- what you are doing, as these machines can be used as power supply for some X-ray tubes). Be careful also with old incandescent lamps and with some modern small lamps, that have high vacuum inside and can produce significant radiation. Most modern incandescent lamps are filled with an inert gas, and don't produce X rays. Fluorescent lamps and neon lamps are also safe. The generation of X rays is always associated with the generation of cathode rays inside the vacuum devices. When these rays (just high-speed electrons) hit the walls of the tube or internal parts with enough speed, X rays are generated. A sign of possible X-ray generation is fluorescence in the glass walls, associated to absence of light in the vacuum volume.

All devices that produce sparks in open air produce also some ozone (O3). All the usual electrostatic machines produce sparks at the connections through points, at the main terminals, and through corona effects. Ozone is irritant to the upper respiratory tract and eyes, causes headaches, and can be poisonous to plants and animals when in high concentrations. It is also highly reactive, and can damage the machine itself, through oxidation and decomposition. Ozone has a characteristic smell, often felt during thunderstorms. Always operate electrostatic machines (or other high-voltage devices) in ventilated areas and don't stay close to an operating machine for long periods, specially if it's a large machine.

The construction of the machines shown in this site requires some hability with tools, and this hability is the insurance against accidents. Be careful and patient, use adequate protection, and ask more experienced persons for assistance when necessary.


If a machine is not working properly, verify:

Humidity must be low. Most electrostatic machines work well only if the relative humidity of the air is low (<70%). Try to operate it in a room with dry conditioned air, expose the machine to the Sun for some time, or use a hair dryer to heat the machine slightly before operation. Be careful to not deform plastic parts with the heat. Blowing the disks from above, while turning them, is the most effective method. Sectorless machines (Holtz, Bonetti) are much more sensitive to humidity than sectored machines, and may not work at all with high humidity.

Insulators must be very good. This means long insulators made of acrylic or some other plastic material in the supports for the terminals/collectors/inductors. Acrylic always works well. PVC tubes are reasonable in dry air. Teflon is an excellent insulator, but is weak mechanically. Nylon is a poor insulator, because it slowly absorbs humidity and can become quite conductive after some time. The classical material, varnished glass, can also be used, but plastic materials are better. Bare glass is usually too hygroscopic, and works well only in very dry air. Pyrex (borosilicate) glass is much better than regular alcaline glass. Terminal support insulators in bipolar machines must have lengths at least greater than 1/2 of the desired spark length, more if possible. The same applies to distances from charged areas in disks to the machine structure. Never use wood, cloth, or similar materials for insulation, as they are too conductive. The insulators must be absolutely clean. Clean the disks, and other insulators with alcohol or similar product, removing any dirt, grease, etc. from the surfaces. The application of some wax (left by some furniture maintenance products) may improve the insulation. Leyden jars must also obey the rule of minimum distance between the plates greater than 1/2 of the desired spark length (when used in pairs), and must be very clean and dry. Some insulation problems are easy to spot in the dark, due to sparking over the insufficient spaces, or corona formation.

No points or sharp corners shall exist in the charged areas, of in grounded structures close to them. Keep all the surfaces of these areas as smooth as possible. If you used screws to fix something there, cover them with something, as plastic beads or keep their heads inside holes. Do the same to points or wire ends. Sectors in the disks must have smooth borders, and be glued perfectly flat in the disks. Clean the conductors to avoid points caused by dust particles. Spheres shall have 1 cm of radius for each 30 kV of voltage on them. Cylindric conductors may have much smaller radius, as most of the charge escapes to the ends, that shall be spheres, or at least closed rings.

Do not varnish terminal balls. Sparks will perforate the varnish easily, but will leave irregular surfaces that produce corona and impede the formation of long sparks.

The distance between inductors and active areas of disks where induction take place must be small, but not too small. Use disks and insulators as thin as possible for mechanical stability. 2.5 mm acrylic plates and spacings of a few mm are adequate for machines up to several tens of cm in size. Small spacing between inducting and inducted surfaces improves the charge generation by induction (influence), but up to a limit. After the limit air breakdown occurs, and there is no further gain. The losses may increase and the efficiency decreases. Large machines shall have more spacing. See [p74] and [p76].

Self-exciting machines may not start if there is imperfect contact in the brush circuits, usually due to brushes not touching the sectors in the disks, broken brushes, or bad contacts between the two sides of neutralizer rods. Machines with fixed inductor plates connected to brushes, as the Voss, Wehrsen, and Lebiez machines, also don't start if the contacts between brushes and inductors are not good.

A trick to obtain long sparks is to "excite" them. I know three methods, based in the same idea: a) Glue a small (3-9 mm) steel ball to one terminal, separated from it by a short (1-3 mm) insulating tube. This easily increases the spark length when the positive terminal is at the side with the small ball. A small ball directly attached to a larger ball works too, but not so effectively. b) While turning the machine, move a piece of paper between the terminals, close to the positive side. Also works with other objects, better if insulated, positioned close to the terminals. A spark from a terminal to the object starts a spark between the terminals. c) Suspend a small steel ball glued to an insulating thread from a fixed support above, so that it can swing between the terminals. You will see abnormally long sparks hitting the small ball and proceeding to the other terminal when the small ball approaches the positive terminal. To see what is the maximum spark length that you can obtain, operate the machine without Leyden jars, and listen to sounds of weak sparks at the terminals (you can see them in the dark). Leyden jars can only intensify them (and maybe increase a little their length).

Return to Electrostatic machines
By Antonio Carlos M. de Queiroz
Created: 1996.
Last update: 1 April 2004

Lamento informar que o Prof. Antonio Carlos Moreirão de Queiroz faleceu há algum tempo.
Sei que esta página é visitada constantemente. Assim, gostaria de saber se temos algum visitante (interessado) que seja da UFRJ. Se for, por favor, envie um e-mail para
Comento que é impressionante ver o que Moreirão foi capaz de fazer. Ele não só projetou os circuitos, mas também fez todo o trabalho de marceneiro (melhor que muitos que já vi e eram profissionais).
Segundo Moreirão contou em uma palestra, ele só levou choque uma vez. Sem querer encostou o dedo médio em um capacitor com alta tensão que se descarregou através do dedo. A corrente ao passar por uma das articulações a danificou e doía sempre que dobrava esse dedo. Mas, segundo ele, já tinha acostumado.

E. Watanabe (ELEPOT)