How to make an electrostatic generator

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HOW
DO
ELECTROSTATIC
GENERATOR

S. A. Khoroshavin

If you introduce a charged ball into the inner metal sphere on a handle made of insulating material, and touch the ball to the inner surface of the sphere, then it separates its ball from the sphere. By repeating this operation many times, you can give the sphere a very large spharid.

Of course, it would take a lot of time and labor to charge the sphere to a high potential in this way. But you can do something better - arrange a transformer of electric charges, for example in the form of a tape glued into a ring. The tape is moving. In one place it receives electrical charges, and in another it gives them to the sphere.

Such an electrical generator was built in 1931 by Vande Graaff. It is arranged as follows (Fig. 1). A tape 3 glued into a ring is stretched between two rollers 1 and 2. It moves and is charged from the third on the brush 4. Electric charges, together with the tape, move upward and end up inside a large metal ball 5. Here the tape touches the brush 6. Electric charges are always placed on the surface of the conductor. Therefore, charges from the tape pass through the brush to the surface of the ball. The tape moves continuously, and therefore returns a large number of charges on the surface of the ball electrode.

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In 1936, such an electrostatic generator was built at the Physico-Technical Institute of the Ukrainian SSR Academy of Sciences in Kharkov. A 10 m diameter ball electrode was installed on a 10 m high support column. Ten belt conveyors charged the ball electrode to a potential of 4 million volts. The electric field created by this generator was used to accelerate charged particles that bombarded atomic nuclei.

You can build a working model of a high-directional electrostatic generator yourself. For example, the one we built in our school physics circle (see photo on page 37). This model allows you to receive a direction of up to 80 thousand volts, and it is designed as follows.

On the basis of 1, the common 2 (Fig. 2) is strengthened. The clip is intended for fastening the support column 3 and the lower roller 4. The support column is rolled up from sheet organic glass. Ours has an outer diameter of 50 mm and a height of 370 mm. In cage 2, roller 4 with a diameter of 17 mm and a width of 26 mm rotates on an axis. To prevent the tape from slipping off the roller, it has low beads. The upper roller 5 is designed in the same way. The distance between the rollers is 320 mm. The rollers are fitted with a ring made of nylon tape 6, 23 mm wide. To connect the ends of the nylon tape, they must be folded together and welded with a heated soldering iron.

The axis of the upper roller can be fixed directly to the support column. But it is better to provide a device 7 that would allow the axis of the upper roller to move along the column, thereby adding the required belt tension.

In our model, charges appear on the tape as a result of the friction of the nylon tape on the polystyrene plate 8. It is pressed against the tape by a flat spring. At the top of the column there is a brass brush 9, which touches the tape and is connected to a ball conductor 10. It has a diameter of 100 mm. We took the ball conductor from a school set on electrostatics. If you put the ball on the column (it has a hole with a diameter of 50 mm), then the upper roller and brush

Fig. 1. Principal diagram of an electrostatic generator.

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ka 9 will be inside the ball. The electrode ball is connected to the brush with a bolt and secured by ball 11, which has a threaded hole.

The lower roller, and therefore the tape, can be turned by hand. But we use an asynchronous single-digit electric motor from an electric player 12 and connected it to the lower roller with a clutch 13.

The electric motor is turned on, the tape runs along the rollers. Along the way, from friction with brush 8, it picked up and deposited electric charges on the ball conductor. The more the tape has to rotate, the higher the potential of the ball will be. But there is a limit here. The fact is that at high electric field strength near the ball, the impact becomes electrically conductive, and charges begin to drain from the ball. The ball’s potential stops growing.

We can calculate to what potential we can charge the ball. The field voltage on the surface of the ball is calculated by the formula $E = \frac{V}{R}$, where $V$ is the potential of the ball, and $R$ is its radius. If $R = 5$ cm, and $E$ for exposure at atmospheric pressure is equal to 30 kV/cm, then the ball can be charged up to 150 thousand volts.

This calculation is correct only for an ideal smooth ball, located very far from all other conducting objects. Therefore, the voltage created by our generator will be lower than the calculated one.

If you manage to make a larger ball, then you can charge it to a higher potential. Above only means that the ball should be very smooth and even.

Now do some experiments with the model.

First of all, determine the sign of the ball’s charge. How to do this is clear. You just have to remember that an ebonite rod rubbed against fur is electrified negatively, and a glass rod rubbed against silk is positively electrified.

Determine how many full revolutions the tape must make in order for the high-voltage electrode to be fully charged. To do this, place a clearly visible mark on the tape and count how many full revolutions the tape makes until the electrode potential increases to its maximum value. Please note that the “generator performance” depends significantly on the position of the upper brush.

If the brush is positioned so that it is close to the place where the tape enters the ball, then the charges from the tape will not be completely removed, and you will have to wait a long time until the high-voltage electrode is charged. By changing the position of the upper brush, find one in which the high-voltage electrode will be fully charged at a minimum number of revolutions of the belt.

Our model uses nylon tape. And Van de Graaff used silk. In the electrostatic generator of the Kharkov Institute, the charging tape was made of rubberized fabric. Try making a charging tape from a different material.

Our charging brush is made from a polystyrene plate; it can be made from limited stack or copper. If you use a wire brush, ground it. The generator begins to work better. Why?

Don’t despair if you fail at first. Remember that even the best existing electrostatic generators produce voltages that are only a third of what they are designed for. All the reasons for this are not yet clear. So there is something to think about and something to invent.

For those who want to learn more about electrostatic generators, we recommend reading the book “Electrostatic accelerators of charged particles”, Gosatomizdat, 1963.